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

Xu Chu, Cin-Ty A. Lee, Rajdeep Dasgupta, & Wenrong Cao

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.

Chu, X., Lee, C-T. A., Dasgupta, R. & Cao, W. (2019). The contribution to exogenic CO₂ by contact metamorphism at continental arcs: A coupled model of fluid flux and metamorphic decarbonation. American Journal of Science 319, 631-657. doi:10.2475/08.2019.01