Serpentinization at Burro Mountain,California

The Burro Mountain ultramafic complex, Monterey County, California, consists of dunites and peridotites which are partially or wholly serpentinized. Primary minerals in both rock types are olivine, enstatite, diopside, and picotite which upon alteration yield chrysotile, lizardite, brucite, magnetite, talc, tremolite, and carbonate. Electron microprobe analyses show that enstatite, En_85.8 to En_90.8, alters to “bastite” composed only of lizardite (5.0–12.0 weight percent FeO), whereas olivine, Fo_90.8 to Fo_91.6, forms lizardite+chrysotile+brucite with or without magnetite. The chrysotile ranges from 3.0 to 5.0 weight percent FeO, the brucite from 16.0 to 43.0 weight percent FeO. As Serpentinization proceeds, the alteration products are enriched in FeO relative to MgO. Serpentinization probably originates in a changing (P_{O_2 })-T environment by two different reactions:

1. (a)
   Olivine+enstatite+H₂O+O₂?Mg, Fe⁺² chrysotile+Mg, Fe⁺³, Fe⁺² lizardite with or without magnetite.