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This study describes the application of new synchrotron X‐ray fluorescence (XRF) and diffraction (XRD) microtomographies for the 3‐D visualization of chemical and mineralogical variations in unsectioned extraterrestrial samples. These improved methods have been applied to three compositionally diverse chondritic meteorite samples that were between 300 and 400 μm in diameter, including samples prepared from fragments of the CR2 chondrite LaPaz Icefield (LAP) 02342, H5 chondrite MacAlpine Hills (MAC) 88203, and the CM2 chondrite Murchison. The synchrotron‐based XRF and XRD tomographies used are focused‐beam techniques that measure the intensities of fluorescent and diffracted X‐rays in a sample simultaneously during irradiation by a high‐energy microfocused incident X‐ray beam. Measured sinograms of the emitted and diffracted intensities were then tomographically reconstructed to generate 2‐D slices of XRF and XRD intensity through the sample, with reconstructed pixel resolution of 1–2 μm, defined by the resolution of the focused incident X‐ray beam. For sample LAP 02342, primary mineral phases that were visualized in reconstructed slices using these techniques included isolated grains of α‐Fe, orthopyroxene, and olivine. For our sample of MAC 88203, XRF/XRD tomography allowed visualization of forsteritic olivine as a primary mineral phase, a vitrified fusion crust at the sample surface, identification of localized Cr‐rich spinels at spatial resolutions of several micrometers, and imaging of a plagioclase‐rich glassy matrix. In the sample of Murchison, major identifiable phases include clinoenstatite‐ and olivine‐rich chondrules, variable serpentine matrix minerals and small Cr‐rich spinels. Most notable in the tomographic analysis of Murchison is the ability to quantitatively distinguish and visualize the complex mixture of serpentine‐group minerals and associated tochilinite–cronstedtite intergrowths. These methods provide new opportunities for spatially resolved characterization of sample texture, mineralogy, crystal structure, and chemical state in unsectioned samples. This provides researchers an ability to characterize such samples internally with minimal disruption of sample micro‐structures and chemistry, possibly without the need for sample extraction from some types of sampling and capture media.

 

Walter et al . issue a number of critical comments on our report about the discovery of davemaoite to the end that they believe to show that our results do not provide compelling evidence for the presence of davemaoite in the type specimen and that the hosting diamond had formed in the lithosphere. Their claim is based on a misinterpretation of the diffraction data contained in the paper, an insufficient analysis of the compositional data that disregards the three-dimensional distribution of inclusions, and the arbitrary assumption that Earth’s mantle shows no lateral variations in temperature, inconsistent with state-of-the-art assessments of mantle temperature variations and with their own published results. 

Calcium silicate perovskite, CaSiO 3 , is arguably the most geochemically important phase in the lower mantle, because it concentrates elements that are incompatible in the upper mantle, including the heat-generating elements thorium and uranium, which have half-lives longer than the geologic history of Earth. We report CaSiO 3 -perovskite as an approved mineral (IMA2020-012a) with the name davemaoite. The natural specimen of davemaoite proves the existence of compositional heterogeneity within the lower mantle. Our observations indicate that davemaoite also hosts potassium in addition to uranium and thorium in its structure. Hence, the regional and global abundances of davemaoite influence the heat budget of the deep mantle, where the mineral is thermodynamically stable. 

Abstract This article is dedicated to the occurrence, relevance, and structure of minerals whose formation involves high pressure. This includes minerals that occur in the interior of the Earth as well as minerals that are found in shock-metamorphized meteorites and terrestrial impactites. I discuss the chemical and physical reasons that render the definition of high-pressure minerals meaningful, in distinction from minerals that occur under surface-near conditions on Earth or at high temperatures in space or on Earth. Pressure-induced structural transformations in rock-forming minerals define the basic divisions of Earth's mantle in the upper mantle, transition zone, and lower mantle. Moreover, the solubility of minor chemical components in these minerals and the occurrence of accessory phases are influential in mixing and segregating chemical elements in Earth as an evolving planet. Brief descriptions of the currently known high-pressure minerals are presented. Over the past 10 years more high-pressure minerals have been discovered than during the previous 50 years, based on the list of minerals accepted by the IMA. The previously unexpected richness in distinct high-pressure mineral species allows for assessment of differentiation processes in the deep Earth. 

