High-pressure phases in an ultramafic rock from Mars

Shock-induced melt pockets in the martian meteorite Chassigny are completely transformed into a set of high-pressure mineral phases previously unknown from ultramafic rocks, the major constituents of the Earth's mantle. Ferropericlase, amorphous (Mg,Fe)SiO(3)-perovskite, ringwoodite, wadsleyite, high-pressure phosphate gamma-Ca(3)(PO(4))(2), and CaTi(2)O(4)-structurecl chromite were formed by solid state transformation and fractional crystallization upon pressure release from >23 to 10 GPa. During shock pressure release the temperatures in the melt pocket dropped to timescales of 10(-2) to 10(-3) S to allow preservation of the high-pressure phases. Formation and preservation of these phases in Chassigny depend on the duration of shock compression, which increases with the pre-impact burial depth of the meteorite below the martian surface. The estimated burial depth of 25 to 250 m is in agreement with the sampling depth of potential source craters for the Chassigny meteorite. (C) 2009 Elsevier B.V. All rights reserved.