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Petrogenesis of a weathering profile peninsular ranges, Southern California
Alford, Dan E.
As part of a larger study aimed at documenting the precision and accuracy of data derived from the new GeoChemical Laboratory at San Diego State University, seven samples were collected from the A, 4 from the C, and 5 from the R horizons of a weathering profile developed on a Cretaceous granodiorite. At the collection site, mean annual precipitation is between ~ 15 and ~ 20 inches, and mean annual air temperatures are between ~ 13° - ~ 15° C. On A-CN-K ternary plots, chemical data cluster with some tendency for specimens from the A horizon to plot slightly above samples from the R and C horizons. Such a relationship implies that Na and Ca may have been preferentially removed during development of the A horizon. On an A-CNK-FM plot samples cluster with the positions of samples from the different horizons overlapping significantly. Hence, A-CN-K and A-CNK-FM data are consistent with earlier petrologic work that indicated that plagioclase was preferentially attacked during development of the A horizon, but that in general the intensity of chemical weathering was not great. Mass balance arguments suggest that at the 95% confidence level, less than about 12% of the original elemental mass of Fe, Ca, Mg, Na, and Sr were removed while P and Ba lost ~23% and ~20% of their original masses. In contrast, both Ni and Zr mass were added. These variations translate into an overall change in bulk mass of-2.1 grams(+/- 1.0 grams)/100 grams.
San Diego State University
Bachelor of Science (B.S.) San Diego State University, 2002
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