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1. Origin of Compositional Gradients with Temperature in the High-SiO2 Rhyolite Portion of the Bishop Tuff: Constraints on Mineral–Melt–Fluid Reactions in the Parental Mush

   https://doi.org/10.1093/petrology/egab087  

   Jolles, Jameson S ; Lange, Rebecca A ( December 2021 , Journal of Petrology)

   Abstract The Bishop Tuff (BT), erupted from the Long Valley caldera in California, displays two types of geochemical gradients with temperature: one is related to magma mixing, whereas the other is found in the high-SiO2 rhyolite portion of the Bishop Tuff and is characterized by twofold or lower concentration variations in minor and trace elements that are strongly correlated with temperature. It is proposed that the latter zonation, which preceded phenocryst growth, developed as a result of mineral–melt partitioning between interstitial melt and surrounding crystals in a parental mush, from which variable melt fractions were segregated. To test this hypothesis, trends of increasing vs decreasing element concentrations with temperature (as a proxy for melt fraction), obtained from published data on single-clast pumice samples from the high-SiO2 rhyolite portion of the Bishop Tuff, were used to infer their relative degrees of incompatibility vs compatibility between crystals and melt in the parental mush. Relative compatibility values (RCVi) for all elements i, defined as the concentration slope with temperature divided by average concentration, are shown to be linearly correlated with their respective bulk partition coefficients (bulk Di). Mineral–melt partition coefficients from the literature were used to constrain the average stoichiometry of the crystallization/melting reaction in the parental mush: 32 % quartz + 34 % plagioclase + 31 % K-feldspar + 1·60 % biotite + 0·42 % titanomagnetite + 0·34 % ilmenite + 0·093 % allanite + 0·024 % zircon + 0·025 % apatite = 100 % liquid. The proportions of tectosilicates in the reaction (i.e. location of eutectic) are consistent with depths of melt segregation of ~400–550 MPa and an activity of H2O of ~0·4–0·6. Temperatures of <770–780 °C are constrained by allanite in the reaction. Evidence that a fluid phase was present in the parental mush is seen in the decreasing versus increasing H2O and CO2 contents with temperature in the segregated interstitial melt that formed the high-SiO2 rhyolite portion of the Bishop Tuff. The presence of an excess fluid phase, which strongly partitions CO2 relative to the melt, is required to explain the compatible behavior of CO2, whereas the fluid abundance must have been low to explain the incompatible behavior of H2O. Calculated degassing paths for interstitial melts, which segregated from the parental mush and ascended to shallower depths to grow phenocrysts, match published volatile analyses in quartz-hosted melt inclusions and constrain fluid abundances in the mush to be ≤1 wt%. The source of volatiles in the parental mush, irrespective of whether it formed by crystallization or partial melting, must have been primarily from associated basalts, as granitoid crust is too volatile poor. Approximately twice as much basalt as rhyolite is needed to provide the requisite volatiles. The determination of bulk Di for several elements gives the bulk composition of the parental leucogranitic mush and shows that it is distinct from Mesozoic Sierran arc granitoids, as expected. Collectively, the results from this study provide new constraints for models of the complex, multi-stage processes throughout the Plio-Quaternary, involving both mantle-derived basalt and pre-existing crust, that led to the origin of the parental body to the Bishop Tuff. 

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2. Post-melting oxidation of highly primitive basalts from the southern Andes

   https://doi.org/10.1016/j.gca.2020.01.042  

   Tassara, S. ( January 2020 , Geochimica et cosmochimica acta)

   null (Ed.)

