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Earth's lower mantle most likely mainly consists of ferropericlase, bridgmanite, and a CaSiO3- phase in the perovskite structure. If separately trapped in diamonds, these phases can be transported to Earth's surface without reacting with the surrounding mantle. Although all inclusions will remain chemically pristine, only ferropericlase will stay in its original crystal structure, whereas in almost all cases bridgmanite and CaSiO3-perovskite will transform to their lower-pressure polymorphs. In the case of perovskite structured CaSiO3, the new structure that is formed is closely related to that of walstromite. This mineral is now approved by the IMA commission on new minerals and named breyite. The crystal structure is triclinic (space group: P 1) with lattice parameters a0 = 6.6970(4) Å, b0 = 9.2986(7) Å, c0 = 6.6501(4) Å, α = 83.458(6)°, β = 76.226(6)°, γ = 69.581(7)°, and V = 376.72(4) Å. The major element composition found for the studied breyite is Ca3.01(2)Si2.98(2)O9. Breyite is the second most abundant mineral inclusion after ferropericlase in diamonds of super-deep origin. The occurrence of breyite has been widely presumed to be a strong indication of lower mantle (>670 km depth) or at least lower transition zone (>520 km depth) origin of both the host diamond and the inclusion suite. In this work, we demonstrate through different formation scenarios that the finding of breyite alone in a diamond is not a reliable indicator of the formation depth in the transition zone or in the lower mantle and that accompanying paragenetic phases such as ferropericlase together with MgSiO3 are needed. [ABSTRACT FROM AUTHOR]

We report the first natural occurrence and single-crystal X-ray diffraction qstudy of the Fe-analog of wadsleyite [a = 5.7485(4), b = 11.5761(9), c = 8.3630(7) Å, V = 556.52(7) A3; space group Imma], spinelloid-structured Fe2SiO4, a missing phase among the predicted high-pressure polymorphs of ferroan olivine, with the composition (Fe2+ 1.10Mg0.80Cr3+ 0.04Mn2+ 0.02Ca0.02Al0.02Na0.01)Σ2.01(Si0.97Al0.03)Σ1.00O4. The new mineral was approved by the International Mineralogical Association (No. 2018-102) and named asimowite in honor of Paul D. Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at the California Institute of Technology. It was discovered in rare shock-melted silicate droplets embedded in Fe,Ni-metal in both the Suizhou L6 chondrite and the Quebrada Chimborazo (QC) 001 CB3.0 chondrite. Asimowite is rare, but the shock-melted silicate droplets are very frequent in both meteorites, and most of them contain Fe-rich wadsleyite (Fa30 -- 45). Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)2SiO4 at high temperature (>1800 K) and pressure are clearly needed. This may also have a significant impact on the temperature and chemical estimates of the mantle's transition zone in Earth. [ABSTRACT FROM AUTHOR]

"Super-deep" diamonds are thought to have a sub-lithospheric origin (i.e., below ~300 km depth) because some of the mineral phases entrapped within them as inclusions are considered to be the products of retrograde transformation from lower-mantle or transition-zone precursors. CaSiO3-walstromite, the most abundant Ca-bearing mineral inclusion found in super-deep diamonds, is believed to derive from CaSiO3-perovskite, which is stable only below ~600 km depth, although its real depth of origin is controversial. The remnant pressure (Pinc) retained by an inclusion, combined with the thermoelastic parameters of the mineral inclusion and the diamond host, allows calculation of the entrapment pressure of the diamond-inclusion pair. Raman spectroscopy, together with X-ray diffraction, is the most commonly used method for measuring the Pinc without damaging the diamond host. In the present study we provide, for the first time, a calibration curve to determine the Pinc of a CaSiO3-walstromite inclusion by means of Raman spectroscopy without breaking the diamond. To do so, we performed high-pressure micro-Raman investigations on a CaSiO3-walstromite crystal under hydrostatic stress conditions within a diamond-anvil cell. We additionally calculated the Raman spectrum of CaSiO3-walstromite by ab initio methods both under hydrostatic and non-hydrostatic stress conditions to avoid misinterpretation of the results caused by the possible presence of deviatoric stresses causing anomalous shift of CaSiO3-walstromite Raman peaks. Last, we applied single-inclusion elastic barometry to estimate the minimum entrapment pressure of a CaSiO3-walstromite inclusion trapped in a natural diamond, which is ~9 GPa (~260 km) at 1800 K. These results suggest that the diamond investigated is certainly sub-lithospheric and endorse the hypothesis that the presence of CaSiO3-walstromite is a strong indication of super-deep origin. [ABSTRACT FROM AUTHOR]

