Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

Liberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little investigation of the metamorphic reactions responsible for sulfur release from subducting slabs. Here, we investigate the relative stability of anhydrite (CaSO4) and pyrite (FeS2) in subducted basaltic oceanic crust, the largest contributor to the subducted sulfur budget, to place constraints on the processes controlling sulfur release. Our analysis of anhydrite stability at high pressures suggests that this mineral should dominantly dissolve into metamorphic fluids released across the transition from blueschist to eclogite facies (~450–650 °C), disappearing at lower temperatures on colder geothermal trajectories. In contrast, we suggest that sulfur release via conversion of pyrite to pyrrhotite occurs at temperatures above 750 °C. This higher temperature stability is indicated by the preservation of pyrite–bornite inclusions in coesite-bearing eclogites from the Sulu Belt in China, which reached temperatures of at least 750 °C.Thus, sulfur may be released from subducting slabs in two separate pulses; (1) varying proportions of SO2, HSO4− and H2S are released via anhydrite breakdown at the blueschist–eclogite transition, promoting oxidation of remaining silicates in some domains, and (2) H2S is released via pyrite breakdown well into the eclogite facies, which may in some circumstances coincide with slab melting or supercritical liquid generation driven by influx of serpentinite-derived fluids. These results imply that the metallogenic potential in the sub-arc mantle above the subducting slab varies as a function of subduction depth, having the greatest potential above the blueschist–eclogite transition given the association between oxidised magmas and porphyry Cu(–Au–Mo) deposits. We speculate that this zoned sulfur liberation might be one of the factors that lead to the apparently redox-influenced zoned distribution of ore deposit types in the Andean arc. Furthermore, given the lack of sulfate-associated sea floor oxidation prior to the second great oxidation event, the pattern of sulfur transfer from the slab to the sub-arc mantle likely changed over time, becoming shallower and more oxidised from the Neoproterozoic onwards

Insights into subduction zone sulfur recycling from isotopic analysis of eclogite-hosted sulfides

Subduction of sulfur in ocean crust makes a significant but poorly understood contribution to the global sulfur cycle. Part of the uncertainty arises from a lack of knowledge about the metamorphic changes that affect subducted sulfur-bearing minerals, and the ultimate source of sulfur that is subducted to depth. Sulfur δ34S varies both as a function of the original sulfur source, and as a consequence of processes subsequent to sulfide crystallisation such as devolatilisation, redox reactions, and fluid loss. To investigate sulfur liberation during subduction, secondary ion mass spectroscopy (SIMS) was used to measure δ34S in grains of pyrite, chalcopyrite and pyrrhotite in eclogites from the Zermatt–Saas zone in the Western Alps, and the Pouébo terrane of New Caledonia. Trace element mapping on selected sulfide grains was also performed. Sulfides in these rocks are generally associated with greenschist retrogression assemblages, but also occur as inclusions in garnet, associated with glaucophane and omphacite, and as polysulfide grains with typicalmagmaticcombinations of minerals. δ34S varies significantly within individual pyrite grains, with striking correlations, insome cases, between Co zoning and changes in δ34SVCDT. δ34SVCDT is, in many cases, greater than 13‰, consistent with derivation from seawater-derived sulfate. The dataset suggests that sulfur isotopes in pyrite experienced little or no post-crystallisation re-equilibration, that pyrite grew under open system conditions with heterogeneous fluid flow on a thin section scale, and that sulfide growth involved sulfur addition. Prograde subduction processes most likely involved sulfur loss. Sulfide growth occurred in some samples at the very earliest stages of exhumation. Therefore these sulfides provide useful information on the fluids present in slabs at great depths

Effects of geodynamic setting on the redox state of fluids released by subducted mantle lithosphere

