PII S0016-7037(00)00544-5 REE mobilization during small-scale high-pressure fluid–rock interaction and zoisite/fluid partitioning of La to Eu AXEL BRUNSMANN, 1,2, * ,² GERHARD FRANZ, 1 and J¨ ORG ERZINGER 2 1 Technische Universita ¨t Berlin, Fachgebiet Petrologie, EB 15, Strasse des 17. Juni, D-10623 Berlin, Germany 2 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany (Received August 2, 1999; accepted in revised form September 14, 1999) 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, REE = 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. Copyright © 2001 Elsevier Science Ltd 1. INTRODUCTION 1.1. REE Partitioning Data Studies on rare earth elements (REE) have important appli- cations to igneous and metamorphic petrology. They are widely used to deduce source regions, tectonic settings and fraction- ation-contamination processes (e.g., Hanson, 1989; Bau and Knittel, 1993). Furthermore, Sm and Nd play an important role in age determination in both igneous and metamorphic petrol- ogy. A crucial point in all applications of REE is the knowledge of partition coefficients between coexisting solid phases and between solid phases and melts or fluids. Closely connected with partition coefficients is the question of mobility or immo- bility of REE under different geological conditions. Many data exist on the partitioning of REE between various minerals and melts (see Table 4.1 to 4.3 of Rollinson, 1993, and references therein; McKay, 1989), and on REE behaviour in hydrothermal systems at temperatures below 350°C (Hermann et al., 1974; Michard and Albarede, 1986; Wood, 1990a,b; Jochum and Verma, 1996), but little is known about REE behaviour under metamorphic conditions. Condie and Sinha (1996) described the mobility of LREE during mylonitization in a shear zone. The mobilization of REE in contact aureoles at 500 to 600°C/0.2 GPa was shown by Giere (1990) and Giere and Williams (1992). Mobility of LREE at eclogite facies conditions is reported by Sorensen and Grossman (1989), Phil- ippot and Selverstone (1991); Getty and Selverstone (1994), and Brunsmann et al. (2000). Sta ¨hle et al. (1987) and Pan and Fleet (1996) postulated mobility of the REE at granulite facies conditions. Despite this qualitative information on mobility or immobil- ity of REE at metamorphic conditions, little is known about REE partition coefficients between minerals and fluid. This is due to the fact that in contrast to magmatic rocks, where both the melt (i.e., the glassy matrix) and the minerals can be analysed, there is normally no information concerning fluid composition. Ayers and Watson (1993) published experimen- tally derived apatite/fluid partition coefficients for REE and Sr at 1.0 GPa/1000°C. Partition coefficients for trace elements including REE between olivine and aqueous fluids at elevated P-T conditions are given in Brenan and Watson (1991). Based on a study of fluid inclusions, Banks et al. (1994) reported LREE mineral/fluid partition coefficients for allanite and titan- ite. Zoisite is a typical mineral in metabasic rocks at blueschist and eclogite facies conditions, which are characteristic of sub- duction zone environments (Peacock, 1993). Like other epi- dote-group minerals, it may contain large amounts of trace elements such as REE and will contribute to the trace element budget in subduction zones. Because dehydration of a down- going slab plays an important role in the generation of arc magmas, the composition of the fluid phase released from the downgoing slab has significant consequences for the composi- tion of the generated magmas (Vidal et al., 1989; Wilson, 1989, and references therein; Morris et al., 1990; Bau and Knittel, 1993). Therefore, the partitioning of REE between zoisite and a fluid will influence the composition of the fluid generated in * Author to whom correspondence should be addressed (abruns@ classic.min.uni-heidelberg.de). ² Present address: Ruprecht–Karls Universita ¨t Heidelberg, Mineralo- gisches Institut, Im Neuenheimer Feld 236, D-69120 Heidelberg, Ger- many. Pergamon Geochimica et Cosmochimica Acta, Vol. 65, No. 4, pp. 559 –570, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 + .00 559