Origin of sulfide replacement textures in lunar breccias. Implications for vapor element transport in the lunar crust

Lunar samples 67016,294, 67915,150, and 67016,297 represent clasts of Mg-suite and ferroan anorthosite lithologies that have interacted with a S-rich vapor. Numerous studies have speculated on the composition and source of these “fluids”, their capability for the transport of vapor-mobilized elements, and the scale and environment under which these types of process occurred. These models all assumed a Moon with a very “dry” mantle, crust, and surface. The olivine in these lithologies is partially to totally replaced by troilite and low-Ca pyroxene. The troilite makes up 30–54 vol% of the troilite + low-Ca pyroxene pseudomorphs after olivine. Other silicates and oxides in the assemblages have experienced post-magmatic reequilibration (pyroxene exsolution, recrystallization, “exsolution” of ilmenite in spinel). The troilite also occurs in veins cross cutting individual phases and metamorphic textures. The sulfide veining and replacement features are restricted to individual clasts and do not cut across the matrix surrounding the clasts, and thus predate the breccia-forming event. The proportion of troilite to low-Ca pyroxene and silicate chemistries indicate that simple reactions (such as olivine + S_2 ↔ low-Ca pyroxene + troilite + O_2) do not adequately represent the replacement process. The sulfides have compositions that are similar to those found in mare basalts. In particular, the sulfides generally are enriched in Co relative to Ni. Exsolution of Ni–Co–Cu in the sulfides is distinctly different between the breccias and mare basalts and suggests a different cooling or crystallization (melt versus vapor) history. The sulfur isotopic composition of the vein and replacement troilite ranges from approximately δ^(34)S = −1.0‰ to −3.3‰. Based on our observations, it appears that the model suggested by Norman et al. (1995) is the most appropriate for the origin of the troilite veining and troilite–pyroxene pseudomorphs after olivine. Our data add significant definition to this model. This process occurs in the relatively shallow lunar crust on a scale that involves vapor interaction with multiple plutonic lithologies of various ages and compositions. These reactions occur at distinct conditions of fS_2, fO_2, and temperature. The reacting vapor is S-rich, and perhaps low in H. The reduction of the oxides in the clasts was not a product of H-streaming as has been suggested for similar textures in lunar rocks, but more likely related to “S-streaming”. These vapors had the capability to transport chalcophile–siderophile elements. However, a proportion of the minor elements making up the troilite (Fe, Ni, Co) did come directly from the olivine being replaced. Further, there is evidence to suggest minor mobility of Mg from the olivine pseudomorphs into the adjacent pyroxene. One of the heat sources driving the transport of elements is closely tied to the emplacement of magmas into the shallow lunar crust. These intrusions were either the source for the S or provided heat to remobilized troilite already in the lunar crust. The process that drove the derivation of the S-rich volatiles was instrumental in fractionating the isotopic composition of S. The enrichment of S^32 in the vapor phase may be attributed to either the stable S species during degassing (COS, S_2 and CS_2) or the high-temperature partial breakdown of troilite in the shallow crust.