VOLATILES AND THE ERUPTIBILITY OF FLOOD BASALT MAGMAS

Individual flood basalt lavas often exceed 10³ km³ in volume, and many such lavas erupt during emplacement of flood basalt provinces. The large volume of individual flood basalt lavas demands correspondingly large magma reservoirs within or at the base of the crust. The overpressure associated with a new injection of magma is inversely proportional to the total reservoir volume, and as a large magma body heats the surrounding rocks thermally activated creep will relax isotropic overpressure more rapidly. Here, we examine the viability of buoyancy overpressure as a trigger for continental flood basalt eruptions. We employ a new one-dimensional model that combines volatile exsolution, bubble growth and rise, assimilation, and permeable fluid escape from Moho-depth and crustal chambers. We investigate the temporal evolution of degassing and the eruptibility of magmas, using the Siberian Traps flood basalts as a test case. We suggest that the volatile budget set during mantle melting controls ascent of magma into the crust, thereby regulating the tempo of flood basalt magmatism. Volatile-rich melts from low degrees of partial melting of the mantle are buoyant, and erupt frequently from Moho-depth chambers, reaching the surface with little staging or crustal interaction. Melts with moderate volatile budgets accumulate in large, mostly molten magma chambers at the Moho. These large magma bodies may remain buoyant and poised to erupt—triggered by volatile-rich recharge or external stresses—for 10⁵ – 10⁶ years. If and when such chambers fail, enormous volumes of magma can ascend into the crust, staging at shallow levels and initiating substantial assimilation that contributes to pulses of large-volume flood basalt eruptions. Our model further predicts that the Siberian Traps may have released 10¹⁹ − 10²⁰ g of CO₂ during a number of brief (~10⁴ year) pulses, providing a plausible trigger for warming and ocean acidification during the end-Permian mass extinction. The assimilation of carbon-rich crustal rocks strongly enhances both flood basalt eruptibility and CO₂ release, and the tempo of eruptions influences the environmental effects of CO₂, SO₂, and halogen degassing. The eruptive dynamics of flood basalts are thus intertwined with their environmental consequences.