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The punctuated distribution of major gold deposits along orogenic belts is poorly understood. In northern Perú, gold deposits cluster along trends that transect the subparallel Phanerozoic belts that accreted onto the western margin of the Archean–Proterozoic Amazonian craton. This suggests the influence of as yet unrecognized pre-Andean basement structures in controlling the localization of Phanerozoic deposits. We report the newly discovered Tomac ophiolite, which represents a missing link in tracing a cryptic basement collisional suture striking obliquely to the Peruvian Andes. The Tomac ophiolite signals the opening of a former Neoproterozoic ocean basin between the Amazonian craton and a continental ribbon of dispersed microterranes. Re-accretion of the continental ribbon resulted in the formation of a collisional belt between ca. 465 and 440 Ma. Rifting and re-accretion occurred parallel to the strike of the ca. 1400–1000 Ma Grenvillian-Sunsás orogen, indicating that structural corridors that transect the Andes at an oblique strike angle reflect inheritance of preexisting lithospheric weak zones. Our study demonstrates that Phanerozoic gold deposits in northern Perú cluster at the intersection between the newly defined basement suture and the superimposed Andean belt and associated well-documented cross faults. Gold mineralization was localized at ancient structural intersections, where enhanced permeability facilitated anomalous energy and mass transfer, promoting major ore accumulation during episodes of accretion.

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261–243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 ± 2.5 Ma, 256.0 ± 1.8 Ma, 247.8 ± 2.7 Ma and 243.2 ± 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/Yb < 10), unradiogenic Sr/Sr (0.7029 to 0.7032) and radiogenic eNd (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (δNd = 233 ppm) compared to global plutonic rocks (δNd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite.

We present new U–Pb ages and Hf and O isotope data for zircon from I-, S- and A-type granites from both the western and eastern edges of the Delamerian Orogen in southeastern Australia. The I-type Tanunda Creek Gneiss contains zircon populations of 507 ± 4 and 492 ± 6 Ma inferred to reflect igneous and metamorphic ages, respectively. The I-type Palmer Granite yielded an age of 509 ± 3 Ma, and the Port Elliot S-type Granite has a magmatic age of 508 ± 7 Ma. Inherited zircon in these granites range from 1092 to 3343 Ma, probably derived from assimilation of Adelaide Group sediments. The Murray Bridge A-type Granite is 490 ± 2 Ma in age and lacks inherited zircon. In the Glenelg River Complex, an S-type migmatite from near Harrow contains a complex zircon population. It is most likely ~500 Ma in age and has inherited zircon of 550–700, 1000–1100 and 2437 Ma, hence matching those from the Kanmantoo Group. From this and detrital zircon ages, we infer that only the Kanmantoo Group extends across the Murray Basin into the Glenelg River Complex. The Wando Tonalite and Loftus Creek I-type granites yielded ages of 501 ± 2 and 486 ± 3 Ma, respectively. Zircon from the Dergholm Granite has suffered Pb loss, and the best age estimate for this granite is 488 ± 5 Ma. Combining all the granite data together, εHft and δ18O in the magmatic zircon range from 5.6 to −10.3 and from 5.8 to 8.1, respectively, and are well correlated. The zircon indicates the same temporal and compositional evolution of granitic petrogenesis across ~300 km of strike, reaffirming the notion that these terranes form part of the same orogen. Westward-directed subduction caused orogenic thickening, heating and increasing amounts of crustal contribution. This was followed by convective thinning of the thickened mantle lithosphere and a return to more primitive magmas lacking significant crustal contributions. It contrasts significantly with inferred granite petrogenesis and tectonic style in the younger Lachlan and New England Fold Belts further east that were not built upon extended cratonic lithosphere.

The peraluminous Foulwind and Windy Point Granites are important components of the Foulwind Suite–a diverse group of high field strength element-enriched Carboniferous granitoids in New Zealand’s Western Province. Four phases are identified in the Foulwind Granite (Seal Point, Siberia Bay, Tauranga Bay, Lighthouse). A new age of 324.0 ± 4.1 Ma has been obtained for the relatively undeformed Siberia Bay phase. The rocks are generally enriched in high-field strength elements and the Tauranga Bay and Lighthouse phases can be classified as ferroan A-type granites, whereas classification of the other phases is ambiguous. Sr isotope compositions were disturbed by the formation of the Paparoa Metamorphic Core Complex. Nd isotopes vary considerably and indicate involvement of multiple sources. The Nd isotope and A-type signatures can be explained by melting of mid-Paleozoic I-type granites with variable involvement of Greenland Group metasediments. The tectonic setting of the Foulwind Suite is unclear, but may be related to processes subsequent to the generation of voluminous mid-Paleozoic granitoids and amalgamation of the Buller and Takaka Terranes.

The 3.49 Ga Dresser Formation has been considered to host evidence of the earliest microbes metabolising sulfur species on Earth. However, previous bulk analyses and in situ measurements conclude disparate metabolisms based on opposite ΔS. This study first established the generations of pyrite growth, and then measured the multiple sulfur isotopes in situ using Sensitive High Resolution Ion MicroProbe-Stable Isotope analyses. Two main generations of pyrite were revealed based on core-rim textures and multiple sulfur isotopic compositions: ΔS-positive Generation One (G1) and δS- and ΔS-negative Generation Two (G2). In the chert-barite unit, the diluted ΔS-positive and ΔS-negative photochemical products were mainly sequestered in G1 and barite, respectively. G2 were formed via the sulfide pathway with sulfur derived from sulfate reduction and magmatic HS. The δS-ΔS-ΔS systematics suggests an abiological origin for G1, and thermochemical and possible (minor) microbial sulfate reduction for G2.

Mega volcanic eruptions associated with the formation of Large Igneous Provinces (LIPS) pump vast amounts of carbon dioxide and sulfur-rich gases into the atmosphere and stratosphere with the potential to totally change the chemistry of the global ocean. Here we investigate the sedimentary pyrite sulfur isotope record of black shales through time and demonstrate two coherent populations termed P1 and P2. Population P1 dominates the Archean pyrites, has a mean of δ34S = +3.7‰, standard deviation of 5.3‰, and is considered to represent S of mantle origin. Population P2 appears toward the start of the Proterozoic, dominates the Phanerozoic, has a mean around +25‰ and standard deviation of 13.5‰, and is considered to represent S of seawater sulfate origin. Population P1 can be identified in sedimentary pyrite at certain times in the Proterozoic and Phanerozoic, which correspond, within error, with the timing of 25 major LIP events. Our data suggest that at regular times through the Proterozoic, coinciding with major LIP events, the oceans contained a mixture of seawater sulfate and dissolved mantle sulfide derived from the LIPs. LA-ICP-MS analyses of the sedimentary pyrite indicate that metals, particularly gold, nickel, cobalt, and PGE were also enriched in the oceans at these times. The long periods between major LIP eruptions enabled the oceans to return to a seawater sulfate background equilibrium with a decrease in the mantle-derived metals.

The Adelaide Rift Complex is arguably one of the most complete and best studied Neoproterozoic to early Cambrian successions worldwide, preserving evidence of the breakup of Rodinia, two Cryogenian glaciations and the interglacial phase, and one of the best documented Ediacaran to early Cambrian biotic transitions. However, the complex and protracted tectono-sedimentary history of this 0.8–0.5 Ga province is still being debated. We present new and published U Pb ages and Hf and O isotope data for detrital zircons from the Adelaide Rift Complex, representing the most complete assembly of such data for this succession. Deposition during initial mid-Tonia extension was largely sourced locally from rift shoulders. As the basin evolved from rift- to sag-phase following continental breakup in the Cryogenian the provenance regions extended to more distal late Mesoproterozoic terranes to west and northwest. New data from Sturtian Glacial Epoch deposits are consistent with termination of this event at 0.66 Ga, with most deposition during deglaciation. Uplift of the Musgrave region during the Ediacaran to early Cambrian Petermann Orogeny led to dominant sediment supply from that terrane at that time in the north. In the south, earliest Cambrian deposition followed local tectonism, initially revitalising local proximal basement sources. An abrupt change in provenance occurred at the base of the Cambrian Kanmantoo Group, the youngest sediment package in the south. Paleocurrent data indicates transports from the south, probably from formerly contiguous Antarctica, possibly reflecting the onset of convergent tectonics and deposition in a foreland basin, consistent with the near depositional age of the dominant detrital zircon population. Whilst several episodes of significant crustal reworking are identified in the Hf and O isotope data, many of the zircon TDM ages lie within 0.5 Ga of the U Pb ages indicating that new additions from the mantle were common in the provenance regions.

