Date of Award
Doctor of Philosophy
Harry Y. McSween
Jamie D. Gilmour, Norbert Thonnard, Larry A. Taylor, Claudia L. Mora
Analyses of Chassigny, Nakhla, and ALH84001 reveal, in addition to a Xe component from the martian atmosphere, a second component loosely attributed to the martian "interior." This appears to be a mixture of solar- and fission- (244Pu) derived xenon components. The proportions are consistent in each meteorite but vary from meteorite to meteorite. The working hypothesis is that this variation reflects different contributions of solar (mantle-derived) and fission (crustal-derived) xenon to each parent melt.
This study focused on mineral separates from two basaltic shergottites, Shergotty and EETA79001 Lithology-B (EETA), chosen to reflect, as far as possible, the extremes of crustal assimilation in the parent melt. Xenon analysis was performed revealing martian interior and atmospheric components. Of the mineral separates examined, the opaque phases in both meteorites have higher atmospheric gas concentrations than maskelynite and pyroxene separates. This is attributed to the adsorption of atmospheric gas on the grains and then shock implantation. The opaques, the smallest mineral phases in the meteorites, provided the most surface area and thus resulted in higher gas concentration.
The interior component is best defined in the pyroxene separates of both meteorites. This interior component is attributed to ambient xenon trapped upon crystallization of the mineral. The 129Xe excess (129Xe/132Xe ~ 1.2) in the interior component of Shergotty is described as admixture of martian atmosphere to the melt, while this component is absent for EETA. The fission-derived xenon of the two meteorites varies in that Shergotty has a higher 136Xe* concentration than that observed in EETA.
A model was constructed to reflect the isotopic evolution of xenon in the martian mantle, crust, and atmosphere during planetary differentiation, outgassing and atmospheric loss. An atmosphere is produced with elevated 129Xexs/130Xe and low radiogenic xenon to excess xenon ratio (136Xe*/129Xexs), along with two interior reservoirs, one consisting of solar xenon with little or no radiogenic xenon and one with a high ratio of fissiogenic 136Xe*/130Xe and a low 129Xexs/136Xe* ratio. These latter ratios are qualitatively similar to those required to produce the interior components of the shergottites.
Stone, Katherine Delene Ocker, "Martian Xenon Components in Basaltic Shergottite Meteorites. " PhD diss., University of Tennessee, 2002.