16-36 Using Carbonate Porphyroblast Textures in Soapstones from Virginia as Natural Analogs of Potential In Situ Carbon Mineralization

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

Ultramafic rocks have been proposed as potential reservoirs for in situ carbon sequestration. There are ultramafic rocks in Virginia, including those from Franklin and Nelson Counties, that are regionally adjacent to carbon emission sources, and the mineralogy of which may serve as end-product analogs of in situ carbon mineralization. These rocks are talc and chlorite dominated schists that are low CaO (~4 wt.%), high Fe₂O₃ (~11 wt.%) and high MgO (27 wt.%). Low Zr (~27 ppm) and Nb (~1 ppm) concentrations indicate these rocks were likely sourced from primitive mantle. These rocks also contain 10-20%, by modal abundance, syn-tectonic to epigenetic, Fe-dolomite to magnesite and siderite porphyroblasts. The porphyroblasts commonly contain minor to trace inclusions of apatite, ilmenite, chromite, and pentlandite. We examined the textures of carbonate porphyroblasts in these otherwise low permeability soapstones to better understand how carbon might be sequestered and mineralized within them. We made mineral maps at different locations within several thin sections. We obtained Raman spectra of all the minerals in the designated area using the JMU Renishaw inVia Raman Microscope with a 532 nm laser. Then, we subtracted baselines from all spectra and peak fitted specific mineral bands. We use these peak intensities to create a false color image map of the distribution of carbonate minerals. In each thin section, we identified at least two carbonate phases. In some thin sections, the carbonates were isolated clasts and some showed radial growth textures. The boundaries of the carbonates ranged from sharp to diffuse. The maps show variable connectivity of the carbonates. The different textures of the carbonates may imply that some soapstones are more amenable to in situ carbon mineralization based on higher potential permeability and connectivity. Furthermore, PerpleX modelling of mineral stability using geochemical data of the talc chlorite schists at pressures <0.05 kb and CO₂/H₂O ~1.5 saturated conditions indicate dolomite and magnesite are stable at ≤180-200°C. This sets an upper temperature limit for potential carbon saturated fluids to fix carbon as stable carbonate porphyroblasts in these ultramafic rocks.

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