Carbon polymorphs in Frontier Mountain ureilitic meteorites: A correlation with increasing the degree of shock?

Ureilites meteorite fragments present different levels of shock classified on the basis of optical observations of shock features in silicates. We have investigated by scanning electron microscopy (SEM), micro X-Ray Diffraction (XRD) and Micro-Raman Spectroscopy (MRS) five ureilitic fragments (FRO 95028, FRO 01089, FRO 97013, FRO 01088 and FRO 01012) with increasing degree of shock (from S2 to S6) with the purpose to observe if there is any correlation between the level of shock recorded by silicates and the presence of the different carbon polymorphs formed by shock. XRD patterns showed that all investigated samples contain nano-graphite. Besides this phase, sample FRO 95028, with S2 degree of shock, contains nanodiamond, while samples from level of shock S3 to S6 contain both nano-and micro -diamond. XRD results support the shock 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 is already allowed at pressure between 5-10 GPa. Temperature estimated by a graphite-thermometer based on MRS data, provided values in the range of 1291 to 1398 degrees C +/- 120 degrees C, revealing that there is not a considerable variation of the graphite temperature with the increasing degree of shock. (c) 2023 Elsevier B.V. All rights reserved

ANTARCTIC FRO90011 LODRANITE - COOLING HISTORY FROM PYROXENE CRYSTAL-CHEMISTRY AND MICROSTRUCTURE

The Antarctic FR090011 lodranite shows an achondritic coarse granoblastic texture with an approximately chondritic composition. The crystal chemistry of Pbca and C2/c pyroxene single crystals and the TEM texture of Pbca pyroxene were investigated to reconstruct the evolutionary history of the meteorite parent body. The structural behaviour of orthopyroxene (En(85)Fs(12.4)Wo(2.6)) is well constrained in the En-Fs trend. Orthopyroxene Fe2+-Mg ordering in the M1 and M2 sites yields a closure temperature (T-c) of 570 +/- 20 degrees C and a cooling rate 10 degrees C/day near the T-c. Clinopyroxene is a low-iron augite (En(51)Fs(5)Wo(44)). Clinopyroxene Fe2+-Mg ordering in the M1 and M2 sites yields a T-c of 513 +/- 50 degrees C, which is compatible with the coaling rate derived from the orthopyroxene crystals. TEM investigation on orthopyroxene showed a few thin lamellae not larger than 20 Angstrom parallel to (100) and rare one unit cell Guinier-Preston zones. TEM-EDS analysis did not reveal significant chemical differences between lamella-rich and lamella-free zones. The above data are interpreted as being due to fast cooling resulting from radiative heat loss of small fragments ejected from a parent body, which were subsequently reassembled into a larger body

TEM TEXTURE AND X-RAY SINGLE-CRYSTAL STUDY OF SERRA-DE-MAGE ORTHO-PYROXENE

A combined Transmission Electron Microscopy (TEM), X-ray single crystal diffraction (XRD) and microprobe study was undertaken on the orthopyroxene of Serra de Mage feldspar cumulate eucrite. Two single crystals, natural and equilibrated at several temperatures, were examined in order to identify the true symmetry of the orthopyroxene and to estimate the cooling history of the meteorite. TEM texture analysis showed augite lamellae larger than 500 A irregularly spaced within the orthopyroxene matrix, with orientation parallel to (100). XRD data were collected using a four-circle diffractometer. Structural refinement carried out in space group Pbca converged to a discrepancy factor of R = 2.81%. Because of the presence of some Okl 'forbidden' reflections that may be explained either by P21ca symmetry of the orthopyroxene or by overlap of reflections from the exsolved C2/c clinopyroxene, structural refinement in space group P21ca was also attempted. The resulting discrepancy factor was very close to that obtained in Pbca; however, the anomalous values of many thermal parameters and cation-oxygen bond distances do not support the choice of the P21ca space group. Calibration or KD (KD = (Fe(2+) + Mn)M1(MgM2/(Fe(2+) + Mn)MZMgM1) vs. 1/T gave a closure temperature Tc for the intracrystalline Fe(2+)-Mg exchange reaction of about 470 C, consistent with a relatively slow cooling rate