Tracing of Cl input into the sub-arc mantle through the combined analysis of B, O and Cl isotopes in melt inclusions

Abstract

The effect that recycling crust and sediments have on the composition of the mantle wedge, in particular in terms of volatiles, is still debated. Chlorine, an important fluid mobile element that has stable isotopes with different concentrations in the terrestrial reservoirs, has the potential to be used to trace slab-derived fluids.Olivine-hosted melt inclusions (OHMIs) provide a first order constraint on the δ37Cl of primary magmas, since they are unaffected by near surface processes. In this study, δ37Cl were coupled with δ11B and δ18O analyses in samples from the Lesser Antilles, Vanuatu, Aeolian, NE Japan and Izu-Bonin arcs. This unique dataset is used to better understand the large δ37Cl variation in melt inclusions from a single sample. OHMIs from the Vulcano (Aeolian arc) and Sukumoyama (Izu-Bonin arc) samples have similar δ37Cl (−2.5 ± 0.5‰ and −2.6 ± 0.8‰, respectively). These are different from δ37Cl in OHMIs from the other three localities (δ37Cl of −0.7 ± 0.6‰ for Aoba (Vanuatu arc) and St. Vincent (Lesser Antilles arc), −1 ± 0.9‰ for Iwate (NE Japan)). Vulcano OHMIs also have statistically different B and O isotope compositions compared to those from the other locations: average δ11B of −5.1 ± 2.9‰ for Vulcano OHMIs, compared to 2.5 ± 3.7‰, 5.2 ± 1.4‰, 7.0 ± 2.2‰, 3.8 ± 7.5‰ for Sukumoyama, Iwate, Aoba and St. Vincent OHMIs, respectively. All OHMIs have δ18O between 4.0 and 7.4‰, except for those from Vulcano, which are significantly different, with δ18O from 7.2 to 9.1‰. Combining these three stable isotope systems suggests that the large variation (>2‰) of δ37Cl in OHMIs from a sample reflects inputs from different sources of Cl rather than heterogeneities in a single main source. Variability between arcs might reflect different major sources of Cl.Comparing OHMIs Cl isotope data from the Aeolian and Izu-Bonin arcs with existing bulk rock Cl isotope data suggest that OHMIs preserve the source signature of Cl input whereas this signal can be lost in whole rocks as a result of Cl isotope diffusive fractionation during Cl degassing. SIMS measurements of Cl isotopes in OHMIs could thus help refine models of Cl cycles in the mantle.