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Magnetic cosmic spherules from the Great Sand Sea (Western Desert, Egypt): A new example of eolian concentration and trapping.
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Carbon polymorphs in Frontier Mountain ureilitic meteorites: A correlation with increasing the degree of shock?
No full textUreilites 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
Zircon megacrysts from basalts of the Venetian Volcanic Province (NE Italy): U-Pb ages, oxygen isotopes and REE data
No full textMafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange-brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2-1:2.5. U-Pb ages of zircon (51.1 +/- 1.5 to 30.5 +/- 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (delta O-18 =5.31-5.51 parts per thousand) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma. (C) 2006 Elsevier B.V. All rights reserved
2007. Zircon megacrysts from basalts of the Venetian Volcanic Province (NE Italy): U–Pb ages, oxygen isotopes and REE data
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