Metasomatism in the Finero Phlogopite Peridotite: New insights from C and N concentrations and δ13C - δ11B signatures

Abstract

The interconnection between element recycling and mantle metasomatism may provide powerful insights to unravel the mechanisms governing the transfer, partitioning and residence times of volatiles among the different Earth's reservoirs. In this study, we provide new constraints on the incorporation of C and N in the rock-forming minerals of the Finero Phlogopite Peridotite (FPP) from the Ivrea-Verbano Zone (Southern Alps, Italy), representing a natural laboratory to shed lights on the amounts of volatiles that may be fixed in a deeply metasomatized sub-continental lithospheric mantle (SCLM). Constraints on the relationships between metasomatic events and volatile element enrichment were given by in-situ LA-ICP-MS and micro-Raman investigations together with C (δ13C) and B (δ11B) isotopes.All rock-forming minerals (olivine, clinopyroxene, amphibole and phlogopite) show enrichments in C (270–690 ppm) and N (7–20 ppm) compared to depleted mantle values. The high volatile contents in clinopyroxene and in amphibole are associated with enrichment in incompatible elements (e.g., Li, B, Rb, Sr, Pb, Th, U) and high LaN/LuN ratios (up to 42.0) thus confirming that the metasomatic agent affecting the FPP derive form a hydrous melt with crustal affinity enriched in C and N. Micro-Raman investigations document the occurrence of graphite, carbonate and N2 inclusions, specifically in olivine and clinopyroxene, that are likely responsible for the high CN concentrations reported here. The δ13C signatures measured in mineral separates range from −12.6 to −22.7 ‰, suggesting an overprinting of the C mantle signature by a 12C-rich crustal metasomatic agent. High temperature decarbonation processes, simulated by heating the samples up to 900 °C, indicate that up to10 ppm of C can be residually stored in the SCLM, whereas N is completely released even at lower temperature. The LA-MC-ICP-MS in-situ δ11B signature of amphibole is −5.4 ± 4.1 ‰ (2σ, n = 18), whereas bulk δ11B of olivine and clinopyroxene is −5.51 ± 0.01 ‰ (2σ) and − 1.82 ± 0.06 ‰ (2σ), respectively. Modelling shows that residual slab melts plus variable amounts of 11B-rich serpentinite-derived fluids may account for these B isotope imprints. A subduction-related metasomatizing event during the FPP's evolution is thus proposed to explain the geochemical features documented in our study, in agreement with previous works. At global scale, our results suggest that the metasomatized SCLM might significantly contribute to the so-called hidden C reservoir invoked to match the global C mass balance calculations.