Geochimica et Cosmochimica Acta, March 1, 2012, Vol.80, p.92(16)
To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2011.11.038 Byline: Lin Ma (a)(b)(c), Francois Chabaux (b), Eric Pelt (b), Mathieu Granet (b), Peter B. Sak (d), Jerome Gaillardet (e), Marina Lebedeva (a), Susan L. Brantley (a) Abstract: To quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions, we analyzed Uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from Basse-Terre Island in Guadeloupe. U, Th, and Ti elemental profiles reveal that Th and Ti behave conservatively during rind formation, but that U is added from an external source to the rind. In the rind, weathering reactions include dissolution of primary minerals such as pyroxene, plagioclase, and glass matrix, as well as formation of Fe oxyhydroxides, gibbsite and minor kaolinite. Rare earth element (REE) profiles reveal a significant Eu negative anomaly formed during clast weathering, consistent with plagioclase dissolution. Significant porosity forms in the rind mostly due to plagioclase dissolution. The new porosity is inferred to allow influx of soil water carrying externally derived, dissolved U. Due to this influx, U precipitates along with newly formed clay minerals and oxyhydroxides in the rind. The conservative behavior of Th and the continuous addition of U into the rind adequately explain the observed systematic trends of (.sup.238U/.sup.232Th) and (.sup.230Th/.sup.232Th) activity ratios in the rind. Rind formation rates, determined from the measured U-series activity ratios with an open system U addition model, increase by a factor of [approximately equal to]1.3 (0.18-0.24mm/kyr) from a low curvature to a high curvature section (0.018-0.12mm.sup.-1) of the core-rind boundary, revealing that curvature affects rates of rind formation as expected for diffusion-limited rind formation. U-series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale. The weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300-2200. Author Affiliation: (a) Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA (b) Laboratoire d'Hydrologie et de Geochimie de Strasbourg, EOST, University of Strasbourg and CNRS, Strasbourg, France (c) Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968-0555, USA (d) Department of Earth Sciences, Dickinson College, Carlisle, PA 17013, USA (e) Laboratoire de Geochimie et Cosmochimie, Institue de Physique du Globe de Paris, Paris, France Article History: Received 28 March 2011; Accepted 21 November 2011 Article Note: (miscellaneous) Associate editor: Eric H. Oelkers
Clay Minerals -- Analysis ; Basalt -- Analysis ; Soil Moisture -- Analysis ; Uranium -- Analysis ; Porosity -- Analysis
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