Figure 2.  Four cross sections show the distribution of LVZs in north Tibet.  All sections are plotted with absolute Vs values.

Contacts:  Xuan-Ce Wang, Zheng-Xiang Li
Funded by:  CCFS ARC ECSTAR funds

Zircon and baddeleyite fingerprint platinum, nickel and copper mineralisation processes at Noril’sk (Russia) 

The ultramafic–mafic Noril’sk-1 intrusion in Polar Siberia (Russia; Fig. 1) hosts one of the world’s major platinum group-element (PGE)-Cu/Ni sulfide deposits.  Despite years of study there is still ongoing debate about the origin of the Noril’sk-type intrusions, especially about the sources of the silicate magmas and ore metals for individual lithological units.  However, it is generally accepted that the mantle-derived ultramafic–mafic magmas and PGE–Ni/Cu deposits of the Noril’sk-Talnakh region in Russia are closely linked, implying that juvenile mantle-derived materials are intrinsic to their petrogenesis.  It is also commonly assumed that the ‘Noril’sk-type’ intrusions are genetically linked to the 250 Ma Siberian flood-basalt volcanism. 

Figure 1.  (A) Schematic map of Siberia, Russia and (B) location of Noril’sk-1 intrusion relative to the Siberian flood-basalt formation (CCFS Publication #215).

Nd-Sr-Os-Pb isotopic studies of the main units of the economic Noril’sk-type intrusions and their ores have contributed to a better understanding of the origin of Noril’sk-type intrusive hosts and associated ores.  The Os- and Pb-isotope compositions of the PGE–Cu/Ni sulfide ores preserve mantle-like values, and it has been suggested recently that staged chambers played an important role in the evolution of the Noril’sk-type deposits.  Whole-rock Nd-isotope data represent the homogenised end product of these processes; they provide cumulative information and do not tell much about magma sources during the evolution of the ultramafic-mafic magmas.  The wide variation of Hf-isotope values in zircon in rocks with homogeneous whole-rock Nd-isotope data has been documented previously (see GEMOC Publications #251, #491), demonstrating that the zircon data provide superior constraints for end-member mixing components.  This emphasises the usefulness of in situ analytical techniques, where the Hf-isotope composition of the zircon records the evolution of the magma chamber, with input of magmas from different sources, and/or progressive contamination by country rocks. 
 

Figure 2.  Examples of zircon in situ in thin sections (A, B; in back-scattered electrons) and mineral separates (C, D; in transmitted light) from rocks of the Noril’sk-1 intrusion (GEMOC Publication #720).  A, B, Assemblages of zircon and rock-forming minerals from olivine gabbro and taxitic pyroxene leucogabbro. Zr, zircon; Cpx, clinopyroxene; Pl, plagioclase.  C, D, Inner structure of polyphase grains, composed of “cores” (Zr1 and Zr2) and “rims” (Zr3 and Zr4).

However, there are few such data on zircon from the Noril’sk-type intrusions.  To fill this gap we have explored the isotope systematics of Hf in zircon (Fig. 2) and baddeleyite from variously mineralised rocks of the economic Noril’sk-1 intrusion (CCFS Publication #215).  In-situ Hf-isotope data of zircon and baddeleyite, combined with whole-rock Nd-isotope results, identify three distinct clusters of Hf-Nd isotope values (Fig. 3) typical of different lithological units (e.g. unmineralised gabbroic rocks, mineralised ultramafic and taxitic-textured rocks with disseminated PGE-Cu-Ni sulfide ores, and gabbro-diorite).  These groupings suggest the interaction of three distinct magma sources during the protracted evolution of the Noril’sk-1 intrusion: (1) a juvenile source equivalent to the Depleted Mantle, (2) a subcontinental lithospheric mantle source and (3) a minor crustal component.  It appears that the Hf-isotope composition of zircon and the Nd-isotope compositions of unmineralised lithologies were decoupled.  These data also suggest that a prolonged period for concentration of the ore components in staged chambers during this interaction is a key factor for the formation of economic deposits, and that the zircons can fingerprint this process as a guide to exploration.

Figure 3.  Plot of zircon εHf(T) versus whole-rock εNd(T), identifying three distinct clusters for the Noril’sk-1 intrusion.  Mantle and crustal arrays according to Vervoort et al. (Geochim. Cosmochim Acta, 63, 1999). 

The zircon and baddeleyite from Noril’sk-1 show isotopic and geochemical features that are not usually expected for mafic and ultramafic rocks.  Models for the origin of the Siberian flood basalts and their relationships to the Norilsk-type intrusions should be further investigated in light of new Hf-Nd isotopic constraints, which indicate a complex geological history for the economic ultramafic-mafic intrusions of the Noril’sk region.

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth Architecture and Fluid Fluxes.

Contacts:  Elena Belousova, Bill Griffin, Sue O’Reilly, Norman Pearson
Funded by:  ARC Future Fellowship, Macquarie University contribution to ARC FT, CCFS Foundation Project 1 (TARDIS)

Giant porphyry copper deposits on the roof of the world flow from deep wet crust 

The Gangdese porphyry copper belt in Tibet, the roof of the world, is the richest porphyry copper system known to have developed in a continental collision zone.  Five porphyry copper – molybdenum (Cu-Mo) deposits are currently being mined in this province, accounting for an estimated 18 million tons (Mt) of contained copper-10.5 Mt of this in the giant Jiama deposit alone (Fig. 1).  Seven additional prospects are under active exploration in the region, and it is clear that the potential of the belt has not been fully unlocked. 

Figure 1.  The giant Jiama porphyry Cu deposit in Tibet.  Photo taken from the elevation of 5000 m.

These porphyry Cu deposits developed in the Miocene, significantly post-dating the collision of India with Asia, so the most common model of porphyry copper genesis in association with oceanic subduction cannot be applied here.  One of the most striking features of the metalliferous deposits is their close association with adakite-like intrusions, felsic igneous rocks with high Sr and Eu contents, and high Sr/Y ratios.  These intrusions are widely attributed to dehydration melting (in the absence of free water) of garnet-amphibolite and/or eclogite facies rocks in the thickened crust below Tibet.  However, the associated mineralised porphyries contain more than 9% water by weight, and cannot be derived through any simple evolution of melts originating from anhydrous high-grade metamorphic parent rocks.  This presents a dilemma in explaining the development of this important endowed province. 

To resolve this dilemma, we are looking at the wider regional context of these porphyries to produce an internally consistent model that can account for their key characteristics, and give a key to developing strategies for further exploration of the Gangdese belt.

In addition to the water problem, dehydration melting of amphibolites and/or eclogites does not produce melts with the chemical characteristics that match the inferred ore-forming magmas.  Such melting would consume the hornblende in the source rocks before plagioclase began to melt, and would not produce the adakite-like melts with high Sr and Eu contents and high Sr/Y.  However, a high H₂O content in the starting material (and the resultant partial melt) stabilises hornblende to higher temperature and lowers the melting point of plagioclase.  If significant amounts of water were added to the mafic lower crust, the chemical problems thus would be solved -- but how can large volumes of water be fluxed up into the lower crust in the Miocene, long after the cessation of oceanic subduction beneath this region?

We suggest that the coeval ultrapotassic lava flows of South Tibet provide the missing link in this puzzle.  These mafic rocks are notably water-rich.  Furthermore, the solubility of water in such ultrapotassic melts increases with increasing pressure, and therefore as the melts ascend, they release free water into the surrounding wall-rocks.  Although they are rare at the surface, significantly larger volumes of ultrapotassic rocks are believed to occur at depth beneath the Tibetan lower crust, and underplating and crystallisation of these mantle-derived rocks may have provided the fluid required to catalyse the development of the fertile Tibetan porphyries in the Miocene.

This conceptual model is illustrated in Figure 2.  We suggest that metasomatism of the lithospheric mantle and Jurassic underplating of arc melts below Tibet were essential to pre-condition the region for the subsequent development of the mineralised porphyries.  In the Miocene, lithospheric thinning – probably due to convective removal of lithosphere during Himalayan orogenesis – caused melting of the water-rich metasomatised mantle domain, producing hydrous ultrapotassic-potassic melts.  In the extensional conditions that prevailed across the western portion of the southern Lhasa subterrane, these mantle-derived magmas were able to ascend into the upper crust – and locally erupt.  In the more compressive contemporary regime to the east of this subterrane however, the ultrapotassic-potassic magmas ponded in the lower crust.  Release of water from the crystallising magmas then triggered melting of the overlying Jurassic arc material to produce the hydrous and adakite-like intrusions and associated porphyry Cu deposits.

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Yong-Jun Lu, Marco L. Fiorentini, Cam McCuaig 
Funded by:  CCFS foundation project 2a, 9, ARC CCFS ECSTAR funds

Figure 2.  A conceptual model for the genesis of the hydrous fertile Tibetan porphyries, as outlined in the text.

Modelling multi-phase reactive flow in the mantle: a bottom-up approach 

The physico-chemical processes that govern the evolution of continental lithosphere and its interaction with the sublithospheric upper mantle occur over very different spatial and temporal scales.  The details of the interactions between these different scales and the resulting integrated macroscopic effects on the evolution of plates are still poorly understood.  In order to understand these multi-scale (geological) processes, we need to be able to accurately simulate/model all the processes involved in natural multi-phase multi-component reactive systems.  Unfortunately, although several groups are making progress towards this end, most modelling platforms available to the geodynamic community do not really deal with all these problems in an internally-consistent manner.  It also can be difficult to scale them to a wide range of problems (e.g. from melting in cm-scale veins to mantle convection). 

To bypass these limitations we take advantage of a novel numerical framework known as the Multi-scale Finite Element Method (MsFEM).  The main idea is to capture the small-scale details of a problem and transfer them to the macro-scale through consistent and robust coupling of the micro- and macro-problems (back and forth).  In particular, the MsFEM replaces the basis functions used in the traditional FEM, with a new set of micro-problem-dependent functions.  This development provides a new numerical platform directly applicable to modelling geological phenomena such as the generation, migration and emplacement of magmas, chemical entrainment, transport of geochemical species, deposition mechanisms of sulfide liquids, the formation of ore bodies in hydrothermal systems, etc.  Importantly, these “small-scale” processes are consistently coupled, in our numerical framework, to the large-scale driving tectonic processes, and that coupling allows us study the complex interactions between these different scales.  So far, the MsFEM has been successfully applied and tested in fields such as composite materials, petroleum reservoirs and groundwater transport, but not yet in the large-scale analysis of the lithosphere-asthenosphere system. 

Multi-phase reactive flow is governed by a multitude of balance and constitutive equations that need to be solved accurately with numerical methods.  Ones of particular importance are the so-called “advection-diffusion-reaction” and the “momentum” equations, governing heat transfer and momentum transfer, respectively.  Stability and accuracy analyses for these equations are well known and have been the subject of thousands of papers in the numerical community.  Although there are many ways to solve these equations, not all of them are appropriate to a multi-scale approach.  Moreover, the robustness, accuracy, and efficiency of many of these algorithms are still debatable when they are applied to geodynamic processes at different scales.  This is due partly to poorly documented tests and benchmarks and partly to the geodynamics community’s lack of any broad interest in multi-phase multi-component reactive flow.

As part of a larger CCFS Foundation Project 4, we are currently developing/testing a number of individual algorithms particularly designed to work under a multi-scale formalism for geodynamic problems.  

In particular, we have developed a new Lagrangian-Eulerian algorithm for advection-diffusion-reaction, which is proving to have remarkable accuracy, scalability, and generality.  This finite-element algorithm combines the best features of fixed-mesh and marker-in-cell methods into a single conservative scheme that can accurately model processes in which there are large advective and reactive components together with isothermal and non-isothermal phase changes.  We have also tested and improved existing multiscale algorithms that may be applicable to specific sub-problems.  In parallel, we have implemented an efficient and fully compressible Lagrangian-Eulerian iterative solver (based on the “Uzawa scheme”) for the momentum (and mass) equation and fast thermodynamic solvers for the computation of phase equilibria/component distributions.  The internally-consistent combination of these three main components into a single thermo-chemical-mechanical algorithm will allow us to simulate a large variety of fluid-assisted geodynamic processes.

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Beñat Oliveira, Juan Carlos Afonso
Funded by:  CCFS Foundation Project 4

Straining to transform from continent to ocean 

The Grisons, in the Eastern Alps (Switzerland) have preserved an ocean-continent transition; numerous studies during the last decade provide an excellent structural framework.  This area represents a section across a zone of exhumed continental mantle.  Sub-continental spinel peridotite is exposed close to the continent (Totalp area) while the mantle further from the continent was infiltrated by melt and equilibrated in the plagioclase stability field (Platta area) (Fig. 1).  The sequence includes highly deformed peridotites (ultramylonites) that allow us to study the evolution of deformation within an ocean-continent transition.  A detailed study of the peridotite textures and geochemistry was combined with the analysis of the crystallographic orientation of minerals using the electron backscatter diffraction method (EBSD).

Figure 1.  Schematic representation of an ocean-continent transition (after Peron-Pinvidic & Manatschal, 2009, Int.J.Earth.Sci, Muntener et al. 2010, J.Pet).

We sampled the Platta-Totalp massifs made of spinel lherzolites and harzburgites, intruded by gabbros and basaltic dykes, and partially covered by ophicarbonates.  In Totalp, blocks of ultramylonites within a cataclastically deformed domain related to mantle exhumation, and ultramylonites parallel to the foliation of amphibole-bearing peridotites, were sampled.  The ultramylonite blocks are formed by unmixed layers of olivine (ol) + orthopyroxene (opx) and clinopyroxene (cpx) + amphibole (Fig. 2a).  Opx and ol grains are elongated, whereas the amphibole is interstitial and replaces the clinopyroxene.  The ultramylonites have a fine-grained matrix (grain size 2-10 µm) composed of ol, opx, cpx, spinel and amphibole with rounded amphibole porphyroclasts (Fig. 2b).  Pargasitic amphibole occurs in the former whereas kaersutitic amphibole characterises the latter.  The kaersutite was related to pre-deformation metasomatism with K2O enrichment from porphyroclasts to neoblasts during localisation of deformation (Fig. 3).  The pargasite formed later than the kaersutite and was probably related to the exhumation of the mantle.  The crystallographic preferred orientations of the kaersutite within the host peridotite are inherited from the porphyroclastic amphibole with a reorientation of <001> axes, to become parallel to the foliation.  In the ultramylonitic matrix, the amphibole grains have elongated shapes and their <001> axes are parallel to the foliation, suggesting either a strong grain rotation or a dynamic recrystallisation.

Figure 2.  (a) Ultramylonite made by unmixed layers.  Layers are clinopyroxene replaced by interstitial amphibole or olivine and orthopyroxene; (b) Ultramylonite contains round porphyroclasts of amphibole within a matrix of olivine, opx, cpx, amphibole and spinel.

The clinopyroxene composition has been analysed for Totalp and Platta samples.  There is no distinct difference among the Totalp samples, but samples from Platta have lower Na₂O, suggesting melt percolation and crystallisation of plagioclase.  Some Platta samples show slightly lower Na₂O suggesting less melt infiltration, and has a more continental affinity (Figs. 1 and 3).  In porphyroclastic clinopyroxene from the host peridotite in Totalp, the activated slip is (010)[100], which is uncommon and marginal in peridotites because of using a large burger vector (Fig. 4a).  In clinopyroxene porphyroclasts <100> axes are parallel to the foliation whereas the recrystallised grains show a reorientation with <001> axes more parallel to the foliation (Fig. 4b, c).  Olivine porphyroclasts within the host peridotite show the activation of two slip systems within a single grain: (001)[100] and (100)[001].  In the fine-grained ultramylonitic matrix the dominant activated slip system is (001)[100] E-type. 

Figure 3.  Chemical composition of amphibole (TiO₂ vs. K₂O wt%) and clinopyroxene (Cr₂O₃ vs. Na₂O wt%) from Totalp and Platta.

The results obtained in 2013 show that the transition from subcontinental to infiltrated subcontinental mantle towards the ocean involved a compositional evolution and a change in deformation mechanisms.  The presence of amphiboles testifies to fluid percolation before deformation, and tracks chemical evolution of the fluid during localisation of deformation.  The amphibole and clinopyroxene crystal preferred orientations in the host peridotite are partially inherited from the primary texture, and overprinted by several mechanisms such as grain rotation, dynamic crystallisation and/or grain boundary sliding when the strain increases.

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Mary-Alix Kaczmarek, Steve Reddy

Funded by:  CCFS, Curtin University (Perth), The Swiss National Foundation (SNF), The University of Lausanne 

Figure 4.  Clinopyroxene individual grains orientation maps and lower hemisphere equal area stereographic projections of crystallographic data.  (a-b) EBSD orientation map colored for cumulative misorientation (in degrees) relative to reference orientation indicated by a red cross, pole figures of major crystallographic planes for every data point indexed on map, and plot misorientation axes for (2–10°) interval between neighbouring data points.  (c) crystal preferred orientations of recrystallised grains of clinopyroxene showing an inherited texture from the porphyroclasts.  The number of analysed grains is n=339.

Mantle dynamics model constrained by plate history 

The temporal evolution of the Earth’s internal structure is very important to geoscience research, however this time-integrated evolution cannot be observed directly by available techniques.  Mantle convection simulations are thus important tools to investigate this problem.  

Subducting slabs descending deep into the mantle will carry, and subsequently release, water: this can promote partial melting in mantle rocks, resulting in volcanoes overlying deep, dewatering subducting slabs.  Such volcanic activity can concentrate incompatible elements from Earth’s interior at the surface, so models that can predict subduction dewatering pathways are of significant value to the mineral-exploration industry.  However, plate boundaries have moved a lot in the past, obscuring the history of magmatism.  We are developing dynamic models that can reconstruct the P-T-t path of minerals in subduction zones, and also the water content of subducting lithologies.  Adopting mineral physics studies on the mantle solidus and partial melting, we will be able to calculate melting percentages and eruption rates to track the history of subduction zone volcanic activity through time, and provide quantitative and temporal maps of the generation and migration over the past few million years on a global scale. 

In our approach, we use the open-source code ASPECT; this is a highly-parallelised and modularised code that uses the adaptive mesh refinement technique to achieve much better resolution than conventional approaches, at reasonable computational costs.  It also supports temperature, pressure and composition - dependent simulation parameters, allowing us to adopt more realistic mineral physics models.  Plate tectonics is the dominant control on the mantle flow pattern, and plate reconstructions from geophysical and geological data provide crucial constraints on surface motions in the past few hundred million years.  In our mantle convection models, we use plate history as a time-dependent surface boundary condition. 

Using this approach, our model successfully recovers the position of major subducting slabs and mantle upwellings, in accordance with present day seismological observations.  Building such a model requires large amounts of computational power, and current models are running on a cluster with a few hundred CPUs, which may not be have the capacity to achieve the required resolution.  A new resource allocation from Intersect, has delivered more computing power that will allow refinements of the current resolution over the next year.

This project is part of CCFS Theme 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Siqi Zhang, Craig O’Neill
Funded by:  CCFS Foundation Project 4 

Figure 1.  Snapshot at 85.9 Ma ago shows subducting slabs and plumes reconstructed by the mantle convection simulation constrained by plate history.  In the left-hand figure, the transparent surfaces show contours of temperature; the present day coast-lines are shown in white.  In the right-hand figure, the arrows show the surface motion constrained by plate reconstruction, and present-day coastlines are shown in black.

Zircon signposts for Gold  

Multi-isotopic maps are a powerful tool for imaging lithospheric blocks of different age, and have been used in the Yilgarn Craton of Western Australia.  Such maps combine in situ zircon U-Pb and Lu-Hf isotopic analyses with whole-rock Sm-Nd data.  As the ancient lithospheric boundaries sometimes cannot be seen in modern seismic images, the isotopic mapping serves as a form of ‘paleogeophysics’ for imaging paleocraton margins through time. 

Figure 1.  Spatial distribution of gold mineralisation and terrane boundaries within the Wabigoon Subprovince, Canada.  (a) Geological map of the Wabigoon Subprovince showing terrane boundaries based on whole-rock Nd isotope data (Tomlinson et al., Precambrian Research 2004).  Different colors represent different geological units.  The inset shows the location of the study area.  (b) Zircon Hf-isotope mapping of the Wabigoon Subprovince with the revised terrane boundaries after Stott, OGS, 2011.  The contour bar shows the Epsilon Hf value of zircons studied.  The rectangles labeled 1 and 2 highlight the two gold camps in Eastern Wabigoon and the Marmion terrane, respectively.  The most economic gold mineralisation discovered to date is represented by red and grey stars. 

There is a strong spatial correlation between lithospheric boundaries and the concentration of a variety of mineral deposit types; these isotopic boundaries mark lithosphere-scale structures that control magma and fluid flux, and thus the location of large mineral systems through time.  However, the only available case study in the Archean is the Yilgarn of WA, and even that is only focused on the centre of the craton.  Therefore, we have tested this hypothesis in the Wabigoon Subprovince in the western part of the Superior Craton of Canada.

The Wabigoon Subprovince previously has been subdivided into four terranes based on whole-rock Sm-Nd isotopic data (Fig. 1a; Tomlinson et al. Prec. Res., 2004). Gold mineralisation is pervasive within the Wabigoon Subprovince, with numerous gold occurrences and prospects (Fig. 1a).  The most economic gold mineralisation, i.e. gold mines represented by red stars, clusters mainly in the Eastern Wabigoon and Marmion gold camps (rectangles 1 and 2, respectively in Fig. 1a).  However, no relationship between these important gold camps and the terrane boundaries is obvious in Figure 1a.  For example, the terrane boundary in Eastern Wabigoon was assumed to trend E-W, whereas the gold mineralisation trends N-S.  The Marmion gold camp trends NE, but is far away from the terrane boundary (Fig. 1a). 

Figure 1b shows new mapping of Hf-isotope compositions in zircon, undertaken in this study.  The warm colours represent relatively juvenile blocks with higher Epsilon (Hf) values, whereas the cold colours represent relatively ancient blocks with lower Epsilon (Hf) values.  The boundaries of these isotopic domains are interpreted as lithospheric boundaries between different terranes.  The revised terrane boundary in Figure 1b is mainly from Stott, OGS, 2011.  It can be seen that the new zircon isotopic mapping results fit well with the revised terrane boundaries (Fig. 1b).  In particular, the boundary between the Winnipeg River and Eastern Wabigoon Terranes now trends NE, consistent with the gold trend (rectangle 1 in Fig. 1b).

