Rock weathering along small mountainous rivers: Sierra de las Minas, Guatemala
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Date
2016-05
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The Ohio State University
Abstract
Studies of chemical weathering in small mountainous rivers have shown their disproportional role in chemical weathering and delivery of dissolved and particulate materials on a global scale. Previous stream water analyses from La Sierra de Las Minas have shown higher than expected Ca/Na ratios in all streams studied. While the stream chemistry is known, it is important to understand the mineralogy, petrology, and lithology of the area to gain insight into the origins of these chemical weathering products. SEM imaging and elemental mapping of stream cobble samples have shown the presence of calcium bearing minerals such as albite, epidote, apatite, garnet, carbonates and allanite. Mineral grains exhibited dissolution textures across a range of samples, mainly consisting of meta-granites, gneisses, and schists. Analyses of these rocks using x-ray fluorescence analysis was used to determine the major oxide content of the stream cobbles. When normalized to upper continental crust all rock samples show a depletion in calcium with the exception of one sample containing calcium carbonate. While the samples were depleted in the majority of major oxides, silica content was similar to that of upper continental crust. A variety of samples show preferential weathering of calcium bearing mineral phases both under microscope and in XRF major oxide analysis results. This suggests stream bed cobbles are not sources of minerals for silica weathering, but do impact the input of calcium into the streams.
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geochemistry, small mountainous rivers, chemical weathering, allanite, mineralogy, epidote, microscopy