The chemical and mineralogical conversion of pyrite to pyrrhotite has been studied in many aspects, but the associated quadruple sulfur isotopic fractionation between these phases has not been investigated previously. In this study, Ruttan pyrite and pyrite from a Neoarchean carbonaceous phyllite drill core were heated to 675 °C, and the released sulfur was reacted with iron metal, forming compact sulfide rims. The products of both desulfidized pyrite and sulfidized iron metal are pyrrhotite, following the equations n(1-x)FeS2 = nFe1-xS + (1-2x)Sn and Sn + n(1-x)Fe = nFe1-xS, respectively. The mean δ34S of the Ruttan pyrite experiment products has shifted slightly with the residue becoming slightly heavier and the sulfidized iron becoming slightly lighter than the starting Ruttan pyrite. However, the shift is small (0.5‰ and 1.2‰, respectively), yet the range of δ34S increases in the pyrrhotite products, suggesting that the evaporation of sulfur during pyrite breakdown is not occurring uniformly. A shift in δ34S between the initial Archean pyrite and resultant pyrrhotite products is likely a real difference in composition of the analyzed pyrite and the pyrite extracted for the experiment. The compositions of the pyrrhotite run products are quite close with the difference in Δ33S and Δ36S being within ca. 0.1‰. They keep constant with the starting Ruttan pyrite while differ slightly from the initial Archean pyrite, which is likely due to heterogeneity in the original pyrite. The experiments indicate that the fractionation in Δ33S and Δ36S during pyrite desulfidation, sulfur evaporation transfer and condensation and iron metal sulfidation is negligible. Therefore the preservation of Δ33S and Δ36S between initial pyrite and released sulfur during pyrite desulfidation is demonstrated, establishing Δ33S and Δ36S as effective tools of tracing sulfur sources of Archean gold deposits and opening up pyrrhotite as an appropriate phase for studying Archean S-MIF.

Made available in DSpace on 2020-12-11T09:37:44Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-03-15 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Australian Research Council - ARC The Jacobina Basin has a well-preserved sedimentary record, including continental and marine deposits, and hosts Au-(U)Py mineralization in metaconglomerate beds similar to the Witwatersrand gold province. Based on petrographic observations, in situ trace-elements, and multiple sulfur isotope analyses (S-32, S-33, S-34, and S-36) on pyrite from alluvial and marine facies, several types of pyrite were recognized. The pyrite grains identified in the alluvial metaconglomerates resemble those found in several pre-GOE gold-bearing metaconglomerates, including detrital and epigenetic varieties. Detrital inclusion-bearing pyrite is enriched in several redox-sensitive trace-metals as well as Au, which indicate an association with carbonaceous shales. On the other hand, the sources of detrital massive pyrite are more variable and include igneous, metamorphic, and hydrothermal sources from the Paleoarchean hinterland of Jacobina Basin. Epigenetic pyrite in metaconglomerates formed during metamorphism by the recrystallization of detrital pyrite with negligible contributions from external hydrothermal fluids to the basin. Diagenetic and epigenetic pyrite are found in marine lithologies. In a metapelite sample, the pyrite grains formed near the sediment-water column interface, with S sourced from the photolytic sulfate (SO42-, D Delta S-33 < 0) dissolved in the water column. A quartzite sample, in turn, has detrital pyrite grains that were likely reworked from continental deposits, as well as pyrite formed by the assimilation of elemental sulfur (S-8, Delta S-33 > 0) that accumulated in sediment pore water during diagenesis. Significantly, the pyrite associated with terrestrial metasediments shows a wide range in delta S-34 values but quite restricted ranges in Delta S-33 and Delta S-36 values, whereas pyrite associated with the marine metasedimentary rocks exhibits limited delta S-34 values but has a wide range in Delta S-33 values and correlated Delta S-36 values close to the Archean array. These data suggest distinct preservation routes for MIF-S from atmospheric SO42- and S-8 in terrestrial and marine environments. Conditions on the terrestrial surface resulted in re-equilibration of distinct S sources, diminishing the amplitude of the Archean atmospheric signal. In contrast, SO42- dissolved in shallow marine settings while S-8 settled to the floor, favoring the preservation of MIF-S isotopic signatures. Such processes may also explain the apparent differences in interpretations of atmospheric conditions derived from uncharacterized pyrites from Archean sources. Our data suggest that the Earth's atmosphere remained anoxic, and terrestrial conditions were such as to allow the syngenetic accumulation of gold, as recently proposed for the Witwatersrand gold deposits. (C) 2020 Elsevier Ltd. All rights reserved. Univ Fed Campina Grande, Unidade Acad Mineracao & Geol, Campina Grande, Paraiba, Brazil Univ Brasilia, Programa Posgrad Geol, Brasilia, DF, Brazil Univ Vale Rio dos Sinos, Programa Posgrad Geol, Sao Leopoldo, Brazil Australian Natl Univ, Res Sch Earth Sci, Canberra, ACT 2601, Australia Univ Fed Sao Carlos, Dept Fis Quim & Matemat, Sorocaba, Brazil Univ Estadual Paulista, Dept Fis, Presidente Prudente, Brazil Univ Estadual Paulista, Dept Fis, Presidente Prudente, Brazil CNPq: 163459/2013-4 CNPq: 202267/2014-8 Australian Research Council - ARC: DP140103393

Oxygen isotope ratios in magnetite can be used to study the origin of iron-oxide ore deposits. In previous studies, only 18O/16O ratios of magnetite were determined. Here, we report triple O isotope data (17O/16O and 18O/16O ratios) of magnetite from the iron-oxide–apatite (IOA) deposits of the Yazd and Sirjan areas in central Iran. In contrast to previous interpretations of magnetite from similar deposits, the triple O isotope data show that only a few of the magnetite samples potentially record isotopic equilibrium with magma or with pristine magmatic water (H2O). Instead, the data can be explained if magnetite had exchanged O isotopes with fluids that had a mass-independently fractionated O isotope composition (i.e., MIF-O), and with fluids that had exchanged O isotopes with marine sedimentary carbonate rocks. The MIF-O signature of the fluids was likely obtained by isotope exchange with evaporite rocks of early Cambrian age that are associated with the IOA deposits in central Iran. In order to explain the triple O isotope composition of the magnetite samples in conjunction with available iron isotope data for magnetite from the deposits, we propose that magnetite formed from magmatic fluids that had interacted with evaporite and carbonate rocks at high temperatures and at variable water/rock ratios; e.g., magmatic fluids that had been released into the country rocks of a magma reservoir. Additionally, the magnetite could have formed from magmatic fluids that had exchanged O isotopes with SO2 and CO2 that, in turn, had been derived by the magmatic assimilation and/or metamorphic breakdown of evaporite and carbonate rocks.

