D30Si systematics in a granitic saprolite, Puerto Rico

Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (delta Si-30) signatures to their solid and aqueous products. Within a saprolite, clay minerals have delta Si-30 values similar to 2.0 parts per thousand more negative than their parent mineral and the delta Si-30 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 delta Si-30 pore water values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates delta Si-30 pore water signatures more positive than granite by up to 1.2 parts per thousand; these reactions are the main contributor of Si to stream water and determine its delta Si-30 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 delta Si-30 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 delta Si-30(pore water). Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, delta Si-30 pore water values are more negative than granite by up to 1.3 parts per thousand near the top of the saprolite.