Quantifying Weathering Advance Rates in Basaltic Andesite Rinds with Uranium-Series Isotopes: Basse-Terre Island, Guadeloupe
Abstract:Weathering of volcanic rocks plays many important roles at Earth’s surface. U-series isotopes (e.g. 238U-234U-230Th and 232Th) have been demonstrated to be a novel chronometer to constrain the formation rates of weathering rinds on tropical volcanic islands [Pelt et al., 2008, EPSL, 276, 98-105; Ma et al., 2012, GCA 80, 92-107].
Here, new U-series isotopes and major element concentrations were analyzed in a basaltic-andesite weathering rind collected from the relatively dry side (northern) of the tropical Basse-Terre Island, Guadeloupe. Core and rind samples were drilled across low, medium and high curvature segments of the core-rind boundary and the measured (238U/232Th) and (230Th/232Th) activity ratios range from 1.024 to 1.390, and 1.007 to 1.186, respectively. Both (238U/232Th) and (230Th/232Th) ratios increase systematically from the core into the weathering rind, similar to those observed previously in the well-characterized weathering rind of the Bras David watershed, located on the central wet part of Basse-Terre [Ma et al., 2012]. We suggest that similar mechanisms controlled the evolution of U-series isotope compositions of the new weathering clast: 1) an influx of soil solution carries dissolved U with (234U/238U) activity ratios >1 into the clast, responsible for the observed gradual increase of (238U/232Th) activity ratios in the rind; and 2) subsequent production and accumulation of 230Th in the rind over time from the decay of excess 234U and 238U account for the observed continuous increase of (230Th/232Th) activity ratios.
Weathering advance rates of this new clast calculated with the U-series activity ratios are ~0.1-0.2 mm kyr-1. These preliminary rates are lower than the clast weathering advance rate (~0.3 mm kyr-1) previously determined for the Bras David watershed. Such a difference may be attributed to less precipitation in the northern region of the island (~1800 mm yr-1), compared to the Bras David watershed (~3000 mm yr-1). This study illustrates a new case study where weathering advance rates of rinds are successfully estimated by using U-series isotopes. Furthermore, the application of U-series chronometry in chemical weathering and rind formation will also be useful in measuring the importance of precipitation on weathering at the clast scale.