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Pressure is one of the key variables that controls magmatic phase equilibria. However, estimating magma storage pressures from erupted products can be challenging. Various barometers have been developed over the past two decades that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. These Jd-in-clinopyroxene barometers have been applied to rift zone magmas from Iceland, where published estimates of magma storage depths span the full thickness of the crust, and extend into the mantle. However, tests performed on commonly used clinopyroxene-liquid barometers with data from experiments on H2O-poor tholeiites in the 1 atm to 10 kbar range reveal substantial pressure-dependent inaccuracies, with some models overestimating pressures of experimental products equilibrated at 1 atm by up to 3 kbar. The pressures of closed-capsule experiments in the 1-5 kbar range are also overestimated, and such errors cannot be attributed to Na loss, as is the case in open furnace experiments. The following barometer was calibrated from experimental data in the 1 atm to 20 kbar range to improve the accuracy of Jd-in-clinopyroxene barometry at pressures relevant to magma storage in the crust: P(kbar) = -26.27+39.16T(K)/104ln [XJdCpx/XNaO0.5liq XAlO1.5liq (XSiO2liq)2]-4.22ln(XDiHdCpx)+78.43XAlO1.5liq+393.81(X NaO0.5liq XKO0.5liq)2. This new barometer accurately reproduces its calibration data with a standard error of estimate (SEE) of ±1.4 kbar, and is suitable for use on hydrous and anhydrous samples that are ultramafic to intermediate in composition, but should be used with caution below 1100 °C and at oxygen fugacities greater than one log unit above the QFM buffer. Tests performed using with data from experiments on H2O-poor tholeiites reveal that 1 atm runs were overestimated by less than the model precision (1.2 kbar); the new calibration is significantly more accurate than previous formulations. Many current estimates of magma storage pressures may therefore need to be reassessed. To this end, the new barometer was applied to numerous published clinopyroxene analyses from Icelandic rift zone tholeiites that were filtered to exclude compositions affected by poor analytical precision or collected from disequilibrium sector zones. Pressures and temperatures were then calculated using the new barometer in concert with Equation 33 from Putirka (2008) Putative equilibrium liquids were selected from a large database of Icelandic glass and whole-rock compositions using an iterative scheme because most clinopyroxene analyses were too primitive to be in equilibrium with their host glasses. High-Mg# clinopyroxenes from the highly primitive Borgarhraun eruption in north Iceland record a mean storage pressure in the lower crust (5.7 kbar). All other eruptions considered record mean pressures in the mid-crust, with primitive clinopyroxene populations recording slightly higher pressures (3.1-3.6 kbar) than evolved populations (2.6-2.8 kbar). Thus, while some magma processing takes place in the shallow crust immediately beneath Iceland's central volcanoes, magma evolution under the island's neovolcanic rift zones is dominated by mid-crustal processes.

We present petrological investigations and mineral chemistry of several Tethyan ophiolites to reveal the occurrence, origin, and fate of water in podiform chromitites. The results show that clinopyroxene and olivine in chromitites have H O contents of 801–366 and 53–17 ppm, respectively. The highest water contents of olivine occur in massive chromitite and the lowest always in the clinopyroxenebearing ores because much of the available hydrous fluids was taken up by the clinopyroxene during crystallization. The major and trace elemental and Li isotopic compositions of clinopyroxene associated with chromite and olivine in podiform chromitites indicate formation from a mixture of surface hydrous fluids on chromite grains and evolved melts from which olivine crystallized. The hydrous fluids initially originated from dehydration of a subducting slab as revealed by Li isotopic compositions of clinopyroxene and olivine in the chromitites. High fluid/rock ratios facilitated concentration of chromite to form chromitite, suppressing crystallization of olivine. The hydrous fluids that were collected on the chromite grain surface during crystallization allowed chromite grains to rise via decreasing density in the form of bubbles, thus promoting their gathering and concentration. The fate of these hydrous fluids depends on ambient physical and chemical conditions. Mostly they hydrate adjacent olivine grains in the chromitite or penetrate the surrounding dunite envelope. In some cases, the fluids dissolve into silicate melts to produce water-bearing clinopyroxene and/or hydrous minerals, such as amphibole, or infiltrate silicate and chromite grains to form inclusions, which may exsolve later in the form of mineral lamellae. Our investigations provide direct natural evidence for the presence and importance of water in the formation and evolution of chromite deposits, as inferred by earlier experimental studies.

