Joint 52nd Northeastern Annual Section / 51st North-Central Annual Section Meeting - 2017

Paper No. 26-17
Presentation Time: 1:30 PM-5:30 PM


MARTIN, Robert Malik, Department of Geology, Augustana College, 639 38th Street, Rock Island, IL 61201,

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.).