Whole-rock and mineral trace element distributions in Alpine and Franciscan eclogites

F.M. Brouwer* & S.S. Sorensen**

* Institut für Geologie, Universität Bern, Switzerland
** Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, USA

Poster presentation at the V.M. Goldschmidt Meeting, Davos, August 2002.
Abstract published as Geochim. Cosmochim. Acta 66 (S1), A105.

Abstract

Fluids not only mobilise a variety of trace elements during both Alpine and Franciscan eclogite-facies metamorphism (e.g. Philippot and Selverstone, 1991; Sorensen et al., 1997), they also enhance rock deformation. To investigate the interplay of deformation and fluid-mediated mass transfer during subduction, we compared the bulk and mineral trace element chemistry within different eclogite terranes, to rock texture. Eclogites from Alpine localities (Voltri Massif, Monviso) represent both different P-T and fluid-rock regimes than those from the Franciscan Complex of California (USA). Two groups of elements show distinct behaviours in both regions: the large ion lithophile elements (LILE) versus the high field strength elements (HFSE) and V.

Results
Ba and K2O data for whole rocks and phengitic white micas suggest that deformation leads to net Ba loss. In the Franciscan, this is also reflected in the Ba-contents of phengite (Phg) in deformed versus undeformed eclogite samples. Phg in Monviso eclogites shows no depletion of Ba and K2O with deformation.
Ba and K2O are both about one order of magnitude higher in Phg than in bulk Franciscan eclogites. In Alpine eclogites, Ba in Phg is about one, K2O more than two orders of magnitude higher than in the bulk. The Alpine eclogites likely have another carrier of Ba, possibly minor amounts of plagioclase, which responds to deformation.
The bulk and FeTi-oxide (Rt, Ilm, Ttn) compositions suggest deformation has mobilised both TiO2 and V. Depletion of V (but not Ti) in Ttn from deformed versus undeformed Franciscan eclogite supports this notion. In contrast, the compositions of Rt and Ilm in Alpine samples show no V depletion with deformation state.

Conclusions
Minor and trace LILE, as well as V, in both phengite and titanite vary with the deformation state of Franciscan eclogites, but not Alpine ones. The latter rocks must contain small amounts of other minerals rich in these elements that respond strongly to deformation.

References
Philippot, P. and Selverstone, J., (1991), Contrib. Mineral. Petrol. 106, 417-430
Sorensen, S.S., et al., (1997), J. Petrol. 38, 3-34.

The research was partially supported by a Smithsonian Postdoctoral Fellowship to FMB.

View poster (PDF, 1 Mb).