Major volatiles in the Earth's mantle beneath mid-ocean ridges and intraplate ocean islands
Résumé
Composition of the Earth's ocean-atmosphere system is maintained over geologic timescale via outgassing of volatile elements from the Earth's mantle. Two major settings where volcanic outgassing takes place from the oceanic mantle are the mid-ocean ridges (MORs) and intraplate ocean islands (OIs). The outgassing flux of various volatiles such as water (H2O), carbon dioxide (CO2), sulfur (S), nitrogen (N), and halogens (e.g., chlorine, Cl) depend on their abundances, storage mechanisms, and fates during mantle partial melting and magma emplacement in the crust. In this chapater, we synthecize existing constraints on the major volatiles in the mid-ocean 18 ridge basalts (MORBs) and ocean island basalts (OIBs) and their mantle sources. We present and discuss various techniques and approaches used to gather concentration, speciation, and elemental and isotopic compositions of carbon (C), hydrogen (H), sulfur (S), nitrogen (N), and halogens in submarine glasses, melt inclusions, and mantle-derived gases. We summarize literature estimates of elemental concentrations and isotopic compositions of the volatiles for MORB and OIB source mantles as well as the flux estimates of these volatiles from these mantle domains. To gain process-level understanding of extraction of the major volatiles from MORB and OIB source mantles, we also evaluate the storage mechanisms in the mantle, and the fates and influence of different volatiles during mantle melting. We discuss the relative efficiencies of release via mantle melting and exsolution for each of the major volatiles and how the redox states of the mantle impact such processes. Finally, we provide a perspective for open questions and challenges in the knowledge of volatiles and volatile-involved melting for the MORB and OIB source mantles from the Hadean to present.
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