Appendix 1: Foundation Project summaries, progress 2011 and plans for 2012
Aims: Platinum Group Minerals in chromite-rich rocks from ophiolites and komatiites can provide a faithful and robust record of the osmium-isotope composition of Earth’s convecting mantle through time; this record contains vital information on Earth’s origins and the overall evolution of its core-mantle system. In this project an international team of leading mantle researchers will use a unique combination of geochemical and isotopic techniques to decode this information and provide new insights into deep-Earth processes. Specific objectives are:
• To define the initial composition and long-term evolution of Earth’s convecting mantle, using the isotopic systematics of platinum-group minerals and sulfides from ophiolites and komatiites
• To understand the origins of ophiolitic chromitites and the processes that concentrate the platinum-group elements in these and other samples of the convective mantle to help unravel Earth’s geochemical evolution
• To understand the Os-isotope heterogeneity observed in samples from the convective mantle, and how this heterogeneity may reflect major events in Earth’s evolution
• To evaluate the evidence for the preservation of large volumes of ancient continental mantle within the ocean basins
Progress during 2011:
During 2011 we acquired a large collection of chromitites from ophiolite localities worldwide; this collection will continue to grow, especially through our active collaboration with groups at the Universities of Barcelona and Granada. The chromites (and in some cases the associated silicates, where preserved) were analysed for major- and trace-elements, and were surveyed petrographically to locate and identify any platinum-group minerals (PGM) or base-metal sulfides (BMS) residing in them. Those that were found were then analysed by LA-MC-ICPMS to determine their Os-isotope compositions. Approximately 500 Os-isotope analyses of PGM have been carried out; a larger number (>2000) of BMS analyses was carried out, with ca 20% containing enough Os to give reliable results.
One major finding has been the common appearance of ancient PGM and sulfides preserved in the mantle section of young ophiolites. These point to the involvement of ancient (continental?) lithospheric mantle in the ocean basins, and their entrainment in ophiolites. Many of the ophiolite sections also retain evidence of multiple sulfide-forming events in the mantle. A second major finding, based on detailed studies of PGM in the metamorphosed ophiolites of Bulgaria, is that Os is far more mobile in the mantle system than currently believed, at least at high temperatures. This has implications for the stability of Os-isotope systems even in Os-rich PGM, and emphasises the need for in situ analysis and very detailed petrographic study. A paper on this has been accepted by Geology.
To provide comparative material for the ophiolite work, we have continued studies of PGM and BMS in xenoliths from the subcontinental mantle, both as xenoliths and in exposed massifs such as Ronda and Ojen. We hosted visits by Professor Carlos Villaseca (Univ of Madrid) and Dr Monica Escayola (Canadian Geological Survey) who carried out studies of the mantle beneath central Spain and the North American Cordillera, respectively. We also completed a study of BMS in xenoliths from beneath Arkansas, showing the presence of an Archean continental fragment beneath the Proterozoic orogenic belt.
Near the end of the year Dr José María González-Jiménez and Dr Marek Locmelis (both ECRs) started an extensive review of the accumulated chromite data (major + trace elements), integrated with a database derived from an earlier PhD thesis at GEMOC. This has suggested a new approach to the treatment of such data, allowing the inference of tectonic setting and petrological evolution. This work is in progress. Dr Locmelis also worked for several months with Dr Mei-Fei Zhou (indepedently funded ECR) for transfer of knowledge about the methodologies she developed for analysis of Li isotopes in ultramafic and mafic systems relevant to fluid flow in lithospheric mantle domains.
In mid-2011 Dr José María González-Jiménez was awarded independent ECSTAR funding from ARC Post-award funds. His position was filled by Dr Jin-Xiang Huang, whose work is focusing on the application of the Mg stable-isotope system to tracking the evolution of lithospheric mantle.
Two PhD students have joined the project: Mr Qing Xiong is a cotutelle student with the China University of Geosciences, Wuhan and began in November. He is working on ophiolites and ultramafic complexes in two parts of Tibet. Ms Nicole McGowan, having completed an Honours degree in Analytical Chemistry at University of Technology, Sydney, will start a multi-isotopic study of chromitites and their host rocks in 2012. This will involve direct collaboration with Dr Rendeng Shi (Institute for Tibetan Research, Chinese Academy of Sciences) who will provide her with key samples from three Tibetan ophiolites. Ms McGowan also will collaborate with Dr José María González-Jiménez on the study of ophiolites in Spain and Turkey.
CCFS Publications #1, 65, 71, 75, 95 (Research Highlights Minute, Spitzbergen, Arkansas)
Aims and work plan for 2012:
The aims of the project remain unchanged for 2012. Further field work will be done in Spain and Turkey, and more extensive collections of ophiolite material will be provided by PI Arai. The work on these will follow the template developed in 2011. The chromite database will be expanded significantly, to test the synthesis achieved late in 2011; this work will be prepared for publication and presentation in international fora.
Mr Xiong will continue his PhD studies of Tibetan ophiolites, and Ms McGowan will begin her project on the ophiolites of Tibet, Spain and Turkey, working together with Dr José María González-Jiménez and Dr Shi. Dr Villaseca and Dr Escayola, each with independent external funding, will return to carry out further investigations of the Spanish continental lithosphere, and the mantle beneath the Cordilleras of both North and South America. Dr Huang will work on the development of techniques for the analysis of Mg isotopes in mantle rocks, and will apply them to both eclogites and peridotites. She also is developing standards for the analysis of O isotopes in mantle-derived, Cr-bearing garnets.
Melange complex enclosing red cherts overlying the green peridotite massif in the Zedang ophiolite, South Tibet.
Fresh harzburgite in the Zedang ophiolite.
This project is designed to (1) provide new knowledge of the character and behaviour of Earth fluids, such as silicate and sulfide melts, brines, vapours, hydrocarbons, supercritical fluids, in mantle rocks at the P-T conditions of the lower lithosphere and asthenosphere, and (2) unravel the complex transport and concentration mechanisms of siderophile-chalcophile elements such as Ni, Cu and PGE in the deep lithosphere.
Despite the significance attributed to mantle-derived fluids as key elements in the transport and concentration of metals within the crust, we lack a robust understanding of the processes through which the mantle evolves and interacts with fluids at sub-crustal depths, in the lithospheric mantle and asthenosphere. The rationale of this multi-scale integrated study is to address this void in our understanding through (1) a set of key experiments to evaluate the chemical behaviour of fluid systems at lithospheric mantle-asthenospheric conditions, and (2) parameterising and testing these experiments through the measurement of rock samples collected from two key areas: (a) the Ivrea-Verbano Zone in northern Italy and (b) the granulite facies terrains of southeastern Greenland.
As well as representing direct exposure of continental lithospheric mantle rocks, these zones host unusual nickel-sulfide occurrences that offer rare insights into how metallogenic fluids behave at such depths. Most known world-class nickel-sulfide deposits were formed far from their primary mantle metal sources, at the surface or in the uppermost levels of the lithosphere. The nickel-sulfide systems in the Ivrea-Verbano Zone and in the granulite terrains of Eastern Greenland, however, were formed in the mid-to-lower crust and upper lithospheric mantle. These systems display petrological, mineralogical and geochemical features that are anomalous in relation to typical orthomagmatic systems. They display pipe-like geometries and contain abundant accessory mineral phases, which are anomalously enriched in large-ion lithophile and highly-incompatible elements, with a strong alkaline affinity and a unique metal paragenesis.
Fieldwork in the areas of interest will be carried out in close association with (1) researchers from the University of Leoben (Austria), where research is currently active (Zaccarini, PhD student) applying a multi-technique mineralogical, petrological and geochemical approach to unravel the origin of ore mineralisation in the mafic-ultramafic rocks of the Ivrea-Verbano Zone, and (2) researchers from GEUS, who are currently focusing on Greenland’s nickel resource potential. These areas thus offer critical insights into the deeper behaviour of mantle-sourced metallogenic fluids that is not currently incorporated into models of deposit evolution and exploration.
As a corollary to these two key research streams, this study will also investigate the optimisation and integration of analytical techniques to constrain the behaviour of fluids and melts at relevant P-T conditions. This approach will provide improved insight into the meaning and significance of the geochemical signature of deep mineral systems. The experimental results from the proposed study will generate new parameters that can be integrated into the predictive modelling of metal reservoirs, contributing to improved exploration models and opening up new exploration search space for nickel-sulfide systems in the deeper portions of the lithosphere.
Progress during 2011:
In 2011, the work was concentrated on southeastern Greenland (Fig. 1), where two UWA Honours students joined a large team of geoscientists from the Geological Survey of Denmark and Greenland in a two-month field expedition. In the area, laterally extensive mafic bands hosting nickel-sulfide mineralisation can be found within orthogneiss lithologies, which largely dominate the relatively unexplored region between Graah Fjord and Bernstorff Isfjord. The mafic bands are dominantly gabbroic in composition and contain a large proportion of volatile-rich phases, ranging locally from gabbros to pyroxene hornblende norites. The mafic bands display some similarities with the mafic complex of the Ivrea-Verbano Zone in Italy, which is thought to represent lower crustal material.
Amphibole-bearing ultramafic bodies occur locally within the mafic bands. The ultramafic rocks are dominantly peridotites with a smaller volume of pegmatitic pyroxenites. It seems most likely that the mafic bands in the study area represent lower crustal material intruded by mantle-sourced ultramafic magmas.
Preliminary observations indicate that the ultramafic rocks can be subdivided into two generations based on their trace-element geochemistry. One generation probably was sourced from a deep, relatively undepleted mantle source, while the other was sourced from a depleted shallow mantle source. The shallow-sourced ultramafic rocks host most of the mineralisation and display evidence for having interacted with a volatile-rich fluid, rich in incompatible elements, S, Cu and Ni. This fluid is interpreted as a key factor in the formation of the mineralisation.
Work on samples from the Ivrea-Verbano zone also started in 2011 by evaluating an existing sample set from the Valmaggia pipe and selecting samples for major-, trace-, and isotope-analysis. Electron-microprobe analysis and laser ablation ICP-MS were used to analyse primary magmatic minerals (olivine, pyroxene) and metasomatic phases (amphibole, phlogopite) to determine the magma composition and the nature of the metasomatic fluid that is inferred to play an important role in the pipe’s genetic history. Sulfide phases were analysed to determine the major- and trace-element composition of the sulfide liquid that formed the nickel-sulfide deposits in the Valmaggia pipe.