Analytical protocols for SHRIMP‐SI oxygen isotope analysis (δ18O) of a suite of zircon reference materials (RMs) are presented. Data reduction involved a robust estimate of uncertainties associated with the individual spot as well as for groups where the spot data are combined. The repeatability of δ18O measurements is dependent on both the analytical conditions and the choice of the primary reference material. Under optimised conditions, repeatability was often better than 0.4‰ (2s) allowing sample uncertainties to be obtained to better than 0.2‰ (at 95% confidence limit). Single spot uncertainty combined the within‐spot precision with the scatter associated with repeated measurements of the primary zircon reference material during a measurement session. The uncertainty for individual spots measured under optimised conditions was between 0.3 and 0.4‰ (at 95% confidence). The analytical protocols described were used to assess a variety of zircon RMs that have been used for geochronology and for which laser fluorination oxygen isotope data are available (Temora 2, FC1, R33, QGNG and Plesovice), as well as zircons that have been used as RMs for trace element or other types of determination (Mud Tank, Monastery, 91500, AS57, AS3, KIM‐5, OG1, SL13, CZ3 and several other Sri Lankan zircons). Repeated analyses over nine sessions and seven different mounts show agreement within analytical uncertainty for Temora 2, FC1, R33, QGNG, Plesovice and 91500, when normalised to Mud Tank. For existing ion microprobe mounts with these materials, an appropriate δ18O can be determined. However, care should be taken when using zircons from the Duluth Complex (i.e., FC1, AS57 and AS3) as reference materials as our data indicated an excess scatter on δ18O values associated with low‐U zircon grains.

Epidosites occur in association with pelagic and quartzofeldspathic metasediments overlying the Pounamu suprasubduction zone ophiolite. They are dominated by epidote with minor diopside, tremolite,...

In this study, we utilize high-alumina pelitic schist samples from the Holenarsipur Greenstone Belt, a component of the ca. 3.30–3.15 Ga Sargur Supergroup in the Western Dharwar Craton, South India to establish a Mesoarchaean metamorphic history in the terrane. On the basis of integrated textural, mineral compositional, metamorphic reaction history and P-T pseudosection modelling studies, garnet-monazite trace element thermometry and coupled monazite electron probe micro-analyzer and sensitive high-resolution ion microprobe U-Pb dating results, we constrain for the first time a Mesoarchaean metamorphism along a hairpin clockwise metamorphic P-T path in the Dharwar Craton. The path reveals a two-stage prograde segment with an initial phase of heating accompanying burial (dP/dT gradient ~ 30 bar/°C) and a later phase of steep burial (∆P ~ 1.1 kbar, dP/dT gradient ~110 bar/°C), peak metamorphism at 7.5 kbar, 640 °C and a post-peak, steep decompression stage (P~6.3 kbar at 635 °C). Chemically zoned monazite grains shielded within garnet or occurring in low-strain matrix domains were formed in two stages during the Mesoarchaean metamorphism: (a) Monazite cores with relatively lower Y concentrations, higher Th/U ratios, a strong negative Eu anomaly; their REE partitioning relationships with the garnet inner rim reveal their crystallization was synchronous with the growth of garnet. (b) Monazite rims with higher Y and uraniferous compositions and lower Th/U ratios which, combined with Y and HREE enrichments in the outer rim of garnet, indicate their formation during the post-peak decompression stage associated with coupled decompositions of garnet and zircon. We correlate U-Pb monazite core and rim ages at 3144 ± 7 Ma and 3105 ± 10 Ma respectively with the timings of peak and retrograde metamorphism. Our findings, when evaluated with the available geological, isotopic, geochronological data of Meso- to Palaeoarchaean extant and extinct felsic crust as well as thermo-mechanical models, predict the ca.3.14–3.11 Ga metamorphism in the Holenarsipur Greenstone Belt as part of a widespread tectono-thermal event in the Western Dharwar Craton. The event marks late stages of sagduction tectonics in the craton as it evolves from a predominantly vertical to a horizontal shortening mode.

Redox-sensitive trace elements and sulfur isotope compositions obtained via in situ analyses of sedimentary pyrites from marine black shales are used to track atmosphere-ocean redox conditions between ∼1730 and ∼1360 Ma in the McArthur Basin, northern Australia. Three black shale formations within the basin (Wollogorang Formation 1730 ± 3 Ma, Barney Creek Formation 1640 ± 3 Ma, and Upper Velkerri Formation 1361 ± 21 Ma) display systematic stratigraphic variations in pyrite trace-element compositions obtained using LA-ICP-MS. The concentrations of several trace elements and their ratios, such as Se, Zn, Se/Co, Ni/Co, Zn/Co, Mo/Co, Se/Bi, Zn/Bi, Ni/Bi, increase from the stratigraphically lower Wollogorang Formation to the Upper Velkerri Formation. Cobalt, Bi, Mo, Cu, and Tl show a consistent decrease in abundance while Ni, As, and Pb show no obvious trends. We interpret these trace element trends as a response to the gradual increase of oxygen in the atmosphere-ocean system from ∼1730 to 1360 Ma. Elements more mobile during erosion under rising atmospheric oxygen show an increase up stratigraphy (e.g., Zn, Se), whereas elements that are less mobile show a decrease (e.g., Co, Bi). We also propose the increase of elemental ratios (Se/Co, Ni/Co, Zn/Co, Mo/Co, Ni/Bi, and Zn/Bi) up stratigraphy are strong indicators of atmospheric oxygenation. Sulfur isotopic compositions of marine pyrite (δ34Spyrite) from these formations, obtained using SHRIMP-SI, are highly variable, with the Wollogorang Formation exhibiting less variation (δ34S = –29.4 to +9.5‰; mean –5.03‰) in comparison to the Barney Creek (δ34S = –13.8 to +41.8‰; mean +19.88‰) and Velkerri Formations (δ34S = –14.2 to +52.8‰; mean +26.9‰). We propose that the shift in mean δ34S to heavier values up-section corresponds to increasing deep water oxygenation from ∼1730 to 1360 Ma. Incursion of oxygenated waters possibly caused a decrease in the areal extent of anoxic areas, at the same time, creating a possibly efficient reducing system. A stronger reducing system caused the δ34S of the sedimentary pyrites to become progressively heavier. Interestingly, heavy δ34S in pyrites overlaps with the increase in the concentration of certain trace elements (and their ratios) in sedimentary pyrites (Se, Zn, Se/Co, Ni/Co, Zn/Co, Mo/Co, Ni/Bi, and Zn/Bi). This study concludes that there was a gradual increase of atmospheric oxygen accompanied by ocean oxygenation through the first ∼400 million years of the Boring Billion (1800–1400 Ma) in the McArthur Basin.

Significance Scientists have been working on the dating of atmospheric oxygenation in Earth’s history for decades. However, most previous studies relied on evidence from sedimentary rocks. Here, we show that igneous rocks can also be linked with surface oxidation by a key geological process: plate subduction. We here make an attempt to apply the Th/U of worldwide arc igneous rocks as an indicator for the timing of atmospheric oxygenation over the Earth’s history. The results are coincident with the previously defined Great Oxidation Event and Neoproterozoic Oxygenation Event., Atmospheric oxygen has evolved from negligible levels in the Archean to the current level of about 21% through 2 major step rises: The Great Oxidation Event (GOE) in the early Proterozoic and the Neoproterozoic Oxygenation Event (NOE) during the late Proterozoic. However, most previous methods for constraining the time of atmospheric oxygenation have relied on evidence from sedimentary rocks. Here, we investigate the temporal variations of the Th/U of arc igneous rocks since 3.0 billion y ago (Ga) and show that 2 major Th/U decreases are recorded at ca. 2.35 Ga and ca. 0.75 Ga, coincident with the beginning of the GOE and NOE. The decoupling of U from Th is predominantly caused by the significant rise of atmospheric oxygen. Under an increasingly oxidized atmosphere condition, more uranium in the surface environment became oxidized from the water-insoluble U4+ to the water-soluble U6+ valance and incorporated in the sea water and altered oceanic crust. Eventually, the subduction of this altered oceanic crust produced the low-Th/U signature of arc igneous rocks. Therefore, the sharp decrease of Th/U in global arc igneous rocks may provide strong evidence for the rise of atmospheric oxygen. We suggest that the secular Th/U evolution of arc igneous rocks could be an effective geochemical indicator recording the global-scale atmospheric oxygen variation.