Water content plays a significant role in magma genesis, ascent rate, and, ultimately, in the style and intensity of volcanic eruptions, due to its control on the density, viscosity and melting behaviour of silicate melts. A reliable method for determining the pre-eruptive magmatic water content is to use phenocrysts of nominally anhydrous minerals (NAMs) which can preserve water as hydrogen configurations in structural defects. The advantage of this method is that eruptive changes such as water loss during magma degassing may be experimentally reconstructed and analysed by infrared spectroscopy. Applying this to clinopyroxene crystals ( n =17) from lava samples ( n =7) from April 2021 of the Geldingadalir eruption, SW-Iceland, reveals parental water contents of 0.69 ± 0.07 to 0.86 ± 0.09 wt. % H 2 O. These values are higher than those expected for typical mid-ocean ridge basalts (MORB 0.3–0.5 wt. % on average) indicating a significant plume (OIB) contribution to the magma source. Moreover, such water concentrations would imply that water saturation in the ascending Geldingadalir magmas was attained only at very shallow levels within the plumbing system. This could explain the at times pulsating behaviour within the uppermost conduit system as being the result of shallow episodic water vapour exsolution in addition to the deep-sourced CO 2 flux.

Magmatic activity of the Emeishan large igneous province (ELIP) of SW China is one of the most significant geological events of the late Paleozoic. The large volume flood basalts plus rare picrites were erupted in Late Permian. Previous studies indicate that the basalts are the derivatives of primary mantle-derived magma by fractional crystallization, but the depths at which this process took place remain unknown. To answer this question, we use phenocryst compositions and mineral-liquid thermobarometers to determine the P-T conditions of the magma reservoirs where crystallization occurred, then use these data to reconstruct the magma plumbing system of the igneous province. Thermobarometric calculations show that most picrite-hosted clinopyroxene phenocrysts crystallized at ∼25 km and 1200-1280°C, whereas most basalt-hosted clinopyroxene phenocrysts crystallized at depths <20 km and temperatures <1200°C. Some picrites containing primitive olivine with Fo up to Fo92 likely formed by eruption of the most primitive magma with composition similar to the primary magma from the deepest reservoir possibly at the Moho. Parental magmas yield mantle potential temperatures of 1740-1810°C, which are the highest such temperatures yet recorded for terrestrial magmas of any age. Less primitive picrites containing both olivine and clinopyroxene phenocrysts formed by eruption of moderately fractionated magma from a reservoir in the middle crust. Basalts and basaltic andesites formed by eruption of the most fractionated magmas from the reservoirs in the upper crust, coinciding with the depths of coeval sulfide ore-bearing and Fe-Ti-V oxide ore-bearing mafic-ultramafic intrusions. The reason that the Emeishan volcanic sequence is dominated by basalts is because most of the mantle-derived magma was trapped in the middle and upper crusts, undergoing variable degrees of crystal fractionation plus crustal contamination before eruption. Primitive picrites are rare because their eruption requires a trans-lithosphere conduit, which is difficult to create and maintain due to increasing lithospheric pressure with depth. The results from this study reveal that magma reservoirs at the crustal levels play a critical role in magma differentiation in a continental setting.

Clinopyroxene-only thermobarometry is one of the most practical tools to reconstruct crystallization pressures and temperatures of clinopyroxenes. Because it does not require any information of coexisting silicate melt or other co-crystallized mineral phases, it has been widely used to elucidate the physiochemical conditions of crystallizing magmas. However, previously calibrated clinopyroxene-only thermobarometers display low accuracy when being applied to mafic and intermediate magmatic systems. Hence, in this study, we present new empirical nonlinear barometric and thermometric models, which were formulated to improve the performance of clinopyroxene-only thermobarometry. Particularly, a total of 559 experimental runs conducted in the pressure range of 1 bar to 12 kbar have been used for calibration and validation of the new barometric and thermometric formulation. The superiority of our new models with respect to previous ones was confirmed by comparing their performance on 100 replications of calibration and validation, and the standard error of estimate (SEE) of the new barometer and thermometer are 1.66 kbar and 36.6 ∘C, respectively. Although our new barometer and thermometer fail to reproduce the entire test dataset, which has not been used for calibration and validation, they still perform well on clinopyroxenes crystallized from subalkaline basic to intermediate magmas (i.e., basaltic, basalt-andesitic, dacitic magma systems). Thus, their applicability should be limited to basaltic, basalt-andesitic and dacitic magma systems. In a last step, we applied our new thermobarometer to several tholeiitic Icelandic eruptions and established magma storage conditions exhibiting a general consistency with phase equilibria experiments. Therefore, we propose that our new thermobarometer represents a powerful tool to reveal the crystallization conditions of clinopyroxene in mafic to intermediate magmas.

