 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; October 2005; v. 33; no. 10; p. 817–820; doi: 10.1130/G21707.1; 1 figure; 1 table. 817  30 Si systematics in a granitic saprolite, Puerto Rico Karen Ziegler* Oliver A. Chadwick Department of Geography, University of California–Santa Barbara, Santa Barbara, California 93106, USA Art F. White U.S. Geological Survey, Menlo Park, California 94025, USA Mark A. Brzezinski Department of Ecology, Evolution, and Marine Biology, and the Marine Science Institute, University of California–Santa Barbara, Santa Barbara, California 93106, USA ABSTRACT Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope ( 30 Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have  30 Si values 2.0‰ more negative than their parent mineral and the  30 Si signature of the bulk solid is determined by the ratio of primary to secondary min- erals. Mineral-specific weathering reactions predominate at different depths, driving changes in differing  30 Si pore water values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates  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 deter- mine its  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 in- fluence of biotite on  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  30 Si pore water . Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz,  30 Si pore water values are more negative than granite by up to 1.3‰ near the top of the saprolite. Keywords: tropical soil, river chemistry, clay minerals, Si isotopes. INTRODUCTION Igneous rocks have  30 Si values that are more negative than river and ocean water, suggesting an isotopic fractionation that increases the relative amount of 28 Si in neoformed clay minerals (Douthitt, 1982; De La Rocha et al., 2000; Ding et al., 2004). Here we build on existing knowledge of granitic weathering in Puerto Rico (White et al., 1998; Dong et al., 1998; Murphy et al., 1998; Schulz and White, 1999; Turner et al., 2003) to integrate weathering pathways, mineralogical transfor- mations, and numerical geochemical simulations with measurements of the stable isotopic composition of Si in solids and pore waters, and use this synthesis to determine mineral provenance for Si in solution and Si incorporated in clays. In so doing, we trace specific Si isotopic changes during weathering in a granitic saprolite. WEATHERING PROFILE With a weathering flux of 8000 mol Si/ha/yr, the Rio Icacos wa- tershed in the Luquillo Mountains in Puerto Rico has one of the fastest weathering rates documented for granitoid rocks (White et al., 1998). As might be expected from such rapid weathering, the saprolite exhib- its only moderate weathering intensity, with some primary minerals still persisting after more than 100 k.y. of weathering (Brown et al., 1995). Weathering profiles located on the Guaba Ridge between two tributaries in the Rio Icacos basin are composed of clay-rich soils as much as 1 m thick, underlain by 6–8 m of saprolite formed by near- isovolumetric weathering of a quartz diorite intrusion from the early *Current address: Department of Earth and Space Sciences, University of California–Los Angeles, Los Angeles, California 90095, USA Tertiary Rio Blanco stock (White et al., 1998; Schulz and White, 1999; Turner et al., 2003; Buss et al., 2004). Much of the Si mobilization from these profiles occurs via plagioclase and hornblende weathering in a vertically narrow transition from bedrock to saprolite (White et al., 1998; Turner et al., 2003). In contrast, quartz and biotite persist in the overlying saprolite. Quartz dissolution, as indicated by etch pitting and solution rounding, occurs throughout the saprolite but is most pro- nounced at shallow depth, indicating increased weathering rates (Schulz and White, 1999). The effects of biotite dissolution become less influential toward the surface because of its progressive transfor- mation to and overgrowth by kaolinite. Transformation of primary minerals to secondary kaolinite occurs via two processes (Murphy et al., 1998; Dong et al., 1998). (1) Com- plete plagioclase dissolution leads to the precipitation of fine-grained (2 m) kaolinite (K1) at the bedrock-saprolite interface. This kaolin- ite is well preserved and present in constant amounts throughout the saprolite, suggesting that it has not participated in weathering reactions since its formation. (2) Biotite actively transforms (via epitaxial over- growth) throughout the saprolite into coarse-grained kaolinite (K2). Alteration mechanisms are layer by layer replacements, both directly and indirectly via an intermediate halloysite phase, with half of the dissolution flux occurring at the bedrock-saprolite interface, and half within the saprolite. The K2 kaolinite concentration and the kaolinite to biotite ratio within individual grains, and in the bulk saprolite, in- creases with decreasing depth, but more than 70% of the transformation occurred in the bedrock-saprolite transition zone. Due to the increasing amount of surrounding kaolinite overgrowth, the primary biotite grain is increasingly shielded from direct contact with pore solutions.