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Fe2O3 produced in a deep magma ocean in equilibrium with core-destined alloy sets the early redox budget and atmospheric composition of terrestrial planets. Previous experiments (≤28 gigapascals) and first-principles calculations indicate that a deep terrestrial magma ocean produces appreciable Fe3+ but predict Fe3+/ΣFe ratios that conflict by an order of magnitude. We present Fe3+/ΣFe of glasses quenched from melts equilibrated with Fe alloy at 38 to 71 gigapascals, 3600 to 4400 kelvin, analyzed by synchrotron Mössbauer spectroscopy. These indicate Fe3+/ΣFe of 0.056 to 0.112 in a terrestrial magma ocean with mean alloy-silicate equilibration pressures of 28 to 53 gigapascals, producing sufficient Fe2O3 to account for the modern bulk silicate Earth redox budget and surficial conditions near or more oxidizing than the iron-wüstite buffer, which would stabilize a primitive CO-and H2O-rich atmosphere. [ABSTRACT FROM AUTHOR]

Some seismic models derived from tomographic studies indicate elevated shear-wave velocities (≥4.7 km/s) around 120–150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high-Vs candidates to explain the observed velocities, but matching the high shear-wave velocities requires either a large proportion of eclogite (>50 vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of ≤20 vol.% eclogite and ~2 vol.% diamond may account for high shear-wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.

We present new models for the activity of iron (γFe) in solid face-centered cubic (fcc) and liquid FePt alloy at high temperature and pressure to facilitate their use as sliding buffer redox sensors under extreme conditions. Numerous experimental studies of γFe in FePt alloy at 100 kPa have produced a wide spread of values. By favoring high-temperature studies that are more likely to have produced equilibrium measurement and excluding experiments for compositions and temperatures that probably encountered ordered or unmixed low-temperature phases, we regress an asymmetric Margules activity–composition model with parameters WFePtfcc=-121.5±2.1 kJ mol -1 and WPtFefcc=-93.3±4.3 kJ mol -1. These values are close to the widely used model of Kessel et al. (2001), but for Pt-rich compositions they predict larger Fe activities and correspondingly more reduced oxygen fugacities. Activity–composition relations in liquid FePt are calibrated from direct measurements of activities and, most sensitively, from the trace of the Fe–Pt liquidus. Together, these yield asymmetric Margules parameters of WFePtliq=-124.5 kJ mol -1 and WPtFeliq=-94.0 kJ mol -1. The effects of pressure on both fcc and liquid FePt alloy are considered from excess-volume relations. Both solid and liquid alloy display significant positive excess volumes of mixing. Extraction of the excess volume of mixing for fcc FePt alloy requires filtering data for ordered low-temperature phases and corrections for the effects of magnetostriction on Fe-rich compositions which exhibit "Invar" behavior. Applied at high temperatures and pressures, both solid and liquid FePt alloys have strongly negative deviations from ideality at low pressure, which become closer to ideal at high pressure. These models provide a provisional basis for the calculation of aFe in high-temperature, high-pressure experiments that, when combined with estimates of aFeO , allow characterization of fO2 under conditions relevant to magma oceans, core formation, and differentiation processes in the lower mantle of Earth or on other terrestrial planets. Improvements in these models require new constraints on the equation of state of FePt fcc alloy and documentation of the high-pressure melting relations in the system Fe–Pt. [ABSTRACT FROM AUTHOR]

Magma mixing is a widespread magmagenic process. However, its significance in the formation of ultrapotassic magmas has been largely overlooked so far as they are commonly thought to originate directly from the mantle and ascend rapidly through the crust. The Hezhong ultrapotassic lavas in Western Yunnan (SW China) are (basaltic) trachy-andesitic in composition. These rocks display porphyritic textures with olivine, clinopyroxene (Cpx), spinel, and phlogopite occurring as both phenocryst and glomerocryst. Disequilibrium textures and complex zonation of crystals are commonly observed. Specifically, based on the textural and compositional characteristics, olivines can be classified into three different populations: two populations are characterized by highly to moderately magnesian olivines with normal chemical core-rim zonation (Fo~94–86 to Fo~89–79 and Fo~91–89 to Fo~86–84, respectively). The third population lacks obvious crystal zonation, but individual crystals exhibit some compositional variety at lower Fo contents (Fo83–76). Similarly, four populations of Cpx and two populations of spinel phenocrysts are recognized in terms of texture and composition. Notably, Cpx with reverse zoning contains a 'green-core' surrounded by a colourless mantle and rim. Hence, based on the variations of mineral assemblage, types of inclusions, and chemical compositions, all phenocryst/glomerocryst minerals can be divided into three groups. Mineral Group I (MG I) consists of high Fo cores of olivine, cores of the zoned spinel, and phlogopite. MG II only includes the green cores of reversed zoned Cpx (green-core Cpx), and MG III is composed of micro phenocrysts without obvious zoning and rims of large phenocrysts. Comparing these mineral groups with relevant minerals occurring in typical temporally and spatially associated igneous rocks, we suggest that the MG I and II could have been derived from magmas with compositions resembling an olivine lamproite and a trachyte, respectively. The overall bulk-rock geochemical and isotopic features of Hezhong lavas also agree with a mixing process between these two endmembers. Hence, we infer that mixing between these two magmas played a key role in the petrogenesis of the ultrapotassic Hezhong lavas and that the MG III crystallized from the mixed magmas. Our study highlights the complex formation of ultrapotassic magmas inferring that caution must be taken when using bulk chemical magma compositions are to deduce source signatures. [ABSTRACT FROM AUTHOR]

