CO2 transport at shallow depths in arc magmas: evidence from unique orbicular dikes in the Jurassic Bonanza arc, Vancouver Island, Canada
Rebecca Morris, Dante Canil
Abstract
A growing body of evidence suggests the interaction between arc magmas and crustal carbonates may play a large role in outgassed CO 2 at arcs. We examine magma–carbonate interactions within the shallow (< 0.2 GPa) crust in the Jurassic Bonanza arc on Vancouver Island, a well-exposed island arc crustal section. Detailed mapping in the Merry Widow mountain area revealed unique, late-stage orbicular mafic dikes that only occur above the stratigraphic level of subsurface carbonates. The orbicular dikes are primitive, show physical and chemical evidence of interaction with limestone, including high CaO/SiO 2 and low REE (~ 10–20× chondrite) that correlate with orbicule abundance, and are geochemically discordant from other rocks of the Bonanza arc. The orbicules have identical intergranular textures to the host melt but markedly higher Ca/Si. Simple binary mixing and MELTS modeling indicates the orbicular dike compositions are a mixture of a primitive hydrous arc basalt with 3–25% limestone addition. Comparisons to published experimental data on basalt–calcite reaction suggest some of the dike compositions result from > 25% calcite assimilation, producing up to 11 wt% CO 2 , orders of magnitude higher than the CO 2 solubility of the parent melt (0.11 wt% CO 2 ). We interpret the orbicules as Ca-rich hybrid melts produced from limestone assimilation that did not homogenize with the host dike magma and underwent crystallization during rapid ascent, possibly propelled by the excess CO 2 . Our results inform on the amount and mechanism of CO 2 transport at low crustal pressures (< 0.5 GPa) in island arcs built on carbonate platforms. We estimate the CO 2 flux of the Jurassic Bonanza arc to have ranged from 0.14 to > 1.16 Tg CO 2 /year during its ~ 34 Myr lifespan.