Chlorine isotope evidence for crustal recycling into the Earth's mantle

Subduction of oceanic lithosphere is a key feature of terrestrial plate tectonics. However, the effect of this recycled crustal material on mantle composition is debated. Ocean island basalts (OIB) provide direct insights into the composition of Earth's mantle. The distinct composition of the HIMU (high (238)U/(204)Pb)- and EM (enriched mantle)-type OIB mantle sources may be due to either recycling of oceanic crust and sediment into the mantle or metasomatic processes within the mantle. Chlorine derived from seawater or crustal fluids potentially provides a tracer for recycled material. Previously reported delta(37)Cl values for mid-ocean ridge basalts (MORB) range from ca. -3.0 to near 0 parts per thousand. In contrast to MORB, we find a larger variation in OIB glasses representing HIMU- and EM-type mantle sources based on replicate SIMS analyses with delta(37)Cl values ranging from -1.6 to + 1.1 parts per thousand for HIMU-type and -0.4 to +2.9 parts per thousand for EM-type lavas. These delta(37)Cl values correlate positively with (87)Sr/(86)Sr ratios for both the HIMU- and EM-type samples. The negative delta(37)Cl values of some HIMU-type lavas overlap with those of altered oceanic lithosphere, which is assumed to be present in the HIMU source. The EM lavas have high (87)Sr/(86)Sr and primarily positive delta(37)Cl values. We hypothesize that subducting sediments may have developed high delta(37)Cl values by expelling (37)Cl-depleted pore fluids, thus accounting for the positive (37)delta Cl values recorded in the EM-type lavas. (C) 2010 Elsevier B.V. All rights reserved.