T147 Earth's Deep Interior: In Honor of Motohiko Murakami, 2011 MSA Awardee

Tuesday, 11 October 2011
8:00 AM - 12:00 PM
Room 102BC

INCREASING THE YIELD OF ABIOGENIC METHANE FROM CARBONATE AT MANTLE PRESSURES AND TEMPERATURES

Paper number: T147-9
Program: Technical Sessions
Time: 10:05 AM – 10:20 AM

Authors

Abstract

We have conducted new experiments designed to further explore abiogenic methane-forming reactions at mantle pressures and temperatures using both resistively and laser-heated diamond anvil cells (DACs). Several previous studies have shown that CaCO3 + FeO + H2O and similar reactions will produce methane at a wide variety of pressures, but there are outstanding questions regarding the potential role played by interactions with the gasket, and the amount of methane produced tends to be low. Here we have attempted to increase the methane yield by utilizing Fe metal, rather than FeO, and we have incorporated Au-lined gaskets to avoid potential interactions with the gasket. As expected, the amount of methane produced appears to be substantially increased as indicated by the relative intensity of the C-H stretching vibration in Raman measurements.

We have also used synchrotron X-ray diffraction to examine crystalline phases upon temperature quench and after the samples have been decompressed to ambient pressure. Notably, the diffraction data from laser-heated samples is surprisingly complex, with closely spaced regions of the sample producing dramatically different diffraction patterns. Specifically, Fe, FeO and Fe3O4 can be found in close proximity, in addition to unreacted CaCO3 and newly formed Ca(OH)2. Furthermore, some diffraction patterns contain peaks from phases that we have been unable to identify, but we suspect these are produced by obscure Fe-Ca oxides. Accordingly, it is clear that laser-heated samples possess dramatic chemical variation as a result of steep temperature gradients, and we are unable to make a definitive determination of the methane-forming reaction. Nevertheless, we observe very strong methane formation at pressures ranging from 2 - 10 GPa and temperatures ranging from 1,000 - 2,500 K, and it is clear that catalytic reactions with the gasket are not necessary for these reactions to proceed.