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Investigating Noble Gas Outgassing Dynamics in MORB and OIB Magmas with a New Lattice Boltzmann Method

Thomas Williams , Christian Huber

Abstract

Noble gas fractionation during magma ascent is governed by ascent rates, volatile concentrations, and the dynamics of bubble growth and nucleation. When degassing occurs rapidly relative to diffusion, kinetic fractionation effects can arise. Because heavier noble gases diffuse more slowly than lighter noble gases, they are more sensitive to these effects, creating noble gas ratios in melts (e.g., 4He/40Ar) that differ from those generated by solubility-controlled equilibrium degassing. However, bubble nucleation during ascent can counteract kinetic fractionation by reducing diffusional length scales. The extent of kinetic fractionation recorded in MORB and OIB magmas remains poorly constrained, but an increased understanding of this process could improve estimates of volatile fluxes from the mantle to the atmosphere (Tucker, 2018). To address this, we developed a Lattice Boltzmann model of bubble growth that incorporates noble gas diffusion, bubble dynamics, and nucleation events. This method is based on statistical mechanics, where continuum equations (e.g., Navier--Stokes, diffusion) are represented by the advection and collision of particle distribution functions. We compare our simulation results to MORB and OIB noble gas concentrations to determine the relative influence of kinetic and equilibrium degassing processes on these melts. This will help constrain undegassed volatile contents within their parent magmas and improve estimates