Natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite

Ghosh S, Tiwari K, Miyahara M, Rohrbach A, Vollmer C, Stagno V, Ohtani E, Ray D

Abstract

Bridgmanite is the most volumetrically abundant mineral of the Earth’s interior, and it is important to understand its formation mechanism to better comprehend the origin and evolution of planetary interiors. Natural bridgmanites have been reported in only a few shocked meteorites; however, the composition of these specimens differs from plausible compositions of terrestrial bridgmanite. This study reports a natural occurrence of bridgmanite, observed in an ordinary chondrite, with a composition closest to the bridgmanite present in the Earth’s lower mantle. The bridgmanite in the Katol chondrite has high Fe3+/ΣFe ratio and agrees with experimental predictions. The Katol chondrite may serve as a unique analogue for crystallization of bridgmanite during the final stages of magma ocean crystallization of the Earth.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.All study data are included in the article and/or SI Appendix.