Weathering Rinds and Soil Development on Basaltic Andesite, Guadeloupe

Monday, 15 December 2014: 3:10 PM
Peter B Sak1, Mariah Murphy1, Lin Ma2, Jacqueline Engel2, Yvette Pereyra2, Jerome Gaillardet3 and Susan L Brantley4, (1)Dickinson College, Carlisle, PA, United States, (2)University of Texas at El Paso, El Paso, TX, United States, (3)Institute de Physique d Globe Paris, Paris, France, (4)Pennsylvania State University Main Campus, University Park, PA, United States
An oriented clast of basaltic andesite collected from the B horizon of a soil developed in a late Quaternary volcanoclastic debris flow on the eastern, windward side of Basse Terre Island, Guadeloupe exhibits weathering patterns like that observed in many clasts from tropical settings. The sample consists of unweathered core material overlain by a ~19 mm thick weathering rind and a narrow ≤ 2mm thick indurated horizon separating the outer portion of the rind from the overlying >10mm of soil matrix material. Elemental variations are constrained by a seven point bulk ICP-AES vertical transect extending from the core, across the rind and ~15 mm into the overlying soil matix and six parallel electron microprobe transections. The porous-hydrated fraction increases from the core to the rind to the surrounding soil from 7±4% to 45±18% to 60±15%, respectively. Like the well-studied clast from the nearby Bras David watershed (Sak et al., 2010) the isovolumetric transformation from core to rind material is marked by a narrow (< 1500 mm) reaction front and elemental depletion. The hierarchy of elemental loss across the core-rind boundary as characterized by open system mass balances varies in order Ca≈Na>Ba>K≈Mn>Mg>Si>Al≈P>Fe»Ti, consistent with the relative reactivity of phases in the clast from plagioclasepyroxeneglass>apatite>ilmenite. Unlike previously studied clasts, the preservation of the rind-soil interface permits characterization of weathering reactions between the weathering clast and surrounding soil matrix. The abrupt (<1000 μm wide) reaction front at the rind-soil interface is marked by the enrichment of Mn, Ba, Al, Mg and K. The enrichment trends may result from soil waters percolating through atmospherically depositioned dust within the upper few meters of the soil profile, as documented in a deep soil profile in the Bras David watershed. The lack of an enrichment signal within the weathering rind suggests that weathering processes active within clasts are distinct from surrounding soil formation processes.