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Tectonic pseudotachylytes are solidified frictional melts produced on faults during earthquakes and are robust markers of seismic slip events. Nonetheless, pseudotachylytes are apparently uncommon fault rocks, because they are either rarely produced or are easily lost from the geological record. To solve this conundrum, long‐lasting (18–35 days) hydrothermal alteration tests were performed on fresh pseudotachylytes produced by sliding solid rock samples at seismic slip rates in the laboratory. After all tests, the pseudotachylytes were heavily altered with dissolution of the matrix and neo‐formation of clay aggregates. Post‐alteration products closely resemble natural altered pseudotachylytes and associated ultracataclasites (i.e., fault rocks affected by fracturing in the absence of melting), demonstrating that the preservation potential of original pseudotachylyte microstructures is very short, days to months, in the presence of hydrothermal fluids. As a consequence, pseudotachylytes might be significantly underrepresented in the geological record, and on‐fault frictional melting during earthquakes is likely to occur more commonly than generally believed. Plain Language Summary: Tectonic pseudotachylytes are solidified melts produced by rapid sliding of faults during earthquakes. A long‐lasting unsolved dispute in earthquake physics regards the abundance of pseudotachylytes in nature and the relevance of frictional melting as a seismic‐related process. Although experimental and theoretical arguments indicate that frictional melts are easily generated at seismic deformation conditions, pseudotachylytes are apparently rare in the geological record and are often related to specific structural settings (i.e., water‐deficient environments and high shear stresses). Such a discrepancy poses the problem whether pseudotachylytes are rarely generated or only rarely preserved in a recognizable form. Here we investigated the preservation potential of pseudotachylytes in the presence of fluids, by performing long‐lasting (18–35 days) hydrothermal tests on fresh pseudotachylytes produced in the laboratory. After all tests, the pseudotachylytes were heavily altered with the generation of microstructures resembling those of very common fault rocks called cataclasites, which are unrelated to frictional melting. We suggest that pseudotachylytes are easily produced during earthquakes but they fade from the geological record in few weeks at most, in the presence of altering fluids percolating along the faults. This implies that frictional melting is a relevant process occurring on faults during earthquakes rupturing crystalline basement rocks. Key Points: We investigated the preservation of pseudotachylytes in the presence of fluids with long‐lasting (18–35 days) hydrothermal experimentsAfter the experiments, pseudotachylytes were heavily altered with dissolution of the matrix and neo‐formation of clay aggregatesHydrothermal alteration can determine the rapid (days to months) fade of pseudotachylytes from the geological record [ABSTRACT FROM AUTHOR]

Abstract: Here we report the first results from elastic geobarometry applied to a kyanite inclusion entrapped within an eclogitic diamond (from Voorspoed mine, South Africa) using micro‐Raman and Fourier transform infrared spectroscopy, electron microprobe analysis, ab initio calculations, and finite element modeling. Application of elastic geobarometry to very elastically anisotropic kyanite inclusions is challenging, as current models do not allow for elastic anisotropy. In order to minimize the effects of anisotropy, we have explored the effects of deviatoric stress on Raman modes via ab initio density functional theory. The results allowed us to select the Raman mode (at ca. 638 cm−1) that is the least sensitive to deviatoric stress. The shift of this band in the inclusion while still trapped within the diamond relative to the inclusion in air (once liberated) was used under hydrostatic approximation to determine a residual pressure on the inclusion of 0.184 ± 0.045 GPa and an entrapment pressure of 5.2 ± 0.3 GPa (~160 km depth) for an FTIR N‐aggregation residence temperature of 1119 ± 50 °C. This is the first geothermobarometric determination for a diamond from the Voorspoed kimberlite. It overlaps with P–T estimates obtained by traditional chemical geobarometry for diamonds from other kimberlites from the Kaapvaal craton, suggesting that the hydrostatic approximation does not introduce significant errors in the geobarometric evaluation. Our protocol of Raman peak selection can be used for geobarometry of further kyanite‐bearing diamonds and may provide a guide for more robust geobarometry of other types of mineral inclusions in diamonds, both eclogitic and peridotitic. [ABSTRACT FROM AUTHOR]

