Temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the CM chondrites Weifu Guo * , John M. Eiler Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Received 10 October 2006; accepted in revised form 16 July 2007; available onilne 11 September 2007 Abstract Aqueous alteration of primitive meteorites was among the earliest geological processes during the evolution of our solar sys- tem. ‘Clumped-isotope’ thermometry of carbonates in the CM chondrites, Cold Bokkeveld, Murray, and Murchison, demon- strates that they underwent aqueous alteration at 20–71 °C from a fluid with d 18 O VSMOW of 2.0‰ to 8.1‰ and d 17 O VSMOW of 0.1‰ to 3.0‰. The d 13 C VPDB values of these carbonates exhibit a negative correlation with the d 18 O VSMOW of their formation waters, consistent with formation and escape of 13 C-depleted CH 4 during aqueous alteration. Methane generation under these conditions implies that the alteration fluid was characterized by an Eh 6 0.67 and pH P 12.5 (or lower at the highest alteration temperatures). Our findings suggest that methane generation may have been a widespread consequence of planetesimal and plan- etary aqueous alteration, perhaps explaining the occurrence of methane on Titan, Triton, Pluto, and other Kuiper-belt objects. Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Aqueous alteration occurred on the parent bodies of the carbonaceous chondrites within the first tens of million of years of solar system history (Endress et al., 1996). The con- ditions of these processes are poorly constrained, both be- cause direct samples of reactant fluids are rarely, if ever, preserved (Zolensky et al., 2004), and because the altered solids consist of complex, fine-grained mixtures of phases that present challenges to equilibrium thermodynamic ap- proaches to calculate temperature, oxygen fugacity, and other relevant variables. Previous estimates of the tempera- tures of CM chondrite alteration range from <20 to <170 °C(Keil, 2000), based on the stabilities of constituent phases (e.g., tochilinite; Zolensky, 1984) or oxygen isotope fractionations between carbonate and phyllosilicate in the matrix (Clayton and Mayeda, 1984). This first approach yields only upper temperature limits; the second is only va- lid if matrix carbonate and phyllosilicate achieved oxygen- isotope exchange equilibrium. Differences in D 17 O between carbonate and matrix in these samples (Benedix et al., 2003) indicate this assumption is not valid. In any event, uncer- tainties regarding the reduced partition coefficient ratios of the relevant phyllosilicate phases (Sheppard and Gilg, 1996) engender large uncertainties in apparent temperatures based on this approach. Baker et al. (2002) recently deter- mined the oxygen isotopic composition of the structurally bound water released from phyllosilicates in Murchison and suggested an alteration temperature of 80 °C for the CM chondrites based on the oxygen isotopic fractionation between this water and matrix carbonate. However, this estimation also assumes mutual isotopic equilibrium be- tween these two reservoirs of oxygen. We used the carbonate clumped-isotope thermometer (Ghosh et al., 2006; Schauble et al., 2006) to determine the temperatures of carbonate precipitation in the CM chondrites. The carbonate clumped-isotope thermometer is based on a thermodynamic equilibrium that orders 13 C and 18 O into bonds with each other within the carbonate lattice, and is independent of the isotopic composition of any co-existing phase. Moreover, this temperature informa- tion, combined with the known temperature-dependence of carbonate-water oxygen isotope fractionation (Kim and O’Neil, 1997), allows us to determine the oxygen isotope composition of waters from which these carbonates grew. 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.07.029 * Corresponding author. Fax: +1 626 683 0621. E-mail address: wfguo@gps.caltech.edu (W. Guo). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 71 (2007) 5565–5575