| dc.description.abstract | 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. | en_US |
