δ 30 Si systematics in a granitic saprolite, Puerto Rico

Abstract

Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (δ 30 Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have δ 30 Si values ∼2.0‰ more negative than their parent mineral and the δ 30 Si signature of the bulk solid is determined by the ratio of primary to secondary minerals. Mineral-specific weathering reactions predominate at different depths, driving changes in differing δ 30 Si pore water values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates δ 30 Si pore water signatures more positive than granite by up to 1.2‰; these reactions are the main contributor of Si to stream water and determine its δ 30 Si value. Throughout the saprolite, biotite weathering releases Si to pore waters but kaolinite overgrowth formation modulates its contribution to pore-water Si. The influence of biotite on δ 30 Si pore water is greatest near the bedrock where biotite-derived Si mixes with bulk pore water prior to kaolinite formation. Higher in the saprolite, biotite grains have become more isolated by kaolinite overgrowth, which consumes biotite-derived Si that would otherwise influence δ 30 Si pore water . Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, δ 30 Si pore water values are more negative than granite by up to 1.3‰ near the top of the saprolite.