Li–Be–B systematics in the ultrahigh-pressure garnet peridotite from Alpe Arami (Central Swiss Alps): implications for slab-to-mantle wedge transfer

摘要

The abundances of lithium, beryllium and boron in mineral grains from the ultrahigh-pressure garnet peridotite of Alpe Arami were studied by secondary ion mass spectrometry. In order to eliminate the ubiquitous B contamination problem, polished thin sections were cleaned twice in an ultrasonic cleaner using ultrapure water from a Milli-Q water purification system (Millipore). Furthermore, a small aperture was used during instrumental analysis so that only the secondary ion beam from the center of the sputtered crater was sampled. Boron abundances measured with the new techniques are generally very low and increase from garnet (10–28 ng/g) via orthopyroxene (25–82 ng/g) and clinopyroxene (26–301 ng/g) to olivine (57–260 ng/g). From these values and the modal composition of the peridotite, a bulk-rock boron content of ～110 ng/g can be estimated. Beryllium abundances are also very low and increase from olivine (～0.1–1.0 ng, in rare cases up to 3.4 ng/g) via garnet (<2 ng/g, in rare cases up to 14 ng/g) and orthopyroxene (7–18 ng/g) to clinopyroxene (38–148 ng/g). Bulk-rock Be abundances are estimated to ～11 ng/g. Our new data, together with additional Li and Be data from an earlier study [J. Paquin, R. Altherr, Contrib. Mineral. Petrol. 143 (2002) 623–640] strongly suggest that the Alpe Arami peridotite was subjected to a metasomatic overprint by aqueous solutions after the peak of Alpine metamorphism (～5.9 GPa/1180 ℃) but before the formation of secondary amphibole (at <3 GPa), most likely during exhumation in a supra-subduction zone setting. While this metasomatism led to a substantial addition of Li to the peridotite, the addition of B was very limited and no detectable enrichment in Be was found. Assuming chemical equilibrium between the rims of clinopyroxene grains and the metasomatizing fluid and using experimentally determined cpx/fluid partition coefficients, minimum and maximum abundances of Li, Be and B in the slab fluid were estimated. We obtained concentrations of 12–175 μg/g Li, 8–206 ng/g Be and 1–12 μg/g B. The results of this study indicate that Li and B were effectively decoupled during percolation of slab-released aqueous solutions through the mantle wedge. While substantial amounts of Li were incorporated into the peridotite minerals, most of the B remained within the percolating fluid, due to its lower bulk-rock/fluid partition coefficient as compared to Li. Most probably, the metasomatizing fluids were derived from subducted altered oceanic crust or serpentinized peridotite.