Secondary Ca,Fe-rich minerals (CFM; diopside-hedenbergite, andradite, wollastonite, kirschteinite) are widespread in the Allende carbonaceous chondrite. About 90 vol% of the total CaO content of the Allende matrix is concentrated in CFM. The conditions and environment (solar nebular or asteroidal) of this alteration are still matters of controversial scientific discussion. Here we present evidence for late-stage, high-temperature processes recorded in Ca,Fe-rich rims around Allende dark inclusions 3529 and IV-1 studied using scanning (SEM) and transmission electron microscopy (TEM), and electron probe microanalysis. The rims show a multilayered structure, with the outermost layer intergrown with the matrix olivines and chondrule fragments of the Allende host, indicative of in situ formation. The central portion of the rim around IV-1 contains several wollastonite polytypes (a polysynthetically twinned polytype of pseudowollastonite, wollastonite-2M, and wollastonite-1T) and an intergrowth of hedenbergite-PM (primitive monoclinic) and augite-hedenbergitess. These findings require temperatures above 1000 °C and fast cooling rates after formation of the central part of the rim. The close occurrences of three different polymorphs of wollastonite suggest that this process was highly localized and may have resulted from shock metamorphism. [ABSTRACT FROM AUTHOR]

Osmium-rich Pt group element (PGE) alloys occur worldwide in association with chromite in ultramafic (peridotite) complexes. It has been suggested that these Os-rich alloys formed under extreme P-T conditions in the lowermost mantle, before the metallic core of the Earth formed, or later, in the outer core, and have been transported to the upper mantle as xenoliths in deep-rooted mantle plumes. Our investigation of syn- and pregenetic inclusions (including silicate and chromite) found in Os-rich alloys from peridotites in northern California and southwest Oregon yield no evidence that these alloys formed under extreme P-T conditions. Instead these inclusions point to a hydrous magmatic origin in the shallow upper mantle, most likely in an arc-environment. Indeed, the common occurrence of Os-rich PGE alloys as primary inclusions in massive (commonly podiform), chromite deposits and, conversely, the occurrence of chromite, olivine, pyroxene, laurite, and siliceous (boninitic) melt inclusions in Os-rich PGE alloys suggest a common origin for all these minerals. Integrating our observations with recent experimental work and with observed field relations, we find support for a model in which massive chromite deposits, olivine, laurite, and Os-rich PGE alloys form in a single magmatic process. In an arc-environment, H[sub2]0-rich fluids and siliceous melts (e.g., boninites) are produced in the mantle wedge above the descending and dehydrating plate. Large differences in interfacial energy between the precipitated chromite and PGE alloys, and the hydrous fluid(s) and siliceous melt(s), cause a strong concentration of chromite and PGE alloys in the hydrous fluid(s). This general scenario is capable of simultaneously explaining all key observations, including: (1) the formation of massive chromite deposits; (2) nodular chromite textures; (3) Os-rich PGE alloys, laurite, olivine, and pyroxene as common inclusions in massive chromite; (4) inclusions of chromite, olivine, ... [ABSTRACT FROM AUTHOR]

Presents a s study which performed a systematic transmission electron microscopy technique on the size distribution of antiphase domains in omphacite from eclogites from the Adula/Cima Lunga nappe. Geological outline; Reconstruction of the T-t path; Results and discussion.