A Global Compilation of MORB CO2 and H2O: Implications for Magma Ascent and Eruption Dynamics

Abstract:

Observations of volcanic morphology across the global mid-ocean ridge system indicate a strong correlation between spreading rate and eruption rate. Recent studies of vesicle population properties within MORB samples suggest a similar correlation between spreading rate and magma ascent rate. This study examines dissolved CO₂ and H₂O analyses in MORB glasses from a comprehensive sample suite – compiled from the literature (n = 300) and from new measurements (n = 100) – spanning nearly the full range of spreading rates to investigate patterns in supersaturation. We find that samples from ridges with spreading rates <6 mm yr^-1 show a distinctly different population of supersaturations and indicate a fundamental change in magma ascent rate and degassing. We explore the proposed mechanisms that control magma ascent and decompression rates in these systems including viscosity variations, tectonic stress variations, and dynamic decompression of melt reservoirs. Our results suggest that variation in ascent path length, controlled by the thermal structure of the ridge most likely exerts a first-order control on magma dynamics and hence degassing in these systems. We use these results to examine variations in the extent of degassing across the global mid-ocean ridge as a means to refine predictions of CO₂ flux.