Crustal carbonate assimilation limits and CO2 production within arc magmas - The Jurassic Bonanza arc, Vancouver Island, Canada
Date
2024-01-02
Authors
Morris, Rebecca
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Abstract
The contribution of crustal-derived CO2 into arc magmas is controversial. Traditional views assume most CO2 outgassed at arc volcanoes is slab-derived, but recent studies show that active and ancient systems where arc magmas intrude carbonate may contribute significant crustal CO2. Quantifying CO2 from magma-carbonate reactions within the crust is important as it has implications on how we understand the long-term C cycle. In addition, the mechanism of carbonate assimilation into arc magmas is understudied, especially in the form of well-constrained field exposures that provide in situ reactions. Within this thesis, I document carbonate assimilation limits, CO2 production, and the mechanisms that drive assimilation for different scales of well-exposed magma bodies (m-scale dikes and sills, km-scale pluton) that intrude limestone within the Jurassic Bonanza arc, on Vancouver Island, Canada.
Magma-carbonate reactions preserved in m-scale dikes is observed in two forms: 1) as unique orbicular dikes that outcrop above the level of stratigraphy of basalt-limestone reactions, and 2) as reacted margins (or ‘boundary melts’) preserved at basalt-limestone contacts in dikes and sills. Orbicular dikes consist of segregated Ca-rich microcrystalline orbicules, where orbicule compositions are similar to hybrid melts produced from basalt-limestone experiments. Binary mixing models of limestone into basalt confirms the orbicules form from assimilating up to 25 wt.% limestone into basalt, and are capable of producing up to 11 wt.% CO2. I interpret that orbicules formed from basalt-limestone reactions at depth, where produced calcic melts were transported upwards from recharging magma feeding the dikes in the lower section. Rapid cooling within the dikes preserved these segregated calcic melts in situ, where homogenization with the host melt was limited due to their varying viscosities.
Reacted margins at basalt-limestone contacts in dikes and sills document boundary melts that are distinctly lighter in colour and texturally unique (glassy ± orbicules). Boundary melt compositions show unique Ca, U, and Sr enrichments, Si depletion, and 87Sr/86Sr that approaches host limestone values. Binary mixing models indicate the boundary melts form from ~20 wt.% limestone assimilation into basalt, suggesting that these reacted margins which make up to 4% of dike and sill volumes are capable of producing ~10 wt.% CO2. Contrasting viscosities between the boundary and interior melts appears to promote uphill diffusion of elements more enriched in the wallrock (U, Sr), and is preserved in these reacted margins that likely cooled in minutes. Extreme Sr enrichment (up to 5500 ppm) far above typical basalt concentrations (~400 ppm) in dike and sill interiors is surmised to occur from meltback of channel walls, where Sr-enriched boundary melts may remelt and concentrate in dike interiors where flow velocities are greatest.
Magma-carbonate reactions from a km-scale gabbro pluton that intrudes limestone shows limited carbonate interaction, with the exception of a thin (<2 m) gabbroic chilled margin with elevated U, Sr, REEs, and 87Sr/86Sr ratios. This chilled margin is also in contact with a wide (~50 – 150 m) metamorphic aureole. Modeling of assimilation + concurrent fractional crystallization (AFC) indicates the chilled margin chemistry and mineralogy can be obtained from the uptake of 20 wt.% limestone into gabbro, suggesting this thin reacted veneer is capable of producing ~10 wt.% CO2. A lack of enriched 87Sr/86Sr ratios shows no indication of crustal enrichment from ~10 to >1000 m from the contact, which further constrains that any reaction with carbonate is limited to the outer ~10 m margin of the pluton, accounting for <1% of the total pluton volume.
Results indicate that an enhanced extent of magma-carbonate reaction and CO2 production is via a network of shallow m-scale dikes and sills versus deeper km-scale plutons. Additional estimates on CO2 produced within the metamorphic aureole indicate that >89% of crustal-derived CO2 is liberated via wallrock decarbonation and <11% is liberated by magma from carbonate assimilation. Estimates of CO2 were extrapolated to calculate a flux for the Jurassic Bonanza arc that is notably lower than some present-day fluxes from arc volcanoes intersecting carbonate-rich lithologies (i.e., Etna, Popocatépetl). These lower estimates are likely due to thinner (<1 km) carbonate sequences in the Bonanza arc crustal substrate. Nonetheless, this work details the limited extent to which plutons assimilate limestone and caution any historical flux estimate where a consistent mass fraction of carbonate was assimilated into plutons. Experimental data from others further indicates that assimilation into km-scale magma bodies in thick transcrustal sections is limited with depth. The results from this study ultimately provide realistic and quantitative limits on arc-derived CO2 from upper crustal wallrock sources.
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Keywords
magma-carbonate interactions, assimilation, arc magmatism, CO2 flux