ROLE OF CHLORINE IN THE PETROGENESIS OF THE NAKHLITES (MARTIAN CUMULATE CLINOPYROXENITES)

The nakhlite (clinopyroxenite) and associated chassignite (dunite) Martian achondrite meteorites preserve evidence of a long history of evolution of a mafic magma chamber on Mars, probably including the exsolution and unmixing of aqueous fluid and a chloride fluid/melt. The deep nakhlite NWA 998 has intercumulus apatite that shows similar Cl enrichment to Chassigny. The middle nakhlites have widely varying Cl/F ratios in intercumulus apatite, and the uppermost nakhlites show intercumulus Cl loss to degassing. Meanwhile, these same uppermost nakhlites have melt inclusions that contain some of the most Cl-enriched amphibole grains ever analyzed, probably indicating the presence of a trapped chloride melt with very low water content.

This picture contains tantalizing holes. Neither amphibole nor apatite grains in melt inclusions in the lower to middle nakhlites have been analyzed for halogen content, but glass compositions could partially compensate for this. Analysis of melt inclusion and intercumulus glass in these meteorites will allow interpretation of the state of the melt that was trapped in inclusions based on current glass compositions.

However, the nakhlites also have been pervasively altered by a low-temperature Martian aqueous brine and contain Cl-rich clay and evaporite minerals. The possibility that this ubiquitous fluid has changed the chemistry, especially the halogen content, of nakhlite glass needs to be checked before using glass compositions to infer igneous processes.

We will examine the range of mineral and mineraloid chemistry in melt inclusions and intercumulus material in the nakhlites, especially glass and clay-rich altered material in Nakhla and Lafayette, 1) to evaluate the degree to which low-temperature alteration has changed the Cl content of magmatic-appearing glass, 2) to test whether olivine and augite in certain nakhlites crystallized from magma with similar Fe and Cl content, and 3) to test whether the variation in apatite chemistry between melt inclusions and intercumulus material in the Chassigny meteorite is reflected in variations in glass chemistry in the nakhlites. These considerations will give us a richer picture of the volatile evolution of the chassignite/nakhlite sequence during uplift, emplacement/eruption, and crystallization.