HR: 09:45h
AN: P31A-08    [Abstracts]
TI: The Crusts of Mars and Earth
AU: * McLennan, S M
AF: SUNY at Stony Brook, Department of Geosciences, Stony Brook, NY 11794-2100, United States
AU: Taylor, S R
AF: Australian National Univ., Department of Earth and Marine Sciences, Canberra, ACT 2601, Australia
AU: Hahn, B C
AF: SUNY at Stony Brook, Department of Geosciences, Stony Brook, NY 11794-2100, United States
AB: The differentiation of terrestrial planets and large moons results in crusts with compositions differing greatly from primitive mantles. Typically, large fractions of incompatible elements, including heat-producing elements, are transferred into the crust. Mechanisms and timing of this process differ greatly from planet to planet. Accordingly, in order to understand planetary evolution, it is necessary to understand the composition and evolution of planetary crusts. Crustal evolution on Earth is perhaps the least representative of the terrestrial planets and large moons of the solar system. Although Earth substantially melted after the giant impact that resulted in the Moon, there is little evidence for the existence of a primary crust suggesting that such crust was recycled and mixed into the mantle during the Hadean. Instead, Earth has a very young, continually recycled basaltic secondary (oceanic) crust and an andesitic tertiary (continental) crust, unique in the solar system, that grew episodically over 4 Gyr, but with an average age of about 2 Gyr. The continental - oceanic crust dichotomy, temporal changes in continental crust composition, role of plume volcanism and continental growth are largely consequences of evolving plate- tectonic processes. Mars provides a valuable comparison to Earth because it is a planet that is, in many ways, intermediate between Earth and planetary bodies, such as the Moon and Mercury, that completed crustal development by about 3 Gyr and have been dormant since. Martian crust is mostly ancient (>3.5 Gyr) but volcanism has persisted, possibly episodically, to 200 Myr or younger. Proposals of early plate tectonics persist, but the weight of evidence suggests Mars is a one-plate planet. The 50 km thick crust constitutes 3.2% of the mass of the planet and, even with modest levels of LILE enrichment (K=0.33%), has had well in excess of 50% of incompatible elements removed from the mantle during early differentiation that likely resulted in a primary crust. Secondary crust in the form of young basalts (e.g., SNC meteorites) is derived from ultra-depleted mantle sources and orbital GRS data are consistent with secular variations in crustal composition.
DE: 1020 Composition of the continental crust
DE: 1021 Composition of the oceanic crust
DE: 1027 Composition of the planets
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly