Greenstone burial–exhumation cycles at the late Archean transition to plate tectonics

Converging lines of evidence suggest that, during the late Archean, Earth completed its transition from a stagnant-lid to a plate tectonics regime,although how and when this transition occurred is debated. The geological record indicates that some form of subduction, a key component of plate tectonics—has operated since the Mesoarchean, even though the tectonic style and timescales of burial and exhumation cycles within ancient convergent margins are poorly constrained.Here, we present a Neoarchean pressure–temperature–time (P–T–t) path from supracrustal rocks of the transpressional Yilgarn orogen (Western Australia), which documents how sea-floor-altered rocks underwent deep burial then exhumation during shortening that was unrelated to the episode of burial. Archean subduction, even if generally short-lived, was capable of producing eclogites along converging lithosphere boundaries, although exhumation processes in those environments were likely less efficient than today, such that return of high-pressure rocks to the surface was rare

Using microanalysis of minerals to track geochemical processes during metamorphism: examples from the Mary Kathleen fold belt, Queensland, and the Eastern Mt. Lofty Ranges, South Australia

Understanding the behaviour of major and trace elements during metamorphism is fundamental for our understanding of the geochemical evolution of the Earth's crust and the formation of orogenic orebodies. Furthermore, it is essential to know how key elements and radiogenic isotopes behave in metamorphic/hydrothermal systems in order to apply them meaningfully to solve important questions in geosciences. Metamorphic/hydrothermal reactions are most evidently preserved at the mineral scale, so in situ microanalytical techniques are best suited for tracing the record of metamorphism or hydrothermal alteration. In this thesis, I outline new analytical developments for in situ analysis of halogens in minerals and fluid samples, and of Sm-Nd isotopes in REE-rich minerals. These techniques, in conjunction with comprehensive bulk rock and mineral geochemistry and element distribution analysis, are then applied to well-characterised metamorphic rocks from the Adelaide Fold Belt and Mt Isa Inlier. Although fluid is an essential ingredient for mass transport during metamorphism, it is often difficult to identify the source of metamorphic/hydrothermal fluids. Traditionally, fluid inclusions have been used to gain insights into the source and composition of fluids. Until very recently, quantification of key elements such as bromine and chlorine in fluid inclusions relied almost solely on bulk rock analyses techniques (i.e., crush-leach). These methods do not allow distinction between different fluid inclusion generations that might hold crucial information on the evolution of a hydrothermal system and associated mineralization. The development of in situ LA-ICP-MS analysis of chlorine and bromine in fluid inclusions now allows for the targeting of individual fluid inclusions of a specific fluid type in a mineral. In this thesis these techniques were further tested and refined, and applied for the first time to a range of natural scapolite group minerals, minerals assumed to reflect the Cl/Br content of the coexisting hydrothermal fluids. The results show that fluid sources can be identified with a ~ 25 μm resolution in Cl and Br bearing minerals. This technique was applied on scapolite minerals from skarns, regional metamorphic rocks and a mineralized shear-zone of the Mary Kathleen Fold Belt in the Mt. Isa inlier. While scapolite minerals in skarns contain Cl/Br ratios typically associated with granitic fluids, metamorphic scapolite indicates that fluids were dominantly derived from basinal brines formed from sub-aerial evaporation of seawater beyond the point of halite saturation. This bittern fluid infiltrated the underlying sedimentary sequences prior to regional metamorphism. Zoned scapolite in the mineralized shear-zone records three discrete pulses of magmatic and metamorphic fluid, and it is suggested that fluid mixing may have assisted mineralization along and around this shear-zone. To investigate element mobility during metamorphism, I studied the Eastern Mt. Lofty Ranges in South Australia. Metamorphic rocks of the Mt. Lofty Ranges have a relatively simple metamorphic history, and metamorphic gradients and widespread up-temperature fluid flow has been documented previously. This allows monitoring of mineral and bulk rock compositional changes (or lack thereof) during metamorphism across a regional metamorphic gradient from ~350–400 ˚C to migmatite grade (~ 650–700 ˚C) at ~0.3–0.5 GPa, in a confined framework. The results show that, despite widespread up-temperature fluid flow, major elements and most trace elements are isochemical during metamorphism. These elements are effectively redistributed into newly formed major minerals or accessory phases. Monazite or allanite and xenotime control the whole rock concentration of REE whereas apatite and titanite are minor REEs hosts. The only non-volatile mobile elements are Zn, Pb, Cs and As whose concentrations decreased with increasing metamorphic grade. The Zn and Pb depletion was progressive with increasing temperature in staurolite-absent psammo-pelites, with losses of ~ 75% of the original Zn and ~ 50 % of the original Pb from the rocks from high-grade metamorphic zones. Microanalysis showed that biotite is a key mineral for Zn sequestration by concentrating >80 % of the Zn in the bulk rock. Zinc and Pb likely partitioned into a Cl-rich hydrothermal/metamorphic fluid that led to the observed depletion of Pb and Zn in the bulk rock. Simple mass balance calculations show that ~27 Mt of Zn and ~2.7 Mt of Pb were mobilized during prograde metamorphism, which is comparable to the amounts of base metals found in world class Pb-Zn deposits. Hence, prograde metamorphism of sedimentary rock packages is a viable base metal source for the formation of some Pb-Zn deposits, provided that the metamorphic fluid contains sufficient Cl to effectively mobilise metals from the metamorphic system into ore-forming environments. The observed As loss is consistent with the recrystallization of As-bearing pyrite to As-poor pyrrhotite, confirming previous studies. Cesium depletion in migmatites can be explained by the incompatibly of Cs in micas in high-grade metamorphic rocks. Significant element mobility during metamorphism is likely only achieved under conditions with high fluid flux. In order to understand crustal evolutionary processes and crustal fractionation via for example melt production in migmatitic systems equivalent to the high-grade zone of the Eastern Mt. Lofty Ranges, geochemists widely rely on radiogenic isotopes. However recent claims of Nd and Sr isotope disequilibrium during anatexis question the reliability radiogenic isotopes. Microanalysis of REE-rich accessory minerals was used to investigate Nd isotope equilibration during metamorphism in order to assess to potential of disequilibrium situations during high-grade metamorphism. The results are used to demonstrate that apatite retains an original, probably detrital, highly variable Nd isotopic signature until at least 500 ˚C, before being isotopically homogenized. In contrast, allanite and titanite are equilibrated at temperatures as low as 350–400 ˚C. REE-rich accessory minerals in high-grade rocks (~600 ˚C) show very similar initial Nd isotope values at the time of metamorphism. I conclude that Nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely. Furthermore, in situ microanalysis of radiogenic isotopes can help to identify external melt components in migmatites that would not be resolvable by conventional bulk rock analysis

