Spatial and Depth‐Dependent Variations in Magma Volume Addition and Addition Rates to Continental Arcs: Application to Global CO₂ Fluxes since 750 Ma

Abstract

Abstract Magma transfer from the mantle to the crust in arcs is an important step in the global cycling of elements and volatiles from Earth's interior to the atmosphere. Arc intrusive rocks dominate the total magma mass budget over extrusive rocks. However, their total volume and rate of addition is still poorly constrained, especially in continental arcs. We present lateral (forearc to backarc) and depth‐dependent (volcanics to deep crust) magma volume additions and arc‐wide magma addition rates (MARs) calculated from three continental arc crustal sections preserving magma flare‐up periods. We observe an increase in volume addition with depth and less magma added in the forearc (~15%) and backarc (~10% to 30%) compared to the main arc. Crustal‐wide MARs for each section are remarkably similar and around 0.7–0.9 km 3 /km 2 /Ma. MARs can be used to estimate CO 2 fluxes from continental arcs. With initial magma CO 2 contents of 1.5 wt.%, global continental arc lengths, and MARs, we calculate changes in C (Mt/year) released from continental arcs since 750 Ma. Calculated present‐day global C fluxes are similar to values constrained by other methods. Throughout the Phanerozoic, assuming equal durations of flare‐up and lull magmatism, calculated continental CO 2 flux rates vary between 4 and 18 Mt C/year with highest values in the Mesozoic. These fluxes are considered minima since the intake of mantle and/or crustal carbon is not considered. Magmatic episodicity in continental arcs and changes in arc thickness and width are critical to consider when calculating MARs through time.