Kendra Gebhart

Kendra Gebhart

Estimating elemental mass changes during the conversion of corestones to saprock: Implications for the mobility and fractionation of the REE in a Mediterranean (hot summer) climate

Kendra Gebhart
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Wednesday, May 8th, 2013
CSL 422, 9:40am

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ABSTRACT pdf
When precipitation ranges from about 50-80 cm/year to ~200 cm/year, the REE are commonly mobilized and fractionated within the regolith. However, little if any work has been done on how the REE are affected by the conversion of corestone to saprock in a relatively dry climate like that of the Peninsular Ranges, southern California. Here, we document for the first time, the REE mobility patterns that accommodated the transformation of corestone to saprock at two sites within the Peninsular Ranges, all lying within a Mediterranean (hot summer) climate. The two sites are informally referred to as Yucca-Perris and Motte Rimrock. At both sites precipitation and average annual temperature vary from 20.8 cm/year and 10.3° – 25.5° C.
Corestones at the two study sites are composed of hypidiomorphic equigranular tonalite. In adjacent saprocks, plutonic textures are disrupted by an array of cracks, and a thin film of light-brown translocated clay material lies along their walls. During point counting between ~2% to ~11% of points landing on plagioclase at both sites were counted as clay alteration.
XRD study shows that the <2 micron fraction at Yucca-Perris is dominated by illite, kaolinite, and smectite. In contrast, the < 2 micron fraction at Motte Rimrock is dominated by illite, kaolinite, and vermiculite.
REE distributions derived from both corestone and saprock samples from each of the two sites cluster on chondrite-normalized plots. This result suggests that the REE have not been greatly affected during the conversion of corestone to saprock. In order to assess the statistically validity of this and other interpretations mentioned above we calculated the elemental mass changes at each site utilizing Ti as a reference frame element. The results of this part of our study showed that there were no detectable changes in elemental mass for the 10 major elements analyzed during this study at Yucca-Perris. In contrast, there was a statistically significant loss of Ca, Na, and K mass at Motte Rimrock. With the exception of a nominal statistically significant gain in the mass of La at Yucca-Perris, changes in the masses of the 14 REE at the two sites are not otherwise statistically significant at the 95% confidence level. Similar results were obtained using Zr as a reference frame. Hence, the results of this study suggest that within Mediterranean (hot summer) climates, the REE are not generally mobilized or fractionated during the conversion of corestone to saprock, a major process involved in the formation of the regolith.