CO2 variability in mid-ocean ridge basalts from syn-emplacement degassing: Constraints on eruption dynamics

Abstract

Basaltic glasses produced during mid-ocean ridge (MOR) eruptions display a wide range of dissolved CO2 concentrations with respect to equilibrium saturation at their eruption depth. This variability is thought to reflect the dynamics of magma ascent and emplacement, with rapid ascent and depressurization leading to supersaturated conditions and slower ascent resulting in equilibrium saturation. In this study we examine a suite of samples from the 2005–06 eruption of the East Pacific Rise (EPR), collected at ~200m intervals along two well-defined flow pathways within a single seafloor eruption to examine syn-emplacement degassing, using measurements of vesicularity, crystallinity, volatile contents, and helium. This new data set is unique because flow pathways of individual MOR lava flows have rarely been mapped and have never been systematically sampled. Here we show that a large range in dissolved CO2 concentrations exists within this single eruption that nearly spans that observed in tens of flows measured along this portion of the EPR crest. This lava flow experienced equilibrium degassing of dissolved CO2 from supersaturated conditions at the vent (and persisting over the first 750m of flow) toward equilibrium with seafloor pressures along a ~2.5km-long flow path. This was accompanied by an increase in vesicularity and characteristic bubble radius along the flow, indicating the importance of bubble growth by diffusion for degassing. Nearly constant total helium concentrations (melt+vesicles) indicate that no bubbles were lost during emplacement. With a model of diffusion-controlled bubble growth constrained by observations from the samples along the largest flow lobe of the eruption, we calculate minimum ascent rates of 0.15m/s, and – for this portion of the eruption – an eruption duration of ~30h and flow rates from 0.02 to 0.12m/s. This study represents the first quantitative assessment of eruption dynamics using volatiles in a mid-ocean ridge lava flow.