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

A zoisite-quartz segregation and its adjacent host metabasite from the Taue rn Window (Austria) have been investigated in detail to study the geochemic al behaviour of REE during high-pressure fluid-rock interaction and the rol e of zoisite. The segregation formed at T = 500 to 550 degreesC/P much grea ter than 0.6 GPa because of decompressional dehydration of the host metabas ite. 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 i n allanite and were leached from the host rock by the fluid phase. This dec oupling and the leaching of La to Tb significantly alter the REE patterns o f the host rock in the immediate vicinity of the segregation from LREE enri ched 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 prec ipitated is calculated. The fluid is LREE enriched with (La/Sm)(N) = 3.7. I ndividual REE contents range from Eu = 2.4 +/- 1.1 ppm up to Ce = 90 +/- 20 ppm, Sigma REE = 190 +/- 50 ppm. Calculated partition coefficients D-REE(z oisite/fluid) are generally <1 and increase from D-La(zosite/fluid) = 0.08 +/- 0.02 to D-Eu(zoisite/fluid) = 0.8 +/- 0.4, indicating a stronger prefer ence 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 b etween 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 w ith zoisite will have a three- to ten-times higher La/Sm ratio than coexist ing zoisite. Copyright (C) 2001 Elsevier Science Ltd.