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

Contacts:  Liene Spruzeniece, Sandra Piazolo
Funded by:  ARC Discovery Project (DP120102060)

Dating kimberlites and mantle evolution below southern Africa

Figure 1.  Cumulative-probability plots and histograms for U-Pb ages on perovskites.  (a) ages >200 Ma; (b) ages <200 ma, showing buildup to main peak; (c) Group I kimberlites with ages <110 Ma (main pulse) showing abrupt cut-off at <80 Ma.

Kimberlite magmas represent our best source of information on the composition and evolution of the deep continental lithosphere, but extracting that information can be difficult, because kimberlites typically are jumbled mixtures of the original magma and debris carried up from the mantle.  However, perovskite (CaTiO₃, with extensive substitution by Sr, U and REE) occurs in many kimberlites as a groundmass phase, early-crystallising in most cases.  Its chemistry allows not only U-Pb dating of the kimberlite’s emplacement, but Sr- and Nd isotopes carry information on the source(s) of the magma (CCFS Publication #466).  We have dated groundmass perovskite by LA-ICPMS U-Pb techniques in 135 kimberlites and related rocks from 110 localities across southern Africa.  Sr and/or Nd isotopes have been analysed by LA-MC-ICPMS in a subset of these; combined with published data this gives 89Sr-age datasets and 85Nd-age datasets.  The age distribution (Fig. 1) shows peaks at 1600-1800 Ma, 1000-1200 Ma, 500-800 Ma and 50-130 Ma.  The major “bloom” of Group I kimberlites at ~90 ±10 Ma was preceded by a slow buildup in magmatic activity that began at ~70 Ma.  The main pulse of the cluster of kimberlites at 120-130 Ma (called “Group II”) was a distinct major episode within this buildup.

The Sr- and Nd-isotope data (Figs. 2 and 3) show that the subcontinental lithospheric mantle (SCLM) sampled by the younger kimberlites was isotopically heterogeneous, and that this heterogeneity reflects a metasomatic refertilisation that may have begun as early as 1.2 Ga ago, but probably was episodic. This metasomatically modified mantle was sampled extensively by the Group II kimberlites that erupted at 120-130 Ma.  However, the latest major bloom of Group 1 kimberlites (~90 ±10 Ma) sampled a much more strongly metasomatised mantle. 

Figure 2.  Sr-isotope data for perovskite in the analysed kimberlites, plotted vs time, to show the increased variability of the isotopic signatures of kimberlites toward the present.  Heavy lines in (b) show the evolution of the radiogenic Reservoir 1 that could have contaminated the younger kimberlites, assuming that the necessary metasomatism occurred at ca 1.2 Ga or ca 2.0 Ga.  Light line in (b) shows the depletion in Rb/Sr (relative to DM) that would be required to produce Reservoir 2 with low ⁸⁷Sr/⁸⁶Sr.  (c) shows details of the data for the last 200 Ma, with an interpretation of the data in terms of the destruction of reservoir (1).

Figure 4 shows the spatial distribution of εNd and εSr overlain on the seismic-tomography image of southern Africa generated by the Kaapvaal Seismic Project (Fouch et al., 2004).  The most striking observation from these maps is that the samples with the lowest εNd and the highest εSr are concentrated in three areas: around the low-Vs feature that marks the intrusion path of the Bushveld Complex, along the Kimberley belt, and in the off-craton “tail” of kimberlites.  The geochemical signature implies long-term geochemical enrichment in the LREE relative to the HREE (and hence low Sm/Nd) and high Rb/Sr.  There is an obvious correlation between this clearly metasomatic signature and low Vs, as would be predicted from studies of xenoliths, where this type of trace-element enrichment is associated with higher Fe/Mg, Ca and Al, and lower calculated seismic velocities (Griffin et al. 2009).

These maps suggest that volumes of metasomatised SCLM with very low εNd and high ⁸⁷Sr/⁸⁶Sr, (the characteristic isotopic signature of Group II kimberlites (~120-130 Ma)), were confined to low-Vs zones along trans-lithospheric structures.  These include the locus of the Bushveld intrusion, major faults and craton boundaries.  Such metasomatised zones existed as early as 1800 Ma, but were only sporadically tapped until the magmatic buildup began at ca 170 Ma, and apparently were mostly gone by ca 110 Ma.  We suggest that these metasomatised volumes resided mainly in the deep SCLM, and that their low-melting-point components were “burned off” by rising temperatures, presumably due to an asthenospheric upwelling that led to SCLM thinning and a well-documented rise in the ambient geotherm between 120 Ma and 90 Ma.  The younger Group I kimberlites therefore rarely interacted with such SCLM, but had improved access to shallower volumes of differently metasomatised, ancient SCLM with low ⁸⁷Sr/⁸⁶Sr and intermediate εNd (0-5).  The kimberlite compositions therefore record the slow evolution of the SCLM of southern Africa, from 1.8 Ga through a refertilisation event around 1 Ga ago, and a major thermal and compositional change at ca 110 Ma.

Figure 4.  Values of εSr (upper) and εNd (lower) superimposed on the Vs tomography for the 150 km depth slice (after Begg et al., 2009), illustrating the association of metasomatic signatures (high εSr and low εNd) with low-Vs zones and trends.  See Figure 2 for discussion of the velocity scales and interpretation of major tomographic features.

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

Contact:  Bill Griffin
Funded by:  CCFS

The growth of the Tibetan Plateau: the last 25 Myr

The spectacular topography of the Tibetan Plateau (Fig. 1) is the result of collision between India and Eurasia over some 50 Myr, but how did the plateau grow to its present size?  Previous work along the eastern Longmenshan margin (LM in Fig. 1) of the Tibetan Plateau suggests a two-stage uplift (thus growth of the plateau) since ~30 Myr: one from 30-25 Myr, and another from 15-10 Myr.  Was this just a local feature, or does it reflect plateau-wide episodic lateral growth?  We have used high-resolution seismic reflection and drill core data from the southern Tarim Basin to analyse the pattern of plateau growth along the West Kunlun margin (WK in Fig. 1) of the northwestern Tibetan Plateau.  The work was carried out through collaboration with Professor Xiao-Dian Jiang of Ocean University of China and published in Nature Communications (CCFS contribution #496).

The continental lithosphere of the Tarim Block underthrusted northern Tibet to its south, with up to 12 km of Cenozoic foreland basin deposits accumulated along the southern Tarim due to the loading of the plateau.  These deposits provide a unique record of the history of the mountain building.  Seismic reflection and drill hole data were acquired in the foothills of the West Kunlun Range and the southwestern Tarim foreland basin by the Shengli Oilfield Company (location of seismic profile shown by red line in Fig. 1), and were made available for our research.  The change in sedimentary environment from a marine carbonate platform in the Oligocene to a clastic tidal flat in the early Miocene marks the beginning of foreland-basin development due to the formation of the proto-West Kunlun Range at the northern edge of the proto-Tibetan Plateau.  The temporal variation in the deposition/subsidence rate is taken as reflecting changes in the orogenic loading on the lithosphere of the Tarim Block, which would have resulted in isostasy-driven surface uplift at the present northwestern margin of the Tibetan Plateau.  Rate estimates for the depocentre (green curve in Fig. 1) indicate that there have been two episodes of rapid uplift along the West Kunlun Range: one in the mid-Miocene (~16-11 Myr), and the in the last ca 5 Myr; the latter has been significantly faster.  Using the differences in thickness between strata on the two sides of a frontal thrust, we calculated the relative uplift rate on the hanging wall as a function of time; this analysis also indicates two episodes of rapid uplift (purple curve in Fig. 1).

