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In this study, we use micro-X-ray absorption near-edge structures (µ-XANES) spectroscopy at the S K-edge to investigate the oxidation state of S in natural magmatic-hydrothermal apatite (Durango, Mexico, and Mina Carmen, Chile) and experimental apatites crystallized from volatile-saturated lamproitic melts at 1000 °C and 300 MPa over a broad range of oxygen fugacities [log(fO2)=FMQ, FMQ+1.2, FMQ+3; FMQ = fayalite-magnetite-quartz solid buffer]. The data are used to test the hypothesis that S oxidation states other than S6+ may substitute into the apatite structure. Peak energies corresponding to sulfate S6+ (∼2482 eV), sulfite S4+ (∼2478 eV), and sulfide S2- (∼2470 eV) were observed in apatite, and the integrated areas of the different sulfur peaks correspond to changes in fO2 and bulk S content. Here, multiple tests confirmed that the S oxidation state in apatite remains constant when exposed to the synchrotron beam, at least for up to 1 h exposure (i.e., no irradiation damages). To our knowledge, this observation makes apatite the first mineral to incorporate reduced (S2-), intermediate (S4+), and oxidized (S6+) S in variable proportions as a function of the prevailing fO2 of the system. Apatites crystallized under oxidizing conditions (FMQ+1.2 and FMQ+3), where the S6+/STotal peak area ratio in the coexisting glass (i.e., quenched melt) is ∼1, are dominated by S6+ with a small contribution of S4+, whereas apatites crystallizing at reduced conditions (FMQ) contain predominantly S2-, lesser amounts of S6+, and possibly traces of S4+. A sulfur oxidation state vs. S concentration analytical line transect across hydrothermally altered apatite from the Mina Carmen iron oxide-apatite (IOA) deposit (Chile) demonstrates that apatite can become enriched in S4+ relative to S6+, indicating metasomatic overprinting via a SO2-bearing fluid or vapor phase. This XANES study demonstrates that as the fO2 increases from FQM to FMQ+1.2 to FMQ+3 the oxidation state of S in igneous apatite changes from S2- dominant to S6+ > S4+ to S6+ >> S4+ Furthermore, these results suggest that spectroscopic studies of igneous apatite have potential to trace the oxidation state of S in magmas. The presence of three S oxidations states in apatite may in part explain the non-Henrian partitioning of S between apatite and melt. Our study reveals the potential to use the S signature of apatite to elucidate both oxygen and sulfur fugacity in magmatic and hydrothermal systems.

Olivine (Fo80-88) from the Shitoukengde deposit exhibits low levels of Ca, Cr, and Al (< 220 ppm) and varying Ni content. The low Ca-Cr-Al contents in olivine and subsolidus temperatures (600–900 °C) indicated by olivine-spinel thermometers align with subsolidus equilibria, emphasizing substantial postcumulus modifications. Therefore, the postcumulus effect must be considered when applying olivine-spinel oxybarometers to intrusive rocks. Back-calculating the spinel Fe–Mg contents to magmatic temperature, the estimated oxidation fugacity (fO2) range between FQM − 1.5 and FQM − 3.0, approximately 0.5 to 1.5 ΔFQM more reduced compared to those calculated from the raw spinel composition. Moreover, the fO2 aligns with results obtained from the olivine-sulfide pair (FMQ − 3.0 to FMQ 0). The considerably reducing state and wide oxidation variation are consistent with the graphite occurrence within the reduced intervals and the systematic fO2 indicated by olivine V/Sc ratios. Combined with the wide olivine Ni range (200–1500 ppm) and the restricted Ni tenor in coexisting sulfides, those findings imply that the olivine-sulfide interaction was predominantly controlled by fO2. Diffusion modeling at magmatic temperatures reveals that the core-level Fe–Ni re-equilibration after crystallization requires hundreds of years. The homogeneous olivine composition suggests that re-equilibrium has been achieved in Shitoukengde. However, in fast cooling systems, olivine may record the status approaching olivine-sulfide equilibration, leading to extensive intragrain Ni variation (up to 1000 ppm). This study highlights that extreme Ni depletion in olivine from sulfide-bearing rocks is a sign of reducing conditions. Strongly Ni-rich olivine, such as those in the Kevitsa deposit, could result from interaction with high-Ni tenor sulfides at oxidizing conditions.

Magma mixing and contamination are among the dominant processes in the building of the isotopic diversity of granite rocks. Felsic microgranular enclaves (FMEs) are remnants of magma mixing and they often carry xenocrysts that also record contamination and assimilation in crystallization fronts disrupted during replenishment events. In addition to isotopic changes, contamination may alter the redox state of the intruding magmas depending on the nature of the country rocks. We investigate the role played by different country-rocks on the redox trajectory of the precursor magmas of two Brazilian occurrences: the Mauá pluton, which intruded sulfide- and graphite-bearing metasediments, and the Salto rapakivi pluton, which intruded orthogneisses.Oxygen fugacity (fO2) was estimated using Eu2+/Eu3+ ratios retrieved from plagioclase chemistry and literature equation that considers melts chemistry and temperature. The model is demonstrated to replicate the fO2 from plagioclase-bearing experimental data within a precision usually better than 1 log unit.Results obtained for the Mauá pluton indicate that contaminated plagioclase cores are significantly more reduced (∆QFM − 1.5; 87Sr/86Sr ~ 0.713) than the xenocryst rims (∆QFM + 0.5; 87Sr/86Sr ~ 0.710). In contrast, results obtained for the Salto rapakivi pluton vary from ∆QFM + 1.0 to ∆QFM + 2.7 and do not coincide with core to rim variations in 87Sr/86Sr. MMEs are comparatively more reduced (− 0.1 ≤ ∆QFM ≤  + 0.7).Our results imply that the redox path registered by plagioclase crystals from different geological backgrounds reflects the nature of their country rocks and the processes that affected their precursor magmas The main advantage of using plagioclase trace element data to model redox conditions of equilibrium magmas is the possibility of determining fO2 paths via spatially controlled trace element analyses of crystals rims and cores. Coupled to the possibility of obtainment of Sr isotope data, the model represents a powerful way to unravel the processes responsible for redox paths of magmatic rocks of varied nature.

