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1. Effects of composition and pressure on electronic states of iron in bridgmanite

   https://doi.org/10.2138/am-2020-7309  

   Dorfman, Susannah M. ; Potapkin, Vasily ; Lv, Mingda ; Greenberg, Eran ; Kupenko, Ilya ; Chumakov, Aleksandr I. ; Bi, Wenli ; Alp, E. Ercan ; Liu, Jiachao ; Magrez, Arnaud ; et al ( July 2020 , American Mineralogist)

   Abstract Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10–50% Fe/total cations, 0–25% Al/total cations, 12–100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4–3.1 andmore »3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.« less
2. Thermal expansivity, heat capacity and bulk modulus of the mantle

   https://doi.org/10.1093/gji/ggab394  

   Stixrude, Lars ; Lithgow-Bertelloni, Carolina ( October 2021 , Geophysical Journal International)

   SUMMARY We derive exact expressions for the thermal expansivity, heat capacity and bulk modulus for assemblages with arbitrarily large numbers of components and phases, including the influence of phase transformations and chemical exchange. We illustrate results in simple two-component, two-phase systems, including Mg–Fe olivine-wadsleyite and Ca–Mg clinopyroxene-orthopyroxene and for a multicompontent model of mantle composition in the form of pyrolite. For the latter we show results for the thermal expansivity and heat capacity over the entire mantle pressure–temperature regime to 40 GPa, or a depth of 1000 km. From the thermal expansivity, we derive a new expression for the phase buoyancy parameter that is valid for arbitrarily large numbers of phases and components and which is defined at every point in pressure–temperature space. Results reveal regions of the mantle where the magnitude of the phase buoyancy parameter is larger in magnitude than for those phase transitions that are most commonly included in mantle convection simulations. These regions include the wadsleyite to garnet and ferropericlase transition, which is encountered along hot isentropes (e.g. 2000 K potential temperature) in the transition zone, and the ferropericlase and stishovite to bridgmanite transition, which is encountered along cold isentropes (e.g. 1000 K potential temperature) in the shallow lowermore »mantle. We also show the bulk modulus along a typical mantle isentrope and relate it to the Bullen inhomogeneity parameter. All results are computed with our code HeFESTo, updates and improvements to which we discuss, including the implementation of the exact expressions for the thermal expansivity, heat capacity and bulk modulus, generalization to allow for pressure dependence of non-ideal solution parameters and an improved numerical scheme for minimizing the Gibbs free energy. Finally, we present the results of a new global inversion of parameters updated to incorporate more recent results from experiment and first principles theory, as well as a new phase (nal phase), and new species: Na-majorite and the NaAlO2 end-member of ferropericlase.« less
3. Mineralogy of the deep lower mantle in the presence of H2O

   https://doi.org/10.1093/nsr/nwaa098  

   Hu, Qingyang ; Liu, Jin ; Chen, Jiuhua ; Yan, Bingmin ; Meng, Yue ; Prakapenka, Vitali B ; Mao, Wendy L ; Mao, Ho-kwang ( May 2020 , National Science Review)

   Abstract Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800–2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2Hx (x≤1) phase. The (Mg, Fe)O2Hx has the pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2Hx is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+, and H+). This important phase has a number of far-reaching implications including the extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM, and an internal source for modulating oxygen in the atmosphere.
4. Detrital Chromite from Jack Hills, Western Australia: Signatures of Metamorphism and Constraints on Provenance

   https://doi.org/10.1093/petrology/egab052  

   Staddon, Leanne G ; Parkinson, Ian J ; Cavosie, Aaron J ; Elliott, Tim ; Valley, John W ; Fournelle, John ; Kemp, Anthony I ; Shirey, Steven B ( December 2021 , Journal of Petrology)