Figure 1.  Location of the West Kunlun Range (WK) at the northwestern margin of the Tibetan Plateau, and time variations in estimated relative subsidence and uplift rates.  The location of the seismic profile is shown as a red line.  LM - the Longmenshan Range.

Our data thus suggest that the uplift of northern Tibet started from near sea level at ~23 Myr; the first episode ended by ~10 Myr, followed by rapid uplift along the present plateau margin over the last 5 Ma.  The contrast in the intensity of the two episodes probably indicates that during the first episode, the front of the proto-West Kunlun orogen at the edge of the plateau was off from its present position, and it only propagated to its present position during the second episode.  This two-episode uplift history is comparable to that reported along the eastern margin of the Tibetan Plateau, suggesting that the growth of the Tibetan Plateau after the Eocene likely has been episodic in nature, and near-synchronous along both eastern and northern margins.

Recent work on both margins also suggests that brittle thickening of the upper crust plays the dominant role in plateau propagation along those margins (see CCFS contribution #223).  There is thus a case for synchronous episodic plateau expansion, dominantly through brittle thickening of the upper crust rather than mechanisms like crustal channel flow. 

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

Contact:  Zheng-Xiang Li
Funded by:  NSFC (40772124 and 41176038), CGS (GZH200900504)

Modelling mantle melting: New horizons

  Figure 1.

One of the great goals of Geosciences is the explanation of geochemical data consistent with models of the Earth.  Models are constrained by physical laws, so successful modelling explanations not only give multifaceted support to our stories about the formation and evolution of rocks, but simultaneously constrain the properties of the deep Earth from which they originate.  Our project attempts to develop a new generation of detailed models for grain-scale microstructural evolution and chemical transport in the framework of classical irreversible thermodynamics.  We have so far applied the current version of the models to the explanation of Rare-Earth Elements (REE) and Uranium-series isotopes in Ocean Island Basalts (OIB’s). Figure 1a illustrates an example of the microstructure showing the 2D distribution of 5 phases (olivine, opx, cpx, garnet, and melt) around the garnet transition, and Figures 1b and 1c show the corresponding distributions of Sm/Yb and ²²⁸Ra/²³²Th ratios throughout the microstructure.  Because different trace elements are characterised by different partition and diffusion coefficients, very different diffusive features can be seen.

One success of our modelling efforts is the explanation of REE ratios in OIBs.  As shown in Figure 2, if REE ratios are plotted against the age of the lithosphere on which OIB’s are erupted, we see step-like features in La/Sm and Sm/Yb ratios, separating low values over young (thin) lithosphere, and higher values on old (thick) lithosphere.  The thin black line is a moving average of the data and the bold green line is our best model compared to the data.  The fit is impressive, and by inspection of the properties of the model, we can infer the origin of the step-like feature.  The step in La/Sm is caused by the onset of anhydrous melting, where melt productivity as a function of depth substantially increases.  

Sm/Yb, on the other hand, reveals a garnet signature: when oceanic lithosphere is about 15 million years old, it is ~50 km thick, which happens to be the depth at which garnet is stable.  As garnet preferentially holds heavy rare earth elements (e.g. Yb) during melting, Sm/Yb is high in magmas derived by melting beneath older lithosphere, and low otherwise.

Figure 1.  Paleogeographic map of the southeastern South China Block (SCB) showing areas studied in this project. 

The South China Block (SCB) constitutes a major continental segment in the Western Pacific region (Fig. 1).  Revealing the tectonothermal history of the region is fundamental to establishing the timing and kinematics of flat-slab subduction, foundering, and the opening of a marginal South China Sea.

We have carried out the first comprehensive thermochronological analysis of four areas in the southeastern SCB, making up a SE-NW traverse from the coast to the inland (A-D in Fig. 1).  Late Triassic muscovite and biotite 40Ar/39Ar ages (220-200 Ma) in the northwest are clearly older than the Late Jurassic ages from the southeast (165-155 Ma; Fig. 2a).  Among forty-one zircon (U-Th-[Sm])/He ages, four ages are pre-Middle Jurassic (253, 245, 220 and 176 Ma) and the remaining ages range from the Late Jurassic (152 Ma) to the Late Cretaceous (75 Ma).  The four pre-Middle Jurassic ages are all from the northwest (area D) and the southeast is dominated by Cretaceous ages (Fig. 2b).  Apatite fission track ages cluster at 70-30 Ma; two older ages (90 Ma) are from samples in the northwest (Fig. 2c).  Like the apatite fission- track dates, apatite (U-Th-[Sm])/He ages form a tight Late Cretaceous-Eocene cluster at 70-30 Ma (Fig. 2d).  

According to the time-temperature trajectories extracted from both forward and inverse modelling of thermochronological data, area D records rapid cooling related to orogenic uplift and related erosion during the Late Triassic.  This orogenic uplift is interpreted as a result of the approaching flat subduction of a paleo-Pacific oceanic plateau from the southeast.  At the same time, regions to the southeast were sagging and receiving terrestrial and shallow-marine sediments during the Late Triassic and Early Jurassic (Fig. 1 and 3).  This may have resulted from the gravitational pull of the eclogitised oceanic flat slab (Li and Li, 2007,  Fig. 3).  Significant magmatic reheating occurred first in area B in the Middle Jurassic and generated a thermal peak of ~300 °C.  Subsequently, in the Late Jurassic, a thermal peak of 250-300 °C was recorded in areas C and A, to the n

Figure 2.  Database of new thermochronological ages in the southeastern SCB from this project.  X-axis is longitude and y-axis is latitude.  Mica ⁴⁰Ar/³⁹Ar = muscovite and biotite 40Ar/39Ar ages, ZHe = zircon (U-Th-[Sm])/He, AFT = apatite fission track ages, AHe = apatite (U-Th-[Sm])/He.  Ages are expressed in Ma as circles; age values are indicated by the colour of circles.  The pattern of ages was obtained by interpolation and smoothing of the data set.  Previous data points are shown as black crosses in interpolated maps.  Note that linking between areas without data points is by interpolation.  The maps show the spatial distribution of ages and general trends; geology and faults are ignored.

Northwest and east of area B, respectively.  This spatial distribution of Jurassic reheating is consistent with slab break-off as in a flat-slab model (Fig. 3). 

The southern SCB was subjected to slow exhumation and cooling during the Cretaceous.  The relatively quiescent, low temperature (200-100 °C) conditions reflect long-term thermal relaxation after the Middle-Late Jurassic magmatic heating.  Such a protracted slow cooling is consistent with lithospheric rebound in the flat-slab model.  Intensive Cretaceous magmatism in the coastal regions during the Cretaceous, interpreted as reflecting the roll-back of the remaining paleo-Pacific oceanic slab (Fig. 3; e.g. CCFS Publication #16), can account for the reheating recorded in area A. 

Figure 3.  Schematic diagram showing the tectonic evolution of southeastern SCB, modified from Li and Li (2007); Li et al. (2012).  Arrow bars on the left show the heat/cooling ages.  ND = northern area D; CD = central area D; SD = southern area.

