PRELIMINARY STUDY OF SERPENTINITE SAMPLES FROM THE INGALLS OPHIOLITE COMPLEX, CENTRAL CASCADES, WASHINGTON

The Ingalls ophiolite complex, central Cascades, Washington, was identified by Krevor et al. (2009) as a potential source of Mg silicates that could be used for mineral CO₂ sequestration. This is because over 66% of the Ingalls consists of variably serpentinized ultramafic rocks. This study uses geochemistry of serpentinite from the Ingalls to identify whether abundant Mg silicate minerals are present. Whole rock geochemistry for 19 serpentinites from across the ophiolite was analyzed by XRF and ICP-MS. The MgO for 16 of these samples range from 40.95 to 45.47 wt.%. 3 samples that were contact metamorphosed by the Mt. Stuart batholith have MgO that ranges between 14.11 and 32.84 wt.%. Major element ratios suggest harzburgite may have been the primary protolith for serpentinites. Trace elements from serpentinite are transitional between mid-ocean ridge and forearc settings as defined by modern peridotites. 7 mesh and hourglass lizardite variety serpentines were analyzed from 2 samples using EMPA at the Florida Center for Analytical Electron Microscopy. Due to the inability to account for the OH and Fe-valance state, structural formulae were calculated using 14 O following Whittaker & Wicks (1970). Mg/Si for these samples range from 1.43 to 1.47 and Si+Al range from 3.99 to 4.04 p.f.u. Si, Mg, and Mg# in the serpentine range from 3.94 to 4.04 p.f.u., 5.67 to 5.86 p.f.u., and 95.96 to 98.23 respectively. Minor substitution of Fe, Ni, and Cr occurs for Mg; while minor substitution of Al for Si occurs. Chemical analysis of serpentinite from the Ingalls indicates that it is rich in Mg silicate minerals. These could be used for mineral CO₂ sequestration. Chemistry also confirms that lizardite is the main serpentine variety in the Ingalls. New data from unaltered Cr-spinels in serpentinites will be used to help constrain the protolith and tectonic setting.