The Chlorine Isotope Composition of Earth's Mantle

Abstract

Chlorine stable isotope compositions (δ^(37)Cl) of 22 mid-ocean ridge basalts (MORBs) correlate with Cl content. The high-δ^(37)Cl, Cl-rich basalts are highly contaminated by Cl-rich materials (seawater, brines, or altered rocks). The low-δ^(37)Cl, Cl-poor basalts approach the composition of uncontaminated, mantle-derived magmas. Thus, most or all oceanic lavas are contaminated to some extent during their emplacement. MORB-source mantle has δ^(37)Cl ≤ –1.6 per mil (‰), which is significantly lower than that of surface reservoirs (∼ 0‰). This isotopic difference between the surface and deep Earth results from net Cl isotopic fractionation (associated with removal of Cl from the mantle and its return by subduction over Earth history) and/or the addition (to external reservoirs) of a late volatile supply that is ^(37)Cl-enriched.