Redox sensitive trace elements in pyrite, including nodules, are increasingly used to infer the chemical conditions of ancient oceans - but considerable uncertainty remains regarding the mechanism and timing of nodule formation. Resolving these uncertainties is important because pyrite nodules must form in connection with the overlying water column, rather than during late diagenesis, to reflect the composition of the global ocean. Existing models for pyrite nodule formation have been specific to pyrite textures from individual sites, and we lack a unified model that can explain the compositional and textural diversity observed in nodules from different localities. In this study we examine ten pyrite nodules from several geological periods (Neoarchean to Carboniferous) using in situ LA-ICP-MS and SHRIMP-SI analyses. We present transects of spot analyses of trace elements (As, Ag, Cu, Co, Ni, Sb and Se) and S isotope ratios for each nodule. The pyrite nodules can be classified according to three main categories: those with (1) little to no trace element or isotopic zonation of the nodule from core to margin, (2) strong zonation from core to margin, and (3) minor zonation near the core but more significant zonation near the margin of the nodule. We further illustrate this zonation with a NanoSIMS element map from an additional pyrite nodule. These results are interpreted to indicate nodule formation along a spectrum between two end-member mechanisms. We suggest that the absence of trace element or isotopic zonation reflects nodule growth by a pathway that is analogous to the pervasive growth mechanism for carbonate nodules. This model involves the production of many nucleation sites that are evenly distributed within the volume that the nodule eventually occupies. Consequently, this mechanism results in a chemically homogenous nodule. Pyrites formed this way are suitable for paleoceanographic reconstruction. The other end-member mechanism is analogous to the concentric growth model for carbonate concretions. In this scenario, the core of the nodule forms first and is followed by the addition of concentric layers - each with a progressively different trace element content and δ34S signature as diagenesis progresses. Despite having limited utility for reconstructing ancient seawater, these late forming nodules may track the evolving availability of bioessential trace elements for the subsurface biosphere with important implications for global biogeochemical cycles. Spatial trends in trace elements and S isotopes thus speak to the mechanisms of pyrite nodule formation and provide a framework for evaluating nodule suitability for a range of paleoenvironmental studies.

Scheelite has been analyzed from the Shizhuyuan and the Xianglushan world-class W deposits from the Nanling W–Sn region and Jiangnan W belt, respectively. The Shizhuyuan deposit consists of proximal skarn and greisen W–Sn–Mo–Bi and distal Pb–Zn–Ag veins. The Xianglushan deposit, contains layer-like sulfide–scheelite and skarn W orebodies on granite cupolas overprinted by W greisen veins. Scheelite in skarn ores from the Shizhuyuan contains higher concentrations of Mo than those in the sulfide–scheelite and skarn ores from the Xianglushan deposit, reflecting differences between oxidizing and reducing magmatic-hydrothermal fluids. Under oxidizing conditions, W is accompanied by Mo partitions into exsolved fluids to form W–Mo garnet skarns, whereas under reducing conditions, little Mo is carried by exsolved fluids to form W pyroxene skarns. Trace element patterns of scheelite from both deposits show negative Ba, Sr, Zr, and Ti, and positive Ta anomalies. Rare earth element (REE)patterns of scheelite within skarns from the Shizhuyuan deposit have negatively inclined and flat M-type tetrad patterns, and scheelite from the greisens displays flat and positively inclined M-type tetrad patterns. We infer that the fluids formed scheelite within the W skarns and greisens inherited parental magma trace element and REE characteristics (depleted Ba, Sr, Zr, and Ti, enriched Ta, negative Eu anomalies, and tetrad effects). Whereas, scheelite from sulfide–scheelite veins and skarns of the Xianglushan deposit also has W- and MW-type tetrad REE patterns. The W-type tetrad REE patterns are complementary to REE patterns from the Renjiashan granite, and the MW-type tetrad REE patterns occur during a single evolutionary stage within a complex hydrothermal environment. Sulfide mineralization can form after or before W skarns (the former like Shizhuyuan deposit and the latter like Xianglushan deposit). The formation conditions of the latter included reducing conditions and sulfide firstly supersaturated in the melt, resulting in sulfide drops which carried W aggregated on the cupolas. W skarns and greisens in both deposits underwent generally successive processes related to water supersaturation in the melt. Following a temperature decrease and crystallization, bubbles carried material changing from Si and metal to Si oxide complexes.

The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, α-implantation, α-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71 Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.

Results of biostratigraphic and geochronological investigations in eastern Nagaland and Manipur, NE India, provide new constraints on the tectonic evolution of the western margin of the Burma microplate. U/Pb zircon ages indicate that the Naga Hills ophiolite developed in a suprasubduction zone setting as part of an intraoceanic island arc developed during late Early Cretaceous (mid-Aptian) time and is younger than similar rocks exposed along the Indus-Yarlung Tsangpo suture zone. Radiolarian microfossils provide Jurassic and Cretaceous age constraints for Tethyan ocean floor sediments that were subducted beneath the forming ophiolite. Timing of the emplacement of these rocks onto the passive margin of eastern India is constrained by Paleocene/Eocene radiolarians in sediments over which the ophiolitic assemblage has been thrust. Previously undated schists and gneisses in the Naga Metamorphics are of Early Ordovician age, and their sedimentary protolith was most likely derived from sources in the south of Western Australian and East Antarctica. After Barrovian-style metamorphism, these rocks were uplifted and eroded becoming an important source of detritus shed into the Eocene Phokphur Formation. This unit also contains abundant clasts sourced from the disrupted basement of the Naga Hills ophiolite, which it overlies. It also contains Permo-Triassic-aged detritus eroded off an enigmatic source that was possibly a continental convergent margin arc system somewhere along the northern margin of Gondwana.

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gaviao Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (S, S, S, and S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with e(t) = −2.47 to −1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δO = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gaviao Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have e(t) = −5.40 to −0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δO = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (ΔS between −1.3‰ to +1.4‰) and a ΔS/ΔS slope of −0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δS range (0 to +2‰), ΔS ~ 0‰, and ΔS ~ 0‰. A carbonaceous schist shows positive δS (2.1‰–3.5‰) and elevated ΔS (1.2‰–1.4‰) values, with corresponding negative ΔS between −1.2‰ to −0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δS, ΔS, and ΔS signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.

C-type asteroids are considered to be primitive small Solar-System bodies enriched in water and organics, providing clues for understanding the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing and on-asteroid measurements with Hayabusa2, but further studies are expected by direct analyses of returned samples. Here we describe the bulk sample mainly consisting of rugged and smooth particles of millimeter to submillimeter size, preserving physical and chemical properties as they were on the asteroid. The particle size distribution is found steeper than that of surface boulders11. Estimated grain densities of the samples have a peak around 1350 kg m-3, which is lower than that of meteorites suggests a high micro-porosity down to millimeter-scale, as estimated at centimeter-scale by thermal measurements. The extremely dark optical to near-infrared reflectance and the spectral profile with weak absorptions at 2.7 and 3.4 microns implying carbonaceous composition with indigenous aqueous alteration, respectively, match the global average of Ryugu, confirming the sample’s representativeness. Together with the absence of chondrule and Ca-Al-rich inclusion of larger than sub-mm, these features indicate Ryugu is most similar to CI chondrites but with darker, more porous and fragile characteristics.

