On the evolution of large ultramafic magma chambers and timescales for flood basalt eruptions

Large igneous provinces are characterized by magmatic activity on two distinct timescales. While these provinces have total active lifetimes of order 10-30 Ma, most of the erupted volume is emplaced within 1 Ma in many cases. The longer timescale is consistent with plume or tectonic models for mantle melting responsible for flood volcanism, but the shorter main stage timescale is enigmatic. We hypothesize that cessation of main stage eruptions reflects shutoff of dike propagation from the deep crust due to the onset of thermally activated creep on a similar to 1 Ma timescale, with intrusive processes and minor eruptions continuing over 10-30 Ma. To test this hypothesis we model deep magma differentiation and the stability of Moho level magma reservoirs. Assuming mantle volatile contents, fractionation results in concentration and deep exsolution of CO2, with the geothermal gradient and melt influx setting the timescale for buoyancy production and thus timing between individual eruptions. Chemical evolution generally occurs rapidly compared to heat conduction from the magma body. Although the viscous response of surrounding rocks depends on lower crustal rheology, we find that thermally induced creep can reasonably prevent dike propagation within 1 Ma of intrusion. However, if melt influx is high or heat transfer from the magma chamber is low, viscous creep may outpace differentiation. In this regime, continued melt influx spreads along the Moho until external stresses provide a destabilizing trigger. The coevolution of large-scale melting and lower crustal rheology may thus control a progression of large igneous province emplacement from largely eruptive to largely intrusive magmatism.