Apparently Oliviine weathering may be the culprit:

http://www.dartmouth.edu/~news/releases/2005/06/07a.html

In their paper published online in May in the American Geophysical Union's journal, Geophysical Research Letters, Sharma and Oze describe how methane on Mars can be made from abiotic, or non-living, sources. When water containing dissolved carbon dioxide comes in contact with olivine, it produces hydrogen, which then combines with carbon dioxide to produce methane. The authors contend that olivine is abundant on Mars at shallow depths, and it could easily react with fluids just beneath the surface.

"Most methane on Earth is produced by bacteria, and methane has been cited as an indicator of life on other planets," explains Sharma. "However, we show in our paper that the mineral olivine can be altered in the presence of water and carbon dioxide, which can produce copious quantities of methane. It's quite easy to do, and there is nothing bacterial about it. If there is life on Mars, I would like to see better evidence than methane."