Internal textures, multiple sulfur isotopic compositions, and contents of gold, selenium, and molybdenum of ore-related pyrites from a Neoarchean carbonate-facies BIF-hosted gold deposit in Quadrilatero Ferrifero were investigated to elucidate the source(s) of sulfur. Sodium hypochlorite etching and BSE imaging revealed pervasive zoning in pyrites. Five different growth zones (Py1a, Py1b, Py2, Py3, and Py4) and six types of zoning (Type A–Type F) were identified. Two pyrite generations were distinguished in ores: G1 (Py1a and Py1b) and G2 (Py2, Py3, Py4). Both G1 and G2 have positive Δ33S, but the magnitudes of G1 are higher. The G1-relevant fluids can be shallow-sourced, whereas the G2-related auriferous fluids are most likely deep-sourced metamorphic fluids derived from devolatilization of the lower succession of the Nova Lima Group (metavolcanics and metasedimentary rocks) during metamorphism, with fluid–rock interactions during fluid ascent and at the depositional site. The negative δ34S, higher selenium contents, and carbonates inclusions of Py1a contrast with the positive δ34S, lower selenium contents, and carbonaceous material inclusions of Py2, Py3, and Py4. The Lamego system possibly started with shallow-sourced oxidized and low-gold fluids (Py1a), closely followed by mixing in of deep-seated reduced auriferous fluids (Py2, Py3, Py4) initiated by tectonic activities. The oscillatory zoning of Py2 with the highest gold contents consists of alternating gold-rich and gold-poor laminae, indicating that fault-valve activity is a trigger of gold deposition. The sulfidation of siderite in BIF (desulfidation of auriferous fluids) related to the formation of ore-related pyrites also contributed to gold precipitation.

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The Collahuasi district of northern Chile hosts several late Eocene-early Oligocene world-class porphyry Cu-Mo deposits, including Rosario, Ujina, and Quebrada Blanca deposits, and associated high-sulfidation epithermal mineralization at La Grande. Mineralization is hosted by intermediate to felsic intrusive and volcanic rocks of the upper Paleozoic to Lower Triassic Collahuasi Group, which experienced lower greenschist facies regional metamorphism prior to mineralization. Extensive hydrothermal alteration zones surround the porphyry and epithermal deposits, associated with hypogene ore-forming processes. However, outside of the observed sulfide halo the limits of geochemical anomalism associated with mineralization are difficult to define due to mineralogical similarities between weak, distal propylitic alteration and regional metamorphism affecting the host rocks.Recent advancements in laser ablation-inductively coupled plasma-mass spectrometry analysis of epidote from hydrothermal alteration zones around porphyry and skarn deposits have shown that low-level hypogene geochemical anomalies can be detected at distances farther from the center of mineralization than by conventional rock chip sampling. Selective analysis of propylitic epidote from the Collahuasi district indicates that anomalous concentrations of distal pathfinder elements in epidote, including As (>50 ppm), Sb (>25 ppm), Pb (>100 ppm), and Mn (>5,000 ppm), were detectable 1.5 to 4.0 km from deposit centers. Significantly, the concentrations of these trace elements in epidote were obtained from samples that contained whole-rock concentrations of

Oxygen is the dominant element in our planetary system. It is therefore remarkable that it shows substantial isotopic diversity both in mass-dependent fractionation, because it is a light element, and in mass-independent fractionation, primarily associated with variation in abundance of 16O. On Earth, the primary variation in isotopic composition is related to temperature-dependent kinetic mass fractionation between hydrosphere and atmosphere. Meteorites provide samples of primitive bodies, that have not experienced melting, and planetesimals that have melted early in their history. Samples of Mars, Vesta, and the Moon are present in the meteorite collections. In meteorites, the cosmochemical fractionation related to the abundance of 16O provides a useful classification scheme. Inclusions in chondrites show a large range in 16O abundances from highly enriched (solar) through to compositions closer to terrestrial (planetary). The variability in 16O appears originally to be related to predissociation and self-shielding of carbon monoxide likely in the primordial molecular cloud. Within the chondrite parent bodies, exchange between 16O-poor fluids and relatively 16O-rich solids created isotopic mixing lines. This model makes specific predictions for isotopic compositions of silicates and water ice throughout the solar system. One prediction, that the Earth should be isotopically heavier than the Sun, appears to be verified. But other tests based on oxygen isotopes within the solar system require either remote analysis or sample return missions. Remote analysis will require new instrumentation and analytical techniques to achieve the precision and accuracy required for three oxygen isotope analysis. Methodologies associated with cavity ring-down spectroscopy appear promising. Sample return appears viable only for the inner solar system including Mars and asteroids. While sample return missions to either Venus or Mercury appear highly challenging, the scientific benefits are immense both in oxygen isotope characterisation, and in a variety of other geochemical analyses. Measurement of three oxygen isotopes throughout the solar system would further our concepts for formation of other solar systems, and give us insight into the general mechanisms of planetary system formation and the role of water in the formation and evolution of the chondrite parent bodies and planets.

Made available in DSpace on 2020-12-10T19:49:26Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-02-05 Australian Research Council (ARC) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) We present protocols for high-spatial resolution measurement of oxygen isotope ratios of goethite (alpha-FeOOH) with the Sensitive High Mass Resolution Ion Microprobe - Stable Isotopes (SHRIMP-SI) and propose a natural sample as a potential goethite reference material (RM) for ion microprobe analysis. We assess the effects of goethite chemical composition, crystallographic orientation, and texture on the accuracy and repeatability of SHRIMP-SI delta O-18 (delta O-18(SIMS)) results. Synthetic goethites evaluated as potential delta O-18(SIMS) RM are powdery, porous, and finely crystalline; they do not yield repeatable results. A dense colloform goethite from the Capao topaz mine, Minas Gerais, Brazil, fulfills major prerequisites: it is stoichiometrically relatively pure, yields repeatable oxygen isotope results, and occurs in abundance to produce a RM for long-term use. We use an average laser fluorination delta O-18(LF-VSMOW) value of 17.3 0.3%o (1SD) obtained for five aliquots of this RM to normalize all delta O-18(SIMS) measurements. Multiple delta O-18(SIMS) analyses of a large fragment of the Capao L4 (CL4) RM analyzed in three different runs yield an overall repeatability of -17.3 +/- 0.5 parts per thousand (25D, n = 294) for all three runs combined. Natural variability and crystal orientation effects are the main reasons for the excess spread of the 8180sims results compared to the spot internal precision (ca. 0.2 parts per thousand). All delta O-18(SIMS) analyses (n = 1027) in various aliquots of CL4, randomly oriented and analyzed in 26 sessions during eight distinct runs, yield an overall repeatability of 17.3 +/- 0.7%0 (2SD), confirming that CL4 is a suitable SIMS RM. After ascertaining its suitability as a RM, we used CL4 to standardize analyses of other natural goethite samples with the SHRIMP-SI and compared delta O-18(SIMS) and laser fluorination results to test the relationship between natural properties (e.g., porosity, minor elements substituting for Fe), preparation procedures (e.g., polish and relief), instrument conditions, and the overall precision and accuracy of the SIMS analyses. Samples containing minor elements substituting for Fe (e.g., Al, Mn, Cu, Zn, etc.) or as contaminants (e.g., Si, P) require significant matrix corrections. Because we could not find homogenous natural goethite samples showing a large range in metal concentrations, we extrapolate our calibration curves beyond the composition of our calibration goethite samples. delta O-18(SIMS) results corrected for instrument mass fractionation (using CL4) and compositionally dependent matrix effects (using several calibration goethites of known elemental composition) are less precise but statistically indistinguishable from their laser fluorination results. However, porous samples are unsuitable for SHRIMP-SI delta O-18 analysis. Dense colloform samples yield repeatable results for individual growth bands, showing that the high spatial resolution, moderate precision, and speed of analysis of the SHRIMP-SI can resolve variations in oxygen isotope composition acquired during sample growth. (U-Th)/He geochronology of equivalent aliquots from the same goethite samples reveal that the combination of the two methods permits the extraction of temporal variation in the isotopic compositions of meteoric solutions in the geological past. Univ Estadual Paulista, Dept Planejamento Terr & Geoprocessamento, BR-13506900 Rio Claro, SP, Brazil Univ Queensland, Sch Earth & Environm Sci, Brisbane, Qld 4072, Australia Australian Natl Univ, Res Sch Earth Sci, Canberra, ACT 2601, Australia CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA Univ Estadual Paulista, Dept Planejamento Terr & Geoprocessamento, BR-13506900 Rio Claro, SP, Brazil Australian Research Council (ARC): DP160104988 Australian Research Council (ARC): LP1401008005 Australian Research Council (ARC): LE0560868

