Regolith Evolution Influences Element Redistribution During Weathering of Volcanic Rocks in Erosional, Sedentary, and Depositional Landscapes: Examples From Hawai'i, Guatemala and Southeastern Australia

This study examines the weathering of volcanic rocks exemplifying each of three landscape/regolith associations (erosional, sedentary/relict/residual, and depositional), and illustrates how the regolith/landscape associations and their geomorphic evolution influence the geochemical evolution of the regolith. In erosional landscapes, the rate of physical erosion exceeds the rate of chemical weathering of rock to altered regolith, and surface materials consist of fresh or minimally weathered bedrock. Recent basalts (<4ka) from Hawai'i have weathered slightly and have accumulated no weathering rinds, saprolite, or allochthonous regolith over their brief exposure history. Whole-rock geochemistry is not affected by the small amount of chemical weathering. Leaching has been insufficient for differential removal of elements, and there are no elemental sources outside of the nearly fresh outcrops from which elements might have been introduced into the exposed volumes. In sedentary/relict/residual landscapes, the rate of chemical weathering equals or exceeds the rate of physical erosion, and surface material consists of deeply weathered saprolite. Some volcanic rocks of Plio-Pleistocene age from Hawai'i and Guatemala have experienced spheroidal or corestone weathering, in which corestones of minimally weathered rock are surrounded by concentric saprolitic shells and saprolite derived from the decomposition of the volcanic rock. Many major elements and some minor elements (REE) are depleted from the saprolitic portions of these regoliths. However, several of these minor elements (REE) are enriched in the inner portions of corestone-shell complexes, suggesting that these minor elements and REE leached from saprolite are transferred within the regolith to secondary minerals formed during incipient weathering of the corestones. In depositional landscapes, the surficial material consists of sediment (colluvial, fluvial/alluvial, or aeolian). Tertiary volcanic rocks of the Monaro Volcanic Province (New South Wales, Australia) were emplaced in fluvial-lacustrine environments and almost immediately covered by fine-grained clastic sediment. The jointed flows weathered spheroidally. Corestones have essentially fresh major element and REE signatures. However, Zr (probably redistributed physically from the fine-grained sediment) exhibits systematic absolute enrichment with progressive weathering in the Monaro corestone-shell complexes. Weathering of volcanic rocks results in geochemical trends that differ systematically with the presence, nature, and extent of development of associated regolith. Geochemical patterns of element depletion and enrichment in individual samples and suites of samples can only be properly interpreted if the regolith/landscape context of the samples is taken into account.