An investigation of mid-ocean ridge degassing using He, CO2, and δ13C variations during the 2005–06 eruption at 9°50′N on the East Pacific Rise

Abstract

We report 3He/4He, He and CO2 concentrations for the dissolved (glass) and vapor (vesicle) phase of basalts, for 23 lavas from the 2005–2006 eruption and 4 lavas from 1991–92 at 9°50′N on the East Pacific Rise. We also determined δ13C and δ18O of CO2 in vesicles in 18 of these lavas. These sample suites provide a rare opportunity to study volatile systematics related to magma recharge at a mid-ocean ridge volcano and to quantify degassing prior to and during the eruptions. Our study covers the spatial and temporal extent of the 2005–06 eruption and is complementary to previous studies of variations in basaltic volatiles with distance from the ridge axis during part of this multi-stage eruption (Soule et al., 2012; Gardner et al., 2016). 3He/4He shows minor variation with a mean value of 8.51 ± 0.03 RA. All but 1 sample lies in a narrow range of 280–420 ppm for total (vesicles + glass) CO2 indicating volatile saturation at 1.2–2.3 km depth in the crust, similar to depths of the seismically imaged magma lenses in the region. Vesicle He and CO2 concentrations vary throughout the lava flow field by factors of 17 and 250, respectively. The vapor phase CO2/He ratio co-varies positively with the fraction of CO2 contained in vesicles due to kinetic fractionation between He and CO2 during vesiculation. Vesicle δ13CPDB co-varies with the fraction of CO2 in vesicles (r2=0.83), ranging from −2.6 to −5.0‰, and straddles the isotope composition of EPR vent fluids. The vesicle 13C-enrichment (δvapor−δmelt) is +3.3‰ and comparable to the experimentally determined fractionation factor for basaltic systems. Collectively the observations indicate closed-system degassing during rapid ascent of melt from the axial magma lens through the conduit to its eruption and emplacement on the seafloor, with minimal bubble loss. In contrast, CO2/Ba systematics and a comparison of δ13C for lavas and vent fluids indicate an earlier period of open-system degassing during which 50% or more of the initial (parental magma) CO2 inventory was lost prior to magma storage in the shallow crust.