Magnetite breakdown during subduction of serpentinised ultramafic rocks may produce oxidised fluids that oxidise the deep Earth and/or the sub-arc mantle, either via direct transport of ferric iron, or via redox reactions between ferric iron and other elements, such as sulfur. However, so far, there is no consensus on the oxidation state of fluids released during subduction of ultramafic rocks, or the factors that control this oxidation state. Subducted samples from a magma-poor rifted margin and a supra-subduction zone geodynamic setting were compared to examine evidence of changes in opaque phase assemblage and ferric iron content as a consequence of subduction, and as a function of geodynamic setting. Thermodynamic calculations in the system Fe–Ni–O–H–S and Fe–Ni–O–S at the pressures and temperatures of interest were used to constrain oxygen activities and fluid compositions. Samples from New Caledonia, which exemplify supra-subduction zone mantle, contain awaruite (FeNi3) and equilibrated with hydrogen-bearing fluids at oxygen activity less than the FMQ (fayalite–magnetite–quartz) buffer. In contrast, samples from the Zermatt Saas Zone ophiolite, Western Alps, which are thought to represent mantle from a subducted magma-poor rifted margin, contain magnetite plus sulfur-rich phases such as pyrite (FeS2), and are inferred to have equilibrated with hydrogen-poor fluids at oxygen activity greater than FMQ. This major difference is independent of differences in subduction pressure–temperature conditions, variation in peridotite protolith composition, or the nature of adjacent units. We propose that the Zermatt Saas Zone samples would have undergone more complete serpentinisation prior to subduction than the supra-subduction zone (SSZ) New Caledonian samples. This difference explains the different fluid compositions, because incompletely serpentinised rocks containing olivine and brucite retain or evolve awaruite-bearing assemblages that buffer fluid compositions to high hydrogen activity (aH2). Ultramafic rocks are associated with two distinctly different fluid compositions during pre-subduction and subduction serpentinisation. Initially, while olivine is in equilibrium with infiltrating fluid, mineral assemblages that include awaruite in the rocks buffer fluids to H2-bearing, low aO2 compositions. Deserpentinisation of incompletely serpentinised rocks in which awaruite is present also produces H2-bearing fluids. Once awaruite is exhausted, H2-poor, high aO2 fluids co-exist with awaruite-absent assemblages, and deserpentinisation of such rocks would produce H2O-rich fluids. Thus, deserpentinisation of ultramafic rocks could produce either hydrogen-bearing fluids that could infiltrate and reduce the sub-arc mantle, or more oxidised fluids, which could transfer redox budget to other geochemical reservoirs such as the sub-arc mantle. Therefore, the redox contribution of subducted ultramafic rocks to the deep Earth and sub-arc mantle depends on the extent of protolith serpentinisation. Pre-subduction settings that promote extensive serpentinisation by oxidised fluids at high fluid:rock ratios in open systems, such as slow and ultraslow spreading ridges, transform faults, oceanic core complexes, and exhumed mantle at rifted continental margins, may produce more oxidised fluids than those associated with less pervasive serpentinisation and fluids that may be rock-buffered to a reduced state

Variation in XANES in biotite as a function of orientation, crystal composition, and metamorphic history

Microscale analysis of ferrous:ferric iron ratios in silicate minerals has the potential to constrain geological processes but has proved challenging because textural information and spatial resolution are limited with bulk techniques, and in situ methods have limited spatial resolution. Synchrotron methods, such as XANES, have been hampered by the sensitivity of spectra to crystal orientation and matrix effects. In an attempt to break this nexus, biotites from Tanzania were characterized with a combination of optical microscopy, electron microprobe, Mössbauer analysis, electron backscatter diffraction (EBSD) and X-ray absorption near edge structure (XANES) spectroscopy. Pre-edge and edge characteristics of the FeKa absorption feature were compared to orientation information derived by EBSD and ferric iron content derived from Mössbauer analysis. Statistically significant correlations between measured spectral features and optic/crystallographic orientation were observed for individual samples. However, orientation corrections derived from these correlations did not reduce the uncertainty in Fe3+/Fetot. The observations are consistent with matrix- and ordering-dependency of the XANES features, and further work is necessary if a general formulation for orientation corrections is to be devised

