Crust-Derived CO2 Production From a Shallow Pluton in Limestone Is Driven by Metamorphic Decarbonation, Not Assimilation

Traditionally, it is assumed that contributions of crust-derived CO₂ emissions at arc settings are minor, but this requires well-constrained field studies to determine the extent of magma-carbonate reaction and concomitant C released. The Jurassic Bonanza arc on Vancouver Island (Canada) was built on a Triassic limestone platform and makes for an ideal setting to examine arc magma-carbonate interactions beneath an island arc. We examine how km-scale magma bodies might react with carbonate from a well-constrained study of a gabbro pluton that intrudes limestone. The pluton shows muted to nil carbonate interaction preserving primary igneous ⁸⁷Sr/⁸⁶Sr values (∼0.703) except for a thin (<2 m wide) marginal gabbro (∼0.706) in contact with a decarbonized metamorphic aureole. A lack of ⁸⁷Sr/⁸⁶Sr or δ¹⁸O_cpx enrichment in gabbro ∼10 to >1,000 m from the contact suggests that any reaction with limestone wallrock is limited to at most the outer ∼10 m of the pluton. More enhanced magma-carbonate interaction and CO₂ production occurs via a network of shallow dikes and sills (<0.2 GPa) compared to deeper plutons, consistent with experimental data showing increased carbonate assimilation at shallower depths (≤0.5 GPa). Plutons are an important heat source to release CO₂ from carbonate wallrock by contact metamorphism, where >89% of crust-derived CO₂ is liberated by wallrock decarbonation and <11% is liberated by magma that assimilated limestone. Nonetheless, we show that neither magmatic nor metamorphic crust-derived CO₂ contributions compare to mantle-derived CO₂. Our study places realistic and quantitative limits on arc-derived CO₂ from upper crustal limestone sources.