SIMS chlorine isotope analyses in melt inclusions from arc settings

Abstract

The relative contribution of volatiles from altered oceanic crust, serpentinized mantle, and subducted sediments on the composition of primary arc magmas is poorly constrained. We measured the chlorine stable isotope composition in olivine-hosted melt inclusions in order to provide a first order constraint on the δ37Cl values of primary magmas, since melt inclusions are not or only little affected by near surface processes. Chlorine isotope analyses were obtained with a CAMECA IMS 1280-HR at the University of Lausanne. A series of six Cl-bearing glass standards with δ37Cl values of −1.1 to +1.7‰, SiO2 between 50 and 76wt% and Cl concentration between 0.15 and 3.25wt% were used for calibration. We determined that SiO2, Al2O3 and K2O in glasses are primarily responsible for the observed variations in instrumental mass fractionation.We obtained a δ37Cl reproducibility on the standard glasses typically better than 0.3‰ (2SD) and a total uncertainty, corresponding to the propagated error of SIMS and Pyro-IRMS uncertainty, of 0.4‰ (2SD). Melt inclusions from Lesser Antilles, Vanuatu and Aeolian arcs display ranges of δ37Cl values from −1.9 to +0.6‰, −1.7 to +0.4‰ and −3.4 to −1.4‰ respectively. Combined with Cl/K2O ratios, these data suggest that Cl addition mainly originates from serpentinites (lithospheric serpentinites or serpentinized mantle wedge) in the Lesser Antilles and Vanuatu arcs. In contrast, Cl added to the mantle wedge beneath the Aeolian Islands may record a higher proportion of fluids derived from the subducting sediments. The observed differences of δ37Cl values in melt inclusions from the Lesser Antilles and Vanuatu arcs and the Aeolian Islands may directly reflect the different subduction geometries present.