Quantifying global melt flux and degassing rate from global mantle convection models with plate motion history

Abstract:

How does the Earth’s deep mantle convection affect surface climate change? Volcanism in various geological settings, including  mid-ocean ridges, volcanic arcs, rift zones and sites with intraplate volcanism, releases volatiles to Earth’s surface. The amount and composition of these volatiles influence the evolution Earth’s ocean, crust and atmosphere, which in turn control the evolution of the biosphere. While there are constraints of Earth’s degassing from the geochemistry of samples  in some localized regions, a quantification of the time evolution of degassing on a global scale remains largely unknown.

In this study, we run geodynamical calculations with a full 3D spherical geometry to explore the amount of partial melting in the shallow part of Earth’s mantle and implied degassing at a global scale. The plate motion history for the last 200 Ma or longer is employed as time-dependent velocity boundary condition for mantle flow. Using the temperature, pressure and composition in mantle convection models, we calculate the degree of partial melting in different geological settings. We show that the melt flux at mid-ocean ridges generally increases linearly with the speed of plates, with some perturbations due to changes of length of mid-ocean ridges. Generally, this melt flux is about 2-3 times in the past 200 million years than that of the present-day Earth. The present-day melt flux is ~20 km³/year, but this value reaches ~40 km³/year at about 80Ma, and ~60 km³/year at about 120-160Ma. Given estimates of volatile content in the source regions where partial melting occurs and the melt flux we calculate, we quantify the evolution of degassing rate (of CO₂) at mid-ocean ridges, hotspots, large igneous provinces, and subduction zones.