Deep Degassing of CO2-rich Basanite Magma at El Hierro, Canary Islands: Insights from Melt and Fluid Inclusions

English Abstract:

Basaltic magmas erupted at mid-ocean ridges, subduction zones and intraplate hotspots transfer matter from the mantle to the surface of the Earth. In recent years, increasing research efforts have focused on constraining CO₂ degassing from the mantle, which is a critical, yet poorly resolved input into the carbon cycle. Such estimates of carbon flux rely partly on knowledge of the CO₂ content of primary magmas, the best archive of which is provided by melt inclusions. Here we report on detailed analyses of quenched basanite lava balloon samples from the recent submarine eruption at El Hierro (Canaries), an intraplate ocean island. Laser ablation ICP-MS and ion microprobe analyses of matrix glasses and melt inclusions reveal high concentrations of incompatible trace elements and dissolved volatiles in this magma, with 100 ppm Nb and up to 1.2 wt% CO₂, 3.0 wt% H₂O and 0.5 wt% S. Recent models (e.g., Shishkina et al., 2014) accounting for the effect of alkalinity on CO₂ solubility predict saturation pressures in excess of 200 MPa for most melt inclusions extending up to 1300 MPa. As most melt inclusions contain bubbles, which may have sequestered part of the originally dissolved CO₂ (e.g., Hartley et al., 2014), measured CO₂ concentrations, and hence calculated pressures, are considered minima. In addition, microthermometry on CO₂-rich fluid inclusions from the same basanite samples yield fluid densities between 590 and 860 kg/m³, corresponding to equilibration pressures of 280-580 MPa at 1150 °C. The El Hierro basanite thus had a particularly high volatile-carrying capacity, began exsolving a CO₂-rich fluid at a depth of at least 20 km (but likely as deep as 40 km) in the upper mantle and may have released 4-6 Tg CO₂. These results have further implications for understanding the eruptive behavior at El Hierro and similar volcanoes elsewhere and the volumetrically small but potentially significant contribution of alkaline magmas to volatile fluxes from the mantle.

Hartley et al. (2014), Reconstructing the deep CO₂ degassing behaviour of large basaltic fissure eruptions, Earth and Planetary Science Letters, 393, 120-131.

Shishkina et al. (2014), Compositional and pressure effects on the solubility of H₂O and CO₂ in mafic melts, Chemical Geology, 388, 112-129.