^1,2Erin L. Walton, ¹Sabrina McCarthy
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12829]
¹Department of Physical Sciences, MacEwan University, Edmonton, Alberta, Canada
²Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
Published by arrangement with John Wiley & Sons

The formation of the high-pressure compositional equivalents of olivine and pyroxene has been well-documented within and surrounding shock-induced veins in chondritic meteorites, formed by crystallization from a liquid- or solid-state phase transformation. Typically polycrystalline ringwoodite grains have a narrow range of compositions that overlap with those of their olivine precursors, whereas the formation of iron-enriched ringwoodite has been documented from only a handful of meteorites. Here, we report backscattered electron images, quantitative wavelength-dispersive spectrometry (WDS) analyses, qualitative WDS elemental X-ray maps, and micro-Raman spectra that reveal the presence of Fe-rich ringwoodite (Fa44-63) as fine-grained (500 nm), polycrystalline rims on olivine (Fa24-25) wall rock and as clasts engulfed by shock melt in a previously unstudied L5 chondrite, Dhofar 1970. Crystallization of majorite + magnesiowüstite in the vein interior and metastable mineral assemblages within 35 μm of the vein margin attest to rapid crystallization of a superheated shock melt (>2300 K) from 20─25 GPa to ambient pressure and temperature. The texture and composition of bright polycrystalline ringwoodite rims (Fa44-63; MnO 0.01─0.08 wt%) surrounding dark polycrystalline olivine (Fa8-14; MnO 0.56─0.65 wt%) implies a solid-state transformation mechanism in which Fe was preferentially partitioned to ringwoodite. The spatial association between ringwoodite and shock melt suggests that the rapidly fluctuating thermal regimes experienced by chondritic minerals in contact with shock melt are necessary to both drive phase transformation but also to prevent back-transformation.

^1,2Jayanta Kumar Pati, ³Wen Jun Qu, ^4,5Christian Koeberl, ^6,7,8Wolf Uwe Reimold, ⁹Munmun Chakarvorty, ⁶Ralf Thomas Schmitt
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12826]
¹Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad, India
²National Center of Experimental Mineralogy and Petrology, University of Allahabad, Allahabad, India
³National Research Center for Geoanalysis, Beijing, Xicheng District, China
⁴Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁵Natural History Museum, Vienna, Austria
⁶Museum für Naturkunde – Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
⁷Humboldt-Universität zu Berlin, Berlin, Germany
⁸Geochronology Laboratory, University of Brasilia, Brasilia, Brazil
⁹Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad, India
Published by arrangement with John Wiley & Sons

The Paleoproterozoic Dhala structure with an estimated diameter of ~11 km is a confirmed complex impact structure located in the central Indian state of Madhya Pradesh in predominantly granitic basement (2.65 Ga), in the northwestern part of the Archean Bundelkhand craton. The target lithology is granitic in composition but includes a variety of meta-supracrustal rock types. The impactites and target rocks are overlain by ~1.7 Ga sediments of the Dhala Group and the Vindhyan Supergroup. The area was cored in more than 70 locations and the subsurface lithology shows pseudotachylitic breccia, impact melt breccia, suevite, lithic breccias, and postimpact sediments. Despite extensive erosion, the Dhala structure is well preserved and displays nearly all the diagnostic microscopic shock metamorphic features. This study aimed at identifying the presence of an impactor component in impact melt rock by analyzing the siderophile element concentrations and rhenium-osmium isotopic compositions of four samples of impactites (three melt breccias and one lithic breccia) and two samples of target rock (a biotite granite and a mafic intrusive rock). The impact melt breccias are of granitic composition. In some samples, the siderophile elements and HREE enrichment observed are comparable to the target rock abundances. The Cr versus Ir concentrations indicate the probable admixture of approximately 0.3 wt.% of an extraterrestrial component to the impact melt breccia. The Re and Os abundances and the 187Os/188Os ratio of 0.133 of one melt breccia specimen confirm the presence of an extraterrestrial component, although the impactor type characterization still remains inconclusive.