OS43A-2016
Experimental serpentinization of dunite cores at 150-200ºC and 150 bar: Importance of open system dynamics for hydrogen generation and stabilization of ferric-rich serpentine
Thursday, 17 December 2015
Poster Hall (Moscone South)
Andrew J Luhmann1, Benjamin M Tutolo2, Brian C Bagley3, David F.R. Mildner4 and William E Seyfried Jr3, (1)New Mexico Institute of Mining and Technology, Earth and Environmental Science, Socorro, NM, United States, (2)University of Oxford, Oxford, United Kingdom, (3)University of Minnesota, Department of Earth Sciences, Minneapolis, MN, United States, (4)NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, United States
Abstract:
Tectonic processes often exhume mantle peridotite to environments near the Earth’s surface, where serpentinization occurs and involves the hydration of peridotite at relatively low temperatures. This process oxidizes ferrous iron in olivine, which produces hydrogen (H2), creating environments that are conducive to abiotic synthesis of organic compounds and H2-based microbial communities. To understand better chemical and physical processes associated with serpentinization, two flow-through experiments (>30 days) were conducted at 150 and 200°C and 150 bar on intact dunite cores. Permeability decreased by a factor of 31 during the 200°C experiment, more than an order of magnitude larger than that at 150°C. Furthermore, H2 and methane concentrations exceeded 600 µmol/kg and 300 µmol/kg during the 200°C experiment, and were one and two orders of magnitude higher, respectively, than the 150°C experiment. H2 was primarily generated during the conversion of olivine to ferric serpentine at 200°C, since vibrating sample magnetometer analysis indicated little to no magnetite production. Secondary mineralization was identified on the core from this experiment, but X-ray computed tomography scans indicated little change. Furthermore, (ultra) small-angle neutron scattering datasets indicated that any change in nano-porosity and surface area was smaller than the natural variability of the dunite. Even though there was little evidence of alteration, the initial stage of serpentinization at 200°C was sufficient to produce a dramatic effect on flow fields in the core. Furthermore, this experiment generated significant dissolved H2 concentrations, while simulating open system dynamics. Thus, the experimental data provide insight on mass transfer processes in open geochemical systems, which effectively prevent highly elevated H2 concentrations due to continual loss. We speculate that this process is responsible for stabilizing unusually ferric-rich serpentine in nature.