The Pacific subduction zone had migrated to the south of the present continental margin of SCB by ~90 Ma (Fig. 3; e.g. CCFS Publication #31).  Rapid cooling from ~150 °C to surface temperatures started as early as latest Cretaceous time and lasted until the late Eocene, coinciding with rifting in the southeastern SCB that has been linked to the rollback of the subducting western Pacific oceanic plates.  This rifting event ultimately led to the opening of the South China Sea (Fig. 3; e.g. CCFS Publication #31). 

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

Contacts:  Ni Tao, Zheng-Xiang Li
Funded by:  CCFS, ARC Discovery Project (DP110104799)

Migrating ridges link the deep and shallow mantle

It has long been recognised that mid-ocean ridges (MORs) migrate in an absolute sense relative to the underlying mantle. Present-day MOR migration rates affect seafloor morphology and the physical state of the upper mantle, and these migration rates correlate with asymmetric seafloor spreading, lava generation at ridge crests, asymmetry in melting depths and geochemical discontinuities, and ridge morphology.  However, the effects of long-term variations in MOR migration and the resulting non-uniform sampling of the upper mantle have not been studied, although there is growing evidence from MORB geochemistry and upper mantle seismic tomography, mantle convection simulations and laboratory models, to support close linkages between slowly migrating MORs and plumes in the present oceans.

Here, we use Large Igneous Provinces (LIPs) that have formed since the Early Cretaceous to demonstrate that ridge-plume interactions can continue for up to 180 Ma and strongly influence ridge migration rates.  These long-standing MOR migration patterns and ridge-plume interactions influence the thermal structure of the upper mantle and the geochemistry of the ocean crust vs spreading segments (Fig. 1a) and their full spreading rates.  To assess the relationships of slowly migrating ridges caused by plume-ridge interactions to the thermal structure of the upper mantle, we have developed and implemented a novel methodology to construct a global map of the relative volume of upper mantle material extracted through partial melting at MORs since the Early Cretaceous.  Our map (Fig. 1b) is a proxy for the Volume of Extracted Mantle (VEM) and is calculated as a function of the residence time (i.e. the duration for which a MOR lies above an area of upper mantle) of the global MOR system and the rate at which material is extracted from the mantle (i.e. spreading rate). 

Figure 1.  (A) Reconstructed mid-ocean ridges for the past 140 Myr in 10 Myr intervals.  Black boxes show regions where ridge migration distances were computed along flow lines.  CATL - Central Atlantic, EPAC - East Pacific Rise, NATL - North Atlantic, NWIR - Northwest Indian Ridge, SEIR - Southeast Indian Ridge, sSATL - southern South Atlantic, SWIR - Southwest Indian Ridge.  LIP formation locations (purple) cluster around hotspots coinciding with slow ridge migration rates.  (B) Estimated upper mantle VEM (km3/km of ridge).  (C) 100-175 km S-wave velocities.

Regions with high VEM values indicate prolonged melt extraction and mantle focusing/processing by a relatively stationary MOR system with moderate to fast spreading rates, e.g. the Eastern Pacific.  In contrast, rapid MOR migration, even with rapid spreading rates, does not result in high VEM values.  We observe particularly slow MOR migration rates over the past 100 Ma at three ridges that currently exhibit ridge-plume interactions, namely the East Pacific Rise, the Southwest Indian Ridge, and the southern South Atlantic Ridge.  On average, these three MOR systems have migrated <500 km in an absolute sense in the last 100 Ma, compared with absolute migration distances of >1500-2000 km for the Central and North Atlantic, and Northwest and Southeast Indian Ridges.  We see evidence for higher upper mantle temperatures beneath slowly migrating ridges through global correlations between VEM and perturbations in both P- and S-wave velocity at depths 100-200 km.  This is consistent with recent observations of higher upper mantle temperatures beneath spreading ridge segments located near deeply-sourced plumes.  Together, patterns of MOR migration, LIP formation, and correlations of VEM with seismic tomography and MOR basalt geochemistry suggest a strong feedback between the dynamics of slowly migrating ridges and deeply-sourced plumes at global scale, which produce a self-sustained system over time scales up to 180 Ma. 

In our proposed model, ridges at intermediate distances from plumes best represent the feedback mechanism.  On the one hand, plume material will preferentially flow laterally towards areas with shallower lithosphere-asthenosphere interfaces (i.e. beneath MORs), with the effect of stabilising the MOR segment.  On the other hand, the suction effect produced by the MOR will drain hot material from the plume and help stabilise the deeply-sourced plume. 

These interactions result in a perturbation of the isotherm directly beneath the MOR, as hotter mantle is drawn towards the surface, a model supported by correlations between VEM, MOR basalt geochemistry and seismic tomography.  Ridge segments located farther from the ridge-plume interaction are not affected by the ridge-plume interaction and are able to migrate more rapidly.  These rapidly-migrating MORs do not perturb the upper mantle structure to any significant extent, nor focus large volumes of upper mantle towards the MOR.  The plumes we have identified as participating in long-standing ridge-plume interactions are probably sourced from Large Low Shear Velocity Provinces.  If these provinces are stable for very long periods of geological time, then it is possible that the relationships we observe between plume ridge interactions and slow MOR migration, and their effect on the thermal structure of the upper mantle are similarly stable.  The stabilisation of MOR over time periods >180 million years, facilitated by ridge-plume interactions has significant implications for the way we model the plate-mantle system, and for understanding observed patterns of ridge morphology and geochemistry.

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

Contacts:  J.C. Afonso, Jo Whittaker, Dietmar Muller
Funded by:  CCFS Foundation Program 4, Two phase flow within the Earth’s mantle.

Deformation behaviour in polymineralic rocks

Deformation in rocks can be mainly localised in large ductile shear zones, broadly distributed into several smaller high-strain zones or dynamically change from one system to the other.  One of the main reasons for strain localisation is a switch from Power-law flow, in which dislocation creep is a dominant deformation mechanism to Newtonian flow, in which deformation is dominated by diffusion or dislocation glide accommodated grain boundary sliding (GBS).  A switch between these two types of flow will result in a marked weakening and strain localisation, because it lowers the effective viscosity of the rock by at least two orders of magnitude.

We have investigated the controls on the deformation behaviour of polymineralic rocks using a granitic pegmatite deformed at medium P and T conditions (CCFS Publication #501).  The chosen sample is an example of a mesoscale system where several minerals with different rheological behaviour were deformed together and strain was localised in a complex system of high-strain zones at different scales.

Figure 1.  General characteristics of the studied sample: (a) hand specimen showing two main domains in pegmatite; (b) close-up of zone dominated by K-feldspar (pink) with several discontinuous microscale shear zones (yellow dashed lines) and two mesoscale zones of strain localisation below and above (white arrows).

The sample shows visible zonation in the distribution and grain size of the minerals (Fig.1a).  In the initially coarse-grained, K-feldspar-dominated lower part of the sample, several microscale shear zones developed; two mesoscale shear zones separate this zone from the upper, quartz-rich part of the sample and the wall rock (Fig. 1b). 