   The oxygen fugacity (fO2) of the Earth’s upper mantle and its melting products is an important parameter in the geochemical evolution of arc magmas and their connection with the continental crustal construction and growth. Several works have focused on the fO2 of peridotite xenoliths, primitive melts in relatively young arc settings, and mid-ocean ridge basalts (MORB) but few studies have attempted to examine the early redox history of primitive magmas in mature arcs. Hence, our understanding of the nature and evolution of fO2 during the subduction cycle remains limited. Here, we investigate the basaltic tephra from the Los Hornitos monogenetic cones in central-southern Chile, which are among the most primitive materials reported in the Southern Andes (olivine Mg#
   92.5, and Ni
   5000 mgg1). These features offer a unique opportunity to explore the fO2 conditions below the Andean arc by studying olivine phenocrysts and their contained crystal and melt inclusions. We integrated EPMA, LA-ICP-MS, and m-XANES analyses to constrain the redox conditions recorded in the basaltic tephra by three different and self-reliant methods. First, we determined the fO2 based on the olivine-spinel equilibrium, yielding average values DFMQ + 1.3 ± 0.4 (1r). Second, we constrained the fO2 conditions of melt inclusions using Fe m-XANES data and the redox dependent olivine-melt vanadium partitioning. After correcting for post-entrapment crystallization and diffusive iron loss, the Fe m-XANES data indicate that the melt inclusions were trapped in average at DFMQ +2.5 ± 0.5 (1r). Results using the olivine-melt vanadium partitioning oxybarometer in melt inclusions are in agreement with Fe m-XANES data, yielding average DFMQ values of +2.6 ± 0.3 (1r). In order to test the potential effects of other postentrapment modifications of the melt inclusions that could have affected the fO2 prior to eruption, we assessed the residence time of these magmas using Mg-Fe interdiffusion modelling in olivine. The short residence times (<200 days) compared to vanadium re-equilibration models strongly suggest that the melt inclusions preserve the prevailing fO2 conditions during their entrapment. Correlations between melt inclusions major element composition and their fO2 determined by Fe m-XANES, as well as V/Sc modelling reveal a case of post-melting oxidation of the LHC magmas. We argue that primitive arc magmas behave as an open system with respect to fO2 during their early geochemical evolution. Our data indicate a complex fO2 early history of primitive melts in the southern Andes and provide a cautionary note on the direct extrapolation of primitive melts fO2 values to that of their mantle source. 

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3. Valences of Ti, Cr, and V in Apollo 17 high‐Ti and very low‐Ti basalts and implications for their formation

   https://doi.org/10.1111/maps.13123  

   Simon, Steven B. ; Sutton, Stephen R. ( June 2018 , Meteoritics & Planetary Science)

   To assess the variability of redox states among mare basalt source regions, investigation of the valence of Ti, Cr, and V and the coordination environment of Ti in pyroxene and olivine in lunar rocks viaXANES(X‐ray absorption near‐edge structure) spectroscopy has been extended to Apollo 17 basalts: two high‐Ti (70017 and 74275) hand samples, and three very low‐Ti (70006,371, 70007,289B, and 70007,296) basalt fragments from the Apollo 17 deep drill core. Valences of Ti in pyroxene of both suites range from 3.6 to 4, or from 40% to 0% Ti³⁺, averaging 15–20% Ti³⁺. Assuming Ti³⁺is more compatible in pyroxene than Ti⁴⁺, then even lower Ti³⁺proportions are indicated for the parental melts. TheVLTpyroxene exhibits a slightly wider range of V valences (2.57–2.96) than the high‐Ti pyroxene (2.65–2.86) and a much wider range of Cr valences (2.32–2.80 versus 2.68–2.86); Cr is generally reduced inVLTpyroxene compared to high‐Ti pyroxene. Valences of Ti and Cr inVLTpyroxene become less reduced with increasing FeO contents, possibly indicating change in oxygen fugacity during crystallization. Olivine in all samples has very low (<20%) proportions of Ti³⁺, with no Ti³⁺and higher proportions of Ti in tetrahedral coordination in theVLTs than in the high‐Ti basalts. Olivine in 74275, including that in a dunite clast, has much higher proportions of Cr²⁺than the pyroxene in that sample, consistent with previous studies indicating that the olivine grains in this sample are xenocrysts and possibly indicating oxidation just prior to pyroxene crystallization. Results for this sample, theVLTs, and previously studied Apollo 14 and 15 basalts all indicate that mare magmas were in reducing environments at depth, as recorded in early crystallization products, and that later, presumably shallower environments, were relatively oxidizing; single, characteristicfO₂s of formation cannot be assigned to these samples. A process likely to account for this feature seen in multiple samples is loss by degassing of a reducing, H‐rich vapor (probably H₂) during ascent and/or eruption, causing oxidation of the residual melt, recorded in relatively late‐crystallized pyroxene.