The world's meteorite collections contain a very rich picture of what the early Solar System would have been made of, however the lack of spatial context with respect to their parent population for these samples is an issue. The asteroid population is equally as rich in surface mineralogies, and mapping these two populations (meteorites and asteroids) together is a major challenge for planetary science. Directly probing asteroids achieves this at a high cost. Observing meteorite falls and calculating their pre-atmospheric orbit on the other hand, is a cheaper way to approach the problem. The Global Fireball Observatory (GFO) collaboration was established in 2017 and brings together multiple institutions (from Australia, USA, Canada, Morocco, Saudi Arabia, the UK, and Argentina) to maximise the area for fireball observation time and therefore meteorite recoveries. The members have a choice to operate independently, but they can also choose to work in a fully collaborative manner with other GFO partners. This efficient approach leverages the experience gained from the Desert Fireball Network (DFN) pathfinder project in Australia. The state-of-the art technology (DFN camera systems and data reduction) and experience of the support teams is shared between all partners, freeing up time for science investigations and meteorite searching. With all networks combined together, the GFO collaboration already covers 0.6% of the Earth's surface for meteorite recovery as of mid-2019, and aims to reach 2% in the early 2020s. We estimate that after 5 years of operation, the GFO will have observed a fireball from virtually every meteorite type. This combined effort will bring new, fresh, extra-terrestrial material to the labs, yielding new insights about the formation of the Solar System., Accepted in PSS. 19 pages, 9 figures

Few palaeoclimate archives beyond the polar regions preserve continuous and datable palaeotemperature proxy time series over multiple glacial-interglacial cycles. This hampers efforts to develop a more coherent picture of global patterns of past temperatures. Here we show that Mg concentrations in a subaqueous speleothem from an Italian cave track regional sea-surface temperatures over the last 350,000 years. The Mg shows higher values during warm climate intervals and converse patterns during cold climate stages. In contrast to previous studies, this implicates temperature, not rainfall, as the principal driver of Mg variability. The depositional setting of the speleothem gives rise to Mg partition coefficients that are more temperature dependent than other calcites, enabling the effect of temperature change on Mg partitioning to greatly exceed the effects of changes in source-water Mg/Ca. Subaqueous speleothems from similar deep-cave environments should be capable of providing palaeotemperature information over multiple glacial-interglacial cycles., Few palaeoclimate archives beyond the polar regions preserve continuous and datable paleotemperature proxy time series over multiple glacial-interglacial cycles. Here, the authors show that Mg concentrations in a subaqueous speleothem from an Italian cave track regional sea-surface temperatures over the last 350,000 years.

Fluorine, Cl, Br and I measurements are reported for eight international rock reference materials (BHVO-2, BCR-2, BIR-1a, RGM-2, AGV-2, GSP-2, JB-2 and JR-1), and new F data are reported for five silicate glass reference materials (NIST SRM 610 and 612, BHVO-2G, BCR-2G and BIR-1G). Fluorine was measured by SHRIMP in the silicate glasses and in Li-borate flux glasses prepared from the rock powders. Chlorine, Br and I were measured in vacuum-encapsulated rock powders by the noble gas method (extended Ar/Ar methodology). The methods yield reliable results: Fluorine has a repeatability of 1–16% in the flux glasses compared with 2–4% in the silicate glasses, which suggests incomplete homogenisation during flux melting, but F was not lost during fusion of flux glasses at 1080 °C. The noble gas method gave repeatability of 1–2% for samples included in a single irradiation and intermediate precision of 2% for Cl, 2–3% for Br and 3–10% for I in samples included in two irradiations. The accuracy of the measurements varies from 5% to 10%, based on the repeatability of our results, the agreement of different calibration procedures and uncertainty in experimentally determined neutron capture cross sections. Our results significantly improve upon the precision with which some of the halogens have been characterised in the selected reference materials previously.

Cenozoic adakitic rocks in the Gangdese changed from barren continental melts to ore-forming slab melts at ~ 23 Ma. The distribution and chemical characteristics of the ore-forming adakites point to an association with the Ninetyeast Ridge. The subduction of the thick, rigid Ninetyeast Ridge changed the geometry and rheology of the eastern Tibetan Plateau lithosphere and asthenosphere, restrained the eastward escape of asthenospheric mantle as well as continental fragments, and promoted the uplift and building of the Tibetan Plateau, which consequently changed the tectonic and climatic regimes in eastern Asia.

This study was supported by National Key R&D Program of China 2016YFC0600408, CASXDB18020000, NSFC91328204 to W.D.S., State Key Development Program of Basic Research of China (2014CB845905), NSFC41274087 to Z.Q.W. 41090371 to X.L. TRI acknowledges a Chinese Academy of Sciences President's International Fellowship for visiting scientists Grant No. 2015VEA003. The oxygen fugacity of the upper mantle is 3–4 orders of magnitude higher than that of the lower mantle and this has been attributed to Fe2+ disproportionating into Fe3+ plus Fe0 at pressures > 24 GPa. The upper mantle might therefore have been expected to have evolved to more oxidizing compositions through geological time, but it appears that the oxygen fugacity of the upper mantle has remained constant for the last 3.5 billion years. Thus, it indicates that the mantle has been actively buffered from the accumulation of Fe3+, and that this is linked to oxidation of diamond to carbonate coupled with reduction of Fe3+ to Fe2+. When subducted plates penetrate into the lower mantle, compensational upwelling transports bridgmanite into the transition zone, where it breaks down to ringwoodite and majorite, releasing the ferric iron. The system returns to equilibrium through oxidation of diamond. Early in Earth history, diamond may have been enriched at the base of the transition zone in the Magma Ocean, because it is denser than peridotite melts at depths shallower than 660 km, and it is more buoyant below. Ongoing oxidation of diamond forms carbonate, leading to relatively high carbonate concentrations in the source of ocean island basalts. Postprint

Previous work on oxygen isotopic compositions of hematite samples by secondary ion mass spectrometry (SIMS) has yielded large oxygen isotopic variability (>3 ‰) within individual samples. Deconvolving instrumental effects from geological variation is of vital importance in correctly interpreting in situ SIMS δ18O isotope compositions recorded in hematite. Here, we demonstrate that the excess scatter in δ18O values acquired with SIMS results from a combination of true sample variability and crystallographically-induced instrument mass fractionation. The natural and crystallographically-induced isotopic variability of hematite was investigated by a combination of SIMS, laser fluorination, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) elemental mapping, and electron back-scattered diffraction (EBSD). We used the Sensitive High Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI) on a museum-quality hematite specimen from the Quadrilatero Ferrifero, Minas Gerais, Brazil, and orientation-induced analytical fractionation was found to contribute up to 5 ‰ to the observed SIMS δ18O isotopic scatter. Laser ablation ICP-MS elemental mapping of crystallographically uniform single hematite crystals revealed distinct chemical zonation patterns that indicate several discrete stages of crystal growth. Crystal zoning additionally coincides with abrupt shifts in δ18O ranging ca. 11 ‰ for a single crystallographic orientation. These results highlight an intimate link between crystal growth mechanism and isotopic evolution of the mineralizing fluid, showing the need for high spatial resolution analysis before the isotopic composition of hematite can be used to infer conditions of mineral precipitation and solution compositions. The results also demonstrate that an understanding of both crystallographic orientation and crystallochemical variability are needed to obviate problems previously encountered in oxygen isotope analysis of hematite by ion microprobe.

The links between large-scale silicic volcanism and plutonism offer insights into the dynamics of crustal magmatic systems and growth of continental crust. In Hong Kong, voluminous silicic ignimbrites and linked plutons record a ~26 Myr period of magmatism from ~164 to 138 Ma. We present data from these linked volcanic-plutonic assemblages at the Lantau and High Island caldera complexes, with an emphasis on the ~143–138 Ma activity from the latter. To track the evolution of these magmatic systems, U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) were carried out on zircons from 21 samples from both volcanic and plutonic samples. The SIMS age data sets divide into two groups across volcanic and plutonic origins: (1) seven samples with unimodal age spectra [five of which have the same mean value as the published Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) age from the same sample]; and (2) 14 samples yielding multiple age components. Age patterns from both groups suggest that the previously separated ~143 Ma Repulse Bay (RBVG) and ~141–140 Ma Kau Sai Chau volcanic groups (KSCVG) instead represent activities over a single ~5 Myr period. Direct linkages previously proposed between some volcanic and plutonic units for this period (e.g., High Island Tuff, Kowloon Granite) are no longer supported, and magmatism represented by exposed plutons continued until 137.8 ± 0.8 Ma (Mount Butler Granite). Under CL imagery, a wide range of zircon textures identified in both volcanic and plutonic samples is indicative of complex processes, some of which are identified through trace element data coupled with textural characteristics. Overall, intra-grain (cores vs. rims; sector-zonation) and intra-sample variations in trace element abundances and ratios are larger than those between samples. Zircon chemistries in both volcanic and plutonic samples fall into two groups during the ~5 Myr history of the High Island caldera magmatic system. One group (RBVG and “cold” granites) includes inherited grains back to 164 Ma and wider ranges in Hf, Y, total trivalent elements, Th and U concentrations and Th/U, Yb/Gd, and U/Yb ratios than the other (KSCVG and “hot” granites). Two possible evolutionary models of the High Island caldera magmatic system are: (1) the system randomly tapped a single crustal domain that fluctuated in temperature as a result of varying interactions of hotter melts, or (2) the volcanic and plutonic records reflect the interplay of two crustal domains with contrasting “low-” and “high-temperature” characteristics. In Hong Kong, some plutonic bodies were comagmatic with large-scale volcanism, while others were emplaced at shallow crustal levels independently of volcanism, matching the current two end-member views of the volcanic-plutonic relationship.

The relationships between the composition of hibonite with the general formula CaAl12-2x-yMgxTi4 +xTi3 +yO19, the oxidation state of Ti (Ti3 +/ΣTi, where ΣTi = Ti3 + + Ti4 +), and oxygen fugacity (fO2) were investigated experimentally. It was found that hibonite can be synthesised with a range of Ti3 +/ΣTi values at constant fO2 and with a constant Ti3 +/ΣTi value for a range of fO2s. It was also found that if hibonite with the formula CaAl12-yTi3 +yO19 (Ti3 +/ΣTi = 1) is equilibrated with a melt of CAI composition at fO2s below the iron-wustite buffer then the resulting hibonite contained Mg, with Mg per formula unit (pfu) ~ 0.8 Ti pfu, and Ti3 +/ΣTi ~ 0.2, irrespective of the fO2. These results suggest that the availability of Mg, rather than fO2, is the key factor that determines Ti3 +/ΣTi of hibonite. The structures of synthetic samples of hibonite with the general formula CaAl12-2xMgxTi4 +xO19, where 0 ≤ X < 1, were determined by Rietveld refinement of X-ray powder diffraction data. The predominant site occupied by Ti4 + was found to change from M2 to M4 with increasing Ti content. The range of Ti concentrations over which the site occupancy changed corresponds to that observed in meteoritic hibonite. This change in the Ti4 + site produces changes in the Ti K-edge XANES spectra, particularly in the intensity of the pre-edge feature, for constant Ti3 +/ΣTi. The observed dependence of the pre-edge on the Ti4 + site was reproduced by ab initio simulations of the XANES spectra. The XANES spectra of natural hibonite with variable Ti content from the Murchison carbonaceous chondrite closely match the spectra of the synthetic samples with similar Ti contents. These differences in the spectra of meteoritic hibonite could be misinterpreted as being due to changes in Ti3 +/ΣTi, but are instead due to differences in ΣTi, which relate to the petrogenetic history. Crystal chemistry exerts a first order control on the Ti site occupancy and Ti3 +/ΣTi value of hibonite. As a result, no simple relationship between Ti3 +/ΣTi and fO2 should be expected. It is unlikely that hibonite will be useful as an oxybarometer for solar processes without Ti3 +/ΣTi standards that are compositionally matched to the unknown.

The early tectonic evolution of the Lhasa Terrane remains poorly understood, although evidence for a substantial prehistory has been reported recently. We have carried out in situ zircon U–Pb dating and Hf–O isotopes of late Early Cretaceous monzogranites and get a surprising package of inherited zircons, not only because of their age profile, but also because the oldest Palaeoarchaean zircons are euhedral. The discovery of Palaeoarchaean euhedral zircons in the region suggests the presence of extremely old rocks in Tibet. Zircons from the Nagqu monzogranite yield five age peaks at ~3.45 Ga, ~2.56 Ga, ~1.76 Ga, ~900 Ma, and ~111 Ma. They have large variations in eHf(t) values (−45.1–9.2) and old Hf model ages (924–3935 Ma), with variable δ18O values of −5.80–9.64. Palaeoarchaean zircons (~3.20–3.45 Ga) are euhedral with magmatic zircon characteristics. One of the grains has negative eHf(t) value (−4.8), old Hf model age (3935 Ma), and high positive δ18O value (7.27), which suggests an ancient crus...

South China is famous for the extensive magmatism and polymetallic mineralization that took place there in the Mesozoic. Shilu is a large porphyry–skarn Cu–Mo deposit in the Yangchun Basin, South China. The lithology of the Shilu intrusion is granodiorite and quartz diorite, both of which are high-K calc-alkaline series, with high Sr (>400 ppm) content along with low Y and Yb contents. Most of the samples have characteristics of adakite except for a few samples that have slightly higher Y and Yb contents, which may be plausibly explained by crustal contamination. Laser Ablation Inductively Coupled Plasma Mass Spectrometry zircon U–Pb dating revealed ages between 106.6 ± 1.3 and 103.9 ± 0.5 Ma, with multiple magmatic pulses. Molybdenite Re–Os isochron age of 102.2 ± 2.9 Ma (MSWD = 9.4) was determined, which is identical to the youngest zircon U–Pb age (103.9 ± 0.5 Ma) within error. The Shilu intrusion has high oxygen fugacity as indicated by high zircon Ce4+/Ce3+ and EuN/EuN* ratios. Considering the geochemical characteristics (high Sr, and low Y and Yb contents), high oxygen fugacity, and copper mineralization of the Shilu intrusion, it was most likely formed by partial melting of a subducted young oceanic slab. Whole-rock Sr–Nd isotope-, zircon Hf isotope-, and whole-rock trace element analyses show that Shilu adakitic magmas may have interacted with type II enriched mantle and/or crustal materials during ascent. South China was affected by the Pacific tectonic regime to the east and the Neo-Tethys tectonic regime to the south in the Cretaceous. Based on the Pacific Plate drifting and rotation history, it is hard to explain how the Pacific Plate would have subducted and melted, forming adakitic rocks in the Shilu region. Considering the tectonic history of Southeast Asia and the South China Sea, the Neo-Tethys trench should have been much closer to the South China Block in the Cretaceous, and thus have had a greater impact on the South China Block. Based on the subduction direction, time of subduction, and distance between the Neo-Tethys subduction zone and the Shilu deposit, subduction of the Neo-Tethys ridge is the best mechanism for explaining the Shilu adakitic rocks and Cu–Mo mineralization.

著者人数: 24名ほか (The OSIRIS-REx Team: 所属. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS): Van wal, S.; 吉川, 真; 渡邊, 誠一郎), Number of authors: 24 and others (The OSIRIS-REx Team: Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS): Van wal, S.; Yoshikawa, Makoto; Watanabe, Sei-ichiro), Accepted: 2019-02-26, 資料番号: SA1180355000

Martian meteorite Northwest Africa (NWA) 8114 – a paired stone to NWA 7034 – provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/ΣFe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 °C for at least 7 days in an oxidising regolith environment.We report an approximate 40Ar39Ar maximum age of 1.13 Ga to 1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/ΣFe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket.High temperatures, above 700 °C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days. Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite.