Microstructural observations, and chemical and EBSD data, show that mesoscale shear zones form at the boundary between zones with different mineral assemblages, especially between the initially K-feldspar-rich and quartz-rich zones.  Microscale shear zones in the initially coarser-grained, feldspar-rich zone form by (i) intense recrystallisation in originally smaller grains of plagioclase, next to initially large K-feldspars deformed in the brittle regime; (ii) fracturing coupled with grain-size reduction through the interface-coupled dissolution and reprecipitation replacement of coarse-grained K-feldspar by fine-grained albitic plagioclase.  Strain is localised in the newly formed, fine-grained plagioclase and causes advanced recrystallisation of adjacent K-feldspar. 

Once the grain size is sufficiently small, grain-boundary sliding becomes the dominant deformation mechanism.  Consequently, once micro- and mesoscale high-strain zones are interconnected, the rheology of the rock is controlled by Newtonian flow.  Phase mixing and continuous high strain rates help to maintain small enough grain sizes to allow Newtonian flow over long periods, “stabilising” the high-strain zones. 

Our study shows that, in polymineralic rocks, the variation in initial and developing grain size, as well as mineral distribution, governs the dynamic rheological behaviour of the rock as a whole. 

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

Contacts:  Daria Czaplinska, Sandra Piazolo
Funded by:  ARC Discovery Project (DP120102060), Future Fellowship to Sandra Piazolo

Tibetan chromitites: Excavating the slab graveyard

Figure 1.  Major element oxide and minor/trace-element plot (normalised to MORB chromite (Page and Barnes 2009)) for chromites from Luobusa, Tibet (purple), the Antalya ophiolite complex, Turkey (grey), and chromite hosted in boninite-lavas from Bonin Island (black; Pagé and Barnes, 2009).  Luobusa (Tibet) chromites have Cr# from 0.60-0.78 and Mg# 0.58-0.73; Antalya (Turkey) chromites have Cr# from 0.59-0.73 and Mg# from 0.55-0.69.  

Large peridotite massifs are scattered along the 1500 km length of the Yarlung-Zangbo Suture Zone (southern Tibet, China), the major suture between Asian and Gondwana-derived terranes.  Diamonds occur in the peridotites and chromitites of several massifs, together with an extensive suite of trace phases that indicate extremely low fO₂ (SiC, nitrides, carbides, native elements) and/or ultra-high pressures (diamond, possible stishovite, coesite).  Physical and isotopic (C, N) studies of the diamonds confirm they are natural, crystallised in a disequilibrium environment, and spent only a short time at mantle temperatures before exhumation and cooling (Howell, Griffin et al., submitted).  These constraints are difficult to reconcile with previous models for the history of the diamond-bearing rocks.  However, new investigations are providing evidence that these peridotite massifs have experienced a remarkable journey in time and space (CCFS Publication #522).

Figure 2.  (a) Inverse ²⁰⁶Pb/²³⁸U vs ²⁰⁷Pb/²³⁵U concordia plot for zircon from Luobusa ophiolite, Tibet (n=7). Zircons from a Luobusa chromitite: (b) BSE image; (c) CL image.  Dark inclusions are F-apatite; prisms of apatite are visible on both grains in (a).

We have uncovered evidence that strongly suggests some of these peridotites have undergone metamorphism in or near the upper part of the Transition Zone, around 400-600 km deep. This includes the discovery that many of the chromites contain exsolved pyroxenes and coesite, consistent with inversion from the high-pressure polymorph (calcium-ferrite structure) of chromite, which is stable in the Mantle Transition Zone.  This high-P polymorph structure, unlike the normal spinel structure of chromite, can accommodate ions such as Ca²⁺ and Si⁴⁺; however, these elements are ejected (exsolved) when chromite is emplaced at shallow, (lower pressure) levels.  This evidence, and the presence of diamonds, coesite and other high-pressure phases recorded in both the peridotites and the chromitites, suggest that the peridotite bodies have indeed risen from the Transition Zone.  But how did they get there?  

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

Contacts:  Takako Satsukawa, Bill Griffin, Sue O’Reilly, Sandra Piazolo
Funded by:  CCFS Foundation Program 1, TARDIS-E 

Mean plate velocity and the frequency of orogens: Secular changes in the supercontinent cycle?

We have used two sets of data for the last 2.5 Gyr to address the question of secular changes in the supercontinent cycle: the timing and locations of collisional and accretionary orogens, and average plate velocities as deduced from paleomagnetic and paleogeographic data.  This analysis has been done in collaboration with Kent Condie (New Mexico Tech), Jun Korenaga (Yale University) and Steve Gardoll (Curtin University) (CCFS publication #468).

Figure 1.  Secular changes in craton collision frequency and average area-weighted plate speed (deg/100 Myr).  Collision frequency between cratons is expressed as number of orogen segments per 100-Myr bin moving in 100 Myr increments.  Lines are linear regression analysis: n = 8.68-0.00224a, r = 0.287; s = 9.927-0.00223a, r = 0.393 (n, number of orogens; s, plate speed divided by five; a, age).  Also shown are supercontinent assembly (blue stripes) and breakup (pink stripes) times.  Major LIP (large igneous provinces) events: red arrows correspond to LIPs associated with supercontinent breakup, black arrows correspond to other LIPs.

One of the main problems in counting orogens is: what constitutes a single orogeny?  Collisional orogens of short strike length could be parts of a longer orogen, now displaced by supercontinent breakup.  Therefore we have used “orogen segments” in our analyses.  In some cases an orogen segment may represent a complete orogen, whereas in others, it may represent only part of a much more extensive orogen.  We also have distinguished between collisional and accretionary orogens.  Accretionary orogens do not always end with a continent-continent collision, but collisional orogens always do.

Paleomagnetic data provide a quantitative tool for paleogeographic reconstructions.  However, the number of high-quality paleomagnetic results is limited, especially for the Early-Middle Paleozoic and the Precambrian.  The most complete and reliable global paleogeographic reconstructions for the last 2 Gyr are constrained by both geological and paleomagnetic data.  To calculate the average angular velocities of tectonic plates, we used published post-1800 Ma global paleogeographic reconstructions that fulfil the following criteria: (i) all major continental block positions are shown in the reconstructions; (ii) evolving block positions are known in time slices or animations; (iii) reconstructions are made using spherical geometry; and (iv) each reconstruction is made with Euler rotation parameters.  Pre-1800 Ma paleogeographic reconstructions are rare and rather controversial.  Unfortunately, only the paleomagnetic data from the Superior craton are sufficient for the estimation of mean angular velocities between 2500 and 1800 Ma.  As the sizes of continents vary significantly, we calculated the mean angular velocity (in degrees/100 Myr) for each 100-Ma bin by normalising to continental area.  We analysed only the movement of continental plates, because the data from oceanic plates are not available for most of the period of interest (≤2.5 Ga).

Peaks in the number of orogens, which probably reflect craton collisions, occur at 1850 and 600 Ma, with smaller peaks at 1100 and 350 Ma (Fig. 1).  Distinct minima occur at 1700-1200, 900-700 and 300-200 Ma.  Angular plate velocities as weighted by cratonic area vary greatly from about 20 to 80 deg/100 Myr with two peaks at 450-350 Ma and 1100 Ma (60-80 deg/100 Myr).  However, the overall trend suggests a gradual speed up of plate tectonics with time.  There is no simple relationship in the the frequency of cratonic collision or average plate velocity between supercontinent assemblies and breakups.  The assembly of Nuna at 1700-1500 Ma correlates with very low collision rates, whereas the assembly of Rodinia and Gondwana at 1000-850 and 650-350 Ma, respectively, correspond to moderate to high rates.  Very low collision rates occur during supercontinent breakup at 2200-2100, 1300-1100, 800-650, and 150-0 Ma.  A peak in plate velocity at 450-350 Ma correlates with early growth of Pangea, and another at 1100 Ma with the initial stages of Rodinia’s assembly following the breakup of Nuna.

