Formation of a martian pyroxenite: A comparative study of the nakhlite meteorites and Theo's Flow

The unusual composition of the nakhlites, a group of pyroxenitic martian me teorites with young ages, presents an opportunity to learn about nonbasalti c magmatic activity on another planet. However, the limited number of these meteorites makes unraveling their history difficult. A promising terrestri al analog for the formation of the nakhlites is Thee's Flow in Ontario, Can ada. This atypical, 120 m-thick flow differentiated in place, forming disti nct layered lithologies of peridotite, pyroxenite, and gabbro. Thee's pyrox enite and the nakhlites share strikingly similar petrographies, with concen trated euhedral to subhedral augite grains set in a plagioclase-rich matrix . These two suites of rocks also share specific petrologic features, minera l and whole-rock compositional features, and size and spatial distributions of cumulus grains. The numerous similarities suggest that the nakhlites fo rmed by a similar mechanism in a surface lava flow or shallow intrusion. Th eir formation could have involved settling of crystals in a phenocryst-lade n flow or in situ nucleation and growth of pyroxenes in an ultramafic lava flow. The latter case is more likely and requires steady-state nucleation a nd growth of clusters of pyroxene grains (and olivine in the nakhlites), ci rculating in a strongly convecting melt pool, followed by settling and cont inued growth in a thickening cumulate pile. Trapped pockets of intercumulus liquid in the pile gradually evolved, finally growing Fe-enriched rims on cumulus grains. With sufficient evolution, the melt reached plagioclase sup ersaturation, causing rapid growth of plagioclase sprays and late-stage mes ostasis growth.