Tracking halogens through the subduction cycle

The flux of halogens into the Earth’s mantle at subduction zones is a critical yet poorly constrained parameter in the geochemical evolution of the planet. Here we report the first ever combined high-precision measurements of chlorine, bromine, and iodine for backarc basin basalt (BABB) and ocean island basalt (OIB) glasses. The measurements were undertaken in order to evaluate the depth and extent of the halogen subduction cycle by comparing: (1) melts formed in the Manus Basin (Papua New Guinea) proximal to a modern subduction zone, and (2) melts formed from enriched mantle (EM) reservoirs that have been linked to ancient subduction recycling [EM1 and EM2 sampled by the Pitcairn and Society seamounts (central Pacific Ocean), respectively]. As expected from previous studies, the BABBs are strongly enriched in chlorine relative to other trace elements and mid-oceanic ridge basalts (MORB); however, the combined Br/Cl and I/Cl data provide additional insights. The BABBs have I/Cl weight ratios of up to 5.3 × 10–4, that are up to five times higher than typical MORB; and the BABBs with the highest I/Cl have Br/Cl ratios of 2–3 × 10–3, that are lower than typical MORB, and significantly lower than either iodine-rich sediments or seawater-derived sedimentary pore fluids. The final breakdown of iodine-rich serpentine is considered the most likely source of the halogen enrichment in the BABB, suggesting that subduction of serpentinized peridotites enables transport of strongly incompatible, fluid-mobile, volatile elements, like iodine, beyond zones of arc-magma generation. The Pitcairn and Society melts exhibit a remarkable correlation between K/Cl and 87Sr/86Sr. The K/Cl ratios vary from MORB-like values of ~15 to maxima of ~40 in the isotopically most enriched EM end members. The trend reflects the lower subduction efficiency of halogens compared to K and other lithophile elements. Melts formed from EM and MORB mantle reservoirs have very similar Br/Cl and I/Cl weight ratios of 3.6 ± 0.8 × 10–3 and 85 ± 42 × 10–6 (2 ) respectively, that could indicate that subducted volatiles have been mixed throughout the mantle.