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

Contact:  Sergei Pisarevsky
Funded by:  CCFS Foundation Program 6: Detecting Earth’s rhythms: Australia’s Proterozoic record in a global context 

Highly siderophile elements in the Emeishan LIP

Highly magnesian lavas (picrite) potentially preserve information about the origin and thermochemical state of the mantle sources of large igneous provinces (LIPs).  We have carried out high-precision analysis of highly siderophile elements (HSE) in picrites from the ca 260 Ma Emeishan large igneous province (Fig. 1). 

Figure 1.  Distribution of the various components of the Emeishan Large Igneous Province, showing sample locations.

The absolute abundances of HSE in the Emeishan picrites are greater than those in MORB and in parental melts of Hawaiian picrites, and similar to the concentrations observed in komatiites (Fig. 2).  The CI chondrite-normalised HSE patterns of the studied samples can be divided into two types (Fig. 3).  Type 1, represented by the Muli picrites, is similar to that of the primitive upper mantle.  Type 2, represented by the Dali picrites, has patterns similar to those of East Greenland and Iceland picrites; more fractionated Pt/Ir (8.6-34.5 with an average of 15.9 ± 8.4) and Pd/Ir (1.3-12.1 with an average 6.6 ±3.0) ratios relative to Type 1.  

Figure 2.  Total PGE contents versus MgO. The reference fields are after Ely and Neal, (2003), the Emeishan basalts and picrites data from Li et al. (2012).

The primary melt compositions of the studied samples were estimated using back addition of equilibrium olivine into selected whole rock compositions (Fig. 4).  The estimated HSE abundances in the parental melts of the Dali and Muli picrites are higher than estimates of Hawaiian parental melts.  The large range of HSE abundances in the picrites reflects the integrated effects of source heterogeneity, plume-SCLM interaction, partial melting and early fractionation of olivine (± chromite).  Together with existing isotopic data, this study shows that the source of the Emeishan plume mantle was chemically heterogeneous.

 Figure 3.  CI-chondrite normalised HSE patterns for (a) Muli and (b) Dali picrites.  Primitive upper mantle (PUM) is thought to be representative of fertile peridotites, prior to the removal of material from the upper mantle (Becker et al., 2006). The reference MORB field is modified after Dale et al. (2008).  The primitive melt for Hawaiian picrites is an average of individual suite parental melts (Ireland et al., 2009).  The normalising values are from McDonough and Sun (1995). Data sources are as follows: east Greenland picrites are from Momme et al. (2006) and Momme et al. (1997); Iceland picrites from Momme et al., (2003); Hawaiian picrites from Bennett et al. (2000), Ireland et al. (2009) and Pitcher et al. (2009).

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

Contacts:  Xuan-Ce Wang, Jie Li (GIGCAS, jieli@gig.ac.cn)
Funded by:  National Science Foundation of China (No.40903007) and CCFS ESTAR Fellowship

Figure 4.  Example of how the HSE content in a parental melt was estimated, using Os abundances of the Muli picrites.  The parental melt is assumed to contain 19 wt% MgO, and its Os abundance is determined by linear regression of the data.  There are three samples that fall off the regression trend. 

From core to ore: Localisation of Earth’s hottest lavas

Figure 1.  3.4 billion year old komatiite flow from the Barberton greenstone belt in South Africa, where these ultra-high temperature lavas were first recognised.  These lava flows consist of two zones; the A-zone (upper) is dominated by fine, needle-like crystals of olivine called ‘spinifex texture’, while the B-zone (lower) consists of a solid matrix of olivine crystals, which mark the base of the komatiite lava river.

Volcanic eruptions are as old as the planet itself and have inspired awe, curiosity, and sometimes fear since the dawn of the first humans.  These dynamic systems are the surface expression of the Earth’s internal heat engine, and demonstrate that our planet is alive internally, as well as externally.

Despite the impressive impact of modern volcanoes, these eruptions and their flows pale in comparison to those that affected our planet in the past.  The rarest and most evocative type of volcano is that of the ancient komatiites.  These lavas are restricted to the early history of Earth around, 3.4-1.8 billion years ago when the mantle was much hotter.  Erupted at temperatures above 1600 °C, they produced hose-like fire fountains, and lava flows that travelled at over 40 kilometres an hour as bluish-white, turbulent lava rivers.  These crystallised to form some of the world’s most spectacular igneous rocks, as well as giant nickel deposits, mainly in Western Australia and Canada.

These volcanic rocks have been studied for over 60 years and have been fundamental in developing our knowledge of the thermal and chemical evolution of the planet.  However, until recently we did not understand why they formed where they did. Komatiites are found in ancient pieces of crust, called cratons, preserved from the Archean Eon 3.8-2.5 billion years ago.  These cratons contain belts of preserved volcanic and sedimentary material, called greenstone belts.  One of the largest cratons is Western Australia’s Yilgarn Craton, which hosts most of the gold and nickel ever mined in Australia.  This craton contains many greenstone belts but only a few contain major komatiite flows.  The uneven distribution of ultra-high temperature lavas is not only a puzzling academic problem, but also is relevant to exploration for nickel ore deposits.

Previous research has demonstrated that komatiites form from mantle plumes - upwelling pipes of hot material that stretch from the outer core to the base of the crust; the modern equivalent is the mantle plume that is forming the Hawaiian island chain.  Around 2.7 billion years ago, in a huge global event referred to as a ‘mantle turnover’, multiple mantle plumes formed, and one impacted the base of the Yilgarn Craton, forming the hottest lavas ever erupted on Earth.  When plumes first hit the base of the lithosphere - the 50-250 km thick rigid outer shell of the Earth - they spread out into discs of hot material more than 1000 kilometres in diameter.  If the plume covered such a large area, why are komatiites confined to specific linear belts?

Figure 2.  This cartoon demonstrates the main findings of our study.  By imaging the older, thicker and younger, thinner areas of ancient lithosphere in the Yilgarn Craton, we were able to map the three-dimensional architecture of the craton and explain why komatiites are localised in specific belts.  Plume melts are ‘channeled’ into the younger, thinner continental areas, resulting in a concentration of komatiites and their associated ore deposits in these areas.

CCFS researchers in collaboration with CSIRO and the Geological Survey of Western Australia, set out to answer this question, not by looking at the komatiites, but by studying the slightly younger granites that make up most of the craton.  They used Hafnium isotopes in zircon to constrain the age of the rocks that were melted to form the granites and whether they had a mantle or a crustal source.  Mapping out the isotopic compositions of the granites revealed a jigsaw pattern in the crust, defined by regions where the granites formed by melting pre-existing, much older crustal rocks, and younger areas where the crust was newly created from sources in the deeper mantle.

Comparing the nature and shape of the ancient continent with the location of the major komatiite events, we found a remarkable correlation. The major komatiite belts and their ore deposits are located at the edge of the older continental regions.  This is because of the shape of the base of the ancient Australian continent.  As the plume rises, it impacts the older lithosphere first; this is thick and as a result the plume cannot generate much magma.  However, the plume flows upwards along the base of the lithosphere, into the shallower, younger areas.  Here, huge volumes of magma are generated at the boundary between the old, thick and young, thin areas of the lithosphere.  Subsequently, komatiites, and their nickel deposits, are located at the margins of Earth’s early continents.  

