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Abundant Late-Stage Andraditic Garnet in Actively Serpentinizing Mantle Rocks in Oman and its Implications for Microbial Habitability
  • Spencer Zeigler
Spencer Zeigler
University of Colorado Boulder

Corresponding Author:[email protected]

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Drilling by the Oman Drilling Project provided a unique opportunity to access partially-serpentinized harzburgite and dunite. These are in contact with alkaline fluids in a subsurface environment that support a microbial ecosystem. In concert with studies of the rock-hosted microbial community, we are characterizing the mineralogy and petrology of the serpentinized mantle rocks that host this ecosystem. Samples of whole-round core were collected and preserved every 10 m from 3 boreholes and split into paired subsamples for microbiology and mineral characterization. Thin sections were analyzed with a petrographic microscope to complete mineral abundance estimations and interpret textural relationships. Raman spectroscopy was conducted on the thin sections to reveal structural/compositional data about mineral phases. Powders were prepared for XRD analysis for quantitative phase identification. The main rock types are altered harzburgite and dunite, and altered veins of gabbro or pyroxenite occur at certain depths. All of the cores have experienced multiple episodes of serpentinization. The observed mineral assemblages include relict olivine, pyroxene and abundant secondary serpentine, brucite, iron sulfide and andradite-grossular garnet. The assemblages are generally expected from partial serpentinization of peridotite, but the widespread distribution of garnet was particularly surprising. Over 50% of the samples contained sufficient garnet to be detected by XRD. Optical and Raman analyses show that garnet occurs in many textural contexts, notably inside mm-scale, late-stage serpentine veins. Andradite garnet in serpentine veins similar to those found here are likely to have formed during serpentinization at temperatures below ~200°C [1,2]. Incorporating Fe3+ into the andradite component could facilitate H2 production, a potent energy source for microbial metabolisms [2]. Its high abundance may provide key insights into H2 production and habitability during late-stage serpentinization of the Oman ophiolite. [1] Ménez et al. (2018) LITHOS DOI: 10.1016/j.lithos.2018.07.022 [2] Plümper et al. (2014) Geochimica et Cosmochimica Acta 141 (454-471).