COMPLEMENTARY X-RAY DIFFRACTION, MöSSBAUER SPECTROSCOPY, AND THIN SECTION PETROGRAPHY OF TERRESTRIAL SERPENTINITES: IMPLICATIONS FOR SERPENTINE SITES ON MARS

Ultramafic rocks (rich in Fe and Mg) outcrop on Earth primarily in ophiolites, which are tectonically uplifted blocks of the ocean crust and underlying mantle onto continental crust, and serve as accessible and direct terrestrial analogs for mineralogically similar rocks detected on the surface of Mars via remote sensing and rover instrumentation, including X-ray diffraction (XRD) and Mössbauer (MOSS) spectroscopy. Terrestrial samples of serpentines and peridotites undergoing serpentinization (the process of ultramafic rocks reacting with hydrous fluids to produce serpentine) from the Coast Range Ophiolite (CRO) in California, USA and the Zambales Ophiolite (ZO) in the Philippines were analyzed using XRD and thin section petrography (TSP) for mineralogy, and MOSS for Fe³⁺ vs. Fe²⁺ analysis. XRD and TSP reveal the dominant minerals span a range of primary and secondary minerals including olivine, pyroxene, spinel, magnetite, serpentine, and chlorite, among others, in both the CRO and ZO. Olivine, pyroxene, magnetite, and serpentine, among others, have also been identified on Mars, particularly in the Nili Fossae region (Ehlmann, 2010). Terrestrial samples contain various percentages of Fe³⁺, Fe²⁺, and magnetite (Fe²⁺Fe³⁺₂O₄). ZO samples contain ~41-58% Fe³⁺ and ~23-34% Fe²⁺ in serpentine and relict minerals along with ~8-30% of the total Fe as magnetite. CRO samples contain ~15-50% Fe³⁺ and ~22-88% Fe²⁺ in serpentine and relict minerals and ~0-52% of total Fe in magnetite. As a general rule, the more Fe³⁺ and magnetite in a sample, the more serpentinization has progressed; in other words, the shifting balance of Fe³⁺and Fe²⁺ inventories can be taken to represent either more or less oxidizing conditions. Taken together, XRD, MOSS, and TSP data indicate that serpentinite sampled at surface exposures (ZO) and shallow coring operations (CRO) are still reactive because the Fe²⁺ budget can still undergo serpentinization and yield chemical energy useful to chemosythetic life, such as H₂, CH₄, and other organic compounds (McCollom and Seewald, 2013). (Ehlmann, Mustard, and Murchie, 2010, Geophysical Research Letters, v.37, L06201, doi:10.1029/2010GL042596; McCollom and Seewald, 2013, Elements, v. 9, pp. 129–134)