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

Contact:  Marco Fiorentini 

Funded by:  This study was started through ARC Linkage Projects (LP0776780, LP110100667) and finished within the framework of CCFS Flagship Program 2: Genesis, transfer and focus of fluids and metals

Zircon signposts for base metals 

Figure 1.  Zircon Hf isotopic mapping of the Lhasa Terrane in Tibet.  The colour bands show the Epsilon Hf value of zircons studied.  The ore deposits and zircon sample localities are also shown.  The warm colours represent relatively juvenile blocks with higher Epsilon (Hf) values, whereas the cold colours represent relatively ancient blocks with lower Epsilon (Hf) values. Abbreviations: IYZSZ = Indus-Yarlung-Tsangpo Suture Zone; BNSZ = Bangong-Nujiang Suture Zone; LMF = Luobadui-Milashan Fault; SNMZ = Shiquanhe-Nam Tso Melange Zone; NLS = Northern Lhasa Subterrane; CLS = Central Lhasa Subterrane; SLS = Southern Lhasa Subterrane.

Zircon multi-isotopic maps are a powerful tool for imaging lithospheric blocks of different age, and have been used in the Yilgarn Craton of Western Australia and the Superior Craton in Canada.  Such maps combine in situ zircon U-Pb and Lu-Hf isotopic analyses.  The isotopic mapping serves as a form of ‘paleogeophysics’ for imaging ancient lithospheric architecture through time, even when it may not be visible in seismic data

There is a strong spatial correlation between lithospheric boundaries and the concentration of gold in Archean cratons.  However, it is unknown whether this is the case for ore deposits in younger orogenic belts such as the world’s largest - the Indo-Asian continental collision zone.  Therefore, we have chosen the Lhasa Terrane in southern Tibet to examine the relationship between lithospheric architecture and various types of ore deposits in such orogens.

The Lhasa Terrane is the most metal-rich tectonic unit within the Indo-Asian collision zone with a variety of base metal deposits: copper, molybdenum, iron, lead and zinc (Fig. 1).  It previously has been subdivided into northern, central, and southern subterranes, based on the differing sedimentary covers and metamorphic basements (Fig. 1).  These three subterranes are separated by the Shiquan River-Nam Tso Melange Zone (SNMZ) and the Luobadui-Milashan Fault (LMF) (red lines on Fig. 1).

The zircon Hf-isotope mapping reveals more complex patterns than that defined by surface geology (Fig. 1).  In the northern Lhasa subterrane, the western and eastern segments have high and low Epsilon Hf values, respectively, suggesting a juvenile block in the west and an ancient block in the east.  The central Lhasa subterrane is dominated by low Epsilon Hf values, consistent with it being an ancient Precambrian microcontinent.  The southern Lhasa subterrane consists of two isotopically juvenile blocks in the west and east, withan ancient block in the middle (Fig. 1).  A time slice map at 215-66 Ma, i.e. the period of orogenic accretion before India collided with Asia, reveals the same pattern for the Lhasa Terrane, suggesting that the lithospheric architecture of the Lhasa Terrane formed in the Mesozoic when two juvenile arcs accreted onto an ancient Precambrian microcontinent (Fig. 2a).  A time slice map at 65-12 Ma, during the collision, shows that the eastern segment of the southern Lhasa subterrane is very juvenile, consistent with underplating of mantle-derived magmas generated during the Cenozoic collision (Fig. 2b).

Figure 2.  Time slices of zircon Hf isotopic mapping at 215-66 Ma (a) and 65-12 Ma (b).  See Figure 1 for Legends.

The Mesozoic subduction-related porphyry Cu-Au deposits and Cenozoic collision-related porphyry Cu-Mo deposits are exclusively located in juvenile crust with high Epsilon Hf values (Figs. 1 and 2).  The granite-related Pb-Zn deposits cluster in the oldest crustal regions or are developed along the margin of the old crustal block bounded by lithospheric faults (Fig. 1).  The porphyry Mo-Cu deposits are localised along the reworked margins of the old crustal block.  Skarn Fe-Cu ore deposits are typically localised along a terrane-boundary fault, through which crustally-derived felsic melts mixed with ascending Cu-rich mantle-derived mafic magmas.  Skarn Fe deposits are exclusively located in the ancient crustal region (Fig. 1).

These zircon Hf-isotope maps show temporal-spatial relationships between lithospheric architecture and the location of ore deposits, and demonstrate that the structure, nature and composition of the lithosphere controlled the localisation of ore deposits and the migration of ore-forming metals into the crust.  Zircon isotopic mapping is a powerful tool to image the lithospheric architecture of an orogenic terrane, and thus may become a robust pathfinder to base-metal deposits.

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

Contacts:  Yongjun Lu, Zeng-Qian Hou, Cam McCuaig 
Funded by:  ARC ECSTAR Fellowship, CCFS Foundation Program 9: 4D Lithosphere evolution, CAGS 

Dissecting a continental collision with thermochronology

How are ultrahigh pressure metamorphic (UHM) rocks exhumed to the surface from mantle or lower-crustal depths?  What happens after two continents collide?  The clue could lie in the cooling history of UHP belts and the surrounding rocks, and one of the best-exposed and best-studied UHP terranes is the Dabie-Sulu orogenic belt in eastern China. 

The Dabie-Sulu orogenic belt formed during the collision of the South China Block (SCB) with the North China Block (NCB) in early Mesozoic time.  However, a lack of unambiguous kinematic constraints has made it difficult to verify competing models for the mechanisms of continental collision and exhumation of the UHP rocks.  We have applied mica and hornblende 40Ar/39Ar, zircon and apatite fission tracks, and zircon (U-Th)/He dating to both the metamorphic rocks and syn- to late-orogenic granitic intrusions in the Sulu UHP belt and the adjacent Jiaobei region.  40Ar/39Ar and zircon (U-Th)/He data show that the UHP rocks experienced a prominent cooling event at ca 210-160 Ma (Fig. 1a).  The Jiaobei region, underlain by the Archean basement of the NCB and located immediately north of the Sulu UHP belt, experienced a localised exhumation at ~260 Ma as well as a Jurassic (196 ± 9 Ma to 164 ± 7 Ma), westward-younging exhumation (Fig. 1b) that overlaps with the exhumation age of the Sulu UHP belt.  The ca 260 Ma exhumation probably reflects tectonic erosion at the southern margin of the NCB in response to the initial continental collision, whereas the Jurassic exhumation may be related to an episode of NW-SE tectonic compression.  Hence, the 210-160 Ma exhumation in both the Sulu UHP belt and the Jiaobei region is interpreted to reflect erosion in response to northward thrusting of the UHP rocks and the Jiaobei Archean basement.  Overall, our results lend strong support to the continental-collision model of Li (1994) in which both the Sulu UHP rocks and the upper crust of the Jiaobei region moved northward along a mid-crustal detachment plane after the UHP rocks were exhumed to upper-crust levels (Fig. 2).

