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- Title
Crust‐Derived CO<sub>2</sub> Production From a Shallow Pluton in Limestone Is Driven by Metamorphic Decarbonation, Not Assimilation.
- Authors
Morris, R. A.; Canil, D.
- Abstract
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 δ18Ocpx 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. Plain Language Summary: Magmas generated at subduction zones interact with crustal rocks prior to eruption or during emplacement. Here, we examine how large (km‐scale) stalled magma bodies react with limestone, a common crustal lithology that can have implications on the amount of CO2 produced from arc volcanoes. We show from a well‐constrained and detailed field study that only a thin margin (<1%) of the pluton documents any evidence of reaction with the limestone wallrock. Our results indicate that CO2 produced from arc magmas interacting with crustal carbonate is minimally produced from the magmas directly (<11%), and is instead largely produced from the decarbonation of wallrock within the metamorphic aureole that surrounds the magma body (>89%). These observations reveal that the dominant crust‐derived CO2 carrier in these systems is transported via fluids in the wallrock, and not by magma. Our study provides a realistic approach for magma‐carbonate interactions for the present and historical C‐cycle, where limestone assimilation into large volumes (>km3) of stalled magma is limited and localized to magmatic margins. Key Points: A km‐scale pluton documents assimilation within a thin margin (<1% of pluton volume) in contact with decarbonated limestone (skarn) wallrockDikes and sills are more efficient than plutons at carbonate assimilation, both of which are restricted with depthCrust‐derived CO2 produced by magma‐carbonate interactions is dominantly sourced by wallrock decarbonation, not magma assimilation
- Subjects
VANCOUVER Island (B.C.); LIMESTONE; ISLAND arcs; SKARN; SILLS (Geology); SUBDUCTION zones; IGNEOUS intrusions
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2024, Vol 25, Issue 6, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2024GC011485