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Magma plumbing systems underlying subduction zone volcanoes extend from the mantle through the overlying crust and facilitate protracted fractional crystallisation, assimilation, and mixing, which frequently obscures a clear view of mantle source compositions. In order to see through this crustal noise, we present intracrystal Secondary Ion Mass Spectrometry (SIMS) δ18O values in clinopyroxene from Merapi, Kelut, Batur, and Agung volcanoes in the Sunda arc, Indonesia, under which the thickness of the crust decreases from ca. 30 km at Merapi to ≤20 km at Agung. Here we show that mean clinopyroxene δ18O values decrease concomitantly with crustal thickness and that lavas from Agung possess mantle-like He-Sr-Nd-Pb isotope ratios and clinopyroxene mean equilibrium melt δ18O values of 5.7 ‰ (±0.2 1 SD) indistinguishable from the δ18O range for Mid Ocean Ridge Basalt (MORB). The oxygen isotope composition of the mantle underlying the East Sunda Arc is therefore largely unaffected by subduction-driven metasomatism and may thus represent a sediment-poor arc end-member. Subduction zone volcanoes are underlain by extensive magma plumbing systems, which can obscure original mantle source signals. Here, the authors show that intra-crystal oxygen isotope analysis of clinopyroxenes from the Sunda arc (Indonesia) reveal the δ18 O value of the sub-arc mantle. [ABSTRACT FROM AUTHOR]

Apatite provides a powerful means to determine the Sr isotopic composition (87Sr/86Sr) of ancient crust as it contains enough Sr to be measured accurately and precisely while typically having little post‐crystallisation radiogenic ingrowth due to its low Rb. Although utilising LG‐SIMS is beneficial for small‐volume targets, for example, inclusions, it is technically challenging to measure Sr isotopes in apatite with good accuracy and precision by SIMS, mainly due to molecular isobars interfering on Sr isotopes that have not previously been thoroughly assessed. This study evaluates all theoretically possible interferences affecting Sr isotope analysis of apatite by SIMS and develops a principle protocol for analysis and data processing that can be applied across a wide range of Sr concentration, even lower than 100 µg g−1. With careful control of the peak shape and applying precise mass centring at an intermediate mass resolution (ca. 4500), only interferences from the apatite matrix‐produced Ca dimers and 40Ca31P16O (present as a low mass tail) were found to be significant for the low trace element reference apatite used in this study, both of which can be precisely measured and corrected for. Despite this comprehensive approach, the Ca2 correction remained inadequate, possibly due to as yet unidentified doubly charged molecules interfering at the reference mass 82 peak (40Ca42Ca). While this can be reduced by employing an energy offset of at least −25 eV, precision significantly worsens in low Sr apatite (ca. 0.0024 in 1SE compared with ca. 0.0010 without offset for < 100 Sr µg g−1 apatite). Use of an empirical correction for the residual mass 82 interference is therefore preferred to an energy offset analytical approach, especially for low Sr apatite. Lastly, a slightly positive offset (ca. +0.0007) of unknown origin in 87Sr/86Sr was consistently observed across sessions with different analytical conditions, which may also be empirically corrected. The final corrected 87Sr/86Sr of the reference materials show good accuracy and precision with repeatability from ca. 0.0010 to 0.0002 in 1SE obtained for Sr contents from < 100 to 1500 µg g−1. [ABSTRACT FROM AUTHOR]

The Jurassic Gangdese arc hosts the giant Xietongmen (Xiongcun) porphyry Cu Au magmatic-hydrothermal centre. In order to understand the tectonomagmatic framework of the arc and its controls on the origin and temporal-spatial distribution of porphyry mineralization we conducted extensive major and trace, and isotopic analysis on a suite of samples collected regionally. Geochemical variations were observed across the Jurassic arc from south to north, controlled by northward subduction of the Neo-Tethyan oceanic plate and overlying Lhasa lithosphere. In the frontal arc (latitude 29°–29.55°N), the magmas had high Ba/La and Ba/Th ratios, high εNd (t) (+ 3 to + 7) values, and their zircons have high εHf (t) (+ 10 to + 18) values, and heterogeneous O isotopic compositions (δ 18 O = + 3.6 to + 6.6‰). The low titanium-in-zircon temperature and zircon saturation temperature (both average = ~ 700 °C) suggest that a greater flux of dehydration fluids from the subducting slab triggered partial melting of the mantle wedge in the southern arc. High zircon Eu/Eu* and Ce/Nd ratios (> 0.4 and > 20, respectively) indicate highly oxidized, fertile magmas, from which the Xietongmen porphyry Cu Au deposit formed. In contrast, the interior arc (north of latitude 29.55°), close to the ancient Tibetan basement, shows low Ba/La and Ba/Th ratios, low εNd(t) (− 0.1 to + 2.5) and zircon εHf (t) (+ 2 to + 12) values, and less heterogeneous zircon O isotopic compositions with higher δ 18 O values. Higher titanium-in-zircon and zircon saturation temperatures (mostly > 750 °C) suggest that less water was involved during the partial melting of mantle wedge in this region. Low zircon Eu/Eu* and Ce/Nd ratios (mostly < 0.4 and < 20, respectively) point towards less oxidized magmatic conditions. We conclude that across-arc geochemical and fluid-flux variations controlled the formation and spatial distribution of Jurassic porphyry deposits in the Gangdese belt. [ABSTRACT FROM AUTHOR]

