Oliver A. Chadwick Arthur F. White Mark A. Brzezinski Karen Ziegler 2005 <p><span>Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (&delta;</span><span>30</span><span>Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have &delta;</span><span>30</span><span>Si values &sim;2.0&permil; more negative than their parent mineral and the &delta;</span><span>30</span><span>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 &delta;</span><span>30</span><span>Si</span>_{pore water}<span>values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates &delta;</span><span>30</span><span>Si</span>_{pore water}<span>&nbsp;signatures more positive than granite by up to 1.2&permil;; these reactions are the main contributor of Si to stream water and determine its &delta;</span><span>30</span><span>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 &delta;</span><span>30</span><span>Si</span>_{pore water}<span>&nbsp;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;</span><span>30</span><span>Si</span>_{pore water}<span>. Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, &delta;</span><span>30</span><span>Si</span>_{pore water}<span>&nbsp;values are more negative than granite by up to 1.3&permil; near the top of the saprolite.</span></p> application/pdf 10.1130/G21707.1 en Geological Society of America δ³⁰Si systematics in a granitic saprolite, Puerto Rico article