Lithium, Beryllium and Boron in High-Pressure Metamorphic Rocks from Syros (Greece)

Abstract

In this study, high-pressure metamorphic rocks from the island of Syros (Greece) that are interpreted as parts of subducted oceanic crust, equilibrated at about 1.5-2.0 GPa and 500 °C were analysed. A first group of samples (Group 1) includes rocks that preserved the parageneses formed at the pressure peak of metamorphism, while a second group of samples (Group 2) represents metamorphic reaction zones formed at the contacts between contrasting lithologies. Additional to bulk-rock analyses, in-situ analyses of Li, Be and B abundances and B isotope ratios were performed using secondary ion mass spectrometry (SIMS). A new method was developed for B analysis at low concentrations using SIMS, which enables a reduction of boron contamination to levels close to or even below the detection limit of 2 ng/g. Group-1 samples contribute information on the impact of dehydration of oceanic crust on whole-rock abundances of different trace elements. The abundances of Li and Be do not correlate with H2O contents and are in the same range as in fresh and altered oceanic crust, suggesting that most of the Li and Be is not released with hydrous fluids. In contrast, B concentrations and B/Be ratios are correlated to the H2O contents of the rocks. Group-2 samples provide information on the effects of metasomatism of rocks during exhumation. Li and Be show high abundances in many samples, suggesting an enrichment during metasomatism. The enrichment of B is controlled by the occurrence of tourmaline. Tur-bearing samples display very high B/Be ratios, while Tur-free samples show low B concentrations and B/Be ratios. These results demonstrate that Li is probably a good tool for tracing metasomatic enrichment processes, while B is enriched only in the case of tourmaline formation. The SIMS study resulted in sets of inter-mineral partition coefficients for Li, Be and B for 15 different minerals, derived on the basis of in-situ analyses of coexisting phases in natural rock samples. These sets provide information on the behaviour of the light elements in different lithologies within subducting slabs, and they are essential for modelling of trace-element and isotope fractionation during subduction and dehydration of oceanic crust. In addition, the hosts of Li, Be and B were quantified with respect to the whole rock budgets. Modelling of trace element release from progressively dehydrating rocks was performed for Li, Be and B, which show contrasting behaviour during fluid/rock interaction processes. In principle, the presented model offers the possibility of a quantification of trace-element release from the slab in any lithology along any reasonable P-T path. Tourmaline grains of two metasedimentary and one metabasic rock were analysed in-situ for their chemical and B isotopic compositions. The ?11B values of prograde and peak metamorphic tourmaline range from -1.6 to +2.8 ‰ and are significantly higher than values reported in the literature for high-pressure meta-sedimentary tourmaline, demonstrating that a clear distinction between altered oceanic crust and oceanic sediments is not possible on the basis of B isotopes. Samples investigated in this study display heterogeneous sedimentary sources of tourmaline detrital grains with ?11B between -10.7 ‰ and +3.6 ‰ in a single sample. High-pressure blocks enclosed on the island of Syros are rimmed by blackwalls containing abundant tourmaline, with an unusually high ?11B values, exceeding +18 ‰ in all investigated samples, reaching a unique value of +28.4 ‰ in one sample. Blackwalls formed during exhumation of the rocks at a depth of 20-25 km. Estimated P-T conditions are ~ 0.6-0.75 GPa and 400-430 °C. They were produced by the influx of external hydrous fluids that probably originated in the subsequently subducting slab. The exceptionally high ?11B values are explained by interaction of the tourmaline-forming fluids with material composing the exhumation channel. The calculated model demonstrates that fluids are rapidly modified in both trace-element and isotopic composition during their migration through the material overlying the subducting slab. The formation of tourmaline at the contact between mafic or felsic high-pressure blocks and ultramafic matrix may also occur on the slab-mantle interface during subduction. If this is the case, the formation of blackwall tourmaline has a significant impact on the geochemical cycle of B in subduction zones, as it is fixing heavy B in large quantities in the slab within a highly stable mineral. Trace elements are only selectively incorporated into the blackwall tourmaline. The light elements Li and Be, the HFSE, the REE, Y and the LILE all show very low concentrations. First row transition metals and Sr, Pb and Ga are incorporated into dravite, demonstrating abundances in the same order as in the respective whole rocks, and in paragenetic rock-forming minerals. Hence, tourmaline is not strongly fractionating these elements in any way.