Temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the CM chondrites

Aqueous alteration of primitive meteorites was among the earliest geological processes during the evolution of our solar system. 'Clumped-isotope' thermometry of carbonates in the CM chondrites, Cold Bokkeveld, Murray, and Murchison, demonstrates that they underwent aqueous alteration at 20-71 degrees C from a fluid with delta O-18(VSMOW) of 2.0 parts per thousand to 8.1 parts per thousand and delta O-17(VSMOW) of -0.1 parts per thousand to 3.0 parts per thousand. The delta(13) C-VPDB values of these carbonates exhibit a negative correlation with the delta(18) O-VSMOW of their formation waters, consistent with formation and escape of C-13-depleted CH4 during aqueous alteration. Methane generation under these conditions implies that the alteration fluid was characterized by an Eh <= -0.67 and pH >= 12.5 (or lower at the highest alteration temperatures). Our findings suggest that methane generation may have been a widespread consequence of planetesimal and planetary aqueous alteration, perhaps explaining the occurrence of methane on Titan, Triton, Pluto, and other Kuiper-belt objects. (c) 2007 Elsevier Ltd. All rights reserved.