Preliminary results show that the Valmaggia pipe was emplaced at a depth of at least 20-25 km and a temperature around 900 ºC, close to the water-saturated peridotite solidus. Phlogopite and amphibole data suggest that the metasomatic fluid was mantle-derived and carbonate-rich. The sulfides appear to be mantle-derived and predominantly consist of pyrrhotite from which the nickel-sulfide pentlandite and the copper-sulfides chalcopyrite and cubanite exsolved during cooling.
Concurrent work on selected samples from the neighbouring Ivrea-Verbano Zone pipes was carried out to determine their age of crystallisation. Peter Kollegger (PhD student at University of Leoben, Austria) visited UWA in late 2011 and separated a large number of zircon grains, which were subsequently analysed by SHRIMP. Preliminary data indicate that all pipes were emplaced coevally in the Permian, at a time consistent with metasomatism taking place in the underlying subcontinental mantle. Work is in progress to further establish the link between mantle metasomatism and the genesis of mineralised pipes.
Preparatory work for the experimental program has included the manufacture of specialised capsules and furnace assemblies that will be suitable for the sulfide-bearing systems to be investigated.
Aims and work plan for 2012:
In 2012, fieldwork will be done in the Ivrea-Verbano Zone (IVZ) and in southeastern Greenland. In May 2012 Marek Locmelis and Marco Fiorentini will join a team of geoscientists from the University of Leoben (Austria) to carry out a comprehensive mapping and sampling campaign in selected areas of the IVZ. In July 2012, John Owen (PhD student) and Brendan Lally (Honours student) will join a large team of geoscientists from the Geological Survey of Denmark and Greenland in a one-month field expedition in the area where reconnaissance was done in 2011.
In 2012, laboratory work on previously and newly collected material will steam ahead. Ultra-pure mineral separates (olivine, pyroxene, amphibole, phlogopite), which are currently being prepared for lithium isotope analysis, will be analysed for the lithium isotopic composition of the magma source where the pipes originated, and to address the link between mantle metasomatism, pipe genesis and metal transfer. Together with lithium isotopes, the research team will carry out a wide range of analyses, including in situ and whole-rock major- and trace-element geochemistry, sulfur isotopes and selected radiogenic isotopes.
High-pressure, high-temperature experiments have been planned for 2012. Specifically, a series of experiments at variable pressures and temperatures (1-3.5 GPa and 1000-1300 ºC) will be used to establish how chalcophile metals partition between sulfide melts (modelled on the Ivrea sulfides) and coexisting hydrous alkaline basalt melts. These will provide constraints on both the compositional characteristics of the melts involved in ore deposition and the role of physical gradients (pressure and temperature) during melt transport, in controlling ore deposition. The study will employ basaltic compositions that have already been well-studied at Macquarie University and for which partitioning data are already available for a wide range of elements and peridotite minerals. This will generate a well-integrated data set for both silicate and sulfide phases that will be applicable to the upper mantle/lower crustal conditions of the Ivrea-Verbano Zone.
Understanding the evolution of continental mantle in rifting environments is one key to understanding plate tectonics. How and why divergence is initiated in extensional zones, and how a continental rift evolves to become an oceanic accretion centre, are poorly constrained processes but fundamental to our understanding of the processes structuring the lithosphere. This project explores the upper mantle beneath the East-African Rift (Marsabit, Kenya) to constrain how the deformation has been initiated, localised and evolved in such geological settings. We characterise the microtextures and the deformation mechanisms recorded by mantle minerals such as olivine and pyroxenes, using EBSD techniques. With this information, we can determine the evolution of the deformation related to the rifting.
Progress during 2011:
We obtained a series of mantle xenoliths from the East-African Rift (Marsabit, Kenya) from Neuchatel University (Switzerland). The xenoliths are spinel peridotite and they display granular, porphyroclastic, mylonitic and ultra-mylonitic textures. We characterised both inter- and intra-crystalline deformation for all samples with EBSD. Results indicate that olivine from the granular peridotites displays a strong alignment of (010) axes, indicating the activation of the (100) glide-system. This glide system is unusual and rarely described but not reported as anomalous. Our microtextural observations combined with thermometry (Kaeser et al. 2006) indicate that this glide-system should be active at high temperature and low strain. The principal glide direction changes from  in the granular peridotites to  in porphyroclastic and mylonitic peridotites, with a dominant activation of the (010) glide system. This glide system is common in the upper mantle and also occurs at high temperature and low strain (e.g. Ben Ismail and Mainprice, 1998; Nicolas et al. 1971). A further inspection of intra-crystalline deformation of olivine porphyroclasts in the mylonite and ultra-mylonite peridotites indicates a range of active glide systems in the same rock. For instance the E-type (001) is the more active slip system in the mylonite, whereas in the higher-strain ultra-mylonite sample, the C-type (100) slip system is more active (e.g. Karato, 1995; Jung and Karato 2001). A decrease in temperature should result in a change in the dominant slip direction in olivine from  to  (Durham and Goetze, 1977; Bai et al., 1991, Tommasi et al., 2009), which is consistent with the decompression and cooling recorded by Marsabit xenoliths (from 1200˚C at 2.5-2.7 GPa to <850˚C at 1.5 GPa; Kaeser et al. 2006).
These new observations reflect the textural heterogeneity of the mantle beneath the East African Rift, related to the extensional setting. The mantle is composed of layers or lenses that are more deformed where the decrease in temperature and grain size acted as principal parameters to localise the deformation.
Aims and work plan for 2012:
In 2012, we will complete the imaging of all phases, including EBSD data on primary minerals and acquisition of EBSD data and cathodoluminescent images on carbonate phases. We will start chemical analyses to fully characterise all the mantle phases (including secondary) using the electron microprobe. Raman spectroscopy will be used to analyse fluid inclusions. Isotope analyses by ion microprobe on carbonates will then follow. These data should provide the composition of the fluids that formed the secondary phases.
The project aims to understand the genesis of the earliest continental lithosphere, including the processes of fluid/melt extraction that stabilise, and thus preserve, Archean cratonic lithosphere. This will involve isotopic studies of zircons from ancient terrains and deep-crustal xenoliths worldwide, to further constrain the nature of the oldest preserved crust, and a continued search for the oldest mantle samples beneath cratonic areas. Targeted studies involving (1) a regional characterisation of the 3.1 Ga Mpuluzi batholith (Swaziland/RSA) and (2) a similar characterisation of the end-Archean granites of the North China Craton, will provide a basis for static and dynamic modelling of the rheology of the crust before, during and after melt extraction.
This project has close linkages with the research of Future Fellow Dr Elena Belousova (Research Highlight). This will involve regional surveys (TerraneChron® approach) of zircons (U-Pb, Hf isotopes, O isotopes) from old continental areas, in an attempt to pick up the signatures of the oldest crust. The project also will study zircons from deep-crustal and mantle xenoliths in basaltic and kimberlitic rocks, to look deeper into the lithosphere. This work will be integrated with Os-isotope analysis of sulfides in the mantle xenoliths (see TARDIS project) to define the origins of the subcontinental lithospheric mantle beneath the old areas where ancient crust is identified, and constrain the role of the SCLM in stabilising ancient crust.
Two targeted areas will be investigated to look at some of the processes of crustal stabilisation. The Barberton Greenstone Belt and Ancient Gneiss Complex in Swaziland and adjacent RSA represent some of best-preserved and most-studied Early- to Mid-Archean (3.6 to 3.2 Ga) crustal remnants. There are several models for their formation, ranging from oceanic to continental settings; all may need revision in light of Sm-Nd data and unpublished zircon-Hf data (A. Kroener, pers. comm.) that indicate the participation of older crust (model ages ≥3.8 Ga). However, it is clear that the tectonic and magmatic processes that produced the greenstones, associated TTG magmatism, sedimentation and (locally high-grade) metamorphism ended abruptly at ca 3.1 Ga, with the emplacement of regionally extensive granitic magmas (Mpuluzi granodiorite, Nelspruit potassic granite, Boesmanskop syenite, Salisbury Kop granodiorite). These form thick sheets extending over at least 10,000 km2. This intrusive episode marks the final cratonisation of the crust in this area; little happened thereafter until ca 2.5 Ga. The sheer volume of these magmas raises several fundamental questions relevant to the generation and stabilisation of ancient crust in general. (1) What sort of materials were melted to produce the magmas? (2) Where did the heat come from? (3) What controlled the formation of regionally extensive sheets, rather than deep-rooted batholiths? (4) How many different pulses of magma were involved, and over what time span?
Collaborators include Professor Alfred Kroener (Mainz, and Beijing), a world-recognised expert on Precambrian geology and in particular that of southern Africa. This incorporates the PhD project of Ms Rosanna Murphy, who began in 2011 with Professor Alfred Kroener as an external supervisor.
The second area to be investigated will be the southern part of the North China Craton, where a similar “granite bloom” at the end of the Archean appears to have been the defining event that stabilised the crust, although studies of deep-crustal xenoliths have already shown that felsic-intermediate crust was already in place by 3.4 Ga.
Progress during 2011:
Dr Belousova starts her Future Fellowship in 2012, but during 2011 she carried out field work to collect samples for the project. We await the identification of an appropriate cotutelle student to undertake the Chinese strand of the project.
In a separate development, new PhD student Yuya Gao (cotutelle with CAS, Beijing, a CCFS Partner) began a thesis analysing Li isotopes in A-type granites, following from the success of Dr Xian-Hua Zhou in establishing this technique during 2011. These granites are typically the latest components in “granite blooms” and this work will be directly relevant, as an analogue, to the aims of the project.
Ms Murphy took part in an international conference in Johannesburg in January 2011; this was followed by a field trip that introduced her to the area and allowed her to carry out a sampling program, assisted by personnel from CCFS and the Swaziland Geological Survey. The samples she returned were partly processed during 2011. She has carried out petrographic studies, whole-rock analysis of major- and trace-elements, and separation of zircons from a large number of selected samples. The zircons have been analysed for U-Pb ages and Hf-isotope composition, and in some cases for trace elements. The results so far show that the degree of compositional homogeneity in the batholith has been somewhat overstated. They have confirmed the narrow spread of intrusion ages around 3.1 Ga, but they also have shown that most samples contain a scattering of older inherited zircons, stretching back to 3.5-3.6 Ga. This is consistent with the Hf isotopes, which give model ages in the same range. There is, thus far, little evidence of a Hadean crust in the area.
Published outcomes 2011:
CCFS Publications #12, 97
Aims and work plan for 2012:
The aims of the project will remain unchanged. Dr Belousova and Ms Gao will begin their analytical work in 2012. Professor Griffin will start a program of analysis of sulfide and alloy phases in the most depleted xenoliths from the Kaapvaal Craton, aimed to test if there are remnants of lithospheric mantle older than previously known.
