Abstract

Compositional mapping of Mars at the 100-metre scale with the Mars Odyssey Thermal Emission Imaging System (THEMIS) has revealed a wide diversity of igneous materials. Volcanic evolution produced compositions from low-silica basalts to high-silica dacite in the Syrtis Major caldera. The existence of dacite demonstrates that highly evolved lavas have been produced, at least locally, by magma evolution through fractional crystallization. Olivine basalts are observed on crater floors and in layers exposed in canyon walls up to 4.5 km beneath the surface. This vertical distribution suggests that olivine-rich lavas were emplaced at various times throughout the formation of the upper crust, with their growing inventory suggesting that such ultramafic (picritic) basalts may be relatively common. Quartz-bearing granitoid rocks have also been discovered, demonstrating that extreme differentiation has occurred. These observations show that the martian crust, while dominated by basalt, contains a diversity of igneous materials whose range in composition from picritic basalts to granitoids rivals that found on the Earth.

1. Department of Geological Sciences, Arizona State University Tempe, Arizona 85287, USA
2. Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA
3. Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii 96822, USA
4. University of Colorado, Boulder, Colorado, USA
5. US Geological Survey, Emeritus, Carson City, Nevada 89703, USA
6. Malin Space Science Systems, San Diego, California 92191, USA

Correspondence to: P. R. Christensen¹ Correspondence and requests for materials should be addressed to P.C. (Email: phil.christensen@asu.edu).

Received 13 November 2004; Accepted 7 April 2005

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