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.