Composition of the first bulk melt sample from a volcanic region of Mars: Queen Alexandra Range 94201

Abstract

Antarctic meteorite Queen Alexandra Range (QUE) 94201 is a 12 g basaltic achondrite dominated by plagioclase (now maskelynite) and zoned low- and high-Ca pyroxene. Petrologic, geochemical, and isotopic analyses indicate that it is related to previously described basaltic and lherzolitic shergottites, which are a group of igneous meteorites that are believed to be from Mars. Unlike previous shergottites, however, QUE 94201 represents a bulk melt rather than a cumulate fraction, meaning it can be used to infer magmatic source regions and the compositions of other melts on Mars. This melt has much more Fe and P than basaltic melts produced on Earth and formed at a much lower oxygen fugacity. This has altered the crystallization sequence of the melt, removing olivine from the liquidus to produce a plagioclase and 2-pyroxene assemblage. If the high-phosphorus and low-oxygen fugacity conditions represented by QUE 94201 are common in magmatic regions of Mars, then olivine may be rare in martian basalts. No solar cosmic ray effects were seen in the concentrations of 10Be, 26Al, and 36Cl with depth in the meteorite, implying at least 3 cm of ablation during entry to Earth. Significant excesses of neutron capture noble gas isotopes (80, 82Kr and 128, 131Xe) suggest that the QUE 94201 sample came from a depth 22 cm in a meteoroid of at least that radius. The meteorite also has very low 21Ne/22Ne, which would often be interpreted to mean little ablation (contradicting above evidence) but, in this case, appears to reflect a very low abundance of Mg (the principal target element for Ne) in the meteorite, consistent with our bulk chemical analyses. The meteorite has a terrestrial 36Cl age of 0.29 +/- 0.05 Myr and a 10Be exposure age of 2.6 +/- 0.5 Myr in a 4-pi geometry, implying an ejection age of 2.9 +/- 0.5 Myr.