REE mobilization during small-scale high-pressure fluid–rock interaction and zoisite/fluid partitioning of La to Eu

Abstract

A zoisite–quartz segregation and its adjacent host metabasite from the Tauern Window (Austria) have been investigated in detail to study the geochemical behaviour of REE during high-pressure fluid–rock interaction and the role of zoisite. The segregation formed at T = 500 to 550°C/ P ≫ 0.6 GPa because of decompressional dehydration of the host metabasite. During the formation of the segregation the REE La to Tb and Dy to Lu were geochemically decoupled. In the metabasite La to Tb are incorporated in allanite and were leached from the host rock by the fluid phase. This decoupling and the leaching of La to Tb significantly alter the REE patterns of the host rock in the immediate vicinity of the segregation from LREE enriched to LREE depleted. Based on a mass-balance calculation for La, Ce, Nd, Sm, and Eu the composition of the fluid from which the segregation was precipitated is calculated. The fluid is LREE enriched with (La/Sm) N = 3.7. Individual REE contents range from Eu = 2.4 ± 1.1 ppm up to Ce = 90 ± 20 ppm, ΣREE = 190 ± 50 ppm. Calculated partition coefficients D REE zoisite/fluid are generally <1 and increase from D La zoisite/fluid = 0.08 ± 0.02 to D Eu zoisite/fluid = 0.8 ± 0.4, indicating a stronger preference of zoisite for MREE than for LREE. To describe the fractionation of La relative to Sm between zoisite and fluid, the La-Sm exchange coefficient between zoisite and fluid is calculated. The derived exchange coefficient K D(La-Sm) zoisite/fluid = 0.2 ± 0.1 suggests that a fluid in equilibrium with zoisite will have a three- to ten-times higher La/Sm ratio than coexisting zoisite.