Introduction: Carbon phases occur in different kinds of meteorites, spanning from undifferentiated to differentiated ones. In particular, carbon phases are common in ureilites, a still enigmatic group of achondrites. Ureilites are ultramafic rocks, mainly composed of olivine and pyroxene (mainly pigeonite, +/-orthopyroxene and augite) and minor (up to ~8.5 wt.%) carbon phases (mostly graphite and diamond) [1, 2, 3]. The origin and history of carbon phases in ureilites are important for understanding their petrogenesis and the distribution of carbon in the early Solar System. Recent studies on ureilites [4, 5, 6, 7] indicate that the coexistence of large monocrystalline diamonds, nanodiamonds together with nano-graphite is consistent with a transformation of graphite enhanced by the catalysis of Fe-Ni phases during impact shock events. Sample and methodology: we analysed five olivine-pigeonite ureilitic samples (FRO 95028, FRO 01089, FRO 97013, FRO 01088 and FRO 01012), with different shock degrees. In particular, the shock level of the investigated samples ranges from U-S2 (low shock degree) to U-S6 (very high shock degree) [8]. In order to obtain information on the carbon-bearing aggregates (diamond, graphite, and minor Fe-Ni alloys), we adopted the same experimental approach reported by [4, 5, 6, 7]. In particular, we performed scanning electron microscopy [equipped with energy dispersive spectroscopy (SEM-EDS)], micro-X-ray diffraction (XRD), and micro-Raman spectroscopy (MRS) to characterize the carbon phases and to observe if there is any correlation between the degree of shock recorded by silicates and the presence of the different carbon polymorphs. Results and discussion: XRD patterns showed that all investigated samples contain nano-graphite. Besides this phase, sample FRO 95028, with the lowest (S2) shock degree, contains nanodiamond, while samples from S3 to S6 contain both nano- and micro-diamond. The powder diffraction patterns of each of the selected fragments revealed (i) on the highest peak of graphite (at d-spacing=3.34Ç°) an asymmetry which, as reported by [9], is indicative of "compassed graphite" and (ii) on the highest peak of diamond (d-spacing 2.06 Ç°) a shoulder at d-spacing 2.18 Ç°, suggesting the presence of stacking disorder; both (i) and (ii) are considered markers of impact event(s) [9, 10]. For all samples, the graphite geothermometer, based on MRS data [11, 12] provided temperatures in the range of 1291 to 1398°C ±120°C. As it is evident by these results, taking into account the error range of these measurements, in our samples [11, 12] we did not observe a considerable variation of the graphite temperature with the increase of the degree of shock. Conclusions: XRD results support the formation of micrometer-diamonds found in FRO 01089, FRO 97013, FRO 01088 and FRO 01012 with the assistance of (Fe, Ni)-alloys as catalysts at pressure >10 GPa (S3 shock level recorded by silicates). The formation of polycrystalline diamond, as demonstrated by the results obtained in FRO 95028 low shock ureilite (S2), is allowed at pressure between 5-10 GPa. Application of graphite geothermometer based on MRS data [11] does not show any obvious correlation between temperature and shock stage between low-medium and high shock ureilitic samples. Acknowledgments: This research was funded by the PNRA 2018 00247-A project to F. Nestola. [ABSTRACT FROM AUTHOR]

Minerals trapped as inclusions within other host minerals will develop non-lithostatic pressures during both prograde and retrograde metamorphism because of the differences between the thermoelastic properties of the host and inclusion phases. There is only a single possible path in P-T space, the entrapment isomeke, along which no residual pressure would be developed in a host-inclusion system; non-lithostatic pressures are developed in inclusions as a result of the external pressure and temperature deviating from the isomeke that passes through the entrapment conditions. With modern equation of state and elasticity data for minerals now available, it is possible to perform precise calculations of the isomekes for mineral pairs. These show that isomeke lines are not straight lines in P-T space at metamorphic conditions. We show that silicate inclusions in silicate hosts tend to have flat isomekes, with small values of [ABSTRACT FROM AUTHOR]

A new micro-furnace equipped with an H-shaped resistance heater has been developed to conduct in situ single-crystal X-ray diffraction experiments at high temperature. The compact design of the furnace does not restrict access to reciprocal space out to 2θ = 60°. Therefore, unit-cell parameters and intensity data can be determined to a resolution of 0.71 Å with Mo radiation. The combined use of mineral phases with well characterized lattice expansion ( e.g. pure Si and SiO2 quartz) and a small-diameter (0.025 mm) K-type thermocouple allowed accurate temperature calibration from room temperature to about 1273 K and consequent evaluation of thermal gradients and stability. The new furnace design allows temperatures up to about 1273 K to be reached with a thermal stability better than ±5 K even at the highest temperatures. Measurements of the lattice thermal expansion of pure silicon (Si), pure synthetic grossular garnet (Ca3Al2Si3O12) and quartz (SiO2) are presented to demonstrate the performance of the device. Its main advantages and limitations and important considerations for using it to perform high-temperature diffraction measurements are discussed. [ABSTRACT FROM AUTHOR]

The crystal structure of the title compound, poly[di-μ(4)-formato-lead(II)], [Pb(HCOO)(2)](n), has been re-investigated. It consists of a three-dimensional polymeric network of Pb(2+) nodes connected by bridging formate anions. Despite having been described previously, the structural information available so far [Harrison & Steel (1982). J. Organomet. Chem. 239, 105-113] is incomplete and the reported Pnma space group is incorrect. In this work, the space-group assignment to P2(1)2(1)2(1) is discussed and a complete description of the structural features of lead(II) formate is provided.