Quantifying Mineral Transformations in Granitic Terrain Across the Santa Catalina Mountain Environmental Gradient.

Granitic terrain comprises a significant area of the earth’s land surface (>15%). Quantifying weathering processes involved in the transformation of granitic rock to saprolite and soil is central to understanding landscape evolution in these systems. This study investigated microscale weathering patterns of primary minerals, specifically micas and feldspars, across an array of sites sampled from the Santa Catalina Mountain environmental gradient. The gradient spans substantial climate and vegetation space, with significant range in both mean annual temperature (>10°C) and precipitation (>50 cm/yr). Electron microprobe analyses of <2 mm desert scrub and mixed conifer surface soils and saprolite revealed microscale changes in elemental weight percent and Si/Al ratios, indicative of mineral transformation and elemental loss. In the desert scrub saprolite, altered biotite exhibited a linear decrease in weight percent potassium (interlayer cation) from the center of the grain (~8%) to the mineral edge (~2%) over a ~100 µm distance. A similar trend was observed in single-grained plagioclase feldspar from the same sample where the weight percent sodium decreased from the center of the grain outward (6.8% to 4.1%). These results suggest that edge and fracture weathering are the initial primary mineral alteration pathways. Higher degrees of mineral transformation were identified in the mixed conifer soils compared to desert scrub (lower Si/Al ratio; increased elemental loss), suggesting that climate controls microscale weathering processes across the gradient. These data aided in the constraint of chemical weathering as related to climate, vegetation and primary mineral composition.