Figure 2.  The crustal detachment model that is compatible with observed exhumation events.  A) With initial continental collision at ~260 Ma, the Jiaodong region underwent crustal thickening and exhumation.  B) By ~210 Ma, the UHP-HP rocks had been exhumed back from the mantle depths to crustal depths.  Continuous compression led the UHP-HP rocks to detach from underlying SCB lower crust and move northward along a horizontal detachment until finally climbing the frontal thrust ramp at ~160 Ma.  This movement also caused folding and thrusting in the Jiaobei region and exhumation of both the Sulu UHP rocks and the upper crust of the Jiaobei region.

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

Contacts:  Li-Ping Liu, Zheng-Xiang Li

Funded by:  ARC (DP110104799) and NSFC (41325009) 

Cryptic chemical fingerprints identify different origins for ancient regions of Western Australia

CCFS, through close collaboration with the Geological Survey of Western Australia, has produced a world-leading isotopic dataset that integrates U-Pb ages and Lu-Hf data from zircons with geological, geochemical, and geophysical information from across Western Australia.  This expansive dataset has refined our understanding of the Proterozoic evolution of Australia.  One of the most critical, yet controversial, issues relating to Proterozoic reconstructions of the Australian Continent is the nature, or even existence, of Mesoproterozoic links between the Musgrave Province and the Albany-Fraser Orogen. 

Schematic continental reconstruction of Proterozoic Australia for the interval 1900-1300 Ma (from Howard et al., 2015 Gondwana Research).

The Musgrave Province lies between the North and South Australian Cratons, whereas the Albany-Fraser Orogen lies along the southern and southeastern margin of the Archean Yilgarn Craton.  These two belts evolved through temporally similar Mesoproterozoic orogenies at 1345-1260 and 1215-1140 Ma, but on completely different basement.  The West and North Australian Cratons amalgamated before the 1345-1260 Ma event, which is interpreted to result from the collision of those combined cratons with the South Australian Craton.

The Paleoproterozoic and Mesoproterozoic history of the Albany-Fraser Orogen involved the reworking of the Yilgarn Craton’s margin, accompanied by significant juvenile mantle input.  The orogen’s profile of zircon ages and its Hf- and Nd-isotopic signature reveals an Archean component incorporated into rocks of all ages, which is unequivocally derived from the Yilgarn Craton.  The age and isotopic signature of the Albany-Fraser Orogen thus suggests it is an indigenous component of the Yilgarn Craton, rather than an accreted terrane.

This program strand has produced well-defined Nd- and Hf-isotope arrays that track the evolution of the Musgrave Province, through distinct 1950-1900 and 1600-1550 Ma crustal-formation events.  Rare non-radiogenic material in the Musgrave Province is only seen in detritus and in magmas that assimilated this material.  This evolved detritus, derived from non-Yilgarn and reworked Archean sources, was added to sedimentary basins developed over the Musgrave Province basement from ca 1400 Ma onward.  However, the pre-Mesoproterozoic isotopic composition and zircon-age profile for the Musgrave Province is distinctly different from that of the Albany-Fraser Orogen.

If the basement to the Musgrave Province is neither Archean nor part of the West Australian Craton, then what is it?  The Madura Province, which lies south of the Musgrave Province, has a radiogenic signature similar to the Musgrave Province and the two provinces probably are contiguous beneath the younger cover sequences. We have found that the crust of the Madura Province contains zircons with 2.0-1.4 Ga mantle extraction ages, and is dominated by juvenile rocks related to oceanic crust that was consumed to the north of the South Australian Craton and east of the West Australian Craton.  It appears that the Musgrave Province was formed by modification of Madura-Province crust along the edge of the North Australian Craton.  Together, the Musgrave Province and the Madura Province represent Proterozoic Australia’s most juvenile crustal remnant. 

The new data indicate that the Musgrave Province developed on a juvenile substrate of Madura Province crust that was subducting beneath, and accreting to, the North Australian-West Australian Craton.  During the Mount West Orogeny / Albany-Fraser Orogen at ca 1300 Ma, the Musgrave Province was not an along-strike equivalent of the Albany-Fraser Orogen.  At that stage, the Musgrave was an active subduction zone, while the Albany-Fraser Orogen was undergoing orogenic collapse following the earlier accretion of the Loongana arc of the Madura Province to the West Australian Craton.

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

Contacts:  Chris Kirkland, Elena Belousova
Funded by:  CCFS  Foundation Program 10b. 

Mobile lead and spurious zircon ages

Zircons from metasedimentary and metaigneous gneisses in the Napier Complex (East Antarctica) preserve zircon U-Pb ages greater than 3.8 Ga (Black et al., 1986; Harley, 1997).  The complex underwent two metamorphic events; high-temperature metamorphism at ca. 2.8 Ga (Kelly and Harley, 2005) and ultra-high-temperature (T >1100 ºC) at ca 2550-2480 Ma (Harley, 2000).  The earliest zircon SHRIMP (Sensitive High Resolution Ion MicroProbe) study of the Napier Complex (Williams et al., 1984)described isotopic disturbance of >3.4 Ga zircons, defined by reversely discordant analyses (i.e. U-Pb ages older than 207Pb/206Pb ages) and within-run instability of Pb during analysis.  These results were confirmed recently by ion imaging utilising a Cameca ims 1280 (CCFS Publications #407, 429).  These studies revealed the patchy distribution of Pb and Ti and identified the presence of unsupported Pb with anomalously high (>4 Ga) ²⁰⁷Pb/²⁰⁶Pb ages. 

Following this discovery, we analysed zircons from three samples: one orthogneiss (from Gage Ridge) and two paragneisses (one each from Mount Sones and Dallwitz Nunatak) by TEM (Transmission Electron Microscopy) applying the site-specific focused-ion-beam (FIB) technique at GeoForschungsZentrum (GFZ) Potsdam in Germany.  TEM images revealed that zircon grains from the Napier Complex contain randomly distributed, spherical metallic lead nano-inclusions 5-30 nm in diameter (Kusiak et al., under revision).  They occur either as individual droplets (Fig. 1) or together with unidentified phases rich in Ti and silica, which together constitute melt inclusions ca 20-80 nm in diameter.  The occurrence of metallic Pb nanospheres in zircons that underwent UHT metamorphism explains the unusual U-Pb behaviour of such grains during SIMS (Secondary Ion Mass Spectrometry) analysis.  Further studies are continuing to determine how the nanospheres may have formed.  See CCFS Publications #407, 429.

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

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

References:

Black, L.P., Sheraton, J.W., James, P.R.  1986.  Late Archean Granites of the Napier Complex, Enderby Land, Antarctica - a Comparison of Rb-Sr, Sm-Nd and U-Pb Isotopic Systematics in a Complex Terrain.  Precambrian Research, 32, 4, 343-368.

Harley, S.L., Black, L.P.  1997.  A revised Archaean chronology for the Napier Complex, Enderby Land, from SHRIMP ion-microprobe studies.  Antarctic Science, 9, 74-91

Harley, S.L., Motoyoshi, Y.  2000.  Al zoning in orthopyroxene in a sapphirine quartzite: Evidence for >1120 ºC UHT metamorphism in the Napier Complex, Antarctica, and implications for the entropy of sapphirine.  Contributions to Mineralogy and Petrology, 138: 293-307.

Kelly, N.M., Harley, S.L.  2005.  Timing of zircon growth during highgrade metamorphism: Constraints from garnet-zircon REE. Geochimica et Cosmochimica Acta, 69. 10, A22-A22.

