Volatiles in Basaltic Glasses from Loihi Seamount, Hawaii: Evidence for a Relatively Dry Plume Component

New H2O, CO2 and S concentration data for basaltic glasses from ‘hotspot’, with temperature differences of 200°C or more between hotter upwelling plumes and the ambient mantle Loihi seamount, Hawaii, allow us to model degassing, assimilation, adiabat (e.g. White & McKenzie, 1989; Campbell, 1998; and the distribution of major volatiles within and around the Davies, 1998). In contrast, it has long been known that Hawaiian plume. Degassing and assimilation have affected CO2 ocean island basalts (OIB) are enriched in volatiles relative and Cl but not H2O concentrations in most Loihi glasses. Water to depleted mid-oceanic ridge basalts (MORB), leading concentrations relative to similarly incompatible elements in Hato speculation that the excess magmatism associated with waiian submarine magmas are depleted (Loihi), equivalent (Kilauea, plumes is related to a mantle ‘wet spot’ (Schilling et al., North Arch, Kauai–Oahu), or enriched (South Arch). H2O/Ce 1980) or a ‘not-so-hot-spot’ (Bonatti, 1990). Thus, there ratios are uncorrelated with major element composition or extent or is still lively debate over the relative importance of ‘hot’ depth of melting, but are related to position relative to the Hawaiian and ‘wet’ in the generation of mantle plumes. plume and mantle source region composition, consistent with a zoned The knowledge that OIB are wetter than MORB, plume model. In front of the plume core, overlying mantle is however, does little to answer the question of the origin metasomatized by hydrous partial melts derived from the Hawaiian of volatiles in plume basalts. If the enrichments of volatile plume. Downstream from the plume core, lavas tap a depleted source elements in OIB are proportional to those of nonvolatile region with H2O/Ce similar to enriched Pacific mid-ocean ridge incompatible elements, then their higher concentrations basalt. Within the plume core, mantle components, thought to can be accomplished through simple mineral–melt fracrepresent subducted oceanic lithosphere, have water enrichments tionation processes. In contrast, if the volatile elements equivalent to (KEA) or less than (KOO) that of Ce. Lower H2O/ are decoupled from major and trace elements, then more Ce in the KOO component may reflect efficient dehydration of the complex processes must take place, including involvement subducting oceanic crust and sediments during recycling into the and possible migration into or out of the plume of a deep mantle. separate C + H + O fluid phase, mixing of source regions having different volatile contents, or shallow-level processes such as assimilation or degassing. In particular,