   Abstract Detrital chromites are commonly reported within Archean metasedimentary rocks, but have thus far garnered little attention for use in provenance studies. Systematic variations of Cr–Fe spinel mineral chemistry with changing tectonic setting have resulted in the extensive use of chromite as a petrogenetic indicator, and so detrital chromites represent good candidates to investigate the petrogenesis of eroded Archean mafic and ultramafic crust. Here, we report the compositions of detrital chromites within fuchsitic (Cr-muscovite rich) metasedimentary rocks from the Jack Hills, situated within the Narryer Terrane, Yilgarn Craton, Western Australia, which are geologically renowned for hosting Hadean (>4000 Ma) zircons. We highlight signatures of metamorphism, including highly elevated ZnO and MnO, coupled with lowered Mg# in comparison with magmatic chromites, development of pitted domains, and replacement of primary inclusions by phases that are part of the metamorphic assemblages within host metasedimentary rocks. Oxygen isotope compositions of detrital chromites record variable exchange with host metasedimentary rocks. The variability of metamorphic signatures between chromites sampled only meters apart further indicates that modification occurred in situ by interaction of detrital chromites with metamorphic fluids and secondary mineral assemblages. Alteration probably occurred during upper greenschist to lower amphibolite facies metamorphism and deformation of host metasedimentarymore »rocks at ∼2650 Ma. Regardless of metamorphic signatures, sampling location or grain shape, chromite cores yield a consistent range in Cr#. Although other key petrogenetic indices, such as Fe2O3 and TiO2 contents, are complicated in Jack Hills chromites by mineral non-stoichiometry and secondary mobility within metasedimentary rocks, we demonstrate that the Cr# of chromite yields significant insights into their provenance. Importantly, moderate Cr# (typically 55–70) precludes a komatiitic origin for the bulk of chromites, reflecting a dearth of komatiites and intrusive equivalents within the erosional catchment of the Jack Hills metasedimentary units. We suggest that the Cr# of Jack Hills chromite fits well with chromites derived from layered intrusions, and that a single layered intrusion may account for the observed chemical compositions of Jack Hills detrital chromites. Where detailed characterization of key metamorphic signatures is undertaken, detrital chromites preserved within Archean metasedimentary rocks may therefore yield valuable information on the petrogenesis and geodynamic setting of poorly preserved mafic and ultramafic crust.« less
5. Deformation and Strength of Mantle Relevant Garnets: Implications for the Subduction of Basaltic-rich Crust

   https://doi.org/10.2138/am-2021-7587  

   Vennari, C ; Lin, F ; Kunz, M ; Akaogi, M ; Miyagi, L ; Williams, Q ( July 2021 , American Mineralogist)

   null (Ed.)

   Garnet is an important mineral phase in the upper mantle as it is both a key component in bulk mantle rocks, and a primary phase at high-pressure within subducted basalt. Here, we focus on the strength of garnet and the texture that develops within garnet during accommodation of differential deformational strain. We use X-ray diffraction in a radial geometry to analyze texture development in situ in three garnet compositions under pressure at 300 K: a natural garnet (Prp60Alm37) to 30 GPa, and two synthetic majorite-bearing compositions (Prp59Maj41 and Prp42Maj58) to 44 GPa. All three garnets develop a modest (100) texture at elevated pressure under axial compression. Elasto-viscoplastic self-consistent (EVPSC) modeling suggests that two slip systems are active in the three garnet compositions at all pressures studied: {110}<1-21 11> and {001}<110>. We determine a flow strength of ~5 GPa at pressures between 10 to 15 GPa for all three garnets; these values are higher than previously measured yield strengths measured on natural and majoritic garnets. Strengths calculated using the experimental lattice strain differ from the strength generated from those calculated using EVPSC. Prp67Alm33, Prp59Maj41 and Prp42Maj58 are of comparable strength to each other at room temperature, which indicates that majorite substitutionmore »does not greatly affect the strength of garnets. Additionally, all three garnets are of similar strength as lower mantle phases such as bridgmanite and ferropericlase, suggesting that garnet may not be notably stronger than the surrounding lower mantle/deep upper mantle phases at the base of the upper mantle.« less