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³⁰Si systematics in a granitic saprolite, Puerto Rico

¹ Department of Geography, University of California–Santa Barbara, Santa Barbara, California 93106, USA
² U.S. Geological Survey, Menlo Park, California 94025, USA
³ Department of Ecology, Evolution, and Marine Biology, and the Marine Science Institute, University of California–Santa Barbara, Santa Barbara, California 93106, USA

Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (³⁰Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have ³⁰Si values 2.0 more negative than their parent mineral and the ³⁰Si 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 ³⁰Si_{pore water} values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates ³⁰Si_{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 ³⁰Si 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 ³⁰Si_{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 ³⁰Si_{pore water}. Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, ³⁰Si_{pore water} values are more negative than granite by up to 1.3 near the top of the saprolite.

Key Words: tropical soil • river chemistry • clay minerals • Si isotopes

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