Rare earths behaviour during the deposition of volcanic sublimates

Abstract

In this study, the REE distribution between volcanic fluids and related solids in fumaroles with temperatures ranging from approximately 100 to 421 °C was investigated in different geological scenarios. The treatment of geochemical REE data was carried out by calculating the REE enrichment factors (EFREE) relative to the volcanic host rocks in studied sites under the assumption that the REE transport takes place as silicate aerosol in volcanic fluids. Shale-normalised REE concentrations in these fluids have been assessed to investigate whether the REE transport as aqueous complexes in water-saturated volcanic gas is reasonable. The REE behaviour in alkaline condensates according to the above mentioned treatments of geochemical data is very similar, being characterised by positive Ce and Gd anomalies and significant W-type tetrad effects. These evidences suggest that the geochemical behaviour of REE in fumarolic fluids is firstly influenced by the sublimate deposition along the fumarolic conduit or around the vents rather than by the transport mechanism of these elements in volcanic fluids.

The Gd enrichment relative to its neighbours Eu and Tb induces the growth of positive Gd anomalies recognised in condensates that in turn results in Gd fluxes from the studied fumarolic systems ranging from 0.01 to 0.92 kg y− 1. This indication represents a novelty in the well-known geochemical Gd behaviour, where recognised positive Gd anomalies have usually been attributed to anthropogenic contamination.