Theo's flow, (Ontario, Canada): A terrestrail analog for the martian nakhlite meteorites

Abstract

Martian meteorites provide our only samples for laboratory investigations of Mars, yet the lack of geologic context severely limits their utility. Strong petrologic similarities between the pyroxenitic layer of a 120-m thick, mafic Archean lava flow in Ontario, Canada, called Theo‟s Flow, to the nakhlite meteorite group may elucidate geologic processes that operated on Mars. Theo's Flow is in the Abitibi greenstone belt, an area well-known as a komatiite location. The type locality, and best outcrop, of Theo's Flow is an upturned (~70°) section stretching eastwest for ~500 m. Theo's Flow can be divided into distinct lithologic units: a thin basal peridotite (0-9 m), a thick pyroxenite (50-60 m), a gabbro (35-40 m), and a hyaloclastic, brecciated top (8-10 m). It is the thick pyroxenitic layer that bears a striking textural similarity to the martian nakhlites. Serpentinization of olivine, chloritization of orthopyroxene, and alteration (e.g., pseudomorphic replacement) of plagioclase and minor phases have transformed the original mineral assemblage, though augites remain largely unaltered and textural relationships are wellpreserved throughout the flow. Variations in iron and minor element abundances in augite cores exhibit typical trends for an evolving melt. Bulk rock analyses exhibit elemental trends consistent with an evolving melt, though they exhibit evidence of elemental re-mobilization by later metamorphism. An average of the peridotite, pyroxenite, and gabbro compositions compare well to that of the quenched top hyaloclastite, suggesting it is a single flow that differentiated by crystal settling. The lithologic diversity within Theo‟s Flow suggests nakhlites may also have complementary lithologies which remain unsampled.