CO2 PRODUCTION FROM DIKES VS. PLUTONS IN CARBONATE WALLROCK: MECHANISMS DRIVING MAGMA-LIMESTONE INTERACTIONS WITHIN THE JURASSIC BONANZA ARC

The contribution of CO₂ from crustal carbonates into arc magmas is debated, as well as its role in the long-term C cycle of the crust-mantle system. To better understand these contributions, studies are required on the mechanism(s) that drive CO₂ production at arc settings from magma-carbonate interactions. The well-exposed Jurassic Bonanza arc on Vancouver Island, British Columbia, was built on a Triassic limestone platform and makes for an ideal field setting to examine magma-carbonate interactions beneath an island arc at various depths. We report results from two regions of the arc (Merry Widow, Iron Hill), where contacts between magma and limestone wallrock can be examined in detail on a variety of scales, from m-scale dikes and sills to km-scale plutons. Vertical exposures offer a view of the extensive mafic diking that intrudes the limestone wallrock. Dikes and sills in contact with limestone display unique reacted margins that are distinctly lighter in colour. In addition, drillcore from the Merry Widow location provides a 3D view of a heterogenous and >100 m wide aureole of skarn and polymict breccia between the pluton margin and limestone.

Our results show strong evidence of limestone assimilation within dikes and sills with ⁸⁷Sr/⁸⁶Sr ratios similar to, or on a mixing line with the limestone wallrock (0.708 ± 0.001). Other chemical parameters (Ca, U, Sr, LREEs) from dike margins further supports limestone assimilation. In contrast, the plutonic rocks show muted to no detectable interaction with limestone, where ⁸⁷Sr/⁸⁶Sr ratios are primary igneous values (0.703 ± 0.001), except for a thin (<2 m) chilled margin with elevated ⁸⁷Sr/⁸⁶Sr ratios (0.706 ± 0.001). Our results show a far more enhanced extent of magma-carbonate interactions, and ultimately CO₂ production, via a network of shallow dikes and sills. While plutons may be an important heat source to release CO₂ from carbonate wallrock (i.e., contact metamorphism), they show little (if any) limestone assimilation. These results are used to place realistic and quantitative limits on arc-derived CO₂ from upper crustal sources.