Degassing of Mid-Ocean Ridge Basalts: Quantifying Bubble Nucleation and Growth in the 2005-06 East Pacific Rise Lava

Abstract:

Mid-Ocean basalts (MORB) typically erupt supersaturated with CO₂. Along the 2005-06 East Pacific Rise (EPR) lava flow, the concentration of CO₂ dissolved in MORB glasses decreases with distance from the vent, and is thought to represent degassing of the lava as it flowed across the ocean floor. An increase in maximum bubble radius and porosity with distance from the vent has also been observed in EPR MORB glasses, suggesting that CO₂ degassing occurs through diffusive growth of bubbles present within the melt upon eruption. Additionally, bubble number density increases from vent (1.8x10⁵ bubbles/cm³) to flow front (3.0x10⁶ bubbles/cm³), indicating that bubble nucleation may also play an important role in degassing of the EPR lavas. This study combines CT and optical analyses of a subset of MORB glasses from along the 2005-06 EPR eruption to examine how MORB lava flows degas. Crystallinity of these glasses increases from vent to flow front, and is composed of spherulites, lath-like plagioclase microlites, and blocky to lath-like olivine microlites. Bubbles within these glasses have diameters ranging from 2-446 µm, and are sometimes found attached to the surface of olivine crystals in all examined samples, but not to plagioclase crystals. Porosity increases from 0.01 % to 0.39 % from vent to flow front, in which 72-94 % of this porosity increase results from the growth of pre-existing bubbles, and the remaining 6-28 % is attributed to the nucleation of new bubbles. These results indicate the olivine is an efficient bubble nucleation site in CO₂ supersaturated MORB lavas, and that plagioclase is an inefficient nucleation site, just like it is not in rhyolite. Based upon the supersaturation of CO₂ in vent samples, a ∆P of ~40 MPa is sufficient to initiate heterogeneous bubble nucleation on olivine. Although degassing of these EPR samples is dominated by the diffusive growth of pre-existing bubbles, the role of bubble nucleation is not entirely trivial, and may be limited by the availability of efficient nucleation sites. Bubble nucleation may therefore play a significant role in the degassing of more crystalline MORB lava flows.