The Effect of Fractures on Weathering of Igneous and Volcaniclastic Sedimentary Rocks in the Puerto Rican Tropical Rain Forest

Abstract

Just as microscopic observations show that minerals dissolve quickly at sites where defects intersect the water-crystal interface and dissolve slowly at zones of perfect lattice structure, weathering rates also vary across a landscape. Specifically, at landscape scale, dissolution (weathering) occurs faster where defects (fractures) intersect the land surface, and slower in unfractured zones. In the rain forest of Puerto Rico, for example, we used ground penetrating radar to document how deep fracture zones allow meteoric waters to accelerate weathering in the Luquillo Mountains. The mountains comprise a metamorphic aureole complex where a quartz diorite pluton intruded into volcaniclastic sedimentary country rock. In the only watershed that has incised deeply into the quartz diorite (Río Icacos), water infiltrates slowly through areas without deep fractures and penetrates more deeply into fractured zones that criss-cross the landscape. These deep fracture zones contain weathered granular material, spheroidally weathered corestones, and waters with higher Si concentrations, [Si]. In the areas between fracture zones, the flowpaths are shallower, intersecting only weathered granular material and perhaps a single corestone, and are characterized by lower [Si]. In contrast, in the volcaniclastic sedimentary rocks, fractures are distributed more homogeneously. Penetration of water in those rocks is thus more homogeneous at the landscape scale. Weathering controlled release rates of Si were estimated using residence times of surface and groundwater based on tritium concentrations, [3H]. The Si weathering release rates are higher for waters that have penetrated to deeper depths as expected if fractures accelerate weathering at depth by providing both preferential hydrologic flowpaths and access to highly soluble Si-containing mineral phases.