• journal_title：Geology
• Contributor：Karen Ziegler ; Oliver A. Chadwick ; Art F. White ; Mark A. Brzezinski
• Publisher：Geological Society of America
• Date：2005-
• Format：text/html
• Language：en
• Identifier：10.1130/G21707.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：33
• issue：10
• firstpage：817

Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (δ³⁰Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have δ³⁰Si values ∼2.0‰ more negative than their parent mineral and the δ³⁰Si 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 δ³⁰Si_{pore water} values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates δ³⁰Si_{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 δ³⁰Si 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 δ³⁰Si_{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 δ³⁰Si_{pore water}. Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, δ³⁰Si_{pore water} values are more negative than granite by up to 1.3‰ near the top of the saprolite.