Description
Compositional data derived from Cretaceous granodiorite at the Santa Margarita Ecological Reserve (SMER), southern California, are used to model a linear compositional weathering trend using non-‐central principal component analysis. Unlike the Chemical Index of Alteration (CIA) typically used to determine weathering intensity, non-‐central principal component analysis provides a translation invariant weathering intensity factor, thereby quantifying the degree of chemical weathering. This model yields a trend similar to that derived from modeling data from the classic Toorongo granodioritic weathering profile, south Australia. Both the SMER and Toorongo trends are directed away from the p(CN) apex and towards the p(A) apex in p(A) -‐ p(CN) -‐ p(K) ternary diagrams. Such trends are likely due to the removal of CaO and Na2O associated with the preferential weathering of plagioclase. Weathering intensity factors (t) of saprock at the SMER site range from 0.11-‐ 0.91 with a mean of 0.32 ± 0.17. In contrast, these factors for the Toorongo site range from 0.03 -‐ 3.94 with a mean of 1.29 ± 0.76. This difference in weathering intensity is likely a result of climatic variations between the two localities. Principal component 1 (PC1) accounts for 90.9% of the total variability of data along the adjusted weathering trend of the SMER site and 99.5% for the Toorongo site.