Combined Hf and Nd isotope microanalysis of co-existing zircon and REE-rich accessory minerals: High resolution insights into crustal processes

The Sm-Nd and Lu-Hf isotope tracer systems, applied to whole rocks or mineral separates, have provided powerful insights into the formation, differentiation, and evolution of the Earth's continental crust. However, some key questions remain, such as how certain igneous rocks form, and how reliable radiogenic isotope tracers are for tracking melt sources. Here, the potential for combining data from the two isotope tracers, Nd and Hf, obtained on a sub-mineral-scale is explored to further understand how crustal rocks receive their sometimes-puzzling geochemical fingerprints. New data, in combination with results from previous studies, reveal one of the key strengths of combining the two independent isotope systems at a micrometre scale, namely identifying open versus closed system metamorphic and igneous processes. Such knowledge is key for understanding how the continental crust formed and stabilized, and for elucidating the role of mantle-derived magmas in the production of granitic rocks, a long-standing issue that is still highly debated. We show how measurement of Nd and Hf isotope ratios in accessory minerals from the same sample has helped to evaluate the fundamentally different models (e.g., magma mixing; crustal assimilation; reactive melt transport vs. incomplete geochemical homogenization of melt at its source) invoked to explain heterogeneous isotope signatures in igneous rocks. Lastly, we discuss how the dual in situ Nd and Hf isotope approach can be used to to evaluate the extent to which metamorphism obscured the primary isotope signature of Archean gneisses derived from felsic igneous protoliths, which has profound implications for our interpretation of early crust formation processes

An experimental study of trace element distribution during partial melting of mantle heterogeneities

Trace elements are widely used to interpret the origin of mantle-derived magmas, yet we lack detailed understanding of how trace elements behave during melting of mantle source components. Here, we present new data on trace element distribution and partitioning between phases from high pressure (3.0 to 5.0 GPa), high-temperature (1230 to 1550 °C) melting experiments on starting compositions that represent altered oceanic crust and metasedimentary protoliths. These compositions are expected to be recycled into the mantle via subduction or delamination to form heterogeneous mantle domains that are implicated in the genesis of intraplate and/or ocean floor magmas. In most of the experiments, the investigated trace elements behave incompatibly, expect for HREE and Y, which are compatible in garnet, and V, Cr and Zn, which partition into both garnet and clinopyroxene. Relative to Nd, P is more compatible in garnet than clinopyroxene, leading to fractionation of P/Nd with melting in some cases. Melt compositions in some experiments with low melt fractions feature distinctive negative anomalies for Nb, and for Sr, Ba and Eu, due to retention of these elements in minor/accessory rutile and feldspar, respectively. We also show that highly incompatible trace element (e.g., Cs, Th, U, LREE) concentrations in melts are strongly controlled by melt fraction, whereas moderately incompatible (M-HREE, Zr) to compatible (Cr, V) element concentrations are controlled by temperature and/or phase composition. Pressure has relatively little influence on trace element behaviour at the investigated conditions. Based on our results, we suggest that partial melting of eclogitic components of mantle domains may ultimately produce magmas with trace element compositions that are unlike peridotite-sourced magmas. Therefore, the trace element systematics of mantle-derived magmas should not only be interpreted in terms of mantle source compositions, but also with consideration to source petrology (e.g., mineral compositions and accessory phase stability) and melting conditions (e.g., melt fraction, pressure, temperature)