Ms Murphy plans another field trip to Swaziland for supplementary sampling, together with a French group who have overlapping interests in the area. The field trip will be preceded by a conference, in which she plans to present her results so far. This will be an excellent chance to develop more international connections, and to interact with a very active group with similar interests. Through the first 2/3 of the year, her analytical program will be completed, expanding it to include the new samples. She will analyse O-isotopes in the zircons by SIMS. She also will obtain whole-rock isotopic data on selected samples, for comparison with the data gained from the zircons. With these results in hand, she will begin a program of thermal/compositional/dynamic modelling (with Dr Craig O’Neill) to constrain the processes that produced these huge volumes of magma in such a short time. One alternative is the in situ heat generated by a K-U-Th-rich deep crust blanketed by a refractory upper crust; another is a deep-seated source such as a mantle overturn.
Rosanna Murphy collecting samples in Swaziland.
4. Two-phase flow within Earth’s mantle: modeling, imaging and application to flat subduction settings
The overarching goal of this project is the development and application of in-house state-of-the-art computational tools to simulate and image complex geochemical-geodynamic processes involving two-phase reactive flow in multi-component deformable media. These tools, in combination with advanced seismic imaging techniques, will be applied to the problem of fluid transport from shallow-dipping to flat subduction settings, in both the western USA and South China in the past.
Progress during 2011:
The project started in mid 2011. Work has included collating a shortlist of candidates for the Postdoctoral and technician positions. The work roles have been filled by casual employment of computer technicians over 2011-2012. A fulltime employee will be appointed in early 2012 for the technician position. A formal advertisement for the postdoctoral position is currently being finalised for printing in Eos.
Visualisation pathways for modelling subduction zones settings have been developed, and code to process existing plate data into input format for geodynamic models has been written and debugged. Preliminary models of global plate-driven subduction in 3D have been finished. High-resolution modelling of these zones, with vizualisation, is currently underway. This modelling will be combined with coding on CitcomS to incorporate fluid hydration and dehydration during the subduction cycle.
Aims and work plan for 2012:
The aim in 2012 is to complete coding for a CitcomS module to track hydration and dehydration of subducting lithosphere, and to begin work on the first phase of a coupled finite-element implementation of a two-phase flow code.
The CitcomS module will enable the rapid development of models for global slab dewatering, which will have immediate science outcomes in the latter half of 2012. The FE code development is part of a larger implementation of MPMCRF that will continue over 2012-2014 with development, benchmarking and testing.
Whole mantle convection for an Earth-like planet. Shown are hot upwellings (yellow) and an equatorial temperature cross section.
1. To define the nature of the first life in the early Archean and links between the early evolution of life and the rise of atmospheric oxygen in the Neoarchean
2. To understand the evolution of the Earth’s oceanic and atmospheric composition during the Archean and Paleoproterozoic
3. To evaluate the links between the evolution of the sulfur cycle and the formation of important Archean submarine ore deposits
Context and rationale: Mass-independent fractionation (MIF) of multiple sulfur isotopes d34S, D33S and D36S in rocks older than 2.45 Ga provides the best evidence for changes in Earth’s early atmosphere and ocean due to the evolution of the sulfur cycle. Recent work indicates that variations in the MIF record have important implications for understanding changing environmental conditions in the evolving Archean Earth system. Prior to 3.1 Ga the MIF record is characterised by significant MIF, with reduced variation between 3.1 and 2.8 Ga and increased variation from 2.75 to 2.5 Ga followed by reduction during the Great Oxidation Event (2.45 and 2.32 Ga). We have the potential to analyse samples that may resolve how these changes may have been linked to tectonic events (e.g. a fluctuating sulfur cycle linked to increased subaerial volcanic gas as continents grew) and the evolution of life. Multiple sulfur isotopes are important for the investigation of biological processes on the early Earth. They have the potential to elucidate the types of life present in Earth’s earliest sedimentary environments, as well as to trace the transfer of sulfur in fluids and gases from the interior of the Earth, via the atmosphere and hydrosphere into the biosphere.
Although compilations of sulfur isotope data from Archean sedimentary rocks indicate the sulfur content of the oceans was very low, Archean greenstone successions contain some of the world’s largest submarine metal sulfide deposits and these commonly have non-zero D33S values. A better understanding of the role of different sulfur sources in Archean mineral systems will provide important insights into the evolution of the early sulfur cycle as well as keys for mineral exploration. The analysis of multiple sulfur isotopes in sulfide inclusions in mantle-derived diamonds and eclogites from kimberlites has the potential to indicate details of recycling of Archean sulfur to depth in the crust and mantle, and sulfide inclusions in Hadean zircons and meteorites have the potential for understanding very early Earth fluids.
One problem that has held back research in the biological field is the spatial resolution at which multiple sulfur isotopes can be analysed. Bulk analyses can lead to artificial homogenisation of sulfur-isotope signals in a sample, and a lack of understanding of processes on the micron scale, where microbial mediation may be observed. For example, recent Nano SIMS sulfur isotope studies have revealed large variations in d34S in both modern and ancient microenvironments. These small-scale heterogeneities in sulfur isotope composition cannot be detected by bulk techniques, emphasising the growing need to study sulfur-isotopes on a spatial scale relevant to microbial processes. Hence, as an extension to studying multiple sulfur isotopes in Archean sediments and mineral deposits we will also investigate the biogeochemistry of sulfur using samples from key times in Earth history (e.g. early Archean to investigate Earth’s earliest life; late Archean to investigate the interplay between sulfur metabolisms and the rise of atmospheric oxygen), and will incorporate new technology available at the CMCA. An increased understanding of the biological S-cycle is important not only for studies of life in the Archean, but also for elucidating the composition of the early atmosphere, the mechanisms for cycling of these elements between the lithosphere, biosphere and atmosphere, and the cycling of other elements essential for life.
Biological applications of multiple sulfur isotopes.
The major aims of the project require analysis of multiple sulfur isotopes from relevant samples, some that we already have and others to be collected soon.
The most appropriate method for bulk multiple sulfur isotope analyses is use of the EA-CF-IRMS at the University of Queensland (key analyses in the 2009 DP) this will be used to see which sedimentary and mineral deposit samples require more detailed spatial analyses. The CAMECA IMS 1280 ion microprobe is capable of measuring d34S and D33S with precisions of better than 0.2‰ and from samples as small as ca15 mm. For D36S it can achieve precisions of better than 0.3‰ from samples as small as ca 40 mm. The CAMECA Nano SIMS 50 extends this capability to samples as small as 2-3mm for d34S. This will allow us for the first time to analyse d34S, D33S and D36S from individual sulfides and sulfates at the correct scale to identify individual microbial processes.
Innovation: This is the first time all three analytical methodologies for multiple sulfur isotopes have been available in Australia and will provide the chance of very important high-impact results and one of the best global databases.
Progress during 2011:
The research plans for this project are to obtain appropriate samples and undertake the analyses to meet our objectives. In this context we have been able to get appropriate sets of samples from Mesoarchean (3.3 to 2.8 Ga) greenstone belts and volcanic-hydrothermal massive sulfide deposits and komatiite-hosted Ni sulfide deposits. Australia is the only place with a wide variety of samples of this key age, which has been a gap in the existing global database. We have got the first excellent results from the 2.9 Ga Lake Johnston Belt and Murchison showing key evidence for variations in the sulfur cycle through this period of time rather than just minor oxygen fugacity variations before the great oxidation event (key data shown in diagrams). These data show that the strongest variations in the Mesoarchean S MIF record are linked to the only preserved evidence for major volcanic events with some subaerial volcanism producing the key volcanic gas. The largest 2.95 Ga deep submarine volcanic massive sulfide deposit, Golden Grove, is evidence for major sulphur recycling after plate tectonics started. These are important results and we will submit manuscripts to key journals in the next few months. The CIs have given invited talks at conferences and the PhD students have given posters. PhD student Carissa Isaac (started 2010), whose initial analytical work was partly supported by the Discovery Grant (VMS and sedimentary S isotopes in the North Eastern Yilgarn), has gotten a large, very appropriate batch of samples for ongoing research in this project.
Completing the planned collection of key samples and their analyses plus putting together a good batch of publications will be the milestones for the next year. We have large batches of samples collected in 2011; some are already analysed and the rest will be analysed in the future in the CoE. We are aware of other key areas and important mineral deposits in the Murchison and West Pilbara that can provide more important samples and there are three good PhD students working on this with us. We thus have the potential for some more important milestones in getting a much better understanding of the variations of the sulfur cycle and its links to tectonics, life and mineral deposits in the early Earth between 3.5 and 2.4 Ga.
Using these different methods has given us the first strong data for how the methods fit together for this important work. We are also collaborating with international leaders in multiple sulfur isotope geochemistry (James Farquar, Boz Wing, Shuhei Ono, Doug Rumble, Sue Golding, Jay Kaufman etc) in 2011 and 2012. We also plan to convene a 2 to 3 days MIF Work Shop in Australia (Sydney) with these international leaders.
Aims and work plan for 2012:
David Wacey has just taken up his CCFS Research Associate position. He is the CMCA person with expertise in early life and related biogeochemistry and astrobiology. He will continue collecting an excellent set of samples and examining them by Nano SIMS and IMS1280 to study early life in 2012. The samples sets include the following
3490 Ma Dresser Formation – putative pyritised microbial mats and microbially- induced sedimentary structures in a MIF world. We will conduct multiple sulfur isotope analysis of microbial-like laminations (putative tufted mats) that have been heavily pyritised (March/April 2012). Are the data consistent with microbially-mediated pyrite – if so, what type of metabolismwas involved? How do the data compare with previous data from other environments (e.g. Dresser barite lithology and Strelley Pool Fm sandstone lithology) at this time? Or are the data more consistent with hydrothermal pyritisation? The data also may provide additional insights into the composition of the atmosphere at this time.
1900 Ma Gunflint chert – pyritised microbial communities in a post-MIF environment. We will conduct multiple sulfur isotope analysis of pyritised microfossils (March/April 2012). Was pyritisation induced by microbial sulfate reduction, a combination of metabolisms, or was it hydrothermal? Focused ion beam and TEM techniques (April 2012) will be used to track the conversion of organic fossil material into pyrite (and later into hematite) at the nano-scale in order to understand more about the mechanisms of fossilization in these different materials. These are also excellent analogues for Archean silicified microfossil assemblages, which will provide a baseline for comparison with Archean data.
