SNC meteorites: Clues to Martian petrologic evolution?

Authors

  • Harry Y. McSween Jr.


Abstract

The shergottites, nakhlites, and Chassigny (SNC meteorites) are apparently cumulate mafic and ultramafic rocks that crystallized at shallow levels in the crust of their parent body. The mineralogy and chemistry of these meteorites are remarkably like equivalent terrestrial rocks, although their ratios of Fe/(Fe + Mg) and certain incompatible elements and their oxygen isotopic compositions are distinctive. All have crystallization ages of 1.3 b.y. or younger and formed from magmas produced by partial melting of previously fractionated source regions. Isotope systematics suggest that the SNC parent body had a complex and protracted thermal history spanning most of geologic time. Some meteorites have been severely shock metamorphosed, and all were ejected from their parent body at relatively recent times, possibly in several impact events. Late crystallization ages, complex petrogenesis, and possible evidence for a large gravitational field suggest that these meteorites are derived from a large planet. Trapped gases in shergottite shock melts have compositions similar to the composition measured in the Martian atmosphere. Ejection of Martian meteorites may have been accomplished by acceleration of near-surface spalls or other mechanisms not fully understood. If SNC meteorites are of Martian origin, they provide important information on planetary composition and evolution. The bulk composition and redox state of the Martian mantle, as constrained by shergottite phase equilibria, must be more earthlike than most current models. Planetary thermal models should benefit from data on the abundances of radioactive heat sources, the melting behavior of the mantle, and the timing of planetary differentiation. Calculated depletion of chalcophile elements in source regions indicates a core dominated by sulfides, and paleomagnetic measurements suggest the presence of a weak magnetic field within the last several hundred thousand years. Concentrations of volatile elements indicate that the SNC parent body was not volatile depleted, and trapped atmospheric components measured in shock melts may be useful in understanding planetary degassing. By providing comparisons for spectral reflectance data and Viking soil analyses, these meteorites may also constrain surface mineralogy and weathering mechanisms.

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