An Archean Yellowstone? Evidence from extremely low δ¹⁸O in zircons preserved in granulites of the Yilgarn Craton, Western Australia

We report the discovery of Archean (2980-2670 Ma) zircons from the Yilgarn Craton in Western Australia that record unusually low delta O-18 signatures (to -0.5%). These zircons occur in cordierite-orthopyroxene granulites that retain the geochemical signature of intense premetamorphic hydrothermal alteration. We propose a model whereby the low-delta O-18 zircons crystallized within protoliths that record multiple stages of high-temperature interaction and hydrothermal exchange between shallow crustal material and O-18-depleted meteoric fluids, in a setting analogous to that of the Yellowstone caldera. Burial and subsequent granulitefacies metamorphism of this crust led to the crystallization of zircon, which acquired and preserved the extremely O-18-depleted signature of the whole rock. The apparent absence of strongly O-18 depleted Archean zircons has been a puzzling feature of the global zircon record, but we suggest this is an artifact of poor preservation potential. Our findings suggest that long-lived, shallow crustal magmatic-hydrothermal systems similar to those operating in modern caldera complexes were also a feature of Archean Earth

Measuring in situ CO2 and H2O in apatite via ATR-FTIR

We present a new approach to determine in situ CO2 and H2O concentrations in apatite via attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Absolute carbon and hydrogen measurements by nuclear reaction analysis (NRA) and elastic recoil detection (ERD) are used to calibrate ATR-FTIR spectra of CO2 and H2O in apatite. We show that CO2 and H2O contents in apatite can be determined via linear equations (r(2) > 0.99) using the integrated area of CO2 and H2O IR absorption bands. The main benefits of this new approach are that ATR-FTIR analyses are non-destructive and can be conducted on polished sample material surfaces with a spatial resolution of similar to 35 mu m. Furthermore, the wavenumber of the phosphate IR absorption band can be used to determine the crystallographic orientation of apatite, which allows for accurate quantification of CO2 and H2O in randomly orientated apatite grains. The limit of quantification of H2O in apatite is similar to 400 ppm and similar to 100 ppm for CO2. Via two examples, one from a carbonatite and one from a metasedimentary rock, we show that this new technique opens up new possibilities for determining volatile concentrations and behavior in a wide range of hydrothermal, igneous, and metamorphic systems.ISSN:0010-7999ISSN:1432-096

Composition and evolution of fluids forming the Baiyinnuo'er Zn-Pb Skarn Deposit, Northeastern China: insights from laser ablation ICP-MS study of fluid inclusions