Figure 2.  Residual total-field magnetic image of the Marmion Terrane.  Warm and cold colors represent positive and negative magnetic anomalies, respectively.  The magnetic character shows a marked change in the Marmion terrane across the NE-trending Finlayson greenstone belt, which can be inferred as a terrane boundary.  The Hammond Reef deposit with 10 Moz Au is proximal to the inferred terrane boundary.  However, this magnetic image probably only shows features down to about 20 km depth, and therefore does not image the deep lithosphere, unlike the isotopic data.  The boundary between the Marmion and Western Wabigoon terranes is west of this map area, as indicated by the white arrow. 

This coincidence supports the hypothesis that the terrane boundaries exert important controls on the location of significant gold mineralisation.  A significant observation is that there are no gold mines within the ancient Winnipeg River Terrane, but instead the gold mines cluster within the Eastern Wabigoon Terrane, which is a juvenile block.  This spatial relationship of gold with more juvenile rocks is similar to that observed in the Yilgarn Craton of Western Australia.

The Marmion gold camp is intriguing as it occurs along a NE-trending zone within the Marmion Terrane (Fig. 1b).  There are currently not enough zircon Hf data on both sides of this gold camp to reveal the isotopic signature of the lithosphere (Fig. 1b).  However, Figure 2 shows a contrast in magnetic anomalies on both sides of this NE-trending structure within the Marmion terrane.  The Hammond Reef deposit with 10 M oz of Au is close to this structure, highlighting the potential importance of this inferred terrane boundary (Fig. 2).  

In summary, it appears that the spatial distribution of gold in the Wabigoon Subprovince is controlled by the terrane character (juvenile) and potentially their boundaries with ancient blocks (paleocraton margins), similar to the scenario in the Yilgarn Craton.  Zircon isotopic mapping is a powerful tool to image the lithospheric boundaries and thus, can become a robust pathfinder to gold deposits. 

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture.

Contacts:  Cam McCuaig, Yong-Jun Lu, Katarina Bjorkman
Funded by:  ARC ECSTAR Fellowship, CCFS Foundation Project 9

Diamond growth at the nanoscale – Mantle fluids at work

Diamondites are polycrystalline aggregates of diamond crystals with heterogeneous grain sizes and random orientation that formed in the Earth’s mantle.  They have a highly porous structure, which indicates that they precipitated from a volatile-rich medium strongly oversaturated in carbon.  Gem-sized diamonds can contain a long history of growth and dissolution resulting in complex zonation patterns (CCFS publication #168).  

In contrast, diamondites may form rapidly, presenting snapshots of diamond formation conditions that complement the information from slowly grown gem-sized diamond.

Figure 1.  Cathodoluminescence image of diamondite showing complex zoning that most likely corresponds to episodic growth from a fluid dynamically changing in composition.

New microanalytical methods have allowed diamond studies to be carried to the next level, as submicron inclusions are now accessible, thus building on a solid foundation from studies of large inclusions, which are usually in the range of 200-300 µm.  In situ analysis of diamonds and their inclusions is now able to span roughly six orders of magnitude in spatial resolution and draw critical complementary information from either end of this range.  Large inclusions in diamondites are mostly intergrown with the diamond crystals rather than included.  They record the general chemical environment of diamond formation, but can be subject to metasomatism that postdates diamond formation (CCFS publication #210).  In contrast, submicron-sized minerals included in the diamond crystals are shielded and contain the fluid from which the diamond precipitated.

To reveal more about the relationship between the diamond fluid and the growth mechanisms of the diamond crystals, we employed Transmission Electron Microscopy, enhanced with Focused-Ion Beam sample preparation, Transmission Kikuchi Diffraction Analysis and Nano-SIMS, enabling detailed analysis at the submicron scale and direct sampling of the diamonds’ parental fluids included in the stones (Jacob et al., Earth Science Reviews, in review)

In situ sampling of diamond fluids reveals a large heterogeneity in redox conditions and chemical compositions at small scale, which is not reflected in the macro-inclusion suite.  However, these fluids have compositions that correspond to the fluid end-members established by studies on fibrous diamonds; this suggests a universally important role for a limited number of basic ingredients, namely carbon, silicon, halogens and water. 

Preliminary studies (Rubanova PhD Thesis, see CCFS Postgraduates) indicate that the crystals in diamondites may have grown not only by the octahedral (spiral/dislocation) mechanism, but also by cuboid (rough, adhesive type) growth.  This suggests that other governing parameters, just beyond extreme carbon supersaturation, play an intrinsic role in their growth.  Both growth mechanisms also occur in gem-sized diamonds, and the study of diamondites may tie these together in an integrated model and help define the impurities, oxygen fugacity, and episodic nature of these deep mantle fluids.  Thus, diamondites could act as “Rosetta Stones” that can provide critical information about diamond growth and fluids in the Earth’s mantle.

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Fluid Fluxes. 

Contacts:  Dorrit Jacob, Dan Howell, Bill Griffin, Sue O’Reilly, Sandra Piazolo 
Funded by:  CCFS

The leading edge in pits: the hole story for laser ablation ICP-MS 

Conceptually, laser ablation is a simple process; material is removed from a solid (or liquid) sample by irradiation with a laser beam.  In this sense laser ablation (LA) can be thought of as the micro-scale equivalent of taking a sledgehammer to an outcrop.  However, the real power of laser ablation is that the sampled material can be directly transported into a plasma-source mass spectrometer, allowing rapid measurement of elemental abundances or isotope ratios.  Researchers at Macquarie were among the pioneers in developing new analytical methods using laser ablation ICP-MS, and established early on, that the quantification of elemental abundances or isotope ratios is rather more complicated than a fantasy-world version of ‘star-wars’ mass spectrometry.  As part of the development of leading edge analytical methods involving new laser hardware, CCFS researchers are revisiting the basics of how a laser beam interacts with minerals.

Figure 1.  The plot of normalised ²⁰⁶Pb/²³⁸U (measured ratio relative to true ratio to remove the effect of age) vs time for selected reference zircons provides a comparison of 193 nm and 213 nm laser systems.

The ablation behaviour of different minerals for specific wavelengths of light is well-documented in the literature.  

The degree of coupling between the laser beam and different minerals is indicated by the amount of material removed from the hole; in quantitative elemental analysis this is corrected by use of an independently determined internal standard (an element of known concentration).  A more significant issue is the progressive change in the ratios of measured signals of certain element pairs or isotope pairs as the laser drills into the sample.  This is referred to as ‘down-hole fractionation’.  Elemental fractionation occurs because preferential ablation of some elements (low-volatility elements) in the sample results in non-stoichiometric sampling and analysis.  Perhaps the best-documented example of laser-induced elemental fractionation is the changes in measured Pb/U and Pb/Th ratios in zircon as drilling proceeds.

Figure 2.  Back-scatter electron images of different laser spot sizes in GJ zircon.  Note the marked development of the ‘crown’ of melt material on each pit, but especially the 15 µm diameter pit.

Down-hole laser-induced fractionation is one of the largest contributions to the uncertainty budget of LA-ICP-MS measurements of trace elements and isotope ratios.  Efforts to improve the accuracy of analyses and to reduce the uncertainties associated with laser-induced fractionation have involved developments in both laser hardware and data-reduction software.  Advances in cell design have improved the quantitative transport of material from the ablation site to the ICP, giving increased sensitivity and reduced fractionation.  Rastering the laser, or using short acquisition times, are also commonly used to minimise downhole fractionation, but these compromise spatial resolution and depth information.  Linear and exponential down-hole models are used in many data-reduction software packages and reflect the basic fractionation response for a wide range of laser specifications and operating conditions.  In most models the fundamental assumption is that matrix-matched standards and samples ablate similarly with consistent time-depth relationships.  However, further software advances are limited by our current understanding of the fundamental processes of ablation.  

Figure 3.  Plot of ²³⁸U signal intensity (counts per second) versus time for 4 different spot sizes (15, 40, 65, 85 µm) in GJ zircon.

An investigation of the ablation characteristics of common minerals is part of the CCFS Foundation Project ‘Frontiers in integrated laser-sampled trace-element and isotopic geoanalysis’.  The aims are to assess the effect of laser wavelength, pulse width, spot size and fluence, in conjunction with laser-cell design and gas composition and flow rate.  In the first phase of the study the focus has been on the ablation of zircon to establish the optimum set of hardware parameters and operating conditions to maximise spatial resolution, and to minimise ablation rate and U-Pb fractionation.  

A distinctive down-hole fractionation of U and Pb is observed in ablation of zircon using 213 nm and 193 nm wavelength lasers (Fig. 1).  The change in ratio from both systems is greatest in the first 150-200 laser pulses (20 to 30 seconds of ablation) but for the chosen operating parameters the 193 nm data show little further change with time, whereas the ratios produced by the 213 nm laser continue to increase with time.

Figure 4.  Plot of measured ²⁰⁶Pb/²³⁸U versus time for 4 different spot sizes (15, 40, 65, 85 µm) in GJ zircon.

Laser fluence and spot size have significant effects on ablation rate and fractionation.  Ablation rate increases with fluence but more slowly for larger spot sizes.  A series of images taken after a set number of laser pulses shows the development of a crater wall that forms from the initial laser pulse and grows in height with time by the addition of melt material extruded from the pit (Fig. 2).  The development of this wall is more pronounced with small spot size and low fluence.  There is also a significant decay in signal intensity with time, especially with small spot sizes, indicating that progressively less material is being removed from the hole and transported to the ICP (Fig. 3).  The net effect is a greater fractionation of ²⁰⁶Pb/²³⁸U with small spot size due to the retention of melt and condensed material in and around the laser pit (Fig. 4).

One of the main drivers for investigating the fundamentals of the ablation process is to get higher spatial resolution but without compromising data quality.  The results of the first stage of the study have produced a better understanding of how zircon ablates and the fractionation of U and Pb using a nano-second pulse laser system.  This will form the benchmark for the next exciting experimental phase using the femto-second laser system.

This project is a Technology Development initiative in CCFS, contributes across all Themes (Early Earth, Earth Evolution and Earth Today), and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Norman Pearson, Bill Griffin, Sue O’Reilly, Will Powell
Funded by:  CCFS Foundation Project ‘Frontiers in integrated laser-sampled trace-element and isotopic geoanalysis’

Are ancient zircon ages real, or due to ancient element movement? 

Zircons from the Tula Mountains, Napier Complex (East Antarctica), attracted scientific attention because they come from some of the oldest rocks on Earth (Black et al., CMP, 1986), which experienced high-temperature metamorphism at 

2.8 Ga and ultra-high-temperature metamorphism at 2.5 Ga, with temperatures estimated at over 1100 ºC (Hokada et al., Prec. Res., 2005), some of the highest temperatures recorded in Earth’s crust.  Isotopic disturbance of zircons >ca 3.4 Ga old from the Napier Complex has been recognised for many years (Williams et al., CMP, 1984).  To investigate this phenomenon further, we analysed zircon grains from three samples: one orthogneiss (from Gage Ridge) and two paragneisses (one each from Mount Sones and Dallwitz Nunatak).  The analysis included U-Pb geochronology, oxygen isotopes, REEs, Raman spectroscopy and scanning ion imaging.  All samples yielded reversely discordant data; the zircons from Mount Sones are significantly younger than those from the other two samples with an age range from 3.0 Ga to 2.5 Ga.  Detrital zircons from Dallwitz Nunatak yield ages between 3.5 Ga and 2.5 Ga.  The Gage Ridge sample contains four age groups with concordant data between 3.6 Ga and 3.3 Ga (Kusiak et al., 2013a) and a discrete population of ca 3.8 Ga zircon with reversely discordant data.  The REE distribution in zircon from all three samples is generally similar, with steep MREE to LREE trends, consistent with an igneous origin.  However, such patterns are also present in metamorphic zircon that grew in the absence of garnet.  The sample from Mount Sones contains garnet that grew during ultra-high temperature (UHT) metamorphism (ca 2.5 Ga), and, like Kelly and Harley (CMP, 2005), we suggest that 2.8 Ga zircon grew during metamorphism under high-temperature and low-pressure conditions in the absence of garnet. 