Experimentally produced modern microbes (Winter/Spring 2012). We will obtain in situ multiple sulfur isotope data at the micron scale from sulfides precipitated by cultures of the sulfate-reducing Archaea Archaeaglobus fulgidus. This will be the first multiple sulfur isotope data set obtained from individual micron-sized sulfides precipitated in controlled microbial experiments. The results will aid in the interpretation of similar micron-scale sulfur-isotope data obtained from the Archean rock record.
We also will do more field work in other key areas, including mineral deposits in the Murchison and West Pilbara, to find important samples and analyse them for S MIF data with the appropriate techniques. It is likely we will start a new international PhD student working in specific areas of multiple S isotope data.
We plan to hold a Workshop on Mass-Independent Fractionation processes in Sydney, involving the international leaders in multiple sulphur isotope geochemistry with whom we collaborate. This will be the first major meeting about this important scientific development and the different analytical approaches and will be a high-impact international CCFS event. We plan to produce a benchmark publication from this for Reviews in Mineralogy and Geochemistry.
We also will do more field work in other key areas and mineral deposits in the Murchison and West Pilbara to find important samples and analyse them for S MIF data. It is likely we will start a new international PhD student working in specific areas of multiple S isotope data.
A Workshop on Mass-Independent Fractionation processes is being planned to be held in Australia (Sydney). This will be the first major meeting about this important science and the different analytical facilities and will be a high-impact international CCFS event. We plan to produce a benchmark publication from this for Reviews in Mineralogy and Geochemistry (see Research Highlight).
The main goals of this project are (1) testing a groundbreaking hypothesis that the birth and death of a supercontinent on Earth’s surface is intimately linked to the spatial and temporal location of superplumes, and that cyclic supercontinent-superplume events and associated fluid events dominate Earth’s evolution; (2) pushing our knowledge of the Earth’s palaeogeographic and geodynamic history back to the Archaean.
Context and rationale:
How plate tectonics on the Earth’s outer shell interacts with its mantle, and what drives plate tectonics, remain a challenge to the 21st century geoscience community.
Does plate tectonics drive mantle convection, or vice versa? What determines the formation, positions and lifespan of hot mantle plumes and superplumes deep in the mantle, and how do plumes/superplumes interact with plate dynamics? Are mantle plumes and superplumes fixed relative to the Earth’s rotation axis? Are the formation and breakup of supercontinents in Earth’s history accidental events, or more regular events determined by some internal mechanisms?
Kong-Yang Zhu ready to take core samples of mafic dykes in the Ravensthorpe region of southeastern Yilgarn Craton.
While working on the evolution of Rodinia, Z.-X. Li and co-workers noticed that the assembly of Rodinia was followed by a >100 million years (Ma) episode of worldwide plume activity that is temporally linked to the breakup of the supercontinent. The sequence of events resembles that during Pangaean time both in its lateral scale and its duration, including a time lag of tens of millions of years between the final assembly of the supercontinent and the onset of plume breakout. More intriguingly, Neoproterozoic palaeomagnetic data suggest that Rodinia and the mantle plumes beneath it may have rotated rapidly between ca 0.81 and 0.75 Ga, confirming that mantle plumes and supercontinents are probably coupled. Did such coupling occur during Earth’s earlier history?
In this project we will test the hypothesis further by analysing global palaeogeography back to the Neoarchaean era. Primary data gathering in the first three years will focus on numerous Paleo- to Mesoproterozoic Australian igneous complexes with studies of palaeomagnetism and geochemistry. We will compare the Australian records with those of the entire world through international collaborations.
Progress during 2011:
We made progress on two fronts in this project during the start- up year of CCFS. First, we appointed two key personnel for the project. Dr Sergei Pisarevsky, a world leading palaeomagnetist with much experience in making global palaeogeographic reconstructions, joined the Curtin Node in late March 2011 as a half-time Senior Research Fellow. He is leading part of the palaeomagnetic investigations in the project. We also secured the continuation of Dr Xuan-Ce Wang, a Senior Research Fellow specialising in geochemistry of mafic rocks and mantle evolution. Xuan-Ce was later awarded one of the CCFS’s ECSTAR fellowships, but will continue to lead the geochemical and petrological aspects of the project as well as carrying out his ECSTAR project. We are still seeking new PhD students to be involved in this project. Key collaborators are Professor D.A.D. Evans (Yale University), Dr R. Ernest (Ernestgeosciences) and Associate Professor E. Tohver (University of Western Australia).
On the scientific front, we carried out the first sampling trip to the southeastern Yilgarn Craton (the Ravensthorpe region) in June, targeting the ca 1.2 Ga and possibly younger dykes for palaeomagnetic, geochronological and geochemical/petrological investigations. The palaeomagnetic analysis is nearly complete; geochronological and geochemical/petrological investigations are ongoing.
Aims and work plan for 2012:
In 2012 we aim to complete all analyses of the Ravensthorpe dykes, and write up the results for publication. We also plan to carry out targeted sampling trips to key parts of Western Australia, and to northern Australia if PhD students become available in time.
Palaeomagnetic sampling trip to mafic dykes in the
Ravensthorpe region of southeastern Yilgarn Craton.
Earth was assembled from solar and planetary debris, and the heat generated during this process would have expelled volatile elements into space. Several models suggest that Earth’s current inventory of water and other volatiles was added by a “late bombardment” of cometary material. However, the scarcity of water on the Moon, and our recent discovery that Earth may have had surface water 4.2-4.4 billion years ago, raise key issues about the sources and role of water during Earth’s early evolution.
Geochemical and micro-structural studies of lunar samples, meteorites, and Earth’s oldest rocks and minerals can tell us how and when Earth became “wet”, and link the geochemical evolution and deformation histories of proto-planetary bodies. The isotopic composition of meteorites will be used to define the types of material that went into the construction of Earth and other terrestrial planets, and to give a better baseline for comparison with Earth. We will use a variety of techniques, including synchrotron-based spectroscopy and high-precision stable-isotope analysis of oxidation/reduction-sensitive elements such as Cr, Fe, O, C and S in minerals from Earth, the Moon and meteorites, to assess planetary oxidation states and give new clues as to how the planets formed. Some of these elements have short-lived isotopes (e.g. 53Mn → 53Cr, half-life 3.5 Ma) and can be used to investigate how oxidation states changed during the accretion and very early evolution of Earth.
Mars is the only planetary body in the Solar System where surface conditions similar to those observed on the Earth (i) may have existed in the past and (ii) can be studied directly via samples of Martian meteorites. As such, Mars provides a unique opportunity to test models of atmospheric, hydrospheric and biospheric evolution, as well as climate variations possibly similar to those developed on Earth. It appears that a relatively early record of this evolution, which is very incomplete on Earth, is preserved in Martian samples. We can potentially utilise information obtained from Martian meteorites to apply new constraints on the initial state of Earth’s atmosphere and hydrosphere. Information about the Martian surface environment can be extracted through analysis of a suite of low temperature minerals, including carbonates, preserved in several of these meteorites. On Earth, these minerals are commonly associated with sedimentation, diagenesis, and precipitation from either surface or groundwater. However, carbonates that occur in several Martian meteorites are included in magmatic rocks and their origin is poorly understood, with models ranging from formation in high temperature hydrothermal solutions to precipitation from low-temperature surface water.
We will utilise information obtained from extra-terrestrial sources with ongoing analysis of Earth’s oldest zircons in order to provide new constraints on the nature, evolution and weathering of Earth’s earliest crustal remnants. Several outstanding questions remain: 1) what is the significance of the inclusion suite in ancient zircons which, where amenable to dating, are always younger than their host; 2) were the earliest zircons derived through melting of a solidified magma ocean or do they require extensive continental crust and oceans; and 3) when was the onset of some form of plate tectonics on Earth that led to transfer of material back into the mantle? These and other related questions provide the inspiration for this project.
The aims are twofold:
Part 1: Characterising the earliest crust on Earth: The primary research objective is to characterise the nature and origin of the earliest crust on Earth. This will be achieved by comparing information already acquired from the 4.4-3.5 Ga detrital zircon suite at Jack Hills with comparable new data obtained from the earliest known rocks on Earth, preserved in Antarctica, Canada, China, Greenland, India and Western Australia. We will document the changes that took place from the formation of the earliest zircons (4.4 Ga) to the time when crust was widely preserved (~3.5 Ga) and evaluate why so few Hadean rocks survived on Earth.
Part 2: Characterisation of minerals and related fluids phases in Extraterrestrial samples: The aim of this study is to compare conditions in the early solar system with those on the early Earth and extends the scope of the original project description. Initial work will focus on the Martian meteorites with the aims of (1) characterising the presence of fluid and its interaction with minerals and (2) obtaining information on primitive mantle deformation mechanisms and processes. This will be done by combining geochemical and microstructural techniques such as SEM imaging, electron and ion microprobes, EBSD and Raman.
Progress during 2011:
Part 1: Zircon from the Napier Complex in Antarctica was recognised in earlier studies as having high U contents and structural complexities. Analysed zircons are typically dark in CL and yield 207Pb/206Pb ages up to 3.9 Ga; some of the oldest grains are reversely discordant. Zircon grains have also been ion imaged by SIMS using a single collector for Ti, Y, Hf, Pb, Th and U and a multicollector for the Pb isotopes. Patchy distribution of Pb and Ti does not correspond to either zonation or crystal imperfections and appears to confirm redistribution of radiogenic Pb (now unsupported).
Preliminary work in the Dharwar Craton of India indicates TTG components as old as 3.3 Ga in the eastern part. However, the major events occurred in the Neoarchean with magmatism at ~2.55 and ~2.15 Ga.
Part 2: We have received 5 thin sections of Martian samples (Zagami, Nakhla and ALH84001) from the Smithsonian Institute, the Johnson Space Centre Curation Office and the WA Museum. We have completed initial work on microstructure characterisation on the Zagami and Nakhla samples using Secondary Electron Microscopy (SEM) and SEM coupled with an Electron Backscatter Diffraction (EBSD) system to determine the crystallographic preferred orientation (CPO) of minerals. The Zagami sample corresponds to the “Normal Zone” (NZ) composed of pyroxenes exhibiting a foliated texture interpreted as mantle deformation prior to shock. The NZ portion is cross-cut by glass veins of shock-melt that seem to follow the alignment of pyroxenes. Our preliminary CPO results indicate that clinopyroxenes in the Nakhla sample are randomly oriented whereas clinopyroxenes in the Zagami sample indicate two different orientations. These orientations are still not well constrained, but might be the result of mantle flow. The major part of this study, related to microtextures, is to characterise the internal features and textural relationships between minerals present in the samples. Preliminary EBSD maps of minerals including pyroxene, olivine, feldspar and apatite indicate weak internal deformation with subgrain formation, and the deformation is apparently independent of the orientation of polysynthetic twinning, which has been caused by shock (Fig. 1). We have also completed initial work aiming to characterise fluid inclusions and secondary phases (carbonates) in some of these samples. In particular, we have identified a number of carbonates in our thin sections of sample ALH84001 and characterised their internal features and textural relationships with other rock forming minerals, using optical microscopy, SEM imaging and x-ray analyses. These are Fe-Mg-Mn-Ca carbonates that are globular and growing in small vesicles (Fig. 2). They are strongly zoned with Mn- and Ca-rich cores and Fe- and Mg-rich rims.
