Carbon, Halogens and Sulfur: Key Volatiles in the Lithosphere

The role of volatiles in the lithosphere, especially in the C-O-H-S-halogens system, is crucial to upper mantle geodynamic evolution, metasomatism, and melting. Although it is clear that halogen (± sulfur)-rich aqueous fluids exert a key influence on the geochemical signature of the lithosphere in subduction zones, the composition and the distribution of fluids and/or volatile-rich melts in the oceanic and continental lithospheric mantle in intraplate and extensional tectonic settings have been taken into account only in recent times. Potential tracers of the nature of volatiles include fluid and melt inclusions in peridotite xenoliths, which represent proxies for mantle volatiles at lithospheric depths. Here, we present petrological and fluid/melt inclusion studies in peridotite xenoliths in intraplate and extensional tectonic regions of active magmatism, that include Ethiopia, Hawaii, and the Canary Islands. Mantle fluids are CO2-rich, but contain significant, although variable, amounts of H2O, halogens, and sulfur species. Chlorine represents the most abundant halogen (mole fraction up to 0.04), and sulfur is present either as H2S, SO2, or sulfates. Such compositions are unknown from intraplate and extensional settings and show unexpected similarities to fluids preserved in kimberlitic diamonds. Data delineate changes in volatile speciation and redox conditions in the lithosphere, and yield improved insights on how ascending hydrous carbonate-rich melts exsolve aqueous-carbonic fluids enriched in halogens and sulfur, which may be locally immiscible. Carbon, halogens and sulfur in lithospheric mantle fluids support an origin that includes incorporation of recycled crustal sediments and basaltic oceanic crust, away from subduction zones. The composition and the distribution of lithospheric fluids suggest a possible role of recycling in transporting predominantly CO2, H2O, and some fluid mobile elements via paleo-subduction events in the convective mantle.