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 olivine-hosted glass inclusions from two pillow lava samples dredged from 4200 m on the southern rift zone of Loihi seamount, Hawaii. Matrix glasses (MgO∼9 wt.%) have H2O, Cl, and B contents considerably in excess of the values expected from mantle melting or fractional crystallization of parental Loihi magmas. Glass inclusions have H2O, Cl, and B contents ranging from the high values of the matrix glasses to lower concentrations that are more typical of Hawaiian magmas. Concentrations of other incompatible elements (e.g., K2O, P2O5, and Be) in matrix glasses and glass inclusions are uncorrelated with their H2O, Cl, and B contents. Glass inclusions show considerable scatter in major element compositions compared to matrix glasses, but except for H2O, Cl, and B, the average glass inclusion composition corresponds well to the matrix glass compositions. We propose that the glass inclusions represent compositionally diverse liquids present within the magmatic 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 to blending and homogenizing and therefore preserves a compositional diversity 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 concentrations in glass inclusions, are best explained by variable extents of assimilation by Loihi magmas of H2O–Cl–B-rich, seawater-derived components prior to eruption. The required assimilants range from material similar in composition to seawater to materials with Cl/H2O and B/H2O ratios much higher than seawater. Our preferred explanation (similar to that suggested for MORB by Michael and Schilling, 1989) [Michael, P.J., Schilling, J.-G., 1989. Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components. Geochim. Cosmochim. Acta, 53, pp. 3131–3143.] is that most of the assimilated materials were brines (or rocks containing brines in inclusions or along grain boundaries) enriched in Cl by high temperature phase separation of seawater in sub-sea-floor hydrothermal circulation systems. Addition of ∼1.0 wt.% of a 15 wt.% NaCl brine can explain the H2O and Cl contents of the matrix glasses. Addition of altered basalt cannot readily account for the Cl and H2O contents of matrix glasses and glass inclusions, but may be required to account for their elevated B contents. The enrichment in Cl and contamination with atmospheric noble gases observed in other samples from Loihi could also result from assimilation of Cl-enriched, seawater-derived components.