Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses

We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85 degrees E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO2-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO2 contents in the melt inclusions extend to higher values (167-1596 ppm) than in the co-existing glasses (187-227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO2 melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (similar to 4 km) and similar to 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25-40 km) and cold (1240 degrees-1325 degrees C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous: this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9-20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO2 is perfectly incompatible, we show that the highest CO2 concentrations of the melt inclusions (similar to 1600 ppm) are consistent with the calculated CO2 concentrations of primary undegassed melts. The highest measured CO2/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO2 content of 134 ppm and would result in a global ridge flux of 2.0 x 10(12) mol CO2/yr. (C) 2009 Elsevier B.V. All rights reserved.