Figure 1. Photomicrograph and EBSD map of clinopyroxenes in Nakhla sample. (a) Cross polarised picture of clinopyroxenes with polysynthetic twinning. (b) Cumulative misorientation maps derived from EBSD data showing internal texture of 7 clinopyroxene grains, from a reference point (red cross) in the blue area to a maximum represented by the red area (up to 7˚).
Aims and work plan for 2012:
Part 1: Work will continue on the Antarctic samples with special emphasis on ion imaging of zircons to determine variations in the elemental and isotopic distribution. For comparative purposes, similar studies will be undertaken on the oldest zircons from Jack Hills. Work will commence on zircons extracted from the oldest rocks in Canada and Greenland, with a possible field excursion to obtain additional samples from key localities. The acquisition and interpretation of oxygen, lithium and lutetium-hafnium data from zircons extracted from the most ancient terrestrial rocks in Antarctica, Canada and Greenland will enable us to make a preliminary evaluation of what changes, if any, took place on Earth between consolidation of a magma ocean and the extensive development of continental crust.
Part 2: We will finish the imaging of all mineral phases, in particular complete EBSD on primary minerals and acquire EBSD and cathodoluminescence images on carbonate phases. We will also undertake chemical analyses to fully characterise the mantle phases as well as the secondary phases, using the electron microprobe. Raman spectrometry will be used to analyse fluid inclusions. Once this is completed we will be able to start isotope analyses on carbonates using the ion microprobe. These data should provide the composition of the fluids that permitted the secondary phases to crystallise.
Figure 2. Optical photomicrograph of carbonate phases (orange) in ALH84001.
The aims of the project are:
- To combine LAM-ICPMS analysis of diamonds, developed at Macquarie, with other types of in situ data to define the nature and evolution of diamond-forming fluids
- To constrain the causes of isotopic variability of carbon, oxygen and nitrogen in diamond-forming fluids; are these primary signatures, or do they reflect isotopic fractionation during diamond growth?
- To understand the links between diamond formation and the redox state of the lithospheric and asthenospheric mantle
- To develop a new exploration/evaluation methodology for application to kimberlites, by defining the trace-element signatures of mantle minerals that have been exposed to diamond-bearing fluids
- To better characterise different types of mantle fluids and their interactions with mantle rocks
- This project engages Postdoctoral Researcher Daniel Howell, and PhD students Ms Ekaterina Rubanova and Ms Yao Yu.
Progress during 2011:
The aims of the project were expanded somewhat during 2011, to characterise other types of fluids in the lithospheric mantle. PhD student Yao Yu (cotutelle with Nanjing University) is carrying out an FTIR-based study of the distribution of water in different types of lithospheric mantle, using xenoliths from basalts and kimberlites. The first phase of her study, involving spinel peridotites from the North China Craton, was accepted for publication, and she has begun preparing samples of a range of previously-studied xenoliths from the kimberlites of the Kaapvaal Craton.
O’Reilly and Griffin wrote up a review and synthesis of metasomatic processes in the mantle, for an online review volume and teaching resource that will be launched at the Montreal Goldschmidt conference in 2012 (CCFS Publication #5).
Dr Howell, working together with Dr Craig O’Neill, completed the development of a software package (DiaMap). This allows the construction of maps derived from the reduction of the huge volumes of FTIR data collected by our state-of-the-art FTIR spectrometer. A full description of the program has been submitted to a journal; once the paper is accepted, the software will be put on the CCFS website as freeware. Dr Howell also completed a study of a unique suite of centre-cross diamonds, revealing new information about the processes of diamond growth; a manuscript on this work is in review, along with another on the development of platelets in cuboid diamond growth. Dr Howell initiated a detailed study of a large parcel of diamonds from the Diavik kimberlites (Lac de Gras, Canada), many of which show several stages of growth, of varying habit. Imaging of these stones is progressing, and some have been analysed for C-isotopes. He also carried out a long series of experiments designed to greatly improve the detection limits for the LAM-ICPMS analysis of diamonds; at year’s end, the preliminary results were exciting. Finally, he has written and submitted a long series of articles on his results from 2010 and 2011 (this is a relinquished project).
Another important development in 2011 was the start of carbon-isotope analysis in diamonds, using the SIMS lab at University of Western Australia. CCFS has provided a position for a technical person in this lab (not filled in 2011) and the laboratory has allocated a significant proportion of its capacity to CCFS projects. Dr Howell and Ms Rubanova have both benefited from this in 2011, making trips to carry out analyses that were previously done in Edmonton (Canada). The results show excellent agreement between the two laboratories, and similar internal precision. The remaining challenge will be to improve the homogeneity of our available standards, which at present constrain the external precision.
Ms Rubanova is studying the relationships between diamonds and silicates in suites of diamondiferous eclogites and polycrystalline diamond-silicate intergrowths. In 2011 she carried out both C-isotope analyses of her diamonds, and O-isotope analyses of the coexisting silicates, to investigate possible covariations of these two systems in the fluids that deposit diamonds. Mentored by Research Associate, Associate Professor Sandra Piazolo, she also has adopted a new technique, Electron Backscatter Diffraction (EBSD) to look at the microstructures of the polycrystalline diamonds. This revealed that, rather than being deposited as fine-grained aggregates, the “diamondites” were originally quite coarse-grained and that the grain size has been reduced by deformation and recrystallisation. A paper on this work is under review.
We continued our collaborations with Russian and Israeli colleagues on the nature of diamond-related fluids, which is directly relevant to questions of diamond growth.
CCFS Publications #2, 52, 135
Aims and work plan for 2012:
The aims of the project, as expanded in 2011, will be followed in 2012.
A new eximer laser will be installed early in 2012, for use with the quadrupole ICPMS. Dr Howell will use this system to carry out ultra-trace analyses of diamonds showing multiple stages of growth and/or mixed-habit growth, to quantify the trace-element characteristics of the fluids from which such diamonds grow. This will then be cross-compared with our extensive database of fluid compositions in fibrous diamonds. The new techniques also will be used to re-evaluate our older data on gem-quality diamonds, where many analyses were near the detection limits available at the time. C-isotope analyses will be done on the same diamonds, to correlate trace-element patterns with isotopic characteristics. Dr Howell also will complete his C-isotope studies on zoned and centre-cross diamonds.
Ms Rubanova will complete the planned program of chemical, isotopic and microstructural analysis of her collection of diamonds, diamondites and diamondiferous xenoliths, and prepare papers for publication as well as the first parts of her PhD thesis. She will present some of this work at the 10th International Kimberlite Conference in India (February 2011).
Dr Piazolo has been awarded a Future Fellowship, and will be building up an EBSD capability at Macquarie-CCFS in 2012, which will facilitate further studies of diamonds and their host rocks.
Ms Yu will carry out FTIR studies of the Kaapvaal xenoliths; samples are now being prepared in Nanjing. This is expected to result in at least one paper.
Photomicrograph (A) of a polished diamondite; (B) EBSD map of the area in (A); (C) and (D) traverses showing gradual (1) amd abroupt (2) changes in crystallographic orientation in single diamond grain.
9. 4D lithospheric evolution and controls on mineral system distribution: The Western Superior-Yilgarn comparison
The project will provide a very well-constrained case study in an Archaean craton outside of the Yilgarn to (1) apply multi-isotopic (U-Pb, Lu-Hf, O) analyses of zircon to map lithospheric architecture in space and time, (2) determine if the distribution of mineral systems (VMS, Fe, NiS, Au) shows strong control by this architecture, as it appears to in the Eastern Goldfields Superterrane of the Yilgarn Craton, and (3) generate mappable exploration criteria for targeting exploration for various Archaean mineral systems at the craton to terrane scale.
Context and rationale:
Recent studies (Champion and Cassidy, 2007; McCuaig et al., 2010; Mole et al., 2010; Begg et al., in press) have demonstrated in the Yilgarn Craton of Western Australia that multi-isotopic maps (Lu-Hf and U-Pb in situ analyses of zircon combined with whole-rock Sm-Nd data) are a powerful tool to map crustal growth and image lithospheric blocks of different age (putative paleocraton margins). Moreover, these studies have pointed to a strong spatial correlation between these lithospheric block edges and the location of large concentrations of several styles of mineral deposits. The interpretation is that these isotopic boundaries mark lithosphere-scale structures that control the flux of mass and energy (and thus the location of large mineral systems) through time.
The western Superior Province of Canada is the perfect place to undertake a comparative study to complement the Yilgarn example. The western Superior is host to major deposits of gold (Red Lake Camp, Musselwhite mine, Hemlo), VMS (Kidd Creek) and diamonds (Victor Mine) as well as the recent discoveries of the world-class chromite and nickel deposits in the Ring of Fire. However, the western Superior Province has important contrasts to the Yilgarn in that it is relatively VMS-rich, and NiS-poor. Moreover, the Western Superior is an exceptionally well-constrained Archaean craton, with well-defined stratigraphy, an abundance of high-precision geochronology to constrain its evolution, and abundant high-quality geophysical data including crustal seismic and potential field datasets. Current thinking is that the southwestern Superior Province comprises largely juvenile lithosphere at ca 2.75-2.65, growing by combinations of arc-plume interactions and accretion of terranes. This contrasts with the Yilgarn Craton, comprising largely evolved lithosphere at that time, and tectonic models that range from intracratonic rifting through to assembly of accretionary terranes. Systematic delineation of lithospheric blocks of different character would radically change the views on assembly of these ancient cratons.
This project will investigate the U-Pb-Lu-Hf-O isotope characteristics of zircons from key units in the Wabigoon subprovince of the western Superior Province, in order to obtain a better understanding of the evolution, architecture and preservation of this complex 3.0-2.7 Ga Archaean terrane and the mineral deposits that formed within it.
Figure 1. Nd model ages with Au (yellow) and Ni (red) deposits in Yilgarn Craton (After Cassidy and Champion, 2007).
