Variations in Elemental Weathering Rates as a Function of Clast Geometry around a Single Basaltic Andesite Clast

Weathering profiles across a single basaltic andesite clast collected from the B horizon of a late Quaternary volcaniclastic debris flow on Basse-Terre Island, Guadeloupe suggest that weathering rinds are thicker where the radius of curvature is smaller. Rind thickness was measured along 38 transects around the clast with a maximum thickness on the bottom (35.8 mm) where the radius of curvature is shortest and a minimum thickness on top (20.6 mm) where the radius of curvature is greatest, and a mean of 24.4 ± 3.1 mm. Open system mass balance calculations constrained by whole rock ICP-MS analyses across the clast show a hierarchy of elemental mobility of Ca≈Na>K≈Mg>Si>Al>Fe≈P from the unweathered core to the rind. The narrow zone paralleling the core-rind boundary where parent core material is completely weathered into rind material is the reaction front. The width of the reaction front is quantified in three different ways: through petrographic analysis (< 2 mm), porosity calculations (1.5-2.5 mm), and chemical analysis (7.8 mm for the top and 3.1 mm for the bottom transects, respectively). All three of these methods show a similar pattern of a thicker reaction front across the top of the clast, where the radius of curvature is greatest and a thinner reaction front on the bottom, where the radius of curvature is smallest. The calculated Si weathering rates show that weathering rates of core material on the bottom of the clast (4.34 x 10^-7 m kg mol^-1) are faster than weathering rates on the top of the clast (1.78 x 110^-7m kg mol^-1). These variations as a function of geometry of the clast suggest that 1D diffusion models are inadequate when the radius of curvature is sufficiently small.