Geomicrobiological controls on light rare earth element, Y and Ba distributions during granite weathering and soil formation

Abstract

In this study we examine the redistribution of trivalent and tetravalent light rare earth elements (LREEs), Ba and Y during chemical weathering of granites from southern New South Wales, Australia. In essentially abiotic zones in the lower weathering profile, primary allanite is dissolved, and apatite is extensively replaced by secondary LREE lanthanide phosphates such as rhabdophane and florencite. This association is attributed to the relatively high concentrations of phosphorus at dissolving apatite surfaces and low solubility products for lanthanide phosphates. Bulk chemical data from the lower profile indicate considerable enrichment of Y, La and Nd. In contrast, in granites weathered in proximity to the soil zone, secondary lanthanide phosphates are rare and phosphate surfaces are often colonized by bacteria and fungal hyphae. Bulk chemical data show that Y, La and Nd decrease in abundance with increasing weathering. Low dissolved phosphate concentration due to microbial uptake of phosphorus suppresses secondary phosphate precipitation and also leads to dissolution of secondary lanthanide phosphates formed prior to colonization. In the most highly weathered rocks and soils, secondary phosphates are extremely rare, and only Ce oxides remain. Bulk chemical data show Ce concentrations in excess of 1200 ppm in some soils. Retention of Ce as Ce-oxides reflects the low mobility of (tetravalent) Ce under oxidizing conditions. These observations explain heterogeneities in LREE abundances in weathering profiles, development of extreme Ce anomalies, and greatly elevated concentrations of trivalent LREEs in some regions.