TY - JOUR
T1 - The effect of curvature on weathering rind formation
T2 - Evidence from Uranium-series isotopes in basaltic andesite weathering clasts in Guadeloupe
AU - Ma, Lin
AU - Chabaux, Francois
AU - Pelt, Eric
AU - Granet, Mathieu
AU - Sak, Peter B.
AU - Gaillardet, Jerome
AU - Lebedeva, Marina
AU - Brantley, Susan L.
N1 - Funding Information:
We would like to thank two anonymous reviewers for their constructive reviews, and Associate editor, Dr. E. Oelkers for his editorial handling and insightful review. This research was funded partially by the National Science Foundation Grant CHE-0431328 to S.L.B. for the Center for Environmental Kinetics Analysis at Pennsylvania State. In addition, S.L.B. acknowledges support for this weathering research from Department of Energy (DOE) Grant DE-FG02-05ER15675 . Additional funding from the University of Strasbourg (France) and from Alsace Region (REALISE work) is graciously acknowledged. Waleska Castro (PSU) assisted in the laboratory, Celine Dessert (IPGP), and Emily Lloret (IPGP) assisted in the field, and L’Observatoire Volcanologique et Sismologique de Guadeloupe (IPGP) provided logistical support. L.M. also acknowledges support from Center of Earth and Environmental Isotope Research at UTEP.
PY - 2012/3/1
Y1 - 2012/3/1
N2 - 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 ( 238U/ 232Th) and ( 230Th/ 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 ~1.3 (0.18-0.24mm/kyr) from a low curvature to a high curvature section (0.018-0.12mm -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.
AB - 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 ( 238U/ 232Th) and ( 230Th/ 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 ~1.3 (0.18-0.24mm/kyr) from a low curvature to a high curvature section (0.018-0.12mm -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.
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U2 - 10.1016/j.gca.2011.11.038
DO - 10.1016/j.gca.2011.11.038
M3 - Article
AN - SCOPUS:84855798372
SN - 0016-7037
VL - 80
SP - 92
EP - 107
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -
