CO2 TRANSPORT IN ARC MAGMAS: EVIDENCE FROM UNIQUE ORBICULAR DIKES IN THE JURASSIC BONANZA ARC, VANCOUVER ISLAND, CANADA

It has traditionally been assumed that outgassed CO₂ at arcs is primarily sourced from the mantle, however increasing evidence suggests the interaction between arc magmas and crustal carbonates may play a larger role. The Jurassic Bonanza arc on Vancouver Island is a well-exposed island arc crustal section, where magmas that fed the arc for >30 Myr interacted with pre-existing Triassic and Permian basement carbonates. Here, we examine magma-carbonate interactions within this arc at the Merry Widow Mountain area of northern Vancouver Island, where accessible and well-exposed contacts between magma and carbonate occur on a variety of scales from km-scale plutons to m-scale dikes. Detailed mapping revealed unique late-stage orbicular mafic dikes that show overwhelming physical and chemical evidence of magmas interacting with subsurface carbonate wallrock (limestone). We test whether arc basaltic dikes can assimilate substantial limestone, and how they might transport a significant carbonate component (or CO₂) within arc systems. We interpret the unique orbicular textures within these dikes to be fluid- and Ca-rich liquids produced from melted wallrock and use experiments on similar basalt-carbonate interactions to support our findings. We show that intense mafic diking within early magmatism of the Jurassic Bonanza arc may have assimilated up to 16% limestone to produce up to 7 wt.% CO₂. Our results help to inform on the amount and mechanism of CO₂ transport in arc magmas.