Beckettite (Ca₂V₆Al₆O₂₀; IMA 2015‐001) is a newly discovered refractory mineral, occurring as micrometer‐sized grains intergrown with hibonite and perovskite, and surrounded by secondary grossular, anorthite, coulsonite, hercynite, and corundum. It occurs within highly altered areas in a V‐rich, Type A Ca‐Al‐rich inclusion (CAI), A‐WP1, from the Allende CV3 carbonaceous chondrite. The type beckettite has an empirical formula of (Ca_1.99Na_0.01)(V³⁺_3.47Al_1.40Ti⁴⁺_0.57Mg_0.25Sc_0.08Fe²⁺_0.04)(Al_5.72Si_0.28)O₂₀, with a triclinic structure in space groupPand cell parametersa= 10.367 Å,b= 10.756 Å,c= 8.895 Å, α = 106.0°, β = 96.0°, γ = 124.7°,V= 739.7 ų, andZ= 2, which leads to a calculated density of 3.67 g cm⁻³. Beckettite’s general formula is Ca₂(V,Al,Ti,Mg)₆Al₆O₂₀and the endmember formula is Ca₂V₆Al₆O₂₀. Beckettite is slightly¹⁶O‐depleted (Δ¹⁷O = −16 ± 2‰) compared to the coexisting hibonite and spinel −24 ± 2‰. Beckettite is a primary high‐temperature mineral resulting from igneous crystallization of an¹⁶O‐rich V‐rich CAI melt together with V‐bearing hibonite, perovskite, burnettite, spinel, and paqueite. Subsequently, beckettite experienced an incomplete isotope exchange with an¹⁶O‐poor aqueous fluid (Δ¹⁷O = −3 ± 2‰) on the Allende parent asteroid.

 

Quartz‐in‐garnet elastic geobarometry (QuiG) pressures in rocks from two Barrovian metamorphic terranes in the western US Cordilleran hinterland exceed pressures determined using chemical thermodynamics by 3–4 kbar. For this study, 135 quartz inclusions from the Funeral Mountains, California, were analysed using QuiG in five garnets from three locations representing metamorphic grades of upper greenschist, lower amphibolite, and middle amphibolite facies. From a second Barrovian terrane, the Wood Hills in northeastern Nevada, 125 quartz inclusions were analysed using QuiG in 14 garnets from a single rock sample metamorphosed to middle amphibolite facies. Pressures determined for rocks in the Funeral Mountains using QuiG and methods rooted in equilibrium thermodynamics yielded consistent pressure differences between locations, but QuiG pressures are higher. Similarly, QuiG pressures determined for rocks in the Wood Hills are higher than pressures determined by equilibrium thermodynamic approaches. Possible explanations for the pressure differences include garnet compositions not reflecting equilibrium, sources of error in thermodynamic calculations such as thermodynamic data or a‐X models, or an unknown source of systematic error that causes QuiG to overestimate pressures of entrapment. To test Raman spectroscopy's ability to reproduce inclusion pressures, pressures were calculated using Raman spectroscopy and synchrotron X‐ray diffraction, which yielded consistent pressures and support the use of the single mode‐shift of the 464 cm⁻¹band of quartz for geobarometry, which simplifies the method by assuming hydrostatic compression of quartz. These results are compared with pressures obtained using Grüneisen tensors and show consistency between these different approaches.

 

A new high‐pressure silicate, (Mg,Fe,Si)₂(Si,□)O₄with a tetragonal spinelloid structure, was discovered within shock melt veins in the Tenham and Suizhou meteorites, two highly shocked L6 ordinary chondrites. Relative to ringwoodite, this phase exhibits an inversion of Si coupled with intrinsic vacancies and a consequent reduction of symmetry. Most notably, the spinelloid makes up about 30–40 vol% of the matrix of shock veins with the remainder composed of a vitrified (Mg,Fe)SiO₃phase (in Tenham) or (Mg,Fe)SiO₃‐rich clinopyroxene (in Suizhou); these phase assemblages constitute the bulk of the matrix in the shock veins. Previous assessments of the melt matrices concluded that majorite and akimotoite were the major phases. Our contrasting result requires revision of inferred conditions during shock melt cooling of the Tenham and Suizhou meteorites, revealing in particular a much higher quench rate (at least 5 × 10³ K s⁻¹) for veins of 100–500 μm diameter, thus overriding formation of the stable phase assemblage majoritic garnet plus periclase.