Other investigators have shown that the development of weathering profiles on the small-biotite facies of the Cretaceous La Posta pluton in both arid and Mediterranean climates resulted in significant degradation and loss of plagioclase. Such results imply that a bulk mass loss may have occurred as the parent plutonic material was reduced to the relatively thin A-horizons typical of weathering profiles in both climatic belts. Here I report the results of work aimed at assessing how much, if any, bulk mass loss occurred during the development of the A-horizons in La Posta Soil Profiles #2 and #3. Bulk mass changes can be calculated from the relationship, T = C; / C -1 , where c; is the concentration of some immobile element in the original parent material and C is the concentration of the same element in the altered material. If most elements are not affected by pedogenesis, then the ratio, c; / C , for all such elements should be similar. In order to calculate the uncertainties in the ratios of c; / C , the bootstrap algorithm developed by G. H. Girty and J. Lawrence was used. On a plot of c; / C! versus Ti, Fe, Mn, K, Rb, Zr, and Pb for soil profile #3, and AhO3, TiO2, FeO, MgO, Zr, Nb and Ga for soil profile #2 all data cluster about a similar value, thus suggesting that these elements are good candidates for a geochemical reference frame. The geometric mean of c;/c for these elements is 0.85 for both sampling suites. Hence, in both La Posta Soil Profile #2 and #3 ~ 15% of the parent plutonic material was removed during development of the A-horizon.
