The important role of fluid chemistry in the hydrothermal alteration of ordinary chondrites: Insights from halite and sphalerite in the Sidi El Habib 001 (H5) meteorite

The important role that aqueous fluids played during the evolution of carbonaceous chondrites (CCs) and the carbonaceous asteroids that they derive from is well documented. In comparison, our understanding of how such fluids affected ordinary chondrites (OCs) and their S-type asteroid parent bodies is less mature in part due to the intense thermal metamorphism that overprinted the records of alteration. Further, that only a small suite of unequilibrated OCs shows evidence of hydration hinders our understanding of the role that fluids played in the evolution of OCs and S-type asteroids. Here we report a microstructural analysis on halite (NaCl) and sphalerite (ZnS) in Sidi El Habib 001 (SEH 001), a H5 OC that provides new insights into the role of fluids on the OC parent bodies. Our data reveal that halite contains alteration relicts of submicron silicates, and that widespread sphalerite spatially correlates with halite. This relationship suggests that sphalerite formed from the same hydrothermal fluid that precipitated halite, consistent with experimental and theoretical work showing that Cl-rich fluids induce complexation of Zn and significantly enhance its mobility. We hypothesize that Cl-rich hydrothermal fluids resulted from melting of locally concentrated HCl hydrate, which produced acidic fluids capable of dissolving chondritic mineral phases. The pH of the fluid presumably varied on a micrometer scale due to different rates of hydrolysis reactions as a function of grain size, as illustrated by the absence of halite in SEH 001 chondrules. Such a fluid-alteration model is attractive because it offers a reasonable explanation for the limited and heterogeneous alteration effects in OCs.