It is well known that trace element sensitivity in electron probe microanalysis (EPMA) is limited by intrinsic random variation in the X-ray continuum background and weak signals at low concentrations. The continuum portion of the background is produced by deceleration of the electron beam by the Coulombic field of the specimen atoms. In addition to the continuum, the background also includes interferences from secondary emission lines, \"holes\" in the continuum from secondary Bragg diffraction, non-linear curvature of the wavelength-dispersive spectrometer (WDS) continuum and other background artifacts. Typically, the background must be characterized with sufficient precision (along with the peak intensity of the emission line of interest, to obtain the net intensity for subsequent quantification), to attain reasonable accuracy for quantification of the elements of interest. Traditionally we characterize these background intensities by measuring on either side of the emission line and interpolate the intensity underneath the peak to obtain the net intensity. Instead, by applying the mean atomic number (MAN) background calibration curve method proposed in this paper for the background intensity correction, such background measurement artifacts are avoided through identification of outliers within a set of standards. We divide the analytical uncertainty of the MAN background calibration between precision errors and accuracy errors. The precision errors of the MAN background calibration are smaller than direct background measurement, if the mean atomic number of the sample matrix is precisely known. For a simple matrix and a suitable blank standard, a high-precision blank correction can offset the accuracy component of the MAN uncertainty. Use of the blank-corrected-MAN background calibration can further improve our measurement precision for trace elements compared to traditional off-peak measurements because the background determination is not limited by continuum X-ray counting statistics. For trace element mapping of a simple matrix, the background variance due to major element heterogeneity is exceedingly small and high-precision two-dimensional background correction is possible.

We present new thermometers and barometers based on clinopyroxene–liquid equilibria specific to alkaline differentiated magmas. The new models were calibrated through the regression analyses of experimental datasets obtained by merging phase equilibria experiments from the literature with new experiments performed by using trachytic and phonolitic starting compositions. The regression strategy was twofold: (1) we have tested previous thermometric and barometric equations and recalibrated these models using the new datasets; (2) we have calibrated a new thermometer and a new barometer including only regression parameters that closely describe the compositional variability of the datasets. The new models yield more precise estimates than previous thermometers and barometers when used to predict temperatures and pressures of alkaline differentiated magmas. We have tested the reliability of the new equations by using clinopyroxene–liquid pairs from trachytes and phonolites erupted during major explosive eruptions at the Phlegrean Fields and Mt. Vesuvius (central Italy). The test yielded crystallization conditions comparable to those determined by means of melt and fluid inclusion analyses and phase equilibria studies; this validates the use of the proposed models for precise estimates of crystallization temperatures and pressures in differentiated alkaline magmas. Because these magmas feed some of the most voluminous, explosive, and threatening volcanic eruptions in the world, a better understanding of the environmental conditions of their reservoirs is mandatory and this is now possible with the new models provided here.

Along with temperature, pressure and melt chemistry, magmatic oxygen fugacity (fO2) has an important influence on liquid and solid differentiation trends and melt structure. To explore the effect of redox conditions on mineral stability and mineral-melt partitioning in basaltic systems we performed equilibrium, one-atmosphere experiments on a picrite at 1200–1110 °C with fO2 ranging from NNO-4 log units to air. Clinopyroxene crystallizes from 1180 °C to near-solidus, along with plagioclase, olivine and spinel. Olivine Mg# increases with increasing fO2, eventually reacting to pigeonite. Spinel is absent under strongly reducing conditions. Mineral-melt partition coefficients (D) of redox-sensitive elements (Cr, Eu, V, Fe) vary systematically with fO2 and, in some cases, temperature (e.g. DCr in clinopyroxene). Clinopyroxene sector zoning is common; sectors along a- and b-axes have higher AlIV, AlVI, Cr and Ti and lower Mg than c-axis sectors. In terms of coupled substitutions, clinopyroxene CaTs (MgSi = AlVIAlIV) prevails under oxidized conditions (≥ NNO), where Fe3+ balances the charge, but is limited under reduced conditions. Overall, AlIV is maximised under high temperature, oxidizing conditions and in slowly grown (a–b) sectors. High AlIV facilitates incorporation of REE (REEAlIV = CaSi), but DREE (except DEu) show no systematic dependence on fO2 across the experimental suite. In sector zoned clinopyroxenes enrichment in REE3+ in Al-rich sectors is quantitatively consistent with the greater availability of suitably-charged M2 lattice sites and the electrostatic energy penalty required to insert REE3+ onto unsuitably-charged M2 sites. By combining our experimental results with published data, we explore the potential for trace element oxybarometry. We show that olivine-melt DV, clinopyroxene-melt DV/DSc and plagioclase-melt DEu/DSr all have potential as oxybarometers and we present expressions for these as a function of fO2 relative to NNO. The crystal chemical sensitivity of heterovalent cation incorporation into clinopyroxene and the melt compositional sensitivity of the Eu2+–Eu3+ redox potential limit the use of clinopyroxene-melt and plagioclase-melt, however, olivine-melt DV affords considerable precision and accuracy as an oxybarometer that is independent of temperature, and crystal and melt composition. Variation of DV and DV/DSc with fO2 for olivine and clinopyroxene contains information on redox speciation of V in coexisting melt. By comparing the redox speciation constraints from partitioning to data from Fe-free synthetic systems and XANES spectroscopy of quenched glasses, we show that homogenous equilibria involving Fe and V species modify V speciation on quench, leading to a net overall reduction in the average vanadium valence. Mineral-melt partitioning of polyvalent species can be a useful probe of redox speciation in Fe-bearing systems that is unaffected by quench effects.

