The contribution to exogenic CO₂ by contact metamorphism at continental arcs: A coupled model of fluid flux and metamorphic decarbonation

Abstract

Recent work has suggested a possible temporal coincidence between greenhouse intervals and enhanced arc volcanism, motivating the hypothesis that magmatic and metamorphic CO₂ emissions from volcanic arcs, particularly those intersecting crustal carbonates, may play a strong role in modulating the long-term carbon budget of the exogenic system. When hot fluid exsolving from arc magmas interacts with carbonate sequences on active margins, contact metamorphism releases CO₂ to metasomatic fluids that transport carbon to shallow reservoirs. To estimate the magnitude of CO₂ release, here we model how the infiltration of silica-saturated magmatic water into a porous medium facilitates the decarbonation reaction in contact aureoles. Analytical scalings and numerical simulations show that the propagation rate of the reaction front scales with the ratio of the infiltration flux to the mass of the rate-limiting reactant, and accordingly the CO₂ flux increases linearly with the infiltration flux. This simple relationship allows for scaling to predict regional and global scale CO₂ release at continental arcs if magma emplacement rate is known. Using the global rate of continental arc magma emplacement, we estimate that the present-day contact-metamorphic CO₂ release range from ∼0.06 to 0.9 Tmol/yr, half-to-one orders of magnitude smaller than the field-based estimates of carbon output in modern arcs (1.5–3.5 Tmol/yr). Yet, the extrapolated CO₂ release from Cretaceous continental arcs via simple infiltration-induced decarbonation is comparable to the release from mid-ocean ridges. CO₂ released from continental arcs amplifies the background flux of CO₂ from direct degassing of the magma, and therefore may have been key in causing the climatic greenhouse interval in the Cretaceous when there was heightened arc activity. Thus, our result supports the hypothesis that global arc flare-ups at continental margins effectively increase CO₂ outgassing coinciding with green-house intervals in the geological past. The contribution by arcs to the tectonic CO₂ input could be significant, which needs field-based studies to revise long-term climate models.