Bulk and Elemental Mass Change in Development of Granitic Corestone and Saprolite near the Elsinore Fault
Department of Geological Sciences
San Diego State University Advisor Dr. Gary Girty
Wednesday, May 12th, 2010
CSL 422, 2:20 pm
ABSTRACT It is well documented that saprolite development is an isovolumetric process, though a majority of these studies have occurred in areas that are not seismically active. However, saprolite samples collected from Santa Margarita Ecological Reserve suggest that the saprolitization process was accompanied by volumetric expansion, a possible result of the nearby Elsinore fault zone. Fourteen samples from the granite saprolite were collected along a ~70 cm traverse with varying distances from the parent corestone. Grain densities were determined for 8 of the samples using a He gas pycnometer, and the bulk densities were determined using the modified wax clod method. Thirteen of the samples were crushed and formed into fused disks and pressed pellets for major- and trace-element analysis. Results show that grain density decreases from 2.71 g/cm3 to 2.65 g/cm3 at ~15 cm and ~85 cm respectively, and bulk density increases from 2.33 g/cm3 to 2.38 g/cm3 at ~15 cm and ~85 cm respectively. Chemical results show a significant change in K2O and CaO, possibly due to the weathering of biotite and plagioclase, and the oxidation of Fe2O3 to FeO. It is speculated that ground shaking events cause intercrystalline cracks to form and grains to rotate. Such rotation would lead to an increase in porosity, allowing fluids to weather biotite and plagioclase to expandable clay minerals.
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