A rapid sample preparation procedure is described to determine trace element compositions of peridotites using LA‐ICP‐MS. Peridotite powders were fused with albite in a molybdenum–graphite assembly to obtain homogeneous glasses. Best conditions for the fusion procedure (heating at 1500–1550 °C for 10–15 min with a sample‐to‐flux ratio of 1:2) were constrained with melting experiments on two USGS reference materials, PCC‐1 and DTS‐2B. Mass fractions of first series transition elements, Ba and Pb, in quenched glasses of PCC‐1 and DTS‐2B are consistent with published data within 10% RSD. Three spinel peridotite xenoliths from eastern China were analysed following both our method and conventional solution ICP‐MS. Compared with solution ICP‐MS, the relative deviations of our method for most elements were within 10%, while for the REE, Ta, Pb, Th and U, the relative deviations were within 20%. In particular, volatile elements (e.g., Pb and Zn) are retained in the glass. Compared with conventional wet chemistry digestion, our method is faster. Additional advantages are complete sample fusion, especially useful for samples with acid‐resistant minerals (spinel and rutile), and long‐term conservation of glasses allowing unlimited repeated measurements with microbeam techniques. The same approach can be used for analyses of other mantle rocks, such as eclogites and pyroxenites. Key Points: A rapid sample fusion method to determine trace element compositions of peridotites using LA‐ICP‐MS.First series transition element mass fractions for PCC‐1, DTS‐2B and three natural peridotites agree well with published data and solution ICP‐MS analysis.Our approach can be applied to other mantle rocks containing acid‐resistant minerals. [ABSTRACT FROM AUTHOR]

To evaluate the accuracy of Fe3+ and Fe2+ ratios in silicate glasses determined by Mössbauer spectroscopy, we examine in detail the temperature (47-293 K) of Mössbauer spectra for two andesitic glasses, one quenched at 1 atm, 1400 °C (VF3) and the other at 3.5 GPa, 1600 °C (M544). Variabletemperature Mössbauer spectra of these two glasses are used to characterize the recoilless fraction, f, by two different methods-a relative method (RM) based on the temperature dependence of the ratios of Fe3+ and Fe2+ Mössbauer doublets and the second based on the temperature dependence of the center shift (CS) of the doublets. The ratio of the recoilless fractions for Fe3+ and Fe2+, CT, can then be used to adjust the observed area of the Mössbauer doublets into the Fe3+/∑Fe ratio in the sample. We also evaluated the contributions of non-paramagnetic components to the Fe in the glasses by determining the influence of applied magnetic field on sample magnetization. Finally, for the VF3 glass, we determined the Fe3+/∑Fe independently by wet chemical determination of the FeO content combined with careful electron microprobe analyses of total Fe. Recoilless fractions determined with the CS method (CSM) are significantly smaller than those determined with the relative method and suggest larger corrections to room-temperature Fe3+/∑Fe ratios. However, the RM determinations are believed to be more accurate because they depend less on the assumption of the Debye harmonic model and because they produce more nearly temperature-independent estimates of Fe3+/∑Fe ratios. Non-linear responses of sample magnetizations to applied magnetic fields indicate that the glasses contain a small (0.4-1.1% for VF3) superparamagnetic component that is most likely to be nanophase precipitates of (Fe,Mg)Fe2O4 oxide, but corrections for this component have negligible influence on the total Fe3+/∑Fe determined for the glass. For the VF3 glass, the Fe3+/∑Fe produced by uncorrected room-temperature Mössbauer spectroscopy [0.685 ± 0.014 in two standard deviation (2σ)] agrees within 3% of that determined by wet chemistry (0.666 ± 0.030 in 2σ). The Fe3+/∑Fe corrected for recoilless fraction contributions is 0.634 ± 0.078(2σ), which is 7.5% lower than the uncorrected room-temperature ratio, but also agrees within 5% of wet chemical ratio. At least for this andesitic glass, the room-temperature determination of Fe3+/∑Fe is accurate within analytical uncertainty, but room-temperature Mössbauer determinations of Fe3+/∑Fe are always systematically higher compared to recoilless-fraction corrected ratios. [ABSTRACT FROM AUTHOR]