Partitioning of minor, trace elements and rare earth elements in bentonite affecting by thermal alteration

Abstract Minor and trace elements, included the rare earth elements (REE), of bentonite that has undergone the natural action of a thermal alteration have been determined. This study has been conducted in the Cala del Tomate outcrop, Cabo de Gata region (Almeria Spain), where an important volcanic dome has intruded in a bentonite mass producing a temperature gradient from the contact area to the exterior. Data are compared with those obtained in bentonites that not were subjected to the thermal effect. No major differences in the REE distribution of bentonite subjected to a thermal contact area in front of a stratigraphic contact zone have been found. On the whole, the contact metamorphism causes element mobilisation, depending on whether there is a temperature gradient. Elements whose behaviour depends directly on temperature, behave as qualitative indicators of element mobility. Rb mobilises significantly near a thermal contact, a pattern that contrasts with its behaviour in a mechanical contact. Other elements that are sensitive to the effect of temperature are lithophile elements. These elements demonstrate a fractionation of Light Elements Lithofile Elements (LILE) compared with High Field Strong Elements (HFSE) near the thermal contact. The main chemical differences with the stratigraphic contact area not subject to the effect of temperature are the incorporation of trace elements with a small radius (HSFE) in the structure of the smectites subject to the effect of temperature, to the detriment of those with a larger radius (LILE).