Abstract
More than two-thirds of the Earth's volcanic output occurs in the ocean basins, much of it along the global mid-ocean ridge system. As no active mid-ocean ridge eruptions have been directly observed, our knowledge of eruption dynamics (eruption rates, flow velocities) in this environment are extremely limited. Eruption dynamics represent a primary parameter controlling length scales of volcanic deposition and provide some of the best indicators of the state (e.g., overpressure) of crustal magma storage reservoirs. Lava flow morphology provides a useful, but qualitative proxy for eruption dynamics. I will describe a novel geospeedometer for mid-ocean ridge eruptions, CO₂ degassing and vesiculation, that allows me to place quantitative constraints on eruption duration and flow rates. We use ion microprobe to measure dissolved volatiles in glasses from a suite of samples collected at ~200 m intervals along a 2.5 km long flow path within the 2005-06 eruption. The range of CO₂ concentrations along the flow path is nearly as large as observed in a collection of individual samples representing tens of flows along the ridge crest. Using vesicle size distribution analyses we find that the range in CO₂ reflects degassing from supersaturated conditions near eruptive vents towards equilibrium at the distal end of the flow by diffusion-controlled vesicle growth. Volatile concentrations and vesicle properties are used to constrain dynamic models of degassing and provide estimates of flow duration and time/distance-dependent flow rates. In addition to CO₂ degassing, this sample suite provides information on the nature of geochemical variability within mid-ocean ridge eruptions and the causal links to physical processes to volcanic emplacement.