CRYSTALLIZATION OF THE ZAGAMI SHERGOTTITE: AN EXPERIMENTAL STUDY

Abstract

The Zagami meteorite is a Martian basalt which exhibits multiple lithologies formed by fractional crystallization. While the rock has been widely studied petrologically, its genesis is poorly constrained from an experimental standpoint. Ongoing debates about the thermal history of this and other Martian meteorites affect our thinking about the environments in which they could have formed (e.g. lava flow vs. subsurface dike) and the types of rocks that surficial Mars exploration are likely to recover. We report on twelve equilibrium and nine dynamic crystallization experiments on a synthetic Zagami composition. Equilibrium experiments run at temperatures of 1260-1050°C yield results similar to those of previous workers [E.H. Stolper, H.Y. McSween Jr., Geochim. Cosmochim. Acta 43 (1979) 1475-1498]. We observe crystallization of ~40 wt.% of magnesian pyroxenes comparable in composition to the most magnesian pyroxene in Zagami, even though petrologic studies suggest that this magnesian pyroxene comprises only ~20 wt.% of the meteorite. Experiments run at a range of fO2 from IW to QFM + 3.9 log units suggest that crystallization of Zagami under conditions as reducing as QFM - 3 log units cannot explain this discrepancy. Resorption of pyroxenes may have been an important factor in explaining this discrepancy. The most likely explanation is that Zagami crystallized from a melt with a higher Fe/Mg ratio than used in this work. Dynamic crystallization experiments run at a range of cooling rates (0.5-10°C/h) and nuclei densities (1-3 vol.%, produced by varying the peak temperature between 1230° and 1250°C) suggests that Zagami formed from a melt with a low nuclei density at a relatively slow cooling rate (<1°C/h). Although these conditions produce a textural match for Zagami, important differences remain in the composition and chemical zoning of the experimental pyroxenes compared to Zagami. These differences might require either the use of a bulk composition with a higher Fe/Mg ratio or a slower (<0.1°C/h) or multi-stage cooling rate.