Contrasts in Dissolved CO2 and Bubbles Between MORB and Submarine Flood Basalts: Insights into the Duration, Scale and Style of Lava Emplacement

Abstract:

CO₃^2- is oversaturated and exsolves as CO₂ bubbles from virtually all ascending basaltic magmas. Bubbles may ascend with the magma, or separate if magma ascent slows or halts. On fast-spreading ridges (FSR), most magmas ascend from an axial magma lens. Their Total CO₂ (bubbles + dissolved) often reflects the lens' depth, suggesting that lavas ascended into the lens, exsolved CO₂ and lost early-formed bubbles. New bubbles form when magmas erupt from the lens. Rapidly emplaced lavas on FSR have glass that is supersaturated with CO₂ and has very few bubbles, while slowly emplaced lavas have more CO₂ in bubbles and less dissolved CO₂, (approaching saturation equilibrium) reflecting a greater time as a liquid at seafloor pressure. MORB from slow-spreading ridges typically have larger vesicles and greater vesicularity and widely variable dissolved CO₃^2-, reflecting final ascent from greater depth with less loss of bubbles from the first stage [2]. Total CO₂varies widely, reflecting variable ascent rate and bubble loss.

In sharp contrast to MORB, Cretaceous flood basalts of Ontong Java Plateau (OJP) have low dissolved CO₃^2- and virtually no vesicles, suggesting that they were saturated or undersaturated with CO₃^2- when they were emplaced. We suggest that these lavas remained liquid on or near the seafloor for a much longer duration so that they degassed to equilibrium levels of dissolved CO₂ and lost all late-stage bubbles. Their dissolved H₂O+CO₂content thus reflects their eruption depth.

The difference between paleoeruption depth and reconstructed basement depth for several sites from OJP and nearby basins varies by up to 2km. It is unlikely that plateau uplift and subsidence accounts for such variations. The observations, together with high Cl and extreme homogeneity of lavas over large distances, are best explained by very large-volume eruptions whose lavas flowed for 100s of km, into deeper water over gentle slopes (0.1-0.5°), so that emplacement depth was shallower than eruption depth. The presence of many glass layers within the ODP cores contrasts with continental flood basalts and suggests the flows were covered by a thick, moving, shifting carapace of solidified lava. They might represent an extreme form of inflated pahoehoe flows, or even a mobile lava lake.

[1] Soule et al. 2012 EPSL [2]Chavrit et al. 2012 EPSL