Quantifying weathering rates on tropical volcanic Basse-Terre Island, French Guadeloupe: Insights from U-series isotopes in weathering rinds
Weathering of volcanic rocks plays many important roles at Earth’s surface such as modifying landscapes, providing nutrients to soil, and regulating atmospheric CO2. U-series isotopes (e.g. 238U-234U-230Th and 232Th) have been established as a chronometer used to constrain the formation rates of weathering rinds in tropical island environments. Here, U-series isotopes and major element concentrations were analyzed in two basaltic-andesite weathering rinds collected from the Deshaies watershed located in the northern, relatively dry (mean annual precipitation (MAP) = 1800 mm) region of tropical Basse-Terre Island, Guadeloupe. The two clasts are referred to as clasts AN-14-7.5 and AN-14-7.6. Elemental mobility trends calculated using open system mass balance (?_(Ti,j)) calculations indicate that elemental loss adheres to the following trend: calcium (Ca) ? sodium (Na) ? magnesium (Mg) ? manganese (Mn) > potassium (K) ? silica (Si) ? aluminum (Al) > titanium (Ti) ? phosphorous (P) ? iron (Fe) ? zirconium (Zr), completely depleted, incompletely (partially) depleted, and immobile profiles, respectively. Electron microprobe (EMP) observations give insight on reactivity of phases that decrease in the order: plagioclase ? pyroxene ? glass matrix ? apatite ? ilmenite. Core and rind samples were drilled across low, medium and high curvature segments perpendicular to the core-rind boundary and were measured for (238U/232Th) and (230Th/232Th) activity ratios. In the core, both clasts have (234U/238U) and (230Th/238U) ratios close to secular equilibrium, as is expected for unaltered parent bedrock with an age greater than 1.3 Mya. In the rind of clast AN-14-7.5, (234U/238U) activity ratios range from 0.983 to 1.011, (230Th/232Th) range from 1.035 to 1.320, and (238U/232Th) range from 1.113 to 1.446. Measured ratios in the rind material of clast AN-14-7.6 have the following ranges, (234U/238U) = 0.987 to 1.013, (230Th/232Th) = 1.026 to 1.189, and (238U/232Th) = 1.135 to 1.391. Both (238U/232Th) and (230Th/232Th) ratios increase systematically from the unaltered core into the porous weathering rind, similar to those previously observed in a well-characterized weathering rind of the Bras David watershed. The Bras David watershed, a major drainage basin, is located on the central wet (MAP = 3400 mm) part of Basse-Terre. Similar mechanisms controlled the evolution of U-series isotope compositions of both of the clasts from this study and the Bras David weathering clast. Influxes of soil solution carries dissolved uranium with (234U/238U) activity ratios greater than one into the clast, and are responsible for the observed gradual increase in (238U/232Th) activity ratios in the rind. In addition, 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 in (230Th/232Th) activity ratios. Weathering advance rates of the new clasts from Deshaies watershed calculated with the U-series activity ratios are ~0.1-0.2 mm kyr-1. These 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 (~3400 mm yr-1). In addition to weathering rind analyses, an acid leaching experiment on a fresh bedrock sample collected from Basse-Terre Island was conducted. The (234U/238U) activity ratio of the parent material was measured and found to remain at secular equilibrium with (234U/238U) of 1.009 ± 0.005. As water-rock interaction increases over the duration of the leaching experiment, produced solutions yield activity ratios that steadily increase from 1.201 – 1.280. After 50 days of leaching, the solution outputs appear to remain constant for the remainder of the experiment with (234U/238U) at 1.280 and 238U of 0.0635 ppm. This controlled lab experiment permits us to observe the dissolution processes of an andesitic bedrock sample and monitor the behavior of (234U/238U) activity in response to mild acid leaching as a function of time (60 days). The results are interpreted to be analogous to natural water-rock interactions. Additionally, the U isotope ratios and major element concentrations in 28 major rivers and streams on Basse-Terre Island were measured in this study, in order to understand the spatial patterns of chemical weathering signatures at the field scale. Soluble elements such as Ca, Na, Mg, and Si have low concentrations (<10-20 ppm) relative to other rivers studied from around the world, and consistent with very diluted river water due to high precipitation in the tropics. This study presents a new case study where weathering advance rates of rinds are successfully estimated by using U-series isotopes. It demonstrates that the application of U-series chronometry in chemical weathering and rind formation is a powerful tool to assess the importance of precipitation on weathering at the clast scale.
Engel, Jacqueline, "Quantifying weathering rates on tropical volcanic Basse-Terre Island, French Guadeloupe: Insights from U-series isotopes in weathering rinds" (2015). ETD Collection for University of Texas, El Paso. AAI10000787.