Trace element mobility during spheroidal weathering of basalts and andesites in Hawaii and Guatemala

Trace element mobility has been studied in spheroidally weathered corestone–shell complexes developed on Late Tertiary and Quaternary volcanic rocks. The parent materials are basalts and andesites related to (1) a subduction zone (Tecuamburro and Moyuta Volcanoes, Guatemala) and (2) a hot spot (Hawaiian Islands of Oahu, Maui and Hawai''i). The initial major element compositions are largely preserved in the corestones; each corestone sample shows major element abundances characteristic of the different magmatic processes through which the lavas were generated. However, the changes in trace element compositions due to weathering show similar patterns in the corestone–shell complexes independent of the tectonic setting and age.Element mobility within the different corestone–shell complex was determined using an isovolumetric reference frame. In this method, bulk density is chosen as an independent measure of the extent of weathering. Based on volumetric concentrations, the notion of element immobility cannot be applied to all the corestone–shell complexes. Elements commonly assumed to be immobile (e.g., Zr, Ti, and Al) vary in volumetric concentration for some corestone–shell complex. Despite petrographic evidence of only minor weathering, two corestones (Oahu and Tecuamburro) have become enriched in REE3+ relative to other lava flows from the same volcanoes, and display negative Ce anomalies. The exfoliated shells from these complexes tend to have lower concentrations of REE3+, and have positive Ce anomalies. The fractionation of Ce from the other rare earth elements (REE) is inferred to be a manifestation of changes in the valence state of Ce from trivalent to tetravalent under oxidizing conditions. In settings where the development of corestone–shell complex is advanced, an acidic solution can mobilize trivalent REE, leach them out of more extensively weathered materials, and transport them to less acidic portions of the regolith undergoing incipient weathering. The precipitation of the REE3+ in the rinds and centers of the corestones are likely due to increases of the pH of the weathering solution. Systematic weathering-induced REE behavior (enrichment of most REE, with lesser Ce enrichment resulting in a negative Ce anomaly) can be discerned even in corestone samples that exhibit major element abundances and petrographic features otherwise similar to unweathered basalts and andesites. [Copyright &y& Elsevier]