QUE94201 SHERGOTTITE: CRYSTALLIZATION OF A MARTIAN BASALTIC MAGMA

Abstract

Basaltic shergottites are commonly described as having crystallized from magmas that contained entrained or cumulus pyroxenes. Complexly zoned clinopyroxenes in QUE94201, a newly recovered Antarctic shergottite, are similar to those formed during continuous growth in some lunar basaltic melts. Pyroxene cores have nuclei of magnesian pigeonite mantled by augite, in turn rimmed by strongly zoned ferroan pigeonite, with outer rims of pyroxferroite. Fe-Ti oxide compositions indicate that this meteorite crystallized under relatively reducing conditions, more like lunar basalts than other shergottites. REE and other trace element abundances in zoned pyroxenes, maskelynite, and phosphates are consistent with continuous pyroxene crystallization from an evolving magma, punctuated by the appearance of new phases on the liquidus. The higher proportion of maskelynite in QUE94201 suggests that its composition may be closer to multiple saturation with pyroxenes + plagioclase than other shergottites, and all pyroxene zones show negative Eu anomalies. Thus, this meteorite may represent the first example of a Martian magma unaffected by crystal accumulation. The Fe/(Fe + Mg) ratio and proportions of incompatible elements (reflected in 4 modal% whitlockite) are high, implying that this is a fractionated melt. The calculated bulk REE pattern shows extreme LREE depletion, suggesting either less contamination with enriched crustal materials or a more LREE-depleted source than other basaltic shergottites.