Variations in Magmatic He, CO2 and δ13 C during the 2005-06 Seafloor Eruption Near 9°50'N on the East Pacific Rise

Abstract:

We report dissolved (glass) CO₂ concentrations, plus vesicle He and C isotope compositions and He and CO₂ abundances, for a suite of 23 basalt glasses from the 2005-2006 eruption on the East Pacific Rise. This sample suite provides a unique opportunity to study magma recharge along the mid-ocean ridge, to quantify degassing prior to and during eruption, and to evaluate the extent to which mantle carbon is incorporated into hydrothermal fluids and ultimately into seafloor ecosystems. Our work covers the full spatial and temporal extent of the months-long eruption (ages from ²¹⁰Po-²¹⁰Pb dating), and is complementary to a previous study of volatile changes with distance from the vent during part of the eruptive episode (Soule et al. 2013; EPSL). Vesicle ³He/⁴He is uniform at 8.56±0.04(1σ) R_A. Vesicle He and CO₂ abundances vary by factors of 17 and 230, respectively. The variability in CO₂/He ratio is evidence for kinetic fractionation of He from CO₂ during vesiculation, and reveals the potential for using vesicle CO₂/He as a relative geo-speedometer for the rate of magma ascent and lava emplacement. Nearly all samples lie in a narrow range of 280-370 ppm for total CO₂ (vesicles+glass) suggesting saturation at a depth of ~1.5 km in the crust, similar to the depth of the seismically imaged magma lens. Vesicle CO₂/He co-varies positively with the fraction of CO₂ contained in vesicles (r²=0.94) and negatively with vesicle δ¹³C (r²=0.83). The δ¹³C ranges from -2.6 to -5 per mil, and straddles the value for CO₂ in EPR vent fluids (M. Lilley, pers. comm.). The C isotope results are consistent with vapor phase ¹³C-enrichment during CO₂ exsolution from a basaltic melt, as determined experimentally (Javoy et al. 1978; Contrib Mineral Petrol). Collectively the observations are explained by quasi-closed-system degassing, where melt ascent from the magma lens was sufficiently rapid that minimal bubble loss occurred during the eruption and flow of lava onto the seafloor.