Abstract:
Mid-ocean ridge basalts (MORBs) exhibit a wide range of CO2 concentrations,
reflecting saturation to supersaturation (and rarely undersaturation) relative to their
emplacement depths. In this study, we explore the mechanisms of CO2 degassing and the
implications this has for estimating the advance rates and durations of seafloor eruptions.
We present dissolved volatile concentrations (mainly of CO2 and H2O) and vesicle size
distributions (VSDs) for a unique suite of MORB glasses collected at the East Pacific
Rise, ~9° 50′ N. These MORB glasses were collected at ~200 m intervals along an
across-axis track over a single flow pathway within the recently emplaced 2005-06
eruption boundaries; systematic sample collection provides one of the first opportunities
to characterize intra-flow geochemical and physical evolution during a single eruption at
a fast-spreading ridge. Compared to measurements of MORB volatiles globally,
dissolved H2O concentrations are relatively uniform (0.10 - 0.16 weight percent),
whereas dissolved CO2 contents exhibit a range of concentrations (154 - 278 ppm) and
decrease with distance from the EPR axis (i.e., eruptive vent). Ion microprobe analyses
of dissolved volatiles within the MORB glasses suggest that the magma erupted
supersaturated (pressure equilibrium with 920 - 1224 mbsf) and in near-equilibrium with
the melt lens of the axial magma chamber (~1250 - 1500 mbsf), and degassed to near
equilibrium (299 - 447 mbsf) with seafloor depths over the length of the flow. The
decrease in CO2 concentrations spans nearly the full range of dissolved CO2 contents
observed at the EPR and shows that the varying degrees of volatile saturation that have
been observed in other MORB sample suites may be explained by degassing during
emplacement. Vesicularity (0.1 - 1.2%) increases with decreasing dissolved CO2
concentrations. We use vesicle size distributions (VSDs)—vesicle sizes and number
densities—to quantify the physical evolution of the CO2 degassing process. VSDs
suggest that diffusion of CO2 into preexisting vesicles, and not nucleation of new
vesicles, is the dominant mechanism of increasing CO2 in the vapor phase. We also use
VSDs, along with estimates of vesicle growth rates, to constrain emplacement time of the
2005-06 eruption to <~24 hours and to resolve variations in advance rate with downflow
distance.
