Investigation of in situ weathering of quartz diorite bedrock in the Rio Icacos basin, Luquillo Experimental Forest, Puerto Rico

The Rio Icacos basin, in Puerto Rico, is the site of the highest measured chemical solute fluxes for a catchment on granodiorite; this is partly attributable to high annual rainfall (4300 mm), high average temperature (23 °C), and moderate relief. The bulk of these fluxes is contributed by dissolution of plagioclase and amphiboles in zones of partially weathered rock (0.5–1.5 m thick) underlying saprolite. These zones are characterized by systems of onion-skin “rindlets” (each 3–10 cm thick) in which porosity development is dominated by weathering of plagioclase to kaolinite. Fe-bearing aluminosilicate minerals hornblende, augite, and biotite persist in the weathered rock after plagioclase has weathered to completion, but hornblende and augite disappear between the porous rindlet systems and saprolite. A new watershed-scale solute mass balance corroborates earlier studies and yields an overall reaction stoichiometry that closely resembles complete weathering of the rock (except quartz and biotite) to kaolinite and Fe3+ oxides. ndings indicate that steady-state assumptions are reasonable and useful in studying weathering at this site. Estimated steady-state sediment yields (based on net solute fluxes and average bedrock composition) of 3.24×10−9 to 3.60×10−9 kg m−2 s−1 are smaller than measured values, indicating that sediment generation is currently accelerated with respect to steady-state; however, sediment generation has recently been closer to steady-state. Since weathering within the rindlet systems occurs on multiple fronts moving upward and downward, two new steady-state conceptual models are developed to account for (1) multiple parallel weathering fronts, and (2) differential rates of advance of weathering fronts due to porosity development. Based on these models, in situ weathering rates for plagioclase at two sample locations of 2.7×10−15 and 5.0×10−15 mol Na g−1 s−1 are calculated, values comparable to BET surface area normalized rates of 2.7×10−16 to 5.0×10−15 mol Na m−2 s−1.