The REE-distribution in minerals of the epidote group is essential for our understanding of their behaviour in high pressure rocks during subduction, dehydration and possible melting. We determined the trace element content of zoisite and clinozoisite by ICP-AES (following dissolution and chromatographic separation) in mineral separates from i) two localities, where pegmatites have been formed by high pressure melting of eclogites, and ii) hydrothermal segregations in garnet amphibolites/eclogites/eclogite facies metasediments.
Accessory zoisites from the quartz-albite-phengite pegmatites (Weißenstein, Germany; Saualpe, Austria) are strongly enriched in LREE (between 102 and 103 x chondrite) and are poor in HREE (ª 10 x chondrite), although the indivudual
patterns are different in detail. Coexisting albite and phengite have REE contents below 1 x chondritic values. Retrograde
clinozoisite (formed by the breakdown of zoisite) shows a similar pattern as its precursor, but is enriched by a factor of ª 1.3.
Zoisite and clinozoisite from hydrothermal quartz-segregations (Südvenediger area, Tauern, Austria) have significantly lower LREE concentrations (100 - 400 x chondrite) than those from the pegmatites. They are also characterized by a slightly postitive Eu-anomaly and HREE (Ho - Lu) below 10 x chondrite. Calcite, chlorite and paragonite have concentrations between 1 and 10 x chondritic. Again, as abserved in the pegmatites, retrograde clinozoisite shows a similar pattern and is enriched in all REE compared to its precursor.
The data confirm that during melting the presence of garnet in the residuum results in a very steep REE pattern in the melt. Zoisite picks up the majority of these elements during crystallization. The melts of such pegmatites are tonalitic-trondhjemitic in composition and their REE-pattern is similar to what is observed in tonalites and trondhjemites. Thus they might serve as models for melting of the lower crust.
Also, during hydrothermal fluid-rock interaction, garnet in the host rock retains the HREE, but especially the LREE are mobile at high pressures (> 10 kbar) and temperatures (> 500 °C) and were obviously transported via a fluid phase. However, during later reaction of zoisite into clinozoisite (+ other phases), the REE seem to be immobile and are passively enriched in the retrograde clinozoisite.