The Baiyinnuo'er skarn deposit is one of the largest Zn-Pb deposits in northeastern China, with 32.74 million metric tons (Mt) resources averaging 5.44% Zn, 2.02% Pb, and 31.36 g/t Ag. The deposit formed in three stages: the preore stage (prograde skarn minerals with minor magnetite), the synore stage (sulfides and retrograde skarn minerals including calcite and minor quartz), and the postore stage (late veins composed of calcite, quartz, fluorite, and chlorite; cutting the above mineral assemblages). In this study we analyzed the composition of single fluid inclusions using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to (1) determine the composition of the fluids and the evolution through the stages, (2) infer the fluid and metal sources, and (3) explore the metal deposition mechanisms. The preore fluids trapped in pyroxene have higher homogenization temperatures (432°–504°C), higher salinity (36.5–46.1 wt % NaCl equiv), and higher concentrations of Zn (~0.9 wt %), Pb (~1.4 wt %), and other elements (e.g., Na, K, Li, As, Rb, Sr, Cs, Ba, Cl, and Br) than synore mineralizing fluids (<370°C, <10 wt % NaCl equiv, ~450 ppm Zn, and ~290 ppm Pb). The postore fluids show lower temperatures (<250°C) and a rather dilute composition (<4 wt % NaCl equiv, ~33 ppm Zn, and ~24 ppm Pb). Geochemically, the fluids of all paragenetic stages in Baiyinnuo'er have magmatic signatures based on the element mass ratios, including elevated K/Na, Zn/Na, and Rb/Na ratios, lower Ca/K ratios, and combined Cl/Br-Na/K ratios, which are distinctively different from basinal brines. Inclusion fluids in preore stage show little variation in composition between ~510° and ~430°C, indicative of a closed cooling system. In contrast, the major components of the syn- and postore fluids, including Cl, Na, and K, decrease and correlate with a drop of homogenization temperatures from ~370° to ~200°C, indicating a dilution by mixing with groundwater. The Baiyinnuo'er mineralizing fluids (trapped in sphalerite) have higher Ca/K mass ratios (avg ~0.78) than other proximal magmatic hydrothermal systems (0.06–0.52) but lower than that of the distal El Mochito skarn (avg ~6.4), probably reflecting a gradually weakened magmatic signal away from the causative intrusions. The metal contents in preore fluids are significantly higher than those in synore fluids, but no mineralization occurred. This confirms that the early fluids were, although enriched in metals, not responsible for ore precipitation, most likely due to their high temperature and high salinities. One important factor controlling Zn-Pb mineralization was mixing with groundwater, which resulted in temperature decrease and dilution that significantly reduced the metal solubility, thereby promoting metal deposition. Another main driving force was the interaction with carbonate wall rock that buffered the acidity generated during the breakdown of Zn and (Pb)-Cl complexes and the precipitation of sulfides. Phase separation occurred in both the preore and the early part of the synore stages, but no evidence indicated that it caused metal deposition. The prograde minerals and retrograde minerals (including ore minerals) coexisting in the same samples could have been caused by two (or more) successive pulses of hydrothermal fluids released from residual melts of a progressively downward crystallizing magma. Each fluid produced a series of proximal high-temperature prograde to distal low-temperature assemblages, with the lower temperature assemblages of later fluids overprinting the higher temperature assemblages at most locations

In situ quantification of Br and Cl in minerals and fluid inclusions by LA-ICP-MS: a powerful tool to identify fluid sources

Bromine and chlorine are important halogens for fluid source identification in the Earth's crust, but until recently we lacked routine analytical techniques to determine the concentration of these elements in situ on a micrometer scale in minerals and fluid inclusions. In this study, we evaluate the potential of in situ Cl and Br measurements by LA-ICP-MS through analysis of a range of scapolite grains with known Cl and Br concentrations. We assess the effects of varying spot sizes, variable plasma energy and resolve the contribution of polyatomic interferences on Br measurements. Using well-characterised natural scapolite standards, we show that LA-ICP-MS analysis allows measurement of Br and Cl concentrations in scapolite, and fluid inclusions as small as 16 µm in diameter and potentially in sodalite and a variety of other minerals, such as apatite, biotite, and amphibole. As a demonstration of the accuracy and potential of Cl and Br analyses by LA-ICP-MS, we analysed natural fluid inclusions hosted in sphalerite and compared them to crush and leach ion chromatography Cl/Br analyses. Limit of detection for Br is ~8 µg g−1, whereas relatively high Cl concentrations (> 500 µg g−1) are required for quantification by LA-ICP-MS. In general, our LA-ICP-MS fluid inclusion results agree well with ion chromatography (IC) data. Additionally, combined cathodoluminescence and LA-ICP-MS analyses on natural scapolites within a well-studied regional metamorphic suite in South Australia demonstrate that Cl and Br can be quantified with a ~25 µm resolution in natural minerals. This technique can be applied to resolve a range of hydrothermal geology problems, including determining the origins of ore forming brines and ore deposition processes, mapping metamorphic and hydrothermal fluid provinces and pathways, and constraining the effects of fluid–rock reactions and fluid mixing

Zn and Pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits

Comprehension of the genesis of Pb-Zn ore systems is currently limited by a poor understanding of where these metals are sourced from. Our study of metal mobility during regional metamorphism in the Mt. Lofty Ranges, South Australia, demonstrates that in staurolite-absent siliciclastic metasedimentary rocks, biotite contains >80% of the bulk rock Zn, as well as a considerable proportion of the total Pb. Fluid flow through these metasedimentary rocks led to a continuous depletion of Pb and Zn on a mineral and bulk rock scale during prograde regional metamorphism. We calculate that ~80% of the bulk rock Zn and ~50% of the bulk rock Pb were mobilised, mainly through reactions involving biotite. These reactions led to a calculated Pb and Zn "loss" of ~2.7 and 27 Mt, respectively, in the high-grade metamorphic zone. Halogen contents of apatite and biotite and bulk rock Zn isotope data provide evidence that Cl-rich metamorphic fluids were important for metal transport. Hence, fluid flow accompanying prograde metamorphism of typical sedimentary rocks can mobilise base metals to the degree required to potentially supply significant Pb-Zn ore systems