Figure 1.  Raman spectroscopy maps of the two analysed grains: A. Gage Ridge, B. Dallwitz Nunatak.  Numbers corresponding to grain A: 1. A medium radiation damaged zone;  2. Amorphous area; 3. Glassy area (after Kusiak et al., 2013 a, b; modified).

To test for U/Pb disturbance during metamorphism we analysed oxygen isotopes in zircon from all three samples.  Igneous zircons from Gage Ridge have lower δ18O values (4.7-6.8‰) than zircons from the paragneisses.  However, the Dallwitz Nunatak zircons give a scattering of ages, and δ¹⁸O varies from 6.8-8.0‰ (average δ¹⁸O – 7.4‰), as might be expected for detrital zircons.  Low scatter in δ18O in zircons from the Mount Sones paragneiss, together with high values (7.2-8.9‰, average δ¹⁸O – 8.1‰), are consistent with growth during the earlier metamorphism at 2.8 Ga. 

A novel high-resolution ion-imaging technique using the Cameca 1280 SIMS was used to generate maps of Pb-isotopic “age” for selected zircons.  These record patchy variations in the isotopic ratios that result in spurious ages, including some that are Hadean (>4.0 Ga).  Raman spectroscopy was used to determine the degree of metamictisation in the same zircon domains previously imaged by SIMS (CCFS Publications #407, 429).  From our results (Fig. 1), it is evident that there is no correlation and that Pb is patchily distributed regardless of the degree of crystallinity. 

From these results we conclude that there is no evidence that oxygen isotopes or REE in zircon were disturbed during the UHT metamorphism at 2.5 Ga.  The reverse discordance in zircon from Gage Ridge, Mount Sones and Dallwitz Nunatak is related to ancient mobilisation of Pb and the most likely cause was polymetamorphism under dry conditions - two metamorphic events, one low-pressure at ca 2.8 Ga and a UHT event at ca 2.5 Ga. 

This project is part of CCFS Theme 1, Early Earth, and contributes to understanding Earth’s Architecture.

Contacts:  Monika Kusiak, Simon Wilde
Funded by:  EU-FP7, Marie Curie grant

The enigma of chromitites in the upper mantle resolved

The mantle sections and the crust-mantle transition zones of many ophiolites contain bodies of chromitite, almost monomineralic concentrations of Cr- and Al-rich spinel.  They are important economically as a source of chromium, and scientifically because these rocks encapsulate information on the nature of ancient upper mantle, young oceanic mantle, mantle melt formation and percolation processes, and on large-scale geodynamic emplacement mechanisms. 

Figure 1.  Backscattered electron images of Ru-Os sulfides and alloys from distinct microstructural positions in the Dobromirtsi chromitites.  A and B: Primary (magmatic) laurite included in unaltered chromite.  C and D: Partially corroded (secondary) laurite and Ru alloys in open fractures in chromite.  E–G: Secondary assemblages showing distinct stages of alteration/replacement of laurite in ferrian chromite rims.  H: Irregularly zoned laurite grain in the contact between a ferrian chromite rim and the interstitial matrix (for more details see CCFS Publication #42).

During the last four decades there has been a hot debate about the origin of these type of chromitites, yielding a huge number of studies with diverse and passionately defended hypotheses.  Forensic-type studies of individual chromitite bodies in isolation have led to a profusion of genetic models indicating that chromitites from different localities formed by different mechanisms.  These models can be grouped into three broad categories:  (1) Fractional crystallisation of basaltic melts in magma chambers or conduits in the upper mantle or around the crust-mantle boundary.  Variants of this model include changing the composition of the melt by an external process such as melt-rock reaction or assimilation of pre-existing mafic rocks.  (2) Mixing or mingling of melts within dunite channels.  (3) Separation of volatile-rich fluid phases with an important role for oxygen fugacity.

Among the questions that these models have been not able to resolve satisfactorily are: (1) how can chromium, a minor element in a peridotite-derived basaltic melt, be concentrated to produce large monomineralic bodies of chromite?; (2) how does a chromitite body start to nucleate and grow?; (3) what factors control the size of individual chromitite bodies? 

A major breakthrough in our understanding of the genesis of the chromitites has come during the development of this research project, through the in situ analysis of Os isotopes by laser-ablation MC-ICPMS analysis of single tiny PGMs (and larger base-metal sulfides), which are usually found as inclusions in the chromitites.  The Os-isotope composition of individual PGMs can now be directly related to their microstructural setting, internal structure and bulk composition (CCFS Publications #198, 334, 348, 349). 

Figure 2.  Formation of ophiolitic chromitites according to the new mechanism of crystallisation proposed in this work.  A network of dunite channels drains melts ascending from different sources (marked with different colours) in the heterogeneous deeper mantle (modified from Kelemen, 2004).  (1) Ascending melts move mainly by porous percolation through a melt-film network; in the zone of intersection between dunite channels mixing of basaltic melts with different SiO2 promotes precipitation of chromite.  The image is not to scale, as grains of olivine are generally from < 0.5 mm to a few centimeters.  (2) Focused flow of melt produces melt-filled channels in dunite allowing the chemical isolation and rapid ascent of the melts.  (3) Chemically isolated melts drained through different channels in the dunite meet and mix to produce hybrid melts able to precipitate volumes of chromitite.  The size of the dunite network and the chromitite bodies can vary from a few metres to many kilometers.

The observation that the primary Os-rich PGMs in the chromitites are highly heterogeneous in terms of Os-isotope composition, even at the scale of inclusions in an individual chromite grain, suggests that the chromitite formed by the mixing/mingling of multiple basaltic melts that sampled different mantle sources, and have undergone variable degrees of fractionation.  In the exposed mantle sections of several ophiolites, meso- and micro-structures of chromite suggest that the intersecting melt-filled dunitic channels were draining ascending basaltic melts in a suprasubduction mantle.  This is the ideal framework to produce chromitites by mixing of basaltic melts of different provenance and degrees of fractionation.  In our new model, the formation of a chromitite body, its size and the type of chromitite microstructures, are inferred to reflect focused melt flow within these high-porosity and high-permeability networks of channels at different melt/rock ratios, and different temperatures of melts and host peridotite.  Melt/rock ratio controls the amount of crystallising chromite and the chromitite microstructures.  Moreover, the observation that many chromitites cross-cut their host peridotites suggest that mixing of melts at the intersection between melt-filled dunite channels is not the unique “physical” trap inducing crystallisation of chromitites.  Also, in relatively cool mantle domains, chromitites may form in hydrofractures penetrating peridotites and infiltrated by interstitial chromite-bearing fluids.  During these events of melt infiltration/injection, PGMs such as laurite or Os-Ir alloys already present in the mantle peridotite, or produced by the breakdown of PGE-bearing sulfides during the melt-rock reactions, can be also incorporated in a solid or crystal/melt mix into the parental melts of chromitites.  This may explain why chromitites hosted in the mantle section of young ophiolites may contain PGM with Os-model ages > 2.5 Ga old. 

On the other hand, the fact that some PGM grains in the chromitites are found along healed fractures cutting the primary chromite suggests that other populations of PGMs may have precipitated from metasomatic fluid/melts that infiltrated existing chromitites.  The formation of this new generation of PGM can also explain the Os-isotopic heterogenity.  The opening of the Os-isotopic system in PGMs associated with secondary chromite (produced by alteration of the magmatic chromite) also suggests that some PGMs that precipitated at mantle conditions, can be partly reacted and recrystallised during the infiltration of post-magmatic fluids.  The recrystallisation of pre-existing PGM during polyphase metamorphism or “recycling” of the chromitite into deeper mantle levels can also explain the presence of micrometric PGMs with distinctly different Os-isotope compositions. 

The results obtained in this project highlight the complex history that can affect the chromitites from their time of formation in the upper mantle, up to their surface emplacement.  Chromitites may form at low pressures in the shallow mantle, but they can be later transported deep into the mantle (> 400 km) by subduction processes, and possibly integrated into the convecting mantle. 

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth Architecture and Fluid Fluxes.

Contacts:  José-María González-Jiminéz, Bill Griffin, Sue O’Reilly, Norman Pearson
Funded by:  ARC ECSTAR Fellowship, ARC CoE CCFS Foundation Project 1 (TARDIS)

Transformation on Earth: the transition from ancient to modern Earth  

Around 2.35 billion years ago (Ga), Earth’s atmosphere and near-surface hydrosphere underwent perhaps the most fundamental revolution in Earth history since the Giant Moon-Forming Impact at 4.5 Ga: the rise in the concentration of atmospheric oxygen.  This was a revolutionary event because of the effects it had on ocean chemistry and weathering: the oceans were scrubbed of iron (and soon after, manganese) and rusting of ferrous minerals in the continents released sulfate (and other elements) to the oceans through oxidative weathering.  The resultant change in ocean chemistry is reflected in the rock record, with the disappearance of banded iron formations and the appearance of sulfates and phosphorites from 2.45-2.2 Ga.  Eventually, the rise of atmospheric oxygen led to the evolutionary development (or at least, widespread flourishing) of eukaryotes, a more complex life form that depends on the extra energy available from oxygen.  The rise of atmospheric oxygen coincided with climatic cooling to the extent that glacial deposits appear for the first time on many continents.

Figure 1.  Stratigraphic section through the transition from the uppermost Boolgeeda Iron Formation of the Hamersley Group to the lowermost Kungarra Formation of the Turee Creek Group, here interpreted to occur at the base of the first non-magnetic unit.  C1-C8 refers to chert layers from base to top across the section. (From Van Kranendonk et al., submitted).

Only a few rock successions on Earth are preserved from across this period of Earth history, and almost all of those are plagued by an incomplete record with one or more unconformities, which are horizons of non-deposition, and/or erosion, where the history of change was not recorded, or preserved.  However, there is one succession where a continuous sedimentary record is preserved across this interval of extraordinary change – the Turee Creek Group of Western Australia.  In this succession, which is up to 4 km thick, we recently discovered the transition from ancient to modern Earth, recorded in a sequence of alternating banded iron formation, shale and chert, all of which gradually become less iron-rich up-section, recording the gradual oxidation of ferrous iron dissolved in seawater (Fig. 1: Van Kranendonk et al., in review with Precambrian Research).  Further up-sequence, the appearance of stromatolites and rippled sandstones denote the progressive infilling of the Turee Creek Basin, and above that, we have discovered two glaciogenic successions, each marked by a period of rapid sea-level fall, when seawater volume was transferred into the developing ice sheets, followed by rapid sea-level deepening, as the glacial ice sheets melted (Fig. 2: Van Kranendonk and Mazumder, in review with GSA Bulletin).

Figure 2.  North (bottom)–south (top) stratigraphic section through the mid-upper Kungarra Formation on the northern limb of the Hardey Syncline, showing two distinct glacio-eustatic cycles. 

