ELEMENT MOBILITY AS A RESULT OF CHEMICAL WEATHERING IN A GRANITOID NEAR VALPARAISO, CHILE
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The chemical breakdown of the geologic environment is a source of both beneficial and toxic elements in biological and water systems. A saprolite is preserved when the chemical weathering rate of bedrock exceeds its erosion rate. Rates and styles of geochemical reactions are important when evaluating the risk of toxicity in near surface ecosystems. A saprolite, developed in a Carboniferous granitoid, in the Chilean Coastal Range, was overlain by Pleistocene granitic fluvial gravels more than 2.2 Ma ago (minimum cosmogenic nuclide exposure age). An impermeable duripan (iron-cemented soil horizon) has developed in the gravels, reducing the meteoric flux and therefore the weathering rate through the saprolite below. The saprolite and overlying gravels were sampled for geochemical analyses at varying depths to 8.3 m, including a weakly weathered sample at the base of the section. The concentration of many mobile elements generally increase with depth, reflecting the greater weathering near the surface. Some elements decreased or remained constant, depending on solubility of their original host mineral or the soil forming properties of the secondary minerals they formed. The continuation of the trends through the Pleistocene nonconformity indicates that weathering continued after the sediments were deposited. Significant deviations from these trends with depth occur at 1.79 and 2.37 m below the surface—the depth corresponding to the duripan layer, just below the zone of most intense weathering. The chemical index of weathering (CIW) and the chemical index of alteration (CIA) show maxima of 77 and 73 (unitless), respectively which is consistent with intense weathering of granodiorite. The results indicate that the third and fourth zones of saprolite alteration are preserved, indicating that there has been a significant loss by erosion of the top portion of the saprolite prior to deposition of the fluvial gravels. REE ratios, and trends in the EU anomaly are opposite for the gravels and bedrock samples, in part reflecting the transport of weathering products such as clays. Net leach rates of the most mobile elements (e.g. Na and Ca) are on the order of 10 mg/g integrated over the entire exposure section. Key words: granite, weathering, geochemistry, element mobility, REE, Santo Domingo Complex, duripan, chemical index of weathering, chemical index of alteration.