New lithologies in the Zagami meteorite: Evidence for fractional crystallization of a single magma unit on Mars

摘要

Zagami consists of a series of increasingly evolved magmatic lithologies. The bulk of the rock is a basaltic lithology dominated by pigeonite (Fs_(28.7-54.3)), augite (Fs_(19.5-35.0)) and maskelynite (Ab_(42-53)). Approximately 20 vol.% of Zagami is a basaltic lithology containing FeO-enriched pyroxene (pigeonite, Fs_(27.0-80.8)) and mm- to cm-sized late-stage melt pockets. The melt pockets are highly enriched in olivine-bearing intergrowths, mesostases, phosphates (both whitlockite and water-bearing apatite), Fe, Ti-oxides and sulfides. The systematic increases in abundances of late-stage phases, Fs and incompatible element (e.g., Y and the REEs) contents of pigeonite, Ab contents of maskelynite, and FeO concentrations of whitlockite all point to a fractional crystallization sequence. The crystallization order in Zagami and the formation of these various lithologies was controlled by the abundances of iron, phosphorus, anc calcium. During fractional crystallization, iron and phosphorus enrichment occurred, ultimately forcing the crystallization of calcium phosphates and olivine-bearing intergrowths. The limited amount of calcium in the melt and its partitioning between phosphates and silicates controlled the crystallization of phosphates, plagioclase, pigeonite, and augite. The presence of these FeO-enriched, water-poor late-stage lithologies has important implications. Discrepancies between experimental and petrologic studies to infer the history of basaltic shergottites may be partially explained by the use of starting compositions which are too FeO-poor in the experimental studies. The water-poor nature of the late-stage melt pockets suggests crystallization from a very dry magma, although whether this magma was always dry or experienced significant near-surface degassing remains an open question. Finally, the presence of fractional crystallization products within Zagami suggests that this may be a relatively common process on Mars.