From unweathered core to regolith in a single weathering andesitic clast: Rates and trends of in situ chemical weathering on a tropical volcanic island (Basse Terre Island, French Guadeloupe)

We conduct X-ray microprobe, chemical and U-series isotope analyses on an oriented weathering clast collected from the regolith of a weathered Quaternary volcanoclastic debris flow on Basse Terre Island, French Guadeloupe. The sample consists of an unweathered basaltic andesite core surrounded by a weathering rind, and an indurated crust that separates the rind from the overlying soil matrix. U/Th disequilibria dating indicates that rind age increases away from the core-rind boundary to a maximum of 66 ka. This translates to a rind-advance rate of ~0.2 mm kyr⁻¹, broadly consistent with rind advance rates calculated elsewhere on Basse Terre Island. The overlying indurated crust is 72 ka, indicating a possible minimum duration of the rind formation. Elemental variations are constrained by a bulk chemical analysis along a vertical transects from the core to the overlying soil matrix and parallel electron microprobe analyses. The hierarchy of elemental loss across the core-rind boundary varies in the order Ca > Na ≈ Mg > K > Mn > Si > Al > Ti = 0 > P > Fe, consistent with the relative loss of phases in the clast from plagioclase ≈ glass ≈ pyroxene > apatite > ilmenite. The abrupt, <900 μm wide, Ca, Na and porosity reaction fronts at the core-rind boundary approximately equal the length of the long dimension of plagioclase phenocrysts observed in the unweathered core. The <1000 μm wide reaction front at the rind-soil interface is marked by an indurated horizon with Fe and Mn enrichment that spans into enrichment of Mn, Ba, Al, Mg and K in the soil matrix. Unlike previously studied clasts, the preservation of the rind-soil interface permits characterization of weathering reactions and material exchanges between the weathering core, the rind, and the surrounding soil matrix, shedding insights into communication between the enveloping weathering rind and host regolith. The lack of an enrichment signal of Mn within the weathered rind suggests that weathering processes active within clasts are distinct from surrounding soil formation processes.