'Sarsen stones in Wessex': a society of antiquaries project contextualised and renewed

This paper reviews the Society of Antiquaries’ Evolution of the Landscape project, which started in 1974, and the project’s Sarsen Stones in Wessex survey. The survey was an ambitious public archaeology project, involving c 100 volunteers led by Fellows of the Society during the 1970s. Its aims, objectives and outcomes are described. The survey’s unique dataset, produced for the counties of Wiltshire, Hampshire and Dorset, has now been digitised. Drawing on the dataset, the paper situates the Evolution of the Landscape project in the context of later-twentieth century British archaeology. It demonstrates the importance not only of individual Fellows, but also contemporary movements in academic and development-led archaeology, to the direction of the Society’s activities in this formative period for the discipline today, and shows how the Society’s research was engaged with some of archaeology’s most pressing cultural resource management issue

Characterisation of uranium-pyrite associations within organic-rich Eocene sediments using EM, XFM-µXANES and µXRD

Sediments that are rich in organic matter (OM) can trap and accumulate metals, including uranium (U), and provide favourable conditions for the formation of minerals such as pyrite. In OM-sediments, pyrite may play an important role in sorption and reduction of U and other metals that are sensitive to redox change. Mulga Rock, located in Western Australia, is an Eocene polymetallic deposit hosting economic uranium within its OM-rich sediments. Our previous work on the Mulga Rock deposits showed that OM may host U(VI) for long periods and suggested that reduction to U(IV) may occur through different pathways. Framboidal-like pyrite, embedded in lignite, displaying U(IV)-rich rims, > 100 µm, were investigated using synchrotron-X-ray fluorescence microscopy-micro-X-ray absorption near edge structure (XFM-µXANES), laboratory micro-X-ray diffraction (µXRD) and electron microscopy. Micro-XRD analyses indicated that the U(IV) phase was mostly coffinite [USiO4] mixed with uraninite [UO2]. Analyses of transects across U-pyrite rims using XFM and XFM-µXANES revealed higher concentrations of U(IV) near the pyrite, and proportionally more U(VI) away from the edge of the pyrite. We infer that within OM environments, pyrite has a local influence on the oxidation state of U, which may be influential when considering U mobility

Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the influence of subduction-related metamorphism

Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed microtextural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of microtextural investigations with d34S values from in situ ion probe analyses within individual pyrite and chalcopyrite grains allowed evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic recrystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the submillimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic reequilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies

Impact of a referral management “gateway” on the quality of referral letters; a retrospective time series cross sectional review

Background Referral management centres (RMC) for elective referrals are designed to facilitate the primary to secondary care referral path, by improving quality of referrals and easing pressures on finite secondary care services, without inadvertently compromising patient care. This study aimed to evaluate whether the introduction of a RMC which includes triage and feedback improved the quality of elective outpatient referral letters. Methods Retrospective, time-series, cross-sectional review involving 47 general practices in one primary care trust (PCT) in South-East England. Comparison of a random sample of referral letters at baseline (n = 301) and after seven months of referral management (n = 280). Letters were assessed for inclusion of four core pieces of information which are used locally to monitor referral quality (blood pressure, body mass index, past medical history, medication history) and against research-based quality criteria for referral letters (provision of clinical information and clarity of reason for referral). Results Following introduction of the RMC, the proportion of letters containing each of the core items increased compared to baseline. Statistically significant increases in the recording of ‘past medical history’ (from 71% to 84%, p < 0.001) and ‘medication history’ (78% to 87%, p = 0.006) were observed. Forty four percent of letters met the research-based quality criteria at baseline but there was no significant change in quality of referral letters judged on these criteria across the two time periods. Conclusion Introduction of RMC has improved the inclusion of past medical history and medication history in referral letters, but not other measures of quality. In approximately half of letters there remains room for further improvement