    

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4. Using Trace Elements in Olivine Phenocrysts to Test Models of Melt Differentiation at Convergent Margins: Constraints from Times Series in Monogenetic Volcanoes in the Central Transmexican Volcanic Belt

   Fleming, W. ; Straub, S.M. ; Gomez-Tuena, A. ; Espinasa-Pereña, R. ; Bindeman, I.N. ; Stuart, F.M. ; Batanova, V.G. ( December 2019 , AGU Fall Meeting 2019)

   Models of subduction zone magmatism ascribe the andesitic composition of arc magmas to crustal processes, such as crustal assimilation and/or fractional crystallization, that basaltic mantle melts experience during their ascent through the upper plate crust. However, results from time series study of olivine-phyric high-Nb basalts and basaltic andesites from two monogenetic arc volcanoes (V. Chichinautzin and Texcal Flow) that are constructed on the ~45 km thick continental basement of the central Transmexican Volcanic Belt (TMVB) are inconsistent with this model. Instead, ratios of radiogenic isotope and incompatible trace elements suggest that these volcanoes were constructed through multiple individual melt batches ascending from a progressively changing mantle source. Moreover, the high Ni contents of the olivine phenocrysts, together with their high mantle-like 3He/4Heoliv =7-8 Ra with high crustal δ18O oliv = +5.5 to +6.5‰ (n=12) point to the presence of secondary ‘reaction pyroxenites’ in the mantle source that create primary silicic arc magmas through melt-rock reaction processes in the mantle [1, 2] . Here we present additional trace element concentration of the high-Ni olivines by electron microprobe (Mn, Ca) and laser-ablation ICPMS (Li, Cr and V) analysis in order to test this model. Olivine Li (2-7 ppm) and Mn (1170- 2810 ppm) increase with decreasing fosterite (Fo89 to Fo75), while Cr (29-364 ppm), V (4-11 ppm) and Ca (825-2390 ppm) decrease. Quantitative modeling shows that these trends in their entirety cannot be controlled by fractional crystallization under variable melt water H2O or oxygen fugacity (fO2), or co-crystallization of Cr-spinel. Instead, the variations support the existence of compositionally distinct melt batches during earliest melt evolution. Moreover, the trace element trends are qualitatively consistent with a model of progressive source depletion by serial melting (shown in olivine Ca, V and Cr) that is triggered by the repetitive addition of silicic slab components (shown by olivine Li). These findings suggest mantle source variations are not eliminated despite the thick crust these magmas pass during ascent. [1] Straub et al. (2013) J Petrol 54 (4): 665-701; [2] Straub et al. (2015) Geochim Cosmochim Acta 166: 29-52. 

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5. Eu speciation in apatite at 1 bar: An experimental study of valence-state partitioning by XANES, lattice strain, and Eu/Eu* in basaltic systems

   https://doi.org/10.2138/am-2022-8388  

   Tailby, Nicholas D. ; Trail, Dustin ; Watson, Bruce ; Lanzirotti, Antonio ; Newville, Matthew ; Wang, Yanling ( May 2023 , American Mineralogist: Journal of Earth and Planetary Materials)

   Abstract Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (fO2; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing fO2. Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: ( E u 3 + ∑ E u ) Apatite  = 1 1 + 10 - 0.10 ± 0.01 × l o g ⁡ ( f o 2 ) - 1.63 ± 0.16 . The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: ( Eu E u * ) D Sm × Gd = 1 1 + 10 - 0.15 ± 0.03 × l o g ⁡ ( f o 2 ) - 2.46 ± 0.41 . The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by ( Eu E u * ) D lattice strain = 1 1 + 10 - 0.20 ± 0.03 × l o g ⁡ ( f o 2 ) - 3.03 ± 0.42 . The Eu valence-state partitioning techniques based on (Sm×Gd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or Sm×Gd techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 ± 0.25 (as determined by lattice strain) to DLu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor fO2. These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts. 

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