Chromium valence ratios in igneous olivine may hold a wealth of redox information about the melts from which they crystallized. It has been experimentally shown that the Cr2+/ΣCr of olivine varies systematically with fO2, therefore measurements of Cr valence in olivine could be employed as a quantitative oxybarometer. In situ synchrotron µ-XANES analyses of Cr valence ratios of individual olivine phenocrysts in thin section have the potential to unlock this stored magmatic redox information on a fine spatial scale. However, there are still obstacles to obtaining accurate XANES measurements of cation valence in crystalline materials, as the results from these measurements can be compromised by anisotropic absorption effects related to the crystallographic orientation of the sample. Improving the accuracy of XANES measurements of Cr valence ratios in olivine by calibrating an anisotropy correction is a vital step in developing Cr valence measurements in olivine as a rigorous oxybarometer. To accomplish this goal, we have used an integrated approach that combined experiments, electron backscatter diffraction analysis, and XANES measurements in olivine to systematically examine how orientation affects the resultant Cr K-edge XANES spectra and the Cr valence ratios that are calculated from them. The data set generated in this work was used to construct a model that mitigates the effects of anisotropy of the calculated Cr2+/ΣCr values. The application of this correction procedure as a part of spectral processing improves the overall accuracy of the resultant Cr2+/ΣCr values by nearly a factor of five. The increased accuracy of the XANES measured Cr valence ratios afforded by the anisotropy correction reduces the error on calculated fO2 values from approximately ±1.2 to ±0.25 log units.

Testing the Ballhaus–Berry–Green Ol–Opx–Sp oxybarometer (BBG) on independent experimental data indicates that it overestimates the oxygen fugacity by 0.6–1.3 log units under mildly reduced conditions (near the C–CO buffer) and by as much as 2–3 log units under reduced conditions (at the IW buffer and below it). A newly developed oxibarometer is suggested to minimize this effect and enhance the capabilities of redoxometry of low-pressure mineral associations, including magmatic melts undersaturated with respect to orthopyroxene ( Opx ). The new empirical equation of the oxybarometer is applicable to a wide range of mafic–ultramafic magmas of normal alkalinity, including terrestrial, lunar, and meteoritic systems under pressures of 0.001–25 kbar and oxygen fugacity ranging from IW–3 to NNO + 1. The derived regression fits the ΔQFM values of the calibration dataset (154 experiments) accurate to ~0.5 log units. The new oxybarometer eliminates systematic errors when redox parameters are evaluated for the reduced region (from IW–3 to C–CO) and for crystallization of magmas without Opx on the liquidus. The efficiency of the suggested model is demonstrated by its application to natural rocks: (1) low-Ti lunar basalts, (2) tholeiites from the Shatsky Rise, (3) Siberian flood basalts, (4) rocks of the layered series of the Yoko-Dovyren intrusion, and (5) mantle xenoliths collected in southern Siberia, Mongolia, China, and the southern Russian Far East. The values yielded by such oxybarometers for intrusive rocks, which underwent long-lasting cooling and postcumulus reequilibration, should be regarded with reserve.

The more oxidized mantle peridotites above subducting slabs than stable continental areas have been attributed to the infiltration of some oxidizing fluids released from the subducting slabs. However, knowledge for the redox states of the slabs itself is very limited. Until now, few oxybarometers can be directly used to constrain the redox states of the subducting slabs.The rutile-ilmenite oxybarometer was proposed and successfully applied to constrain the oxygen fugacity of mantle assemblages.However, its application to rocks equilibrated at crustal P-T conditions has been hampered by some uncertainties in an early solid solution model of ilmenite. With a newly-released solid solution model for the ilmenite, we have conducted high-P experiments（at 3 and 5 GPa, and 900–1300°C） to test the accuracy of this oxybarometer. The experiments were performed with their oxygen fugacities controlled by the CCO buffer（i.e., C＋O2=CO2）. We demonstrated that the oxygen fugacities calculated for our high-P experimental products by using the rutile-ilmenite oxybarometer were in excellent agreement with the fO2 dictated by the CCO buffer, suggesting a wide applicability of this oxybarometer to crust rocks. As examples, the rutile-ilmenite oxybarometer has been used to constrain the oxygen fugacities of some metamorphic rocks such as eclogite, granulite and amphibolite usually observed from the subduction zones.

The paper represents an algorithmic implementation of the SPINMELT-2.0 model designed to simulate Cr-spinel–melt equilibrium, and provides a description of its petrologic options. The properties of the SPINMELT-2.0 model were studied by modeling the topology of the liquidus surface of spinel and its dependence on pressure, redox potential, and concentrations of major components (including Cr2O3 and H2O) in the melt. Reference simulations were carried out for primitive MORB tholeiite. The spinel composition is demonstrated to depend on variously (and often oppositely) acting factors. Providing an accurate estimate of a parental magma composition, the SPINMELT-2.0 program allows one to evaluate a range of P–T–fO2–H2 O parameters responsible for the composition of an original magmatic spinel. The SPINMELT program makes it possible not only to effectively correlate available independent petrological estimates but also to consciously correct them, which is particularly important when the composition of the model melts should be estimated. This is illustrated by the application of the model to data on the composition of rocks and minerals of two young volcanoes in Kamchatka: Tolbachik and Gorely.