We report new U-Pb zircon ages, geochemical and isotopic data for Mesozoic igneous rocks, and new seismic interpretations of mostly submerged South Zealandia (1.5 Mkm 2 ). We use these data, along with existing geological and geophysical data sets, to refine the extent and nature of geological units. Our new 1:25 M geological map of South Zealandia provides a regional framework to investigate the rifting and breakup that formed Zealandia, Earth's most submerged continent. Samples of prerift (pre-100 Ma) plutonic rocks can be matched with on-land New Zealand igneous suites and indicate an east-west strike for the subduction-related 260 to 105-Ma Median Batholith across the Campbell Plateau. The plutonic chronology of formerly contiguous plutonic rocks in West Antarctica reveals similar pulses and lulls to the Median Batholith. Contrary to previous interpretations, the Median Batholith does not coincide with the 1,600-km-long Campbell Magnetic Anomaly System. Instead we interpret the continental magnetic anomalies to represent a mainly mafic igneous unit, whose shape and extent is controlled by synrift structures related to Gondwana breakup. Correlatives of some of these unsampled igneous rocks may be exposed as circa 85 Ma alkalic volcanic rocks on the Chatham Islands. Extension directions varied by up to 65° from 100 to 80 Ma, and we suggest this allowed this large area to thin considerably before final rupture to form new oceanic crust. Synrift (90–80 Ma) structures cut the oroclinal bend in southern South Island and support a pre-early Late Cretaceous age of orocline formation.

The Stolzburg Complex is one of several early Archaean ultramafic-mafic layered complexes situated along the northern edge of the Barberton Greenstone Belt on the eastern Kaapvaal Craton. In order to constrain timing and mode of emplacement, as well as post-emplacement processes affecting this igneous complex, we present whole rock trace element compositions as well as isotopic (Usingle bondPb, O, Hf) and REE-Ti concentration data for zircon and titanite extracted from three representative meta-gabbroic samples from the central and western portions of the complex. Zircon (SHRIMP) and titanite (LA-MC-ICPMS) analyses yield precise and identical Usingle bondPb ages of 3247 ± 3 Ma, 3252 ± 16 Ma and 3259 ± 6 Ma (2SE), respectively. Cathodo-luminescence imaging of zircon reveals remnant, faint oscillatory and convolute zoning that is cross-cut by luminescent, irregular fracture fillings. Zircon crystals from one meta-gabbro sample yield oxygen isotopic compositions (δ18O) of 250 °C) soon after magma emplacement. Some recent metamictization may have also contributed to the low δ18O component identified in the magmatic zircon, but is difficult to disentangle from hydrothermal effects. Based on zircon trace element systematics, the gabbros appear to have formed in an oceanic setting, consistent with the lack of whole rock geochemical evidence for assimilation of more evolved felsic crustal material. Considering analytical uncertainty, zircon eHf values are relatively uniform and point to derivation of the mafic magma from a Depleted Mantle source. Titanium-in-zircon temperatures (assuming αSiO2 = 0.7, αTiO2 = 1.0) range from 840 to 1020 °C. Although such apparent crystallization temperatures do neither constrain magma petrogenesis nor tectonic setting uniquely, they match temperatures recorded by modern mid-oceanic gabbros that experienced extensive hydrothermal alteration. Thus, the meta-gabbros of the ca. 3.25 Ga Stolzburg Complex provide a rare record of fluid-rock interaction at high temperatures within mafic crust on an ancient seafloor.

We report the development of a SIMS analytical method for in situ Sr isotope analysis of the mineral apatite at an analytical scale of 15 μm or less. Experiments were conducted on a series of reference apatites of widely differing Sr contents at three different mass resolution settings: low (3000), moderate (4200) and very high (16000), using a large-geometry CAMECA 1280 ion microprobe retrofitted with a HyperionTM oxygen plasma ion source. Peak-stripping algorithms tailored to the different mass-resolution settings were developed to correct for significant interferences on the target strontium isotope species in the mass spectra, namely Ca dimers, CaPO, Rb and SrH, and for instrumental mass fractionation. The most consistently accurate and precise 87Sr/86Sr ratios were achieved at the 3000 mass resolution setting with appropriate energy filtering and use of a cold trap. On average, corrected 87Sr/86Sr ratios by this method were elevated by ~0.8‰ above equivalent TIMS and solution ICPMS data, with consistent behaviour across the range of Sr concentrations. Individual spot precision of ± 0.001 to 0.004 (2SE) was achieved, depending on Sr content. The key advantages of SIMS analysis compared to existing LA-MC-ICP-MS methods are the high spatial resolution, low sample consumption, and the high mass resolution that allows for the elimination of problematic isobaric interferences. Our approach is particularly useful for the analysis of small or valuable targets, such as mineral inclusions or meteoric samples, and for apatite with high REE contents. However, the much larger volume of material consumed during LA-MC-ICP-MS analysis allows for substantially better precision than the SIMS method presented here. The highly variable trace element composition of natural apatites, involving both cationic and anionic substitutions, necessitates careful characterisation of prospective targets prior to any in situ analysis, in addition to matching to appropriate reference materials.

We report U–Pb zircon ages of c. 700–550 Ma, 262–220 Ma, 47–38 Ma and 15–14 Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700–550 Ma and 262–220 Ma are assigned to inherited zircon. Available geochemical data suggest that the 15–14 Ma metamorphic rims grew in situ in the amphibolites, corresponding to a high-grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40 Ma zircon rims also grew in situ, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late-Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40 Ma zircon rims would have grown in situ, and (ii) emplacement in the Miocene as a result mafic underplating during large-scale extension. In this case, only the 15–14 Ma metamorphic outer rims would have formed in situ in the amphibolitic host rocks.

The Late Mesozoic Qianlishan granitic complex in the western Nanling Range, South China is associated with the Shizhuyuan giant W–Sn–Mo–Bi polymetallic deposit. It mainly consists of three phases of intrusions, P-1 porphyritic biotite granite, P-2 equigranular biotite granite and P-3 granite porphyry. All three phases of granite contain quartz, plagioclase, K-feldspar and Fe-rich biotite. They have geochemical affinities of A-type granites, e.g., high FeOT/(FeOT + MgO) ratios (0.84–0.99), total alkali (Na2O + K2O, 7.50–9.04 wt.%), high Ga/Al ratios (10,000*Ga/Al > 2.6) and high Zr + Nb + Y + Ce concentrations (> 350 ppm). High Y/Nb ratios (> 1.2) suggest that the Qianlishan complex belongs to A2-type granite. Zircon U–Pb ages indicate a short age interval decreasing from 158–157 Ma, to 158–155 Ma and to 154 Ma for the P-1, P-2 and P-3 granites, respectively. These ages are similar to the mineralization age of the Shizhuyuan tungsten polymetallic deposit, within error. The Qianlishan granites were generated at low oxygen fugacity conditions based on the low values of zircon Ce4 +/Ce3 + ratios (1.53–198) and significantly negative Eu anomalies (EuN/EuN*, 0.03–0.13) in apatite. New zircon eHf(t) values for the P-3 granite range from − 13.0 to − 4.4, similar to those previously obtained for the P-1 and P-2 granites. Both the granite and apatite grains therein are characterized by high F but low Cl concentrations, suggesting the influx of a high F/Cl component. The P-2 granites especially contain higher F contents (1840–8690 ppm) and W (7–158 ppm) and Sn (6–51 ppm) concentrations and with stronger evolution features. Positive trends between F and W and Sn of Qianlishan complex indicate that high F source is crucial for mineralization of W and Sn. We consider that the lithospheric mantle source may have been metasomatized by subduction fluids in the far end of subduction zones to produce the A2 feature of the Qianlishan granite and the fluorine was introduced through breakdown of phengite in the oceanic slab. Partial melting of the lithospheric mantle was induced by upwelling of the asthenospheric mantle as a consequence of slab rollback. Tungsten and tin have been enriched in reduced granitic magmas through fractionation enhanced by high F contents, forming the giant polymetallic deposits.