Mn‐Precipitates Found in a Martian Crustal Rock

Abstract

AbstractAlteration minerals in one of the Martian meteorite nakhlites, Yamato (Y) 000802, were studied to understand the alteration process and conditions. Mn‐precipitates are discovered between altered plagioclase grains in Y 000802. Mn‐precipitates consist of hausmannite (), manganite (γ‐Mn3+OOH), rhodochrosite (Mn2+CO3), and a trace amount of Mn4+O2 mineral. Jarosite ) is also found. Mn2+ dissolved from olivine contributes to the formation of Mn‐precipitates. A weakly acidic‐neutral fluid containing a trace amount of altered the olivine, and Mn2+ was dissolved into the fluid. The fluid also reacted with plagioclase and probably induced dealkalization of plagioclase, causing a local strong alkaline environment. Plagioclase was altered to ferroan saponite‐nontronite + amorphous SiO2 under alkaline conditions. Simultaneously, Mn2+/3+‐precipitates were formed from the Mn2+‐containing fluid in the interstices between the altered plagioclase grains under the strong alkaline reducing environment. These alterations occurred in the deep part of the nakhlite body, where they are isolated from Martian subsurface water, including strong oxidants. The formation of Mn2+/3+‐precipitates may have been triggered by the melting of permafrost caused by an impact event around ∼633 Ma. Later, the nakhlite body was probably excavated by another impact, making it susceptible to water including strong oxidants. Pyrrhotite was dissolved and a highly acidic oxidizing fluid was formed, which would induce the formation of jarosite and the Mn4+O2 mineral between ∼633 Ma and ∼11 Ma.