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. • Development of new SIMS method for in situ measurement of 87Sr/86Sr in apatite. • Isobaric interferences overcome by high mass resolution, peak-stripping algorithm. • High spatial resolution ideal for targeting mineral inclusions. [ABSTRACT FROM AUTHOR]

This study reports in–situ sulfur isotope analyses (32S, 33S, 34S and 36S) of pyrite in strongly sulfidized stromatolites from the ~3.48 billion–year–old Dresser Formation, Pilbara Craton, Australia. These data shed light on sulfur reservoirs and sulfide precipitation processes and provide clues for the contribution of sulfur–cycling microbes to sulfidization. Sulfur isotope signatures derived from mass dependent fractionation (MDF; monitored by δ34S) and mass independent fractionation (MIF; here Δ33S and Δ36S) of pyrite in stromatolites, and of microscopic pyrite within associated barite, allow for the identification of distinctive sulfur sources: i) magmatic–hydrothermal sulfide (H 2 S) with δ34S and Δ33S ~ 0%; ii) magmatic–hydrothermal sulfate (SO 4 2−) with a MDF signature (MDF–SO 4 2−; δ34S ~ 10‰ and Δ33S ~ 0‰; iii) photochemically–derived sulfate with a MIF signature (MIF–SO 4 ; δ34S ~ −6‰ and Δ33S ~ −3.0‰); iv) photochemically–derived elemental sulfur (S0) with δ34S ≪ 0 and Δ33S ≫ 0‰. The sulfur isotope data suggest that sulfidization was largely driven by reduction of intermixed MDF–SO 4 2− and MIF–SO 4 2− (bulk signature of δ34S ~ 5‰ and Δ33S ~ −1.4‰), and dilution of produced H 2 S (δ34S ~ −12‰ and Δ33S ~ −1.4‰) by native H 2 S in magmatic–hydrothermal fluids. The δ34S shifts (up to ~17‰) generated by sulfate reduction are consistent with both thermochemical reactions and influence of sulfate–cycling microbes, the latter which may have facilitated rapid pyrite precipitation and preservation of microbial remains that are entombed within the petrogenetically earliest pyrite generation of stromatolites. Collectively, our data are consistent with ancient stromatolite growth in proximity to shallow marine hydrothermal vents, where hydrothermal fluids contributed to sulfidization that may have been further influenced by sulfur–cycling microbes. [ABSTRACT FROM AUTHOR]

• Hydrothermal pyrites are classified into three types (Py1, Py2c and Py2r). • Two distinct stages of mineralization are revealed with diverse source materials. • The sources of the early and main stages show lower crust and mantle contributions. The Shanggong lode deposit represents one of the largest Mesozoic orogenic gold deposits in the East Qinling metallogenic belt of central China. Here we investigate the texture, trace element and sulfur isotopic composition of pyrite from this deposit to gain insights in the ore genesis. Three types of pyrite are recognized by its texture: Py1 is represented by coarse-grained cubic and pentagonal dodecahedron belonging to the stage I quartz-pyrite vein; Py2 comprises fine-grained disseminated pentagonal dodecahedron belonging to stage II quartz-sulfides vein; Py2 can be further divided into core and rim: porous cores (Py2c) carrying abundant, visible mineral inclusions and significantly higher arsenic concentration, and clean rims (Py2r) with low arsenic. Systematic LA-ICPMS trace elements and SIMS sulfur isotopic analyses reveal two distinct stages of mineralization with diverse source materials. 1) The early stage is represented by Py1 which contains 0.65–70 ppm Au (average 11.6 ppm), with relatively low and variable concentrations of Cu, Ag, Te, Pb, Zn, and Bi. The δ34S values of Py1 range from −2.7‰ to 7.2‰, suggesting that the sulfur source for the early stage fluid was mainly derived from the Taihua and Xiong'er Groups and that the depleted δ34S values reflect changes of pH, f O2 and fluid-rock interaction. 2) The main stage is represented by Py2 which contains Au in the range of 0.14–189 ppm (average 39.7 ppm). The Py2 carries significantly higher contents of compatible elements (Ni, Co, V, and Cr) and ore metals (e.g. Au, Ag, Pb, Zn, As, Te) than those in Py1, with the ore metals occurring as invisible lattice bound or nanosized particles and visible micron-sized inclusions. The δ34S values of Py2 show marked depletion with a range of −16.3‰ to −11.7‰, attributing to the fractionation between barite and sulfide minerals. Together with previous studies and trace element characters in Py2, we identify a contribution of Au from mantle source into the ore-forming fluid system during the main stage. We propose that the two stages of gold mineralization in the Shanggong deposit, in response to the collisional assembly of Qinling Orogenic Belt, preserve evidence for diverse metal sources with the early stage derived from partial melting of the lower crust (Taihua and Xiong'er Groups) in the compressional stage, and the main stage characterized by mantle contribution in a post-collisional extensional setting. Our findings may have important implications for the exploration of regional Triassic gold and molybdenum deposits. [ABSTRACT FROM AUTHOR]