Natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite

1Sujoy Ghosh1Kishan Tiwari,2Masaaki Miyahara,3Arno Rohrbach,3Christian Vollmer,4Vincenzo Stagno,5Eiji Ohtani,6Dwijesh Ray
Proceedings of the National Academy of the United States of America (in press) Link to Article [https://doi.org/10.1073/pnas.2108736118]
1Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India;
2Graduate School of Advanced Science and Engineering, Hiroshima University, Hiroshima 739-8526, Japan;
3Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany;
4Department of Earth Sciences, Sapienza University of Rome, Rome 00185, Italy;
5Department of Earth and Planetary Materials Science, Graduate School of Sciences, Tohoku University, Sendai 980-8578, Japan;
6Planetary Sciences Division, Physical Research Laboratory, Ahmedabad 380009, India

Bridgmanite, the most abundant mineral of the Earth’s lower mantle, has been reported in only a few shocked chondritic meteorites; however, the compositions of these instances differ from that expected in the terrestrial bridgmanite. Here, we report the first natural occurrence of Fe-bearing aluminous bridgmanite in shock-induced melt veins within the Katol L6 chondrite with a composition that closely matches those synthesized in high-pressure and temperature experiments over the last three decades. The Katol bridgmanite coexists with majorite and metal-sulfide intergrowths. We found that the natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite has a significantly higher Fe3+/ΣFe ratio (0.69 ± 0.08) than coexisting majorite (0.37 ± 0.10), which agrees with experimental studies. The Katol bridgmanite is arguably the closest natural analog for the bridgmanite composition expected to be present in the Earth’s lower mantle. Textural observations and comparison with laboratory experiments suggest that the Katol bridgmanite formed at pressures of ∼23 to 25 gigapascals directly from the chondritic melt generated by the shock event. Thus, the Katol L6 sample may also serve as a unique analog for crystallization of bridgmanite during the final stages of magma ocean crystallization during Earth’s formation.

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