Timescale of spheroidal weathering of a 293Â kyr-old basaltic lava from Réunion Island, Indian Ocean

Spheroidal weathering that forms corestone-shell complexes (hereafter, CSCs) is a common form of chemical weathering of rocks. In this study, we combine mineralogical and geochemical measurements with K-Ar dating and uranium series (U-series) analysis with the goal of constraining the rate of spheroidal weathering in three CSCs collected from a 3 m-deep basaltic lava flow profile from the Piton de la Fournaise volcano (Réunion Island). The K-Ar calculated age is 293 ± 12 kyr. Three zones with increasing degrees of chemical weathering can be identified, ranging from slightly weathered corestones, to gray and red shells. We show that weathering is isovolumetric. The red shells are characterized by a 100% loss of sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg) and a 50% loss of silicon (Si); however, iron (Fe) and thorium (Th) are immobile and the red shells are enriched in uranium (U). To a lesser extent, the cores and the gray shells are also depleted in Na, K, Mg, Ca, Si and U. In the largest CSC (#3), the flux of U lost from the cores and the gray shells is similar to the flux of U gained in the red shells. Different gain-loss mass balance models based on U-Th disequilibria are tested to calculate the chemical weathering rate. Chemical weathering was considered to occur on a timescale of 293 ± 12 kyr; that is, since the deposition of the lava flow. Compared with U-series-based investigations of other spheroidal weathering systems under similar climatic conditions, the average estimated weathering rates (1-3 mm/kyr) in CSC (#3) are an order of magnitude higher than the lowest estimates (0.2 mm/kyr) and an order of magnitude lower than the highest estimates. These results suggest that the propagation of porosity resulting from the selective dissolution of the finely crystalline groundmass of these basalts occurred at lower velocities than the propagation of the fractures controlled by the oxidation of biotite in a quartz diorite at a Puerto Rico site. However, weathering on Réunion Island was faster than the weathering of andesite in a transitional core-rind from Costa Rica. Weathering of CSCs from Réunion may illustrate a different type of weathering that operates based on specific mechanisms of porosity formation. These mechanisms may be controlled by the development of vertical fractures during the formation of basaltic prisms. Comparing the weathering velocities estimated in this study with estimates of stream solute fluxes suggests that the extrapolation from the scale of the basalt profile to the scale of the island requires further attention.