Cs-Rb-Ba systematics in phengite and amphibole: an assessment of fluid mobility at 2.0 GPa in eclogites from Trescolmen, Central Alps

Eclogites from Trescolmen that contain abundant hydrous minerals (phengite, amphibole, paragonite, zoisite, talc, apatite) show petrographic evidence for fluid infiltration under conditions of 2.0 to 1.8 GPa, 650 degreesC. La rge ion lithophile elements (LILE, e.g. Cs, Rb, Ba and Sr) were analysed by in-situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid-mobile elements at high pressure. In-situ analysis of carefully-chosen metamorphic assemblages circumvents the problem of part ial late-stage alteration, which can severely influence the calculated elem ent budgets of whole-rock samples. Phengite is the dominant host for Cs, Rb , and Ba in both eclogite and adjacent garnet mica schist samples, and inco rporates > 90% of the budgets of these elements in whole rocks. LILE conten ts of phengites in phengite-rich rocks are likely to record the Cs/Rb and B a/Rb ratios of their host rock protoliths. The LILE patterns of eclogite ar e consistent with protoliths derived from basalt that underwent seafloor al teration, whereas those of mica schist are almost identical to average uppe r continental crust. In contrast, LILE patterns of eclogite samples that la ck phengite, but do contain amphibole, are unlike any plausible protolith, but are identical to those of amphibole in phengite-bearing samples. This o bservation points to homogenization of the LILE in different lithologies, w hich we correlate with petrographic evidence for fluid infiltration. Becaus e phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to Cs, Rb, and Ba, homogenization of amphiboles is best explai ned by fluid infiltration from the surrounding metapelites into eclogite bo dies, implying at least metre-scale fluid mobility. The amphibole homogeniz ation can be most easily modelled by a pervasive open-system fluid flux thr ough the eclogites, possibly facilitated by ductile deformation during the early stages of uplift. Simple calculations give minimum fluid-rock ratios of similar to0.001 to 0.004. Demonstration of the mobility of very small vo lumes of fluid through eclogite is an important prerequisite of many subduc tion zone models that try to explain across-are variations in trace element geochemistry. The low fluid-rock ratios from this study are not in contras t with oxygen isotope heterogeneities reported from other eclogite localiti es. Fluid mobile elements such as Cs, Rb and Ba are more sensitive indicato rs of small volume, fluid-rock interaction and are therefore potentially va luable for understanding fluid infiltration processes in systems where oxyg en isotope shifts are not large enough to be detectable.