Kusiak, M.A. et al., under revision.  Metallic lead nanospheres discovered in ancient zircons.  Proceedings of the National Academy of Sciences of the United States of America.

Williams, I.S., Compston, W., Black, L.P., Ireland, T.R., Foster, J.J.  1984.  Unsupported Radiogenic Pb in Zircon - a Cause of Anomalously High Pb-Pb, U-Pb and Th-Pb Ages.  Contributions to Mineralogy and Petrology, 88, 4, 322-327.

The Hadean - Archean crust-mantle revolution: How should we divide the Archean?

When and how did the continents on which we live form and stabilise in the torrid early history of our evolving planet?  We have used the mineral zircon as time capsules to record the history of events in the Earth’s crust from trace-element and isotopic data (U-Pb and Lu-Hf).  Sulfide minerals and metal alloys from mantle-derived samples (brought to the Earth’s surface by magmas from 50 to 600 km deep), record the history of fluid, melting and formation events in the mantle using the Re-Os isotopic system.  The combination of these two powerful tools allows time travel back to early Earth times and has produced some rare insights, changing our understanding of the timing of significant ancient upheavals that shaped Earth’s crust.  

Figure 1.  εHf vs age for zircons worldwide, divided by region.  The database (n=6699) builds on the data of Belousova et al. (2010) updated with >2000 analyses published, and/or produced in our laboratories, after 2009.  The conventional subdivision of the Hadean and Archean is shown along the X axis.  (a) zircon age distribution shown as a histogram and a cumulative-probability curve.  (b) distribution of Hf-isotope data.  The “juvenile band”, defined as ±0.75% of the εHf of the Depleted Mantle line at any time, is shown as a grey envelope.  Evolution lines from 4.5 Ga are shown for reservoirs corresponding to typical mafic rocks (¹⁷⁶Lu/¹⁷⁷Hf = 0.024), the average continental crust (¹⁷⁶Lu/¹⁷⁷Hf = 0.015), and zircons, approximated by ¹⁷⁶Lu/¹⁷⁷Hf = 0.  (c) shows these evolution lines.

We have integrated a worldwide compilation of the ages of crust and mantle events from 2 billion years ago until nearly the time of formation of planet Earth at ~4.5 billion years, allowing us to track patterns of evolution of the crust, and interaction between the mantle and crust.  These data show that from about 4.5 to 3.4 billion years ago, the Earth’s crust was essentially stagnant (like a lid on a hot cauldron), and was mainly basaltic in composition.

Figure 2.  (a) Cumulative-probability curve and age histogram for all zircons with εHf falling within 0.75% of the Depleted Mantle curve in Figure 1.  Inset uses an expanded scale to show detail in the oldest datasets. 

Data suggest that some zircons crystallised from magmas that were possibly generated by impact melting from bombarding planetary bodies (meteorites large and small).  Pulses of magmatic activity at about 4.2, 3.8, and 3.3-3.4 billion years, possibly representing mantle convective overturns or rising mantle plumes, broke this quiescent state. 

Between these pulses, there is evidence of resetting of zircon U-Pb ages (by impact?) but the Hf-isotope data (Fig. 1) allow us to see back through such events and imply that there was no further generation of new crust.  There is thus no evidence of plate-tectonic activity, as described for the Earth in the state we know it today (i.e. through the Phanerozoic Era), before about 3.4 billion years ago.  Previous modelling studies indicate that the early Earth may have been characterised by an episodic-overturn, or a stagnant-lid, regime.  New thermodynamic modeling (CCFS Publication #488) confirms that an initially hot Earth could have a stagnant lid for ca 300 million years, and then experience a series of massive overturns at intervals on the order of 150 Ma, until the end of the EoArchean Period (earliest Archean time).  The lack of older Os model ages (Fig. 2) suggests that subcontinental lithospheric mantle (SCLM) sampled on Earth today did not exist before about 3.5 billion years ago.  

Figure 3.  Summary of crust-mantle evolution.  Grey histogram shows distribution of all zircons in the database for the time period shown; red line shows the probability distribution of these ages.  Green line shows distribution of Re-Os TRD model ages for sulfides in mantle-derived xenoliths and xenocrysts.  The revised distribution of meteorite bombardment intensity (Bottke et al., 2012) and a generalized summary of older interpretations of the “Late Heavy Bombardment” is shown as LHB.  The homogenization of PGE contents in komatiites from 3.5-2.9 Ga (Maier et al., 2010) may mark the major mantle-overturn/circulation events discussed in the text.  The O-isotope shift noted by Dhuime et al. (2012) marks the beginning of a quiescent period, or perhaps the destruction of the crust during the major overturns from 3.5-2.9 Ga.

A lull in crustal production around 3.0 billion years coincides with the rapid buildup of Mg0-rich, buoyant SCLM, which peaked around 2.7-2.8 billion years; this pattern is consistent with one or more major mantle overturns.  The generation of continental crust peaked later in two main pulses at about 2.75 and 2.5 billion years ago (Belousova et al., 2010).  The age/Hf-isotope patterns of the crust generated from 3.0-2.4 billion years are similar to those in old tectonic regions of the Gondwana supercontinent, implying the existence of plate tectonics at the time of assembly of the ancient supercontinent (Kenorland) ca 2.5 billion years ago.  We have demonstrated a clear link in these data between the generation of the SCLM and the emergence of modern plate tectonics; we consider this link to be causal, as well as temporal. 

The International Geologic Time Scale sets the Hadean-Archean boundary at 4.0 Ga and divides the Archean into four approximately equal time slices (Fig. 1): Eo-Archean (4.0-3.6 Ga (Ga = billion years ago)), Paleo-Archean (3.6-3.2 Ga), Meso-Archean (3.2-2.8 Ga) and Neo-Archean (2.8-2.5 Ga).  There is no apparent evidence in the present datasets for these divisions, and we suggest a simplified subdivision, based on the changes in tectonic style (Fig. 3) at identifiable times.

In the absence of better data, we accept the Hadean-Archean boundary at 4.0 Ga, although we suggest that the stagnant-lid regime may have continued for another 500-800 million years.  We propose that the term Eo-Archean be discarded, since there is little preserved evidence of magmatic activity between 4.2 and 3.8 billion years, and that the term PaleoArchean be used for the period 4.0-3.6 billion years; this timespan contains the oldest preserved crust, and inferred evidence for one major overturn.  MesoArchean could be applied to the period 3.6-3.0 billion years, during which overturns became more prominent, ending with the buildup toward the major 2.8-2.55 billion years, magmatism that accompanied the building of the bulk of the Archean SCLM.  NeoArchean can be usefully applied to the period 3.0-2.4 billion years, which marks a new tectonic style with frequent plume activity, the beginning of some form of plate tectonics, and the preservation of large volumes of continental crust. The zircon data suggest that this activity continued up to about 2.4 billion years ago, and then ceased quite abruptly, marking a natural end to the Neo-Archean period, and heralding a new geotectonic regime for Earth.  

This project is part of CCFS Themes 1 and 2, Early Earth and Earth Evolution, and contributes to understanding Earth’s Architecture.

Contacts:  Bill Griffin, Elena Belousova, Suzanne O’Reilly
Funded by:  CCFS Foundation Program; TARDIS: Tracking ancient residues distributed in the silicate Earth, Future Fellowship to Elena Belousova