Progress during 2011:
Yongjun Lu was appointed as a research associate in October 2011 to undertake this project. The access to zircon samples allows a value-add on well-constrained samples. Collaboration with Peter Hollings at Lakehead University, Don Davis and Kirsty Tomlinson at Ontario Geological Survey have given us access to zircon samples that have already been characterised in terms of location and context, whole rock geochemistry, U-Pb geochronology and, in many cases, whole rock Sm-Nd.
Regarding the task of analysing 70 samples in the first year, there have been ca 50 samples available for SHRIMP U-Pb dating and Hf-O isotopic analysis. All available zircon samples have been sent to Minsep in Denmark, WA for preparation of zircon mounts. The first two mounts (6 samples) have been returned and are scheduled for SHRIMP dating on March 2nd and 9th 2012. Other mounts are expected to be finished by mid-March. Subsequent oxygen and hafnium analysis will be scheduled once SHRIMP dating is done.
Aims and work plan for 2012:
One aim is to obtain SHRIMP U-Pb ages, oxygen and hafnium isotopes of ca 50 samples which are already in preparation by August 2012.
A field trip is scheduled in mid-2012 for additional selective sampling of critical units across interpreted terrane boundaries in order to ascertain the location of these boundaries in a whole-lithospheric context.
One manuscript is expected to be submitted on the first batch of SHRIMP U-Pb and Hf-O isotopes by the end of 2012.
This project is a major initiative by the Geological Survey of Western Australia (GSWA) to integrate the results of 1:100,000 scale regional mapping, geochronology and geochemistry with newly acquired high-quality geophysical data, including gravity and aeromagnetic data, and deep seismic and magnetotelluric surveys.
The aims of the project are to integrate the data into a 4-dimensional, integrated model of crustal evolution for the western Yilgarn Craton, and to derive a better understanding of the mineralisation processes for this region, how the crust links to underlying lithospheric mantle, and what relationships this piece of lithosphere has with the more highly endowed, eastern part of the craton (Eastern Goldfields Superterrane (EGS)).
Context and rationale:
The Yilgarn Craton of Western Australia is a large and highly complex piece of Archean crust with a long history extending from 4.4–2.6 Ga. It is locally well endowed with a variety of mineral deposits, particularly gold and nickel in the EGS. Previous work has identified a number of lithostratigraphic terranes interpreted to have had distinct crustal histories prior to tectonic amalgamation at ca 2.65 Ga. However, recent work by the GSWA in the northwestern part of the craton has identified a long-lived, autochthonous history of crustal development there, including episodes of volcanism, granitic magmatism, shearing and gold mineralisation that are similar in composition and temporal development to those further east, in what has been interpreted as the accreted, younger part of the craton (EGS). This, together with a number of other features of the surface geology, suggests there are significant problems with current models of crustal development through arc-accretion tectonics.
This project incorporates 2-D and 3-D imaging, modelling and analysis of newly acquired deep seismic, magnetotelluric, gravity, and aeromagnetic data along three linked transects totalling 700 km in length across the northwestern part of the Archean Yilgarn Craton, Australia. These geophysical data will be synthesised in conjunction with new geological data to develop an integrated lithospheric model of the Narryer and Youanmi terranes, and their relation to the Eastern Goldfields Superterrane. Additional studies will include integrated analysis of Lu-Hf in zircons from dated surface samples in order to better understand the roles of juvenile mantle additions to the crust and crustal recycling, and when these different, but possibly linked, factors occurred. The project involves close collaboration between staff of the Geological Survey of WA and researchers at Macquarie and UWA.
The proposed research project will apply well-established, existing research technologies to the previously poorly studied, and underexplored, northwestern part of the Yilgarn Craton: it is the first study of this kind to cover over more than half of the craton. The project is further innovative in developing an integrated research framework from mantle through to crust, involving a team of researchers with different expertise at a variety of scales.
Progress during 2011:
The project will commence in 2012.
Aims and work plan for 2012:
The aims for 2012 are to develop a preliminary 3D model of key areas along the Youanmi seismic reflection lines YU1, YU2 and YU3. This will be achieved by collaborative research between GSWA (Gessner), UWA (Gorczyk), UNSW (van Kranendonk) and Macquarie University (TBA, Belousova, Yang, Afonso, O’Neill). A new Research Associate (employed through Macquarie) will be appointed and based in the Centre for Exploration Targeting at The University of Western Australia in Perth. This researcher will, in collaboration with Macquarie geophysicists, conduct 2-D and 3-D imaging, modelling and analysis of newly acquired deep seismic, magnetotelluric, gravity, and aeromagnetic data along three linked transects totalling 700 km in length across the northwestern part of the Archean Yilgarn Craton, Australia. These geophysical data sets will be used in conjunction with newly acquired geological data with the aim to develop an integrated lithospheric model of the Narryer and Youanmi terranes, and their relation to the Eastern Goldfields Superterrane (see Research Highlight).
Modern geochronology has led to substantial advances in understanding the Precambrian geological evolution of Western Australia. However, in many cases it is unclear how rocks or terranes of similar age might be related. This project will obtain and integrate zircon Lu–Hf isotope data with other geological, geochemical and geophysical information to understand the evolution of continental crust in specific areas of Western Australia.
Zircon Lu–Hf analyses provide insight into the relative contributions of juvenile sources and recycled crust to the continental crust through time, and ‘Event Signature’ curves permit the evolution of different crustal domains to be visually compared. Integrating these constraints with other isotopic and geochemical information, geological mapping, and recently acquired geophysical datasets will advance our understanding of geodynamics and test hypotheses of tectonic evolution. Efforts will be aimed at addressing specific geological questions in key areas, particularly along the new geophysical transects, as well as in underexplored regions (e.g. the Albany–Fraser and Capricorn Orogens) where the new information will also improve the targeting of mineral exploration.
Measurement of hafnium isotopes in zircon crystals allows determination of the Hf isotope ratio at the time of zircon crystallisation. The ability to generate large amounts of data for zircons of a range of ages (within single crystals, single samples, and by compiling data from several samples) means that time-space variations in crustal evolution can be readily evaluated. The project will include analysis of detrital zircons in sedimentary rocks as well as zircons with multiple growth stages in both igneous and metamorphic rocks. In collaboration with GEMOC, isotope analyses will be performed by laser-ablation ICPMS on zircons selected from the GSWA archive of >1000 samples dated using SHRIMP. This is the first time in Western Australia that Lu–Hf isotope data obtained on this scale can be integrated with geological and geochemical data and newly acquired gravity, aeromagnetic, seismic and magnetotelluric datasets.
Progress during 2011:
Samples for Lu–Hf were chosen from the Gascoyne and Musgrave Provinces, the Albany–Fraser Orogen, the Pilbara Craton, the Murchison and Southern Cross Domains and Eastern Goldfields Superterrane of the Yilgarn Craton, and the Edmund, Officer, Ashburton and Bresnahan Basins. The results were integrated with existing isotope data acquired by GSWA, and used in refereed journal articles and GSWA Publications.
The results have already demonstrated the considerable value of this program in understanding geological evolution in key areas of Western Australia. Data for the Albany–Fraser orogen indicate that the earliest Biranup Zone magmas represent reworked Archean crust, and show progressive addition of juvenile mantle through time. This isotopic evolution is compatible with the Biranup Zone representing a Mesoproterozoic active back-arc rift on the Yilgarn margin, rather than being an exotic terrane. Lu–Hf data have confirmed that the Glenburgh Terrane, which forms basement to the Gascoyne Province, has a distinctly different crustal history to the Yilgarn and Pilbara Cratons. Data from the Yilgarn Craton reveal an overlap of the most juvenile Hf values for the Southern Cross Domain with those of the Eastern Goldfields Superterrane. This could suggest a shared crustal source, and that the Eastern Goldfields Superterrane may not be exotic to the western Yilgarn Craton, as previously suggested. Hf isotope data from zircons in intrusive rocks of the west Musgrave Province indicate apparent crustal reworking following juvenile input events at ca 1900 and 1600–1550 Ma.
Aims and work plan for 2012:
The project will continue to generate Lu–Hf isotope data, and integrate them with geological and geochemical data as well as geophysical datasets. The research will be focused in ‘greenfields’ areas where little information presently exists. Based on results obtained, it is likely that new samples will be collected during the normal course of GSWA fieldwork to address specific geological problems.
1. CAMECA Ion Microprobe development: Maximising the quality and efficiency of CCFS activities within the UWA Ion Probe Facility
The Centre for Microscopy, Characterisation and Analysis at UWA is home to two state-of-the-art Secondary Ion Mass Spectrometers: the CAMECA IMS 1280 large-radius ion microprobe, for the high-precision analysis of stable isotopes in minerals, and the CAMECA NanoSIMS 50 for imaging mass spectrometry at the sub-micron scale. In addition to the analytical capabilities located at the other nodes, the CCFS is poised to become a world-leader in in situ stable isotope analysis, and it is therefore essential that the data and interpretations be of the highest quality.
This project provides a dedicated Research Associate for the development of CCFS activities utilising the CAMECA Ion Microprobes at UWA, thereby increasing the capacity of the facility, enabling a higher degree of interaction and participation on projects, and allowing greater synergy with other CCFS node facilities. The Research Associate will play an integral role in experimental design, planning, sample preparation, and the acquisition, processing and interpretation of data. The complex nature of the Ion Microprobes demands a high-degree of technical ability, while an understanding of the aims of the individual projects requires a deep understanding of geological and geochemical processes. This position is fundamental to the generation of high-quality, in situ, elemental and isotopic data for a diverse range of projects and, as such, represents a significant investment into the overall success of the CCFS.
Progress during 2011:
The recruitment process began in September 2011, when funds became available. An offer has been made as this Report is being prepared.
Aims and work plan for 2012:
Training for the appointee will commence when the appointment is taken up (estimated mid 2012). Active participation in relevant Centre projects and targeted technique and method development will follow training.
The project aims to enhance the world-class facility for in situ isotopic and elemental analysis in the Geochemical Analytical Unit (GAU) at GEMOC (see section on Technology Development, p. 91), in order to maintain Australia’s LAM-ICP-MS capabilities at international standards, and to advance beyond it in some aspects. The advances will be based on femto-second laser sampling and the coupling of instruments for simultaneous analysis.
