Biotite controlled chemical alteration of granodioritic–tonalitic saprock: Exploring the use of sieving to enhance compositional linear trends

The Peninsular Ranges, southern California, USA, commonly contains eroded sections of regolith composed of (1) saprock enclosing corestone or (2) alluvial deposits overlying plutonic/metamorphic basement. In this paper, we present a study focused on evaluating the chemical weathering patterns exemplified by type 1 regolith. In an attempt to enhance the coverage of altered samples about calculated compositional linear trends, the <45μm fractions were separated from saprock samples and were chemically analyzed.Though Alfisol is common within the study area, due to erosional exhumation, such soil is absent at the specific site sampled during this investigation, and colluvium directly overlies saprock and corestones. Mean annual precipitation ranges from 25cm–51cm with a mean annual air temperature of 13°C–18°C.In centered p(A), p(CN), and p(K) ternary space, A, CN, and K are compositions representing molar percentages of Al2O3, CaO*+Na2O, and K2O, respectively, in the sum of A, CN, and K. CaO* is the molar percent of CaO following correction for CaO in apatite. Centering preserves the geometrical relationships between plotted samples, but moves them to the center of the ternary diagram.Saprock samples plot about a linear compositional trend directed away from the p(K) apex. Using orthogonal projection, the average weathering intensity value, t, of the bulk saprock samples is 0.09 (±0.04). When the sieved <45μm fractions are considered along with the bulk saprock samples, a similar linear compositional trend is calculated, but sieved fractions plot closer to the A–CN join, and have an average t-value of 0.89 (±0.04). Hence, inclusion of the sieved fraction extended the coverage of altered samples about the weathering trend.Using Na as a reference frame, a statistically significant loss of 10% (+9/−8%) and 14% (+6/−5%) K and P mass, respectively, occurred during the development of saprock. The loss of K mass is attributed to the alteration of biotite to hydroxy-interlayered vermiculite and mixed-layer biotite/vermiculite, while the loss of P mass is due to the dissolution of apatite. In addition, statistically significant increases in Si (6%±4%) and LOI (56% (+37/−30%)) mass occurred. The increase in Si mass reflects either, or both, illuviation of small amounts of kaolinite into the prominent crack system observed at the study site, or the addition of pedogenic silica. Such increases are apparently the result of processes that were operative in the now erosionally removed section of Alfisol. The results of the above elemental mass changes produced a statistically significant increase in bulk mass of 5%±3%.The results of our work suggests that the inclusion of <45μm fraction chemical data can significantly enhance the coverage of calculated linear compositional trends, and could be a useful investigative tool for future studies.