In order to understand and quantify the nature of changes across this important interval, and to assess how the biosphere responded to this global climate change, Martin J. Van Kranendonk of the CCFS and his colleague from Paris, Professor Pascal Philippot, in collaboration with the Geological Survey of Western Australia, undertook a diamond drilling program to obtain fresh samples across each of the major transitions (Fig. 3).  Three drill holes were obtained across different levels of the stratigraphy, including the basal contact with banded iron formations of the underlying Hamersley Group (ancient Earth), a full section through the lower of two glacial deposits and underlying stromatolites, and one through the upper part of the Turee Creek Group stratigraphy where the first sulfates might be expected to have been deposited, immediately above coastal to terrestrial sandstones of the Koolbye Formation (Mazumder et al., in review, Precambrian Research). 

Figure 3.  View looking over moderately dipping rocks of the Turee Creek Group, with the drillrig located on the flat ground in middle left.  Lower slopes are glaciogenic diamictite.

Preliminary results of drillcore inspection reveal that stromatolites have been intersected at several levels, and pyrite, of both authigenic and detrital origin, is common throughout the succession (Fig. 4).  The basal transition from banded iron-formation to non-ferruginous chert was successfully recovered.  A team of scientists from around the world will be analysing the cores to unlock their secrets and reveal more about the evolution of the ocean reservoir and the biosphere across the Great Oxidation Event.

Figure 4.  Left: Authigenic pyrite in fine-grained sandstone.  Right: Rounded grains of detrital pyrite in medium-grained sandstone.  Both views are 4 cm across.

This project is part of CCFS Theme 1, Early Earth, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contact:  Martin J. Van Kranendonk, University of New South Wales  m.vankranendonk@unsw.edu.au
Funded by:  The University of New South Wales, Institute de Physique du Globe de Paris, the Agouron Institute 

Isotopes reunite lost relatives in western Australia

The Arunta Orogen of central Australia (Fig. 1) comprises a large part of the southern margin of the North Australian Craton and records episodic tectonic and thermal activity spanning almost 1.5 billion years of Earth’s history, from the Paleoproterozoic to the Devonian.  The 1690–1630 Ma Warumpi Province forms the southernmost part of the Arunta Orogen and is separated from the older, 1860–1700 Ma Aileron Province to the north by the Central Australian Suture (Fig. 2a). Both are unconformably overlain by Neoproterozoic to Paleozoic basins.

Figure 1.  a) Location of the Warumpi Province on the southern margin of the Arunta region.

The Warumpi Province has been considered to be exotic to the North Australian Craton.  However, Hf-isotope evidence for mixing between 1690–1630 Ma mantle-derived melts and older sources indicates that the history of the Warumpi Province involved interaction with significantly older crust of at least Mesoarchean age (Fig. 2b).

Figure 2.  a)  Simplified interpreted basement geology of the west Arunta region in Western Australia, showing the locations of analysed samples.  b) εHf evolution diagram showing analyses of zircons from the Aileron (red circles) and Warumpi (yellow diamonds) Provinces. Probability density curves show the age distributions of Aileron Province (red) and Warumpi Province (unshaded) zircons.  The light red field, which shows the crustal evolution of older components of the Warumpi Province, overlaps the compositional evolution of the Aileron Province, indicating a common source.  See CCFS publication #436.

The ages and Hf-isotope compositions of inherited zircons in the 1677 Ma Pollock Hills Formation are consistent with this older crust being part of the Aileron Province.  This suggests that the Warumpi Province represents a slice of Aileron Province crust rifted away from the southern margin of the North Australian Craton at, or prior to, 1690 Ma.  The Warumpi Province then acted as a locus for the voluminous production of mantle-derived (juvenile) 1690–1660 Ma crust, both outboard of the southern Aileron Province margin, and during its accretion back onto the Aileron Province during and following the 1640–1635 Ma Liebig Orogeny.

Detrital zircons from Neoproterozoic units, including the Amadeus and Murraba Basins, which unconformably overlie the Aileron and Warumpi Provinces in the west Arunta region, record a history of distinct changes in erosional sources during their early depositional histories.  Changes in detrital age components with stratigraphic level in the >840 Ma Kiwirrkurra Formation, the 1040–820 Ma Heavitree Quartzite, and the probably younger Munyu Sandstone, demonstrate marked changes from Warumpi-dominated to Musgrave-dominated detritus.  This change in provenance over time probably relates to changes in tectonism in the region during the Neoproterozoic, possibly associated with breakup of the Rodinia supercontinent.  See CCFS publication #436

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture. 

Contacts:  Chris Kirkland, Michael Wingate
Funded by:  CCFS, GSWA

Tracking the birth and growth of the Central Asian Orogenic Belt 

Most of the existing continental crust was generated by the end of the Archean (CCFS Publication #342), and the production of new crust has decreased markedly since that time (Belousova et al., Lithos 2010), except for some sporadic bursts of activity.  One of these is the Central Asian Orogenic Belt (CAOB), a complex tectonic collage of microcontinental blocks, island arcs, and remnants of oceanic crust between the Siberian Craton to the north and the Tarim and North China cratons to the south.  The CAOB is the world’s largest Phanerozoic accretionary orogenic belts and is the most important site for juvenile crustal growth in the Phanerozoic.  The challenge is to unravel the major magmatic events in the generation of the juvenile crust from this Phanerozoic accretionary orogenic belt, and to answer several major questions. 

Figure 1.  Histogram of Early Mesozoic zircon U-Pb ages in the CAOB. Only SHRIMP and LA-ICPMS zircon U-Pb data are used.

Why was the CAOB the locus of such extensive juvenile crustal growth in Phanerozoic?  Is this related to the massive amount of oceanic crust that was subducted during closure of the Paleo-Asian Ocean, to subduction of the Mongol-Okhotsk arcs, or to the Siberian mantle plume?  What was happening at the different continental margins (e.g. Siberian margin and North China Craton margin)?  Was the pre-existing Paleozoic accretionary crustal architecture modified largely by Mesozoic magmatism?  Our recent work (Li et al. Earth-Science Reviews, 2013, Wang et al. American Journal of Science, 2014) is beginning to provide plausible answers to these questions, from detailed regional geological, petrological and geochronological studies on the subduction-related, collision-related and intraplate extensional magmatism in the CAOB, tracing granite sources and origins to pinpoint crustal architecture and growth in the Early Mesozoic.

Figure 2. (a) Schematic space-time diagram showing major tectonic events affecting each belt in the CAOB. (b) Pre-Permian and (c) Early Mesozoic cross-section showing schematically the oceanic and arc/continent domains.

The numerous Early Mesozoic granitoids in the CAOB can be broadly classified into two groups on zircon U–Pb ages (Fig. 1): an early group from Early to Middle Triassic (250–230 Ma) and a late group emplaced during Late Triassic and Early Jurassic time (230-190 Ma).  Early (250-230 Ma) granitoids are mainly distributed in the western Central Mongolia–Erguna Belt (CMEB), the western Altai Belt (AB), the South Mongolia–Xing’an Belt (SMXB) and the Beishan–Inner Mongolia–Jilin Belt (BIJB).  They are mainly quartz-diorites, granodiorites and monzogranites, mostly of I-type, with minor mafic intrusions; some have adakite-like signatures and S-type features.  Late (230-190 Ma) granitoids mainly occur in the North Mongolia–Transbaikalia Belt (NMTB), the eastern CMEB (Erguna massif) and the eastern Altai Belt (AB).  They are mainly syenogranites, monzogranites and syenites, associated with many alkaline granites and mafic intrusions and are A-type and transitional I–A type or highly fractionated I-type granites.

Whole-rock Sr-Nd and zircon Hf isotopic data have been compiled for regional isotopic mapping.  The εNd(t) values show large variations from −7.0 to +7.4 and Nd model ages (TDM) from 0.46 Ga to 1.43 Ga; the initial Sr isotopic ratios (Sri) range from 0.7023 to 0.7174.  The zircon εHf(t) values vary from -4.6 to +15.3 and give two-stage Hf model ages (TDM₂) from 0.30 Ga to 2.09 Ga.  The extremely large variations of whole-rock Sr-Nd and zircon Hf isotopes imply heterogeneous source regions mainly dominated by juvenile components but with significant older crust as well.  In the CAOB, the isotopic signatures (Nd-Hf) of the Early Mesozoic granitoids are similar to those of the Paleozoic granitoids in the same belt, indicating the Early Mesozoic crustal architecture retained its original Paleozoic continental signatures, which had not been modified by much juvenile mantle-derived material. 

Figure 3.  Schematic paleogeographic reconstruction of the Early Mesozoic ocean-continent framework of the CAOB and adjacent regions.  The pink areas broadly represent the Early-Middle Triassic (a) and Late Triassic (b) granitoid belts in the CAOB.

The older continental terrane was not thrust onto, or subducted beneath, the younger juvenile accretionary terranes, as seen in some collisional orogens.  The crustal architecture in the CAOB is typical of an accretionary orogen that is characterised by horizontal accretion, not by vertical superposition of terranes.  In contrast to the generation of massive juvenile crust in the Paleozoic accretionary stages of orogenic development, crustal recycling plays a more substantial  role in the post-accretionary stages.

The generation of the Early Mesozoic granitoid magmas in the NMTB and the CMEB was dominated by the ongoing closure of the Mongol-Okhotsk Ocean and some were probably related to a mantle plume (Fig. 2).  They may have been derived from melting of subducted materials or juvenile components with some probable contributions from ancient continental crust.  Early Mesozoic granitoid magmas in the SMXB, the AB and the BIJB were generated in a post-/non-orogenic setting after the closure of the Paleo-Asian Ocean and were the results of partial melting of crustal components in response to underplating of mantle-derived magmas, most likely linked to lithospheric thickening, delamination and asthenospheric upwelling (Fig. 2).  Early Mesozoic granitoid magmatism provides critical information on the Mesozoic post-accretionary tectonic evolution of the Paleo-Asian Ocean and transitional tectonic regimes from Early Mesozoic subduction to Late Mesozoic closure of the Mongol-Okhotsk Ocean as well as post-accretionary continental growth (Fig. 3).

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture.

Contacts:  Shan Li, Tao Wang, Simon Wilde
Funded by:  National Basic Research Program of China, National Natural Science Foundation of China, China Geological Survey

Ghosts of oldest bacterial colonies haunt Western Australia

Identifying and reconstructing Earth’s earliest biosphere is challenging.  Earth’s oldest sedimentary rocks are not only rare, but almost always have been heated and deformed by tectonic activity so that former signs of life are destroyed or modified beyond recognition.  A new study, however, has revealed the well-preserved remnants of a complex bacterial ecosystem in a 3.5 Ga sedimentary sequence from the Pilbara region of Western Australia (CCFS publication #365). 

The ancient core of the Pilbara terrane constitutes one of the rare geological areas that provide insights into the early evolution of life on Earth.  Mound-like deposits (stromatolites) created by ancient bacteria, and rare microfossils of bacteria, have previously been described from this region.  However, a phenomenon called microbially induced sedimentary structures, or MISS for short, had not previously been seen in rocks of this great age.  Before our discovery the oldest MISS were ~ 3.2 billion years old.

Figure 1.  Microbial mat on rock surface from the 3.48 billion year old Dresser Formation, Pilbara, Western Australia.

MISS are created by communities of microorganisms, living in microbial mats, as they respond to changes in physical conditions such as erosion or sediment deposition in their immediate environment.  A common example would be the binding together of sediment grains by microbes to prevent their erosion by water currents.  MISS demonstrate not only the presence of life, but the presence of whole microbial ecosystems that could co-ordinate with one another and influence their environment.

eroded fragments of microbial mat, are on the millimetre to metre scale and can be seen with the naked eye.  Other MISS are microscopic and typically include remnants of microbial filaments wrapping around sediment grains, or miniature versions of tufts (Fig. 2) and mat fragments.  We have found at least ten distinct types of MISS from the 3.48 Ga Dresser Formation and demonstrated their close similarity in both morphology and preservation style to MISS found in the younger geological record.  We also found that the structure and isotopic composition of carbon preserved in these MISS are consistent both with a biological origin and the known age of the samples.

Figure 2.  Cross-section through a tufted microbial mat from the Dresser Formation, viewed down the petrographic microscope.  The black material is carbon and pyrite. 

MISS come in varied shapes and sizes.  Some, such as crinkles and tufts on rock surfaces (Fig. 1), and As well as extending the geological record of MISS by almost 300 million years, our work shows that relatively complex mat-forming microbial communities probably existed almost 3.5 billion years ago.  While it is arguable whether these MISS are the earliest signs of life on Earth, they certainly provide a new and robust source of evidence showing that large volumes of organisms were alive and well in these inhospitable conditions on the early Earth.

New discoveries of ancient life on Earth such as these can also be informative for the search for life on other planets, since the early histories of Earth and Mars are thought to have been similar.  MISS could now be considered a prime target for Mars rovers, especially given that some MISS are of the centimetre scale and larger and may be visible to a rover camera.

This project is part of CCFS Theme 1, Early Earth, and contributes to understanding Earth’s Fluid Fluxes.

Contact:  David Wacey
Funded by:  ARC CCFS CoE

Trace elements in olivine are scouts for subducted continental crust in the source regions of magmas

Oceanic crust produced by sea-floor spreading at mid-ocean ridges is returned to the mantle by the process of subduction.  This can be recognised by geophysical measurements that image sloping blocks of ocean crust beneath the surface, diving down beneath  deep-sea trenches.  For thirty years now, it has been debated how deeply these blocks are thrust into the mantle, and it is widely believed that some of this material reappears as chemical components in ocean islands such as Hawaii – thousands of kilometres from the position of any current subduction zone. 

However, models for this recycling of crust usually deal with the basalts and gabbros that make up a thickness of about 

7 km above the main mass of the ocean plate.  It is much more difficult to characterise the recycling of the few hundred metres of sediments that were deposited on the ocean crust after its formation or during the collision process.  Trace-element and isotopic compositions of volcanic rocks directly above the subduction zone indicate sediment subduction in some places, but in others the sediments are probably completely scraped off in the collision process, and do not follow the slab down into the subduction zone.  The volumes of sediment involved are too small for the geophysical measurements to see.

Trace elements in olivine are a promising tool to solve this dilemma.  Abundances of trace elements such as nickel, cobalt and manganese have been used to suggest the presence and melting of olivine-free rocks that originated from recycled oceanic crust beneath ocean islands (Sobolev et al., 2007, Science), but alternative explanations for these elements have also been put forward.  No indicator for continental crust has been proposed as yet. 

Figure 1.  Both Li and Zn are enriched in olivines in Mediterranean post-collisional volcanic rocks (yellow and green circles) from source regions containing recycled continental crust, which is present in the source as phlogopite-bearing assemblages.  Hawaiian olivines from sources containing recycled ocean crust lie in the grey box with variable Zn but low Li (1-2 ppm).  Mantle olivines have almost exclusively less than 6 ppm lithium.  From Foley, Prelevic, Rehfeldt and Jacob (2013) Earth and Planetary Science Letters Frontiers.

Our recent measurements in post-collisional volcanic rocks in the Mediterranean area have changed this.  Many crustal blocks around the Mediterranean formed by a different subduction process, one that involved the collision and scrunching together of small ocean basins and blocks of continental crust – there is no deep subduction involved.  These blocks are imbricated together and, after the lateral collision stops, the crust and lithosphere rise and melt, resulting in volcanoes that erupt 20-40 million years after collision ceases.  These volcanic rocks contain large olivine crystals; some crystallised as the first minerals from the melts, and others were already present in the mantle and were torn off by the passing melt.  These olivines have high concentrations of lithium and zinc (Fig. 1) - ten times as much lithium as is typical for mantle olivines, but similar to abundances seen in continental sediments.  Coupled with previous results for lead isotopes in the volcanic rocks, which indicated input from continental crust, these lithium analyses in olivine clearly demonstrate the involvement of subducted continental sediments in the source regions of the melts.  This process is widespread; the yellow points in Figure 1 are from Spain in the western Mediterranean, whereas the green points are from as far east as Turkey.

We will be pursuing research on this theme in the CCFS.  The next questions are: [1] What are the olivines really “seeing”?  We suspect that the lithium is concentrated by the formation of a different, intermediate rock type rich in mica and pyroxene, and this is what the olivines are really scouting for.  [2] How important was subduction of continental crust earlier in Earth history?  The suspicion here is that the accretionary type of collision involving small oceans and continental blocks, now seen in areas such as the Mediterranean and Indonesia, was much more widespread in the Archean, before all the continents had been formed.

This project is part of CCFS Theme 2, Earth Evolution, and contributes to understanding Earth’s Fluid Fluxes.

Contact:  Stephen Foley
Funded by:  Geocycles Research Centre, University of Mainz - State of Rhineland Palatinate (Germany)

Auditioning zircons for perfecting performance

Zircon is a common U-rich accessory mineral recognised as a key geochronometer and premier geochemical tracer in terms of Hf and O isotopes because of its resistance to alteration and post-magmatic metamorphism.  However, zircons with high uranium (U) contents commonly yield anomalous older apparent ages in ion microprobe analysis (commonly called the “high-U matrix effect”) (White and Ireland, Chem Geol, 2012).  Moreover, zircons with such discordant U-Pb ages usually also have disturbed oxygen-isotope compositions (Booth, GCA, 2005).  Although these isotopic disturbances are commonly considered to be due to zircon metamictisation (the destruction of the crystal lattice by radiation damage), the discussion of data quality has mostly been qualitative.  It is difficult to define a specific minimum U content that causes the “high-U matrix effect” or metamictisation, because the radiation damage is accumulated through time.  This qualitative evaluation also has led most workers to overlook the different stability of zircon’s U-Pb isotope system and oxygen-isotope system.  For zircons in which the U-Pb system remains apparently undisturbed, the robustness of the oxygen isotope data is rarely questioned. 

Figure 1.  Raman spectra of zircons with different Ddpa values.

Raman spectra show that zircons with increasing degrees of radiation damage have gradually changing peaks and higher values of half-width height (Fig. 1), and the effects of radiation damage can be quantified, as displacements-per-atom, Ddpa (Palenik, Am Mineral 2003).  We have used this technique to assess the effect of varying U contents on the ion microprobe U-Pb and oxygen isotopic signatures of zircon-quartz pairs from the U-rich A-type granites of Suzhou (southern China).  The results show that zircons with U concentrations ([U]) greater than 1500 ppm show a negative correlation between [U] and δ¹⁸O values, while zircons with [U] greater than 3000 ppm show a positive correlation between [U] and U-Pb ages (Fig. 2).  Oxygen-isotope analyses of coexisting quartz provide constraints for interpretation of zircon oxygen-isotope values, because the isotopic partitioning between quartz and zircon is well known.  This relationship allows us to recognise anomalous δ¹⁸O values (Fig. 3), which are clearly common in high-U zircons.

Figure 2.  Plot of zircon U (or Ddpa) vs ²⁰⁶Pb/²³⁸U age (or δ¹⁸O); In-situ zircon U-Pb age and δ¹⁸O data are obtained from SIMS analysis.

The high U contents of the zircons thus produce two independent effects, linked by the radiation damage.  Anomalously high SIMS U-Pb ages are a direct result of the matrix effect caused by metamictisation, whereas the low δ¹⁸O values result from interaction with infiltration of OH-bearing fluid and fluid-facilitated diffusion in the radiation-damaged areas.  Previous oxygen-isotope studies of bulk zircon separates from the Suzhou granites reported low δ¹⁸O ratios; it is now clear that these represent anomalous values due to effects of radiation damage to the zircons. 

Figure 3.  Histograms of δ¹⁸O values for zircon-quartz pairs from the four representative Suzhou granite samples

This study shows how essential it is to evaluate the degree of radiation damage before/after carrying out dating or oxygen-isotope analysis on zircons with high U contents.  Raman spectroscopy can be used to measure the degree of radiation damage of zircons, but, it is not always available, especially for pre-existing data and data from the literature.  In the absence of Raman spectra, Ddpa calculated from the [U], [Th] and age can be used to estimate the highest degree of radiation damage and screening for reliable zircon U-Pb geochronology and oxygen isotope analysis (Fig. 4).  The results indicate that for the Suzhou pluton, Ddpa <0.03 is a robust discriminant threshold to identify zircons with primary oxygen-isotope ratios and Ddpa<0.08 is a robust discriminant to screen for reliable U-Pb dating.  These different screening values reflect the evidence that oxygen-isotope compositions show disturbance at a lower level of post-crystallisation zircon lattice disturbance than does the U-Pb system.

Figure 4.  Correlation between U concentration and U-Pb ages, along with assigned Ddpa of 0.03, 0.08, and 0.5. Dashed lines Th/U = 0.1; solid lines Th/U = 1. The area between dashed and solid lines with same Ddpa, including all the zircons with Th/U = 0.1 ~ 1. Data sources for standard zircons: Penglai – Li, Geost. Geoan. Res, 2010; Qinghu – Li, Sci China Ser D, 2009; Plešovice – Slama, Chem Geol, 2008; Temora – Black, Chem Geol, 2003; R33 – Black, Chem Geol, 2004; M257 – Nasdala, Geost. Geoan. Res, 2008; SL-13 – Lee, Nature, 1997; GJ-1 – Jackson, Chem Geol, 2004; Mud tank – Black and Gulson, BMR J Aust Geol Geophys, 1978; 91500 – Wiedenbeck, Geostandards Newsletter,1995; AS3 – Paces and Miller, JGR,1993. 

This project is part of CCFS Theme 2, Earth Evolution, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Yuya Gao, Xian-Hua Li (P, IGGCAS, Beijing), Bill Griffin
Funded by:  EPS postgraduate fund, iMQRES, MQ PGRF, CCFS, IGGCAS

Why hasn’t the Mediterranean Basin closed?

 The Mediterranean Sea is the vestige of a large ocean that was closed by the northward movement of the African plate, creating the Alps as the leading edge of the African plate was subducted beneath Europe.  But why did it stop there?  Why does the Mediterranean exist at all?  A growing body of data on U-Pb ages of magmatic zircons, Hf model ages of inherited zircons and whole-rock Nd model ages of crustal igneous and metamorphic rocks has provided many constraints on crustal evolution within the Mediterranean basin.  However, there has been little robust information on the age of its underlying mantle, that would allow us to understand the deeper geological framework of the Mediterranean. 

A synthesis of Re-depletion model ages (TRD) for both whole-rock samples and in situ analyses of individual sulfides from mantle-derived rocks, including xenoliths and peridotite massifs that are widespread across the Mediterranean area, demonstrates the existence of different mantle domains (CCFS Publication #234). A maximum TRD age of 1.8 Ga is common to sulfides in mantle-derived peridotite xenoliths beneath Western Europe (Calatrava Volcanic Field, Spain; Languedoc and Massif Central, France) and to whole-rock samples from Azrou (North Africa) and the Pyrenees (France). A maximum at <1.4-1.3 Ga is seen in whole-rock samples from Central Europe (Bohemian and Rhenish Massifs) as shown in Figure 1. 

Figure 1.  Distribution of maximum TRD model ages across the Mediterranean region.  The tagged line represents the Maghrebide-Appenine-Betic tectonic front.  Red areas indicate possible Archean micro-plates.

In contrast, Os-bearing sulfides in xenoliths and sulfides and platinum-group minerals in peridotite massifs from the inner Mediterranean region (Hyblean Plateau in Sicily and Kraubath Massif in Austria) all show an oldest TRD peak at ~ 2.3 Ga, equivalent to the oldest whole-rock TMA of 2.2 Ga for rocks of Beni Boussera in northern Morocco and the 2.4 Ga peak in sulfides from peridotites of the internal Ligurides (Italy).  A peak at 2.6 Ga is defined by sulfides in mantle xenoliths from the Tallante Volcanic Field in southern Spain.

These Re-Os data identify the existence of a common Paleo-Proterozoic (1.8 Ga) mantle on both sides of the Mediterranean realm, and an older (2.2-2.6 Ga) lithospheric mantle sitting inside the more recent Maghrebide-Appenine-Betic front generated during the Alpine-Betic orogeny.  Thus, the Mediterranean basin may contain several buoyant Archean microplates, which could act like bollards, impeding the northward movement of Africa, and protecting the Mediterranean from closing. 

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth Architecture and Fluid Fluxes.

Contacts:  José María González-Jiménez, Bill Griffin, Sue O’Reilly
Funded by:  CCFS ARC ECSTAR funds

Surface structures influence reaction rates of minerals 

The rate at which minerals are dissolved and grow is crucial in our understanding of how Earth materials have changed in terms of physical and chemical properties through time.  In recent years, it has been realised that many reactions that transform one mineral to another require fluid involvement.  The first and often crucial step in this transformation is dissolution.

We have investigated the dynamics of dissolution and the time evolution of the (often assumed) parameter, dissolution constant, that determines at which rate a mineral dissolves under specific conditions.  Classically, dissolution rates are calculated based on models of dissolution kinetics as a function of surface area, which is assumed to remain constant.  However, this assumption ignores the changes occurring on the surface during the dissolution process.  We examined how the topography of natural fluorite surfaces with different crystallographic orientations changed during up to 3200 hours of dissolution.  The results were analysed in terms of changes in surface area, surface reactivity and dissolution rates.  

We found that the dissolution rate is strongly time-dependent, while the value of the dissolution constant varies depending on the crystallographic orientation of the dissolving surface.  All surfaces studied show rapid changes in topography and rapid dissolution rates during the initial 200 hours of dissolution.  The factors  controlling the development of topography are the stability of the step edges forming the initial surface and its inclination to the closest stable planes, which are specific for each surface orientation.  The surface dynamics are accompanied by a significant decrease of dissolution rates after the initial rapid-dissolution regime.  The time-dependent variation of dissolution rates is attributed to a decrease in the density of step edges on the surface and the continuously increasing exposure of more stable planes with time.  During a second dissolution regime, some surfaces continue to show significant changes in topography, while on others the topography tends to remain approximately constant.

Change in dissolution rate, surface structure and topology / roughness as a function of time and surface orientation. (A) & (B) Examples of SE images showing topography of one studied surface after 1600 hrs and 3200 hrs dissolution. (C) Graphs showing that dissolution rates are initially high, while roughness/topography is low, subsequently dissolution rates decrease while roughness / topography increases.  Two examples are shown, (left) surface (102), (right) surface (110).

Our observations suggest that the development of topography during the initial dissolution regime is kinetically driven, and therefore the surface may only reach a metastable state.  The height and size of the topographic features initially developed, and the types of planes and step edges that constitute them, determine the changes during the second dissolution regime.  Dissolution transforms the metastable surfaces formed during the initial dissolution stage towards a more stable topography.  Importantly, our results show that, contrary to classic theory,  dissolution rates decrease even though the total surface area increases.  Therefore, calculations of dissolution rates that assume dissolution rates are directly proportional to surface area are not valid for the surfaces studied.  Instead, to develop accurate kinetic-dissolution models and more realistic stochastic dissolution simulations the surface reactivity, as determined by the relative stability of the planes and type of edges that constitute a surface, must be taken into account. 

Even though experiments were performed using fluorite, the same principles are predicted to hold for other crystal structures.  This study may be the first step toward a quantitative characterisation of the often heterogeneous, fluid-mediated replacement of different minerals in the Earth’s crust and Mantle (see CCFS publication #364). 

This project contributes to the CCFS Theme 3, Earth Today, and furthers our understanding of Earth’s Fluid Fluxes.

Contact:  Sandra Piazolo
Funded by:  ARC Future Fellowship, European Research Council

Different continent configurations around 800 – 500 million years ago give new clues about extreme climatic variations 

The Neoproterozoic was one of the most dynamic times in Earth history, featuring the formation and breakup of the supercontinent Rodinia (e.g. CCFS contribution #117), repeated low-latitude glaciation at sea level (a Snowball Earth?), rapid oxidation of the atmosphere, and the end of the Precambrian explosion of complex life.  To better understand the relationships between these dramatic events, we developed a set of revised global palaeogeographic maps for the 825–540 Ma time interval, accompanied by a compilation and graphical illustration of global data on sedimentary facies (CCFS contribution #314; see Fig. 1 for examples).  These maps form the basis for an examination of the relationships between known glacial deposits, paleolatitude, positions of continental rifting, relative sea-level changes and major global tectonic events such as supercontinent assembly, breakup and superplumes. 

Figure 1.  Revised global palaeogeographic reconstructions for 720 Ma and 635 Ma, with facies distributions featuring the occurrence of the “Sturtian” (ca. 720 Ma) and “Marinoan” (ca. 635 Ma) glaciations, respectively. 

Our analysis reveals several fundamental palaeogeographic features that will help inform and constrain models for Earth’s climatic and geodynamic evolution during the Neoproterozoic.  First, whereas the latitudinal distribution of carbonate rocks appears to be in accord with a normal zonal global climate similar to that of the Phanerozoic (Fig. 2c), thus validating the palaeogeographic maps, glacial deposits at or near sea level appear to extend from high latitudes into the deep tropics for all three Neoproterozoic ice ages (720 Ma, ca 635 Ma, and ca 580 Ma), although the 580 Ma interval remains very poorly constrained in terms of both paleomagnetic data and global lithostratigraphic correlations (see Figs. 1 and 2b for latitudinal distributions of all three glacial events).  This appears to be consistent with the predictions of the Snowball Earth model (e.g. Hoffman et al., 1998).  Second, continental sedimentary environments were dominant in epicratonic basins within Rodinia (from >825 Ma to 750 Ma; Fig. 2a shows the low proportion of deep-marine facies for that time), possibly resulting from both plume/superplume dynamic topography and lower sea-level due to dominantly old oceanic crust.  This was also the case at ca 540 Ma (Fig. 2a), but at that time the pattern reflects widespread mountain ranges formed during the assembly of Gondwanaland and the increasing mean age of global ocean crust.  Third, deep-water environments were dominant during the peak stage of Rodinia break-up between ca 720 Ma and ca 580 Ma (Fig. 2a), probably indicating a higher sea level due to increased rate of production of newer oceanic crust, and perhaps the effect of continents drifting away from a weakening superplume.  Such a geodynamic control of first-order global sea-level changes has also been observed during the evolution of the youngest supercontinent Pangaea (e.g. Muller et al., 2008 ), and agrees well with the hypothesised dynamic supercontinent-superplume coupling (see Li and Zhong PEPI, 2009  for a review of the model).  Finally, there is no clear association between continental rifting and the distribution of glacial strata, contradicting models that restrict glacial influence to those particular tectonic environments. 

Figure 2.  (a) Variations in the number of data points showing deep marine environments among seven palaeogeographic snap shots.  (b) Paleolatitudinal distribution of glacial deposits, and (c) paleolatitudinal distribution of carbonate deposits according to the revised palaeogeographic reconstructions with facies distributions (see Fig. 1 for example).

This project is part of CCFS Theme 2, Earth Evolution, and contributes to understanding Earth’s Architecture.

Contact:  Zheng-Xiang Li
Funded by:  CCFS Foundation Project 6

The first Martian crust

Mars is the closest planet to Earth after the Moon and mankind has long speculated about possible life on Mars.  The science-fiction novels from the ‘fifties depicting human colonies on the red planet certainly fed this curiosity and contributed to the modern development of the small robots now roving the surface of Mars (like NASA’s Opportunity and Curiosity).  These remotely-controlled rovers have been providing stunning images and data from the Martian surface and have massively improved our knowledge of the planet.  The other way we can study Mars is by finding and collecting meteorites, first launched from its surface during impacts, and then delivered to Earth.  While the rovers and their range of high-tech instruments concentrate on the study of the surface, Martian meteorites provide insight into the deeper part of the planet.  Most of them are mafic and ultramafic rocks originating either from the mantle or from the young volcanic plains covering the northern hemisphere of Mars.  The southern hemisphere of Mars, known as the Southern Highlands, is believed to be much older and to hold clues to the planet’s early differentiation.  Yet, so far, no samples are available from this part of the planet. 

A Martian meteorite recently recovered from the north-western African desert and called NWA 7533 is strikingly different from the other known Martian meteorites: it is a fragment of Mars’ regolith breccia, comprising a fine mixture of fragments of igneous rocks and clast-laden impact melt.  Superficially, it appears quite similar to some Apollo samples but has the unique Martian compositional signature (as identified by oxygen isotopes). It also contains (i) high abundances of meteoritic siderophile elements (like Ni and Ir) suggesting it formed as a result of impact and (ii) fine-grained material with composition similar to soil analysed from the Gusev crater by the Spirit rover, suggesting the meteorite mainly consists of soil particles; hence its classification as a regolith breccia.  NWA 7533 also contains small magmatic clasts, enclosing zircon grains. 

Our part in this exciting project was to image and date these zircon grains, achieved by in situ U-Pb dating using the SHRIMP (ion microprobe) in the John de Laeter centre at Curtin University.  The small meteorite slab we studied contained seventeen zircon grains; ten were large enough to be dated, and seven grains, representing eighteen ion-microprobe analyses, could be combined to define a Discordia line.  The upper intercept age at 4426 ± 23 Ma (million years) is interpreted as the crystallisation age of the zircon grains.  The lower intercept age at 1718 ± 82 Ma represents the time where the U-Pb system was disturbed, most likely by an impact.  The discovery of such ancient zircon grains on Mars implies that the planet’s early crust differentiated at about the same time as the Earth and the Moon crusts ( CCFS Publication #370  ). This is of particular interest as it may point toward a similar mechanism for the differentiation of terrestrial planets, regardless of their present shape and overall mass. 

This project is part of CCFS Themes 1, Early Earth, and contributes to understanding Earth’s Architecture.  

Contacts:  Alexander Nemchin, Marion Grange 
Funded by:  CCFS (Marion Grange salary)

Tracking deep fluid processes where continents collide: magmatic segregations in a Tibetan orogenic peridotite 

Arc magmatism in subduction zones is a prime process controlling element recycling and continental growth at the convergent margins in the shallow Earth, but the early stages of magmatic processes at mantle depths remain unclear.  Many previous studies have shown an andesitic average for the composition of the continental crust in contrast with the basaltic nature of primitive melts in plate collision zones, suggesting there is a complementary rock-type component at depth.  Mafic to ultramafic pyroxenites hidden within the arc roots have been suggested as this complement (e.g. Kay and Kay, GCA 1988; Dhuime et al., J Pet. 2009). Thus, knowledge of the genesis and sources of these “hidden” pyroxenites is critical to understanding the dynamic processes of generation and migration of primitive arc magmas beneath active margins, and to linking the recycling of material between subduction slabs and erupted volcanics within a complete framework of plate convergence.  This study examines the origin and history of pyroxenite dykes (interpreted to be magmatic segregations; Fig. 1) in the Shenglikou peridotite massif (North Qaidam Orogen, NE Tibet, China).

Figure 1.  Thin-section photomicrographs of the Shenglikou pyroxenites, North Qaidam, NE Tibet. 

Major- and trace-element compositions of the pyroxenites indicate an origin as high-pressure (garnet-facies) cumulates and/or trapped melts, and the derivation of the parental magmas from the melting of a deep peridotite-dominated source.  Bulk-rock trace-element patterns also show strong enrichment in fluid-mobile elements (Cs, Rb, Ba, U, Pb and Li) and marked negative anomalies in the high-field-strength elements relative to rare earth elements; overall they are geochemically similar to melts from a volatile-rich arc-type mantle.  Initial Sr isotopic ratios (0.7152-0.7105) and Nd isotopic compositions (εNd(t) = -11.6 to -4.4) of clinopyroxene and garnet confirm the contribution of subducted sediments.  However, the Hf isotopes of clinopyroxene, garnet and zircon show depleted-mantle isotopic values at 430 Ma; one garnet sample with depleted-mantle model age of 846 Ma shows minimum assimilation of the host peridotite during the pyroxenite intrusion.  Oxygen isotopic signatures of garnet and zircon imply uncontaminated upper-mantle sources.  These decoupled isotopic features further suggest that the melt source was a peridotite-dominated convective mantle wedge (controlling the Hf and O isotopes) that had been pervasively metasomatised by fluids from the subducted slab (controlling the Sr-Nd isotopes and highly-incompatible elements). 

Figure 2.  Cartoon representing our interpretation of the formation and evolution processes of the Shenglikou pyroxenite dykes within a tectonic framework of an early Paleozoic active continental margin, NE Tibet. 

This study illustrates a detailed dynamic process of arc magmatism within the mantle wedge beneath an active continental margin (Fig. 2).  A strongly metasomatised convective mantle moved as corner flow resulting from the early Cambrian subduction of the Proto-Tethys oceanic slab.  The decompression and flux melting generated magmas that intruded into the overlying lithospheric mantle wedge as pyroxenite dykes.  We suggest this is a significant process to allow the mafic-ultramafic complements of continental-arc lavas to escape delamination back to the convective mantle.

This project is part of CCFS Theme 2, Earth Evolution, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Qing Xiong, Sue. O’Reilly, Bill Griffin, Norman Pearson, Jian-Ping Zheng (China University of Geosciences, Wuhan)
Funded by:  ARC Discovery , Centre of Excellence Grants (S.Y.O’R and W.L.G.), NSFC (J.P.Z.), CSC scholarship, IMQRES, MQ PGRF, EPS postgraduate fund

How soft is the lower crust? More ingredients to test the “jelly sandwich” model 

Today, ancient rifting environments are important zones for the production of oil and minerals.  Observations of rifted basement rocks, under the sediments that accumulate in the rifts, show they have experienced both shearing and extensional strain, often at lower-crustal depths.  This project aims to develop a quantitative tool to determine the relative viscosity or “resistance to flow” of different rock types at lower-crustal levels.  For such a study, “boudins” (French: a specific type of sausage; elongate sausage-shaped pieces of one rock layer aligned like a chain of pearls) can be used.  Boudins occur where a harder, more competent layer in a weaker matrix is deformed by pure and/or simple shear, and  at microscopic to landscape scales; they develop a variety of symmetric and asymmetric shapes.  The temperature and pressure in the lower continental crust mean the rocks can flow and deform in ways unseen at the Earth’s surface, and different rock types have different physical characteristics, melting temperatures, hardness, flow characteristics and viscosity.  Our study adds more information about the rheological characteristics of the lower crust and will provide new data to assess details of the “jelly sandwich” model (e.g. Jackson GSA Today, 2002).

In this project, we combine detailed field work with numerical simulations.  The Anita Shear Zone (ASZ) in Fiordland, New Zealand has been chosen as a field example; it is an excellent example of landscape scale, asymmetric ultramafic boudins formed at depth.  The boudins are surrounded to the east by the Milford Orthogneiss and to the west by the Thurso Paragneiss.  In the field, the first-order relative viscosities of the lithologies were determined, where the ultramafic layer is inferred to be the most viscous, followed by orthogneiss and then paragneiss.  Field work uncovered a ~160m “soft”, high-strain zone between the orthogneiss and the ultramafics and a much wider high-strain zone between the paragneiss and the ultramafics.

Simulated boudin formation where paragneiss:ultramafic:orthogneiss viscosity ratio is 1:5:3 with high strain zones with viscosity of 0.5 and paragneiss to orthogneiss zone width ratio of 4:1.  The ASZ ultramafic boudins digitised from Google earth have been overlaid onto the simulated ultramafic boudins for comparison showing good agreement with the simulated shapes.

To investigate the impact of viscosity on boudin formation we used the Underworld software (Moresi et al., J. Comp. Phys, 2003) with, initially, three layers undergoing Newtonian flow with pure (extensional) shear, keeping the ultramafic:orthogneiss viscosity ratio at 5:3 and varying the paragneiss viscosity.  Boudins started to form where the ultramafic:paragneiss viscosity ratio was 5:1, with better boudins forming as the ratio increased (5:0.5, 5:0.25 etc.).  Subsequently, high-strain zones with a viscosity of 0.5 were added on either side of the more viscous ultramafic layer and the width of the high-strain zone on the orthogneiss side was varied.  Boudin shapes similar to those of the ASZ were developed when the width of the paragneiss:orthogneiss high-strain zone was 4:1. 

In general, the simulations show that individual boudins are more asymmetric where the paragneiss:orthogneiss high strain zone ratio is wider (4:1 cf 2:1) and the viscosity is asymmetric either side of the more viscous, boudinaging layer.  A wider symmetric high-strain zone formed fewer, longer, more symmetrical boudins.  This study highlights the importance of both high-strain zones and the relative viscosity of the lithologies for asymmetric boudin formation.  Comparison of the symmetry and length of boudins measured in the field and those developed in numerical models allows quantitative determination of the relative viscosity ratio of the matrix and boudinaging layers.

This project is part of CCFS Theme 3, Earth Today, and contributes to understanding Earth’s Architecture and Fluid Fluxes.

Contacts:  Robyn Gardner, Sandra Piazolo
Funded by:  ARC Future Fellowship, MQ infrastructure funding, MQRES

Is the Moho the Crust-Mantle Boundary? Evolution of an idea

The concept that the Mohorovicic Discontinuity (Moho) detected seismically does not necessarily coincide with the base of the continental crust as defined by rock-type compositions was introduced in the early 1980s (see Griffin and O’Reilly, Geology 1987 and references therein). This had an important impact on understanding the nature of the crust-mantle boundary by integrating information from geophysics and the petrology and geochemistry of deep-seated samples brought to the surface either as fragments in magmas (xenoliths) or by tectonic activity.  

Figure 1.  The geotherm generated by data from garnet+two-pyroxene xenoliths in alkali basalts of Eastern Australia.  (a) Distribution of mantle peridotites beneath western Victoria, compared with the seismic reflection profile across the area (O’Reilly and Griffin, Geology, 1994); the crust-mantle boundary, as defined by the lowest temperature/depth estimates of abundant peridotite xenoliths, lies in the middle of the package of reflectors above the “reflection Moho”  “EMAC” is the Eastern Margin of the Australian Craton.  (b) Crust-mantle stratigraphy reconstructed from xenoliths and thermobarometry calculations beneath the Bullenmerri locality, compared with the seismic-velocity profile of Finlayson et al. (BMR Jnl. 1979).

The use of empirically-determined xenolith geotherms to plot the locus of the pressure-temperature curve with depth for specific lithospheric sections demonstrated that there is a variety of geotherms depending on tectonic environment.  These geotherms range from very high near rift zones, through highly inflected but lower geotherms in active tectonic regions with basaltic volcanism (higher than “oceanic” geotherms), to very low geotherms in the most ancient cores of coherent cratons.  These xenolith geotherms provided a robust framework for the construction of lithological sections through the lower crust and lithospheric mantle and revealed that the crust-mantle boundary is commonly transitional, over a depth range of 5 - 20 km or more, in off-craton regions.  Early seismic-reflection data revealed that layering is common near the Moho and this correlates well with the petrological observation of multiple episodes of basaltic intrusion around the crust–mantle boundary.  Petrologically, the crust-mantle boundary is defined as the depth at which peridotitic (mantle) wall rocks become dominant over mafic granulites and other deep-seated crustal rock types, and this boundary commonly lies 5 - 15 km above the seismically defined Moho, especially in off-craton regions.  

Recent developments in seismological techniques, petrophysical laboratory measurements on natural rocks, and experimental petrology have refined interpretation of the formation of lower crust and uppermost mantle domains.  The expansion of in situ geochronology (especially U-Pb ages and Hf-isotope composition of zircon and Os-isotope compositions of mantle sulfides) has allowed the recognition of linked tectonic events that have affected whole crust-mantle sections.  These types of data reveal that the crust-mantle boundary has commonly changed in depth and thickness through time (CCFS Publication #235).  

Figure 2.  Crust-mantle section beneath the Xinyang area, on the southern edge of the North China Craton (after CCFS Publication #163).  Histograms show U-Pb ages of zircons extracted from granulite, pyroxenite and peridotite xenoliths.  The original 3.5 Ga crust was overplated at ca 2.85 Ga, and then underplated by mafic magmas at 1.9-2.0 Ga, and again at ca 1.6-1.8 Ga, lowering the crust-mantle boundary by ca 15 km.  This lower crust and the subjacent upper mantle have been repeatedly intruded by mafic magmas from Paleozoic to Cenozoic time. 

The main process effecting Moho depth variation is over- and under-plating of basaltic magmas at this major rheological boundary between peridotite and lower crustal rock types.  This process is responsible for major crustal growth, resulting in a juvenile lower crust of mafic composition (which can then undergo melting and reworking to produce granitic magmas).  In the alkali-basalt province of SE Australia the accumulated addition is approximately 5 km thick and occurred at a rate of about 900 m/Ma.  This magma input is responsible for producing the characteristically high and inflected geotherm associated with such basaltic provinces before thermal decay occurs over about 30 million years. 

However, the nature of the crust-mantle boundary in cratonic regions still remains enigmatic, mainly due to lack of key xenolith samples or exposed sections.

The observation that the Moho defined by seismic data may lie significantly deeper than the crust-mantle boundary has important implications for modelling the volume of the crust, especially in off-craton regions.  Mapping of this crust using seismic techniques alone, without consideration of the petrological problems, may lead to an overestimation of crustal thickness by 15 – 30% and up to 50% in some areas.  This will propagate to large uncertainties in the calculation of elemental mass balances relevant to crust-formation processes and the composition of the convecting mantle.  For more details, refer to CCFS Publication #235.

This project is part of CCFS Themes 2 and 3, Earth Evolution and Earth Today, and contributes to understanding Earth’s Architecture.

Contacts:  Sue O’Reilly, Bill Griffin
Funded by:  ARC CoE CCFS Foundation Project 1 (TARDIS)