Record of S-rich vapors on asteroid 4 Vesta: Sulfurization in the Northwest Africa 2339 eucrite

In the Northwest Africa (NWA) 2339 eucrite, a few high-Fe pigeonite fragments were partially replaced by fine-grained troilite, Mg-rich augite, silica, with minor ilmenite and Mg-rich ferrosilite. The relict pigeonite has distinctly lower Fe/Mn values (17–18) than those (∼30) of augite and Mg-rich ferrosilite in the fine-grained replacement textures and other eucritic pyroxenes in NWA 2339. The few plagioclase grains associated with these Fe-rich pigeonite fragments contain high albite components (Ab17–31). Similar replacement textures are also present in a lithic clast consisting of pyroxene and Na-rich plagioclase (Ab20–25). In this lithic clast, high-Fe subcalcic augite was replaced by troilite, Mg-rich hedenbergite, silica, and minor ilmenite whereas Mg-rich pigeonite remains intact. Troilite also occurs as tiny inclusions in porous regions of plagioclase. Moreover, the porous regions of plagioclase are enriched in Si and depleted in Al, Ca, and Na, compared to relict plagioclase. These textures indicate that the formation of troilite-rich mineral assemblages is related to the reaction between FeSiO3 component in pyroxene and S-rich vapors; whereas, the formation of tiny troilite in porous regions of plagioclase is related to the reaction between Fe2+ in plagioclase and S-rich vapors. X-ray mapping results reveal that during sulfurization of pyroxene and plagioclase, lithophile elements Mn, Al, Ca, Na, and Mg were removed to various extents, very likely by the S-rich vapors. Based on our observations, the S-rich vapors could be dry S–O–P vapors, which most likely have been formed by volatilization of pre-existing S,P-bearing materials due to impact heating on the surface of asteroid 4 Vesta.