Research in CCFS depends critically on world-class geochemical and high-pressure experimental infrastructure. The ongoing improvement and refinement of the geochemical methodologies and techniques is driven by the CCFS research program and the acquisition of new instrumentation. In 2010 GEMOC was awarded a $1.5M LIEF grant, to purchase a femto-second laser sampling system, a new quadrupole (Q-)ICP-MS and a single collector sector-field (SF-)ICP-MS. This project uses this infrastructure boost to advance the capabilities of the GEMOC GAU. The current project aims to investigate ways of linking these new instruments in different combinations with laser sampling, to achieve simultaneous analysis of two isotopic systems or of trace-element data together with isotopic data. Combinations of instruments can be achieved by splitting the ablation gas into two lines downstream from the ablation chamber, and using different instruments to analyse the two gas fractions. This will allow a range of innovative analytical approaches, including (but not restricted to):
1. U-Pb dating of zircon and other U-bearing minerals + Hf-isotope analysis
2. Os-isotopes of sulfide grains + trace elements (Q-ICP-MS) or S-isotopes (SF-ICP-MS)
3. Sr-isotopes (MC-ICP-MS) + Pb isotopes (SF-ICP-MS) in feldspars
4. “Non-conventional” stable isotopes (e.g. Li, Si, Mg, Ca, Fe, Ga, Cu, Zn, Mo, Se, Tl and other still unexplored systems; MC-ICP-MS) + trace elements (Q-ICP-MS)
With this enhanced capability, the plan is to expand the stable-isotope program, including:
- Mg, Fe, Si and Li isotopes in olivine from mantle-derived rocks as tracers of mantle processes: melting, metasomatism and the recycling of crustal material
- Mg, Ca and Si isotopes in chondrules, Ca-Al inclusions and other silicate phases in meteorites, as tracers of cosmogenic processes and their timescales
- Mg and Li isotopes in speleothems and microfossils to constrain past climate change
- Si isotopes in quartz from crustal rocks to study hydrothermal processes, and in SiC from kimberlites, to understand the fractionation of Si and C in mantle processes
- Cu, Fe, Zn, Mo, Se, S isotopes in ore systems, including sea-floor “black smoker” chimneys, to investigate biological and other low-T fractionation processes
In each case the collection of trace-element and isotopic data from the same analytical spot will provide better constraints on each type of data than if they were collected from different spots.
The development of new methodologies and applications in this project will provide more analytical options for CCFS researchers and create new research opportunities across all CCFS themes. This facility and the innovation that it represents will help maintain the high profile that Australian geoscience has enjoyed internationally, making it easier to attract high-quality researchers, postgraduate students and industry-related research funding.
Progress during 2011:
Planned innovations in the Geochemical Analysis Unit (GAU) at Macquarie University in the first term of the Centre of Excellence Core to Crust Fluid Systems are based on new instruments purchased with the recent LIEF funding. This equipment allows the development of the ‘split-system’ techniques in which two mass spectrometers (a MC-ICP-MS and an ICPMS) will be connected to a common laser source for novel simultaneous measurement of geochemical parameters. The current status of this new instrumentation is as follows:
Q-ICP-MS: An Agilent 7700cx Q-ICP-MS has been installed, joining the existing stable of Q-ICPMS (3) and MC-ICPMS (2) instruments. The second instrument has arrived and awaits installation.
Femtosecond Laser Microprobe: At the time of the LIEF application Photon Machines (California) manufactured the only commercially available femtosecond laser ablation microprobe system. In 2010 New Wave Research (UK) released a femtosecond system, which incorporated a new design of their universal platform (UP) beam delivery system. Our assessment of the two laser systems extended into 2011 and following extended negotiations to obtain our required performance specifications an order was placed for a Photon Machines fs198 laser system. This instrument is scheduled for delivery in April-May 2012.
SF-ICP-MS: Following the order of the femtosecond laser, an order was placed for the Nu Attom high-resolution SF-ICP-MS. The features of this instrument include continuously variable high-resolution capabilities, fast electrostatic scanning/jumping and a fully laminated, high-scan-speed magnet. Fast data acquisition is critical in the measurement of the transient signals produced by laser ablation and the performance and operation of the magnet in fast scanning mode during evaluation of the Nu Attom proved to be critical for the types of applications for which it is to be used. The Nu Attom is due for delivery in July-August 2012.
In addition to the new laser systems on order, two of the existing New Wave UP-213 laser ablation microprobes underwent upgrades in 2011 to enhance the system capabilities. The modifications incorporated refurbishment of laser sources and new sample cell designs. These produced improved stability, sensitivity and sample throughput, all of which resulted in increased efficiency and data quality.
While the new instruments are being built, several parallel method development programs for the measurement of Li and Mg isotopes have been underway. These programs have led to significant advances in the separation of these elements from complex rock and mineral matrices, as well as in the mass spectrometer measurements. This work is also contributing to the development of well-characterised reference materials that are essential for in situ analysis by laser ablation ICP-MS or SIMS.
Advances have been made in the application of lithium isotopes as a potential tracer of fluid-rock interaction in the deep Earth. These advances have centred on the extraction and purification of lithium isotopes from a range of different rock types and the development of analytical procedures to determine high precision isotope ratios by plasma source mass spectrometry. Specific developments include:
- the refinement of methods for basaltic rocks;
- the adaptation of these procedures to ultramafic rocks from the Earth’s mantle that have extremely low Li abundances;
- the analysis of Li isotopes in zircon (in collaboration with Partner Institution CAS (Beijing));
- the analysis of Li from corals (collaboration with Dr Mei-Fei Chu, National Taiwan University).
The method development program has been undertaken by a postdoctoral researcher (supported by the NSW SLF) and protocols have been established to enable the transfer of the methodology to other researchers. To date two higher degree research students (both international cotutelle) and one postdoctoral fellow have commenced programs in CCFS to apply and enhance the methods in their specific projects.
The pioneering work on the high-precision measurement of Mg isotopes in mantle olivine by Pearson et al. (2006) demonstrated how mass-dependent isotopic fractionation at high temperature could be used as a tracer of deep earth processes. The extension of this approach to other mantle minerals is complicated by more complex chemical compositions. In 2011 a project was initiated to analyse the Mg isotopic composition of mantle-derived garnets from a suite of eclogites from the Roberts Victor kimberlite in South Africa. So far the work has involved the development of new extraction and purification techniques specifically for Ca-Mg-Fe garnet compositions and preliminary measurements indicate the robustness of the new chemical procedures.
Aims and work plan 2012:
- Delivery, installation and commissioning of femtosecond laser system (due April-May, 2012).
- The appointment of a Research Associate (Postdoctoral position) with existing expertise in laser sampling and ICP-MS analysis to undertake method development on the femtosecond laser system.
- Establish and undertake the first phase of an experimental program to investigate fundamental properties of femtosecond ablation processes in geological materials, focusing on laser-induced isotopic fractionation.
- Delivery, installation and commissioning of Nu Attom high resolution SF-ICP-MS (due July-August, 2012).
- Transfer of in situ methodologies for trace-element analysis and U-Pb isotope measurements from Q-ICP-MS to Nu Attom.
- Refinement of Li isotope methodologies and expansion to other applications.
- On-going development of the procedures for the measurement of Mg isotope composition of garnet.
Liberation and recovery of accessory mineral components of any type of rock, for geochemical and geochronological analysis.
The aims of the CCFS projects require the separation of accessory minerals from a range of different rock types. There are several major issues with these processes: breakage of grains, potential laboratory contamination, and the concentration and separation of extremely fine-grained phases. These problems can now be reduced if not eliminated by using new technology and newly developed procedures: (1) electrostatic pulse disaggregation (EPD); (2) the use of disposable sieves; (3) hydroseparation procedures for ultrafine material.
The first selFrag instrument in Australia was installed in GEMOC in May 2010. The selFrag uses EPD to break rock samples into their component phases and produces better liberation of mineral phases, especially accessory minerals, than conventional crushing procedures. Because disaggregation proceeds along grain boundaries, it greatly increases the proportion of unbroken grains. Disaggregation takes place inside a large Teflon-lined container, which is easily cleaned to prevent cross-contamination.
- Zircon separation: Increasing the yield of zircon crystals for geochronology. The liberated zircon crystals are virtually unbroken and the surfaces are very clean. In contrast to mechanically crushed samples, no remnants of other minerals such as biotite have been found on the zircon surfaces.
- Separation of Platinum Group Minerals from chromite ores: Conventional mechanical crushing of compact chromite samples produces multi-mineral grains and excessive amounts of dust. The recovery of PGMs is very difficult. Selective fragmentation using the selFrag, on the other hand, produces only small amounts of fines and no dust. Breakage occurs preferentially along grain boundaries and inclusions, thus liberating the platinum minerals.
- Separation of minor components from complex rocks: e.g. kimberlites and diamondites
Progress during 2011:
Since its installation the selFrag has been used for a range of applications including zircon separation, the analysis of grain size and shape of phenocrysts in volcanic rocks, and the liberation of trace minerals from a range of mantle-derived and crustal rocks. In 2011 the selFrag was used to process more than 160 samples for 30 different research projects, including CCFS research projects (TARDIS; PhD projects; Honours), TerraneChron® and users from other institutions (e.g. ANU, Wollongong University, NSW Geological Survey). Users are trained in the operation of selFrag and mineral separation procedures, and the training program is an important aspect of the facility’s operations and achievements.
Each new sample processed on selFrag requires the development and refinement of experimental procedures depending on grain size, mineralogy and the amount of sample. As experience is gained through the processing of different rock types, a handbook of experimental conditions is being compiled, benefiting all users and improving the efficiency of the facility.
A facility for mineral separation is being established at GEMOC to process the products of the selFrag and in 2011 the following separation methods were developed and/or refined:
• sample sieving using disposable plastic/nylon sieves to prevent (cross-) contamination of samples
• heavy liquid mineral separation using aqueous solutions of the nontoxic chemical sodium polytungstate (SPT) for heavy-mineral separation
• magnetic/paramagnetic separation using a Frantz® Magnetic Barrier Laboratory Separator for separation of dry materials according to magnetic susceptibility, exploiting either paramagnetic or diamagnetic properties => output fractions: magnetic, paramagnetic and non-magnetic
• micropanning equipment is available for further concentration of phases with densities slightly different from their matrix – best suited to grains >200 microns
A hydroseparator (CNT-HS-11, manufactured by CNT Corporation, Canada) was purchased in 2011 and is currently being installed. This device processes samples of extremely fine-grained (down to a few microns) water-insoluble particles/grains to produce “heavy-mineral concentrates” from material of similar physical properties. The aim is to take disaggregated material from the selFrag and process theses using the hydroseparator to concentrate rare accessory phases (e.g. alloys in mantle peridotites, platinum group minerals in chromitites). Dr José María González-Jiménez visited the University of Barcelona in September 2011 to undertake training in the use of the hydroseparator and this experience is helping to set up the equipment and establish the technique.
Other ancillary equipment purchased in 2011 for the mineral separation facility included a large-volume drying oven, binocular microscope, top-loading balance and glassware for heavy liquid separation.
Aims and work plan for 2012:
The principal objectives for 2012 are to continue to produce high-purity mineral separates for CCFS research programs and to train users in the latest mineral separation procedures.
Planned development and refinement of procedures and protocols for selFrag include:
• Expansion to new rock types
• Use of small volume cell to extract mineral inclusions
• Improve zircon yield from ultra-fine materials
A priority in 2012 is to finalise the installation and commissioning of the hydroseparator, and initial method development will focus on the separation of PGMs from chromitite.
The following projects are supported by ARC Post-Award funds allocated mid 2011 for early-career researchers. These are ARC ECSTAR Fellowships (Early Career Startup Awards for Research). The two appointees in 2011 are Dr José María González Jiménez and Xuan-Ce Wang.
ECSTAR Project 1. Platinum-group minerals: monitors of deep Earth processes
José María González-Jiménez: Supported by ARC CCFS ECSTAR (commenced 2011)
This project interfaces with Foundation Project 1, The TARDIS-E Project: Tracking Ancient Residual Domains In the Silicate-Earth. The Re-Os isotopic system in Platinum-Group Minerals (PGM) from Earth’s mantle potentially provides the most robust record of long-term interactions between distinct regions of Earth’s interior. However, the reliability of this approach needs further testing, because the chemistry of many PGM hosted in mantle-derived rocks has been modified by hydrothermal alteration or metamorphism during excavation of the mantle rocks from deep Earth to surface. The fact that PGM of different suites of mantle rocks exhibit variable scales of heterogeneity of Os suggests that post-magmatic alteration could also disturb the Re-Os compositions of these minerals. This project uses a combination of classical mineralogical methods and novel micro-analytical techniques for isotopic measurements to test the robustness of the Re-Os system in PGM from mantle-derived rocks affected by variable degrees of hydrothermal alteration and metamorphism, and thus to constrain the interpretation and applications of Os-isotope data.
Progress during 2011:
We have found significant differences in 187Os/188Os between primary and secondary PGM from metamorphosed ophiolite chromitites of the Dobromirtsi Ultramafic Massif, in the Central Rhodope Metamorphic Core Complex of southeastern Bulgaria. Primary (magmatic) PGM hosted in unaltered chromite cores have 187Os/188Os from 0.1231 to 0.1270 and 187Os/188Os ≤ 0.002. TMA and TRD model ages, calculated relative to the Enstatite Chondrite Reservoir cluster around three main peaks: ~ 0.3, 0.4 and 0.6 Ga. Secondary PGM, produced by alteration of magmatic PGM, have a wider range of variation (187Os/188Os = 0.1124-0.1398, 187Re/188Os ≤0.024); these grains yield TMA and TRD model ages from -1.7 up to 2.2 Ga. The larger range in 187Os/188Os in the secondary PGM is interpreted as due to reactions between the primary PGM and infiltrating metamorphic-hydrothermal fluids with a range of Os-isotope compositions. This redistribution of Os in PGM during metamorphism has significant implications for the interpretation of both whole-rock and in situ Os-isotope data in mantle-derived rocks since Os-isotope compositions of Platinum-Group Minerals (PGM) in ophiolite chromitites have been commonly regarded as resistant to fluid-related processes. Thus, the fact that secondary PGM in the metamorphosed chromitites of Dobromirtsi yield 187Os/188Os within the range of depleted to enriched mantle sources suggests that much of the Os-isotopic variability previously reported for PGM taken out of their microstructural setting (e.g. mineral concentrates or detrital grains collected from streams), and interpreted as a magmatic feature, may instead be related to secondary alteration processes. Therefore, interpretations of mantle events based on the analysis of PGM nuggets from placers may be need to be re-considered. On a more positive note, the Os-isotope data from the secondary Os-bearing phases in ophiolites can give a wider perspective on the sources and evolution of the host mantle peridotite (Geology, CCFS Publication # 42).
Aims and work plan for 2012:
The basic approach of the ECSTAR I project requires obtaining a statistically useful number of Os-isotope analyses of PGM from a large number of sample localities with a range of types and degrees of alteration. The strategic planning for 2012 aims to expand the set of PGM analysable for in situ Re-Os isotopes by sampling PGM-bearing rocks from ultramafic bodies with well-established P-T conditions of post-magmatic alteration. It will involve visiting ophiolite sequences exposed in the Coolac Serpentinite Belt in southern Australia (low-temperature ocean-floor serpentinisation), the Vizcaino Peninsula in northern Mexico (mid-grade metamorphism) and the Jakovitsa, Avren and Golyamo Kamenyane Ultramafic Massifs in southern Bulgaria (mid-to-high grade metamorphism). Another set of PGM-bearing rocks from ophiolites and other ultramafic complexes with different settings and with potential for finding PGM will be provided by overseas collaborators: ophiolite of Tehuitzingo (central Mexico) to be supplied by Dr Joaquín Proenza (University of Barcelona Spain), transitional ophiolite-subcontinental mantle Kohistan paleo-Arc (Pakistan) to be supplied by Dr Carlos Garrido (Consejo Superior de Investigaciones Científicas, Granada, Spain) and Dharwar Craton (India) to be supplied by Dr Sisir K. Mondal (Jadavpur University, Kolkata, India).
It also is planned in 2012 to expand the set of PGM potentially useful for Re-Os isotopes by obtaining concentrates of PGM from whole-rock samples using the combination of selFrag electrostatic rock disaggregation plus hydroseparation, both facilities being currently developed at the Geochemical Analysis Unit at CCFS/GEMOC. The new development of hydroseparation at CCFS/GEMOC will be carried out in collaboration with two Spanish labs from the University of Barcelona and Granada which already have experience in the use of this technique. With the aim to constraint better constrain the Re-Os isotopic data obtained from the PGM it is also hoped that this technique will assist in searching for zircons in PGM-bearing rocks. Thus it is aimed to determine distributions of major, minor and trace elements as well as radiogenic isotopes (U-Th-Pb and Lu-Hf series). This new research line in the project will be carried out in close collaboration with CCFS/GEMOC and overseas collaborators. The project aims for 2012 mesh with current research projects of undergraduate, post-graduate and PhD students under the supervision of the project leader. These include students from different Earth Sciences departments of overseas Universities: National University of Mexico (UNAM, Mexico), Universities of Zaragoza and Barcelona (Spain) and the Geological Institute of the Bulgarian Academy of Science (BAS, Bulgaria). Communication of results is planned to be mainly through international scientific journals (e.g. Chemical Geology, Contributions to Mineralogy and Petrology, Lithos) and presentations in high-profile international geoscience conferences: Cordilleran Section, Geological Society of America 108th Annual Meeting, to be held 29-31 March 2012 in Querétaro City (Mexico); XXXII Reunión Cientifica de la Sociedad Española de Mineralogía to be held 27-30 June 2012 at Bilbao (Spain); 22nd Goldschmidt conference to be held 24-29 June 2012 at Montréal (Canada); 34th International Geological Congress to be held 5-10 August at Brisbane (Australia); International Earth Science Colloquium on the Aegean Region 2012, to be held 1-5 October 2012 in Izmir (Turkey).
ECSTAR Poject 2. Establishing the links between plate tectonics and mantle plume dynamics: message from the late Cenozoic Leiqiong basalts in SE Asia
Xuan-Ce Wang: Supported by ARC CCFS ECSTAR funding and NSFC (National Science Foundation of China) Project grant (commenced 2011)
Whether mantle plumes and plate subduction are genetically linked is a fundamental geoscience question that impinges on our understanding of how the Earth works. Late Cenozoic basalts in Southeastern Asia are globally unique in relation to this question because they occur above a seismically-detected thermal plume adjacent to major deep subducted slabs. The main goal of this project is to examine the petrogenesis of late Cenozoic continental flood basalts (CFBs) that are located directly above a plume-like mantle seismic structure, and spatially close to major subduction zones in southeastern Asia. We will take a multidisciplinary approach, determining the chemical composition of the primary melts of the basalts, characterising the chemical compositions of their mantle source, and examining the temporal-spatial variations in the geochemical characteristics of the Leiqiong CFB. We will also test the geological and thermochronological evidence for lithosphere uplift. The results will be employed for testing major predictions of plume models as well as other end-member geodynamic models for such unique CFBs. This study will advance our understanding of (1) the thermochemical state of the deep Earth where a lower-mantle-rooted plume-like seismic structure exists unusually close to subducted slabs; (2) relationships between geophysical and geological manifestations of mantle plume activities at mantle downwellings; (3) the nature and origin of enriched mantle source regions; and (4) the behaviour of mantle plumes at plate boundaries and inter-relationships between mantle plume dynamics and plate tectonic processes. Knowledge obtained in this project will help to address one of the most fundamental questions in geodynamics: how the two major processes of whole-mantle convection, i.e., deep subduction that drives mantle downwelling, and mantle plumes that drive mantle upwelling, relate to and interact with each other.
Progress during 2011:
We have established the mantle thermal state beneath Hainan Island, based on the estimated primary melt compositions. The estimated mantle potential temperature varies from about 1500 to 1580° C with a weighted average of 1541 ± 10° C (Wang et al., Journal of Petrology, 2012). This provides independent support for the Hainan plume model that has previously been proposed largely based on geophysical observations. Pb-isotope analyses demonstrate that the basalts were derived mainly from an ancient (4.5–4.4 Ga) primitive mantle reservoir preserved near the core-mantle boundary (CMB). Their Nd- and Os-isotope compositions also suggest a lower-mantle origin. These new findings, along with existing evidence for high melting temperatures, confirm that the plume-like mantle seismic velocity structure indeed represents a plume from the lower mantle fuelled by an ancient reservoir at the CMB. The fact that this young plume is surrounded by deep subduction zones further suggests that they are genetically linked to each other, thus providing the first observational support for dynamic linkages between deep subduction and mantle plume generation.
Aims and work plan for 2012:
1) In-depth characterisation of the mantle heterogeneity beneath the Leiqiong flood basalt by examining geochemical and petrological variation with increasing depth of the lava plateau. This analysis is based on drill core samples.
2) To conduct He isotope analysis of olivine in order to examine the relationship between He isotopes and other elements and isotopes
3) Construction of a 3-D model of the geometry of the Hainan Plume within the upper mantle to evaluate the effect of later subduction-driven mantle flow; this is important for understanding the potential relationship between plate tectonics and mantle plumes
4) To undertake an in-depth analysis of the effect of dehydration of stagnated Pacific slabs on the generation of the Cenozoic intraplate volcanism in eastern Asia
José María González Jiménez and Carlos Villaseca (Universidad Complutense de Madrid) in the field.