Arc magmatism is fundamental to the generation of new continental or island arc crust. However, the mechanisms that add to the chemical complexity of natural calc-alkaline magmas ranging from basaltic to rhyolitic compositions are debated. Differentiation mechanisms currently discussed include magma mixing, assimilation, crustal melting, or (fractional) crystallisation. In this contribution, the differentiation of arc magmas by decompression-driven crystallisation is investigated. We performed a set of equilibrium crystallisation experiments at variable crustal pressures (200–800 MPa) on a hydrous high-Al basalt (3.5 wt.% of H2O in the starting material) with run temperatures varying from near-liquidus conditions (1110 °C) to 900 °C. Oxygen fugacity was buffered at moderately oxidising conditions close to the NNO equilibrium. Combining these novel experiments with previous polybaric fractional crystallisation experiments (Marxer et al., Contrib Mineral Petrol 177:3, 2022) we demonstrate the effects of pressure on the crystallisation behaviour of calc-alkaline magmas with respect to liquid and cumulate lines of descent, mineral chemistry, and phase proportions. Decompression shifts the olivine-clinopyroxene cotectic curve towards melt compositions with higher normative clinopyroxene and enlarges the stability field of plagioclase. This exerts a key control on the alumina saturation index of residual liquids. We argue that near-adiabatic (or near-isothermal) decompression accompanied by dissolution of clinopyroxene entrained during residual melt extraction in the lower crust keeps arc magmas metaluminous during crystallisation-driven differentiation thereby closely reproducing the compositional spread observed for natural arc rocks.

Altered oceanic crust (AOC) is the largest contributor to the subducted sulfur (S) budget and its recycling modulates the redox evolution and S distribution in the mantle. However, the role of AOC in the deep cycling of S remains poorly constrained. Here we probe the primary S isotopes of Cenozoic intraplate basalts in eastern China by investigating sulfide inclusions in magmatic clinopyroxene megacrysts. These basalts were derived from the deep mantle metasomatized by melts derived from recycled AOC but show MORB‐like S isotopes (−0.9–0.9‰), suggesting that AOC‐derived melts transfer negligible sulfate and hardly change the δ34S and redox state of the deeper mantle. This contrasts with the generally high δ34S values of mantle wedge peridotites and primary arc magmas that reflect the slab addition of sulfate, indicating that S species and isotopes released from the subducted slab and associated fO2 are not constant and vary with subduction depth. Plain Language Summary Subduction‐driven recycling of S plays a key role in the redox evolution of the mantle, the formation of ore deposits, and climatic impact through volcanic emission of SO2. Altered oceanic crust (AOC) is the largest contributor to the subducted S budget but the role of AOC in the deep cycling of S remains poorly constrained. Here we present high‐precision in‐situ S isotopes of sulfide inclusions in magmatic clinopyroxene megacrysts captured by Cenozoic intraplate basalts in eastern China, which were sourced from the deep mantle affected by melts derived from recycled AOC within the Pacific slab. The sulfide inclusions are not affected by magmatic differentiation or degassing and reveal the primary δ34S of intraplate basalts and their mantle sources. The mid‐ocean ridge basalts (MORB)‐like δ34S values in deep mantle sources indicate that AOC‐derived melts are mainly sulfide‐saturated and could not modify the δ34S and redox state of the deep mantle significantly. This is in contrast to the high δ34S values of mantle wedge peridotites and primary arc magmas that require the addition of sulfate from the slab or sulfur disproportionation at sub‐arc depth, suggesting that S species and isotopic composition of materials released from the subducted slab vary with subduction depth. Key Points Sulfur isotopes of sulfide inclusions in clinopyroxene megacrysts reveal the primary δ34S of intraplate basalts Deeply subducted oceanic crust hardly changes the δ34S and redox state of the mantle Sulfur species and isotopes released from the subducted oceanic slab and associated fO2 are not constant and vary with subduction depth