Natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite

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 inter-growths. We found that the natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite has a significantly higher Fe3+/Sigma Fe 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 similar to 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.

Automated summary

This study reports the first natural occurrence of Fe-bearing aluminous bridgmanite in the Katol L6 chondrite, with a composition closely matching those synthesized in high-pressure experiments. The mineral coexists with majorite and metal-sulfide intergrowths, and has a significantly higher Fe3+/Sigma Fe ratio compared to coexisting majorite, in agreement with experimental studies. Textural observations and comparisons with laboratory experiments suggest that the Katol bridgmanite formed directly from chondritic melt generated by a shock event at pressures of around 23 to 25 gigapascals.