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Description
In 1999, A.L. Fortin studied a weathering profile in the large-biotite facies of the Cretaceous La Posta Pluton. The weathering profile was subdivided into horizons R, C, and A. A suite of samples from each horizon was analyzed for trace elements using standard XRF methods at the GeoAnalytical Laboratory of Washington State University. A similar suite of samples from the same location was collected in Spring 2002 as part of a larger study focused on assessing chemical patterns associated with weathering in different microclimatic belts of the Peninsular Ranges. For this study, 5 samples from the R horizon, 4 from the C horizon, and 7 from the A horizon were analyzed using a newly installed XRF instrument and a newly established pressed-powder technique at SDSU. Five replicates of powder derived from one specimen collected from the R horizon were used to assess the precision of the new XRF procedures and instrument at SDSU. The results of replicate analyses indicate the following precisions, expressed as the 95% confidence interval about the mean, for 8 trace elements analyzed during this study. Zr =± 5.2, Sr =± 1.8, Ba =± 10.2, Rb =± 1. 1, Y = ± 0.7, Sc = ±1. 1, Th =± 0.0 (5 replicates reproduced the value of 6 ppm), Ni = ±1.5. Unfortunately, the USGS does not currently know how many samples went into compiling the recommended composition of G-2, the standard that is close to the composition of samples analyzed during this study. However, they do report the lcr uncertainty. The expected values and lcr uncertainties for G-2 are Zr = 309 ± 35, Sr = 478±2,Ba = 1882±23,Rb = 170±3, Y = 11 ±2, Sc = 3.5 ±0.4, Th = 24.7±2,Ni = 0 ± 0.0. Results obtained during this study indicate the following trace element composition and lcr uncertainties for G-2: Zr = 307.8 ± 4.1, Sr = 480.5 ± 1.4, Ba =1875.1 ± 8.2, Rb = 172.6±0.9,Y = 11.1 ± 0.5, Sc = 4.4 ± 0.9, Th = 23.5 ± 0.0, Ni = 0 ± 0.0. As a final check of the precision of the newly established methods and instrument, a comparison of the results obtained by Fortin from WSU and those obtained during this study were made. These results indicate that data obtained from the SDSU lab are comparable to those obtained from the WSU laboratory. Hence, the laboratory at SDSU appears to provide precise and relatively accurate trace element analysis.
