Assimilation of seawater-derived components in an oceanic volcano: evidence from matrix glasses and glass inclusions from Loihi seamount, Hawaii

We report major element, H2O, Cl, B, and Be analyses of matrix glass and ol ivine-hosted glass inclusions from two pillow lava samples dredged from 420 0 m on the southern rift zone of Loihi seamount, Hawaii. Matrix glasses (Mg O similar to 9 wt.%) have H2O, Cl, and B contents considerably in excess of the values expected from mantle melting or fractional crystallization of p arental Loihi magmas. Glass inclusions have H2O, Cl, and B contents ranging from the high values of the matrix glasses to lower concentrations that ar e more typical of Hawaiian magmas. Concentrations of other incompatible ele ments (e.g., K2O, P2O5, and Be) in matrix glasses and glass inclusions are uncorrelated with their H2O, Cl, and B contents. Glass inclusions show cons iderable scatter in major element compositions compared to matrix glasses, but except for H2O, Cl, and B, the average glass inclusion composition corr esponds well to the matrix glass compositions. We propose that the glass in clusions represent compositionally diverse liquids present within the magma tic plumbing system at Loihi that were mixed and homogenized to produce the liquid that quenched to the matrix glass on eruption. This range of liquid compositions present at depth was trapped by crystallizing olivine prior t o blending and homogenizing and therefore preserves a compositional diversi ty not present in erupted whole rocks. The high H2O, Cl, and B contents of matrix glasses and some glass inclusions, and the range of H2O, Cl, and B c oncentrations in glass inclusions, are best explained by variable extents o f assimilation by Loihi magmas of H2O-Cl-B-rich, seawater-derived component s prior to eruption. The required assimilants range from material similar i n composition to seawater to materials with Cl/H2O and B/H2O ratios much hi gher than seawater. Our preferred explanation (similar to that suggested fo r MORE by Michael and Schilling, 1989) [Michael, P.J., Schilling, J.-G., 19 89. Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawat er-influenced components. Geochim. Cosmochim. Acta, 53, pp. 3131-3143.] is that most of the assimilated materials were brines (or rocks containing bri nes in inclusions or along grain boundaries) enriched in Cl by high tempera ture phase separation of seawater in sub-sea-floor hydrothermal circulation systems. Addition of similar to 1.0 wt.% of a 15 wt.% NaCl brine can expla in the H2O and Cl contents of the matrix glasses. Addition of altered basal t cannot readily account for the Cl and H2O contents.