CHEMICAL AND THIN SECTION ANALYSIS OF THE COAL CREEK SERPENTINITE

Serpentinite formation is associated with subduction zones where low to intermediate pressures and temperatures yield various polymorphs of serpentine. Serpentinite, in its purest form, consists of combinations of pyroxene, olivine and water. The serpentine group polymorphs that are created at different pressures and temperatures form various minerals such as lizardite, antigorite, and chrysotile. Serpentinization occurs through two processes: constant-volume and constant-chemical reactions. Constant-volume reactions (5Mg₂SiO₄ + 10H₂O à 2Mg₃Si₂O₅ (OH₄) + 4Mg²⁺ + 8(OH) + H₄SiO₄), require the loss of 4 magnesium ions in the reaction which results in a consistent volume of reactants and products(Frost). Constant-chemical reactions (3Mg₂Si₂O₄ + H₄SiO₄ + 2H₂O à 2Mg₃Si₂O5), require an extra addition of silica which increases the total volume of the resulting products in the reaction (Frost). In order to interpret the probable parent rock and the conditions of serpentinization, serpentinite samples were collected in the Coal Creek Domain of the Llano Uplift in Gillespie County, TX and examined in thin section and XRF spectroscopy.

Through XRF spectroscopy, the results show high percentages of MgO and SiO­₂ and low amounts of CaO and Al₂O₃. The samples range from roughly 3%-8% in Fe₂O₃ and the oxidized material can be seen in some of the thin section samples. The chemical compositions suggests that the parent rock is serpentinized harzburgite that consists of predominantly antigorite, lizardite, and cross-cutting chrysotile as verified through thin section analysis. The cross-cutting nature of the chrysotile suggests that the antigorite formed at intermediate temperatures (400-550℃) that formed at depths of >50km (Jung et al.).