The eruptibility of magmas at Tharsis and Syrtis Major on Mars

Magnetic and geologic data indicate that the ratio of intrusive to extrusive magmatism (the I/E ratio) is higher in the Tharsis and Syrtis Major volcanic provinces on Mars relative to most volcanic centers on Earth. The fraction of magmas that erupt helps to determine the effects of magmatism on crustal structure and the flux of magmatic gases to the atmosphere and also influences estimates of melt production inferred from the history of surface volcanism. We consider several possible controls on the prevalence of intrusive magmatism at Tharsis and Syrtis Major, including melt production rates, lithospheric properties, regional stresses and strain rates, and magmatic volatile budgets. The Curie temperature is the minimum crustal temperature required for thermal demagnetization, implying that if the primary magnetic mineral is magnetite or hematite, the crust was warm during the intrusive magmatism reflected in Tharsis and Syrtis Major I/E ratios. When wall rocks are warm, thermally activated creep relaxes stresses from magma replenishment and regional tectonics, and eruptibility depends on buoyancy overpressure. We develop a new one-dimensional model for the development of buoyancy in a viscous regime that accounts for cooling, crystallization, volatile exsolution, bubble coalescence and rise, fluid egress, and compaction of country rock. Under these conditions, we find that initial water and CO2 contents typically <1.5wt % can explain the observed range of intrusive/extrusive ratios. Our results support the hypothesis that warm crust and a relatively sparse volatile budget encouraged the development of large intrusive complexes beneath Tharsis and Syrtis Major.