Trace element and isotopic compositions of exhumed high-pressure (P) mafic rocks are an important archive to investigate chemical processes in subduction zones. Here we report the B isotope (δ11B) composition of eclogitic mafic rocks enclosed in high-P serpentinite from the Voltri Massif, Ligurian Alps (Italy). Combined with bulk δ18O values, 87Sr/86Sr ratios and trace element data, the δ11B of eclogitic mafic rocks were investigated to test oceanic inheritance vs. subduction-related processes and to provide inferences on the timing of B uptake in eclogitic metagabbros.
Petrographic observations along with major and trace element data indicate that the eclogite facies mafic rocks derived from primitive Mg-Al-bearing gabbros (Erro-Tobbio eclogites) and differentiated Fe-Ti-bearing oxide gabbros (Vara eclogites). Metarodingite from Vara shares similar REE pattern to that of the Erro-Tobbio eclogites, suggesting a plagioclase-rich gabbroic protolith. The studied rocks show variable enrichments in fluids-mobile elements (e.g., Cs, Ba, Li, Sr; high Li/Y, B/Nb). Boron concentrations range between 2.3 and 7.6 ppm, with no significant differences among the different samples. Oxygen isotope compositions (1σ ± 0.1‰) for the Vara and the Erro-Tobbio eclogites range from +5.4 to +6.4‰ and from +3.1 to +5.3‰, respectively, whereas the 87Sr/86Sr ratios range between 0.7036 and 0.7042 for the Vara eclogites and from 0.7030 to 0.7034 for Erro-Tobbio metagabbros. The B isotope compositions for the Vara eclogites range between −3.2 to +0.9‰, which are in striking contrast with the positive δ11B signatures of the Erro-Tobbio eclogitic metagabbros ranging from +4.3 to +8.9‰, the highest positive δ11B signatures observed in eclogitic metagabbros so far. Metarodingite from Vara has a δ18O of +3.7‰ and 87Sr/86Sr of 0.7046 and a δ11B value of +11.5‰.
We argue that the δ18O signatures and the 87Sr/86Sr ratios of metamafic rocks from the Voltri Unit mainly reflect inherited signatures from the seafloor, whereas subduction-related processes are mainly traced through δ11B variations. Progressive B isotope fractionation due to dehydration processes during subduction is responsible for the dominantly negative δ11B signatures of eclogites from the Vara area and elsewhere, whereas the positive δ11B of the high-P metarodingite is compatible with an inherited oceanic signature, although interaction with fluids released by the surrounding serpentinites at peak conditions cannot be ruled out. We discuss three scenarios to explain the positive δ11B imprint reported for the Erro-Tobbio eclogites: (i) inheritance of oceanic signatures, (ii) records of B isotope fractionation from an extreme 11B-rich protolith, or (iii) interaction with 11B-rich slab fluids derived from serpentinite hydration at high-P. Based on these scenarios, two potential implications of global relevance are presented: (1) the transfer of largely unmodified oceanic signatures up to eclogite facies conditions, and (2) the formation of newly (subduction-related) 11B-rich reservoirs. We propose that the protolith composition of the mafic crust controls the development of hydrous phases at high-P thus enabling the preservation of high δ11B signatures. This mechanism of chemical transfer may generate a 11B-rich reservoir that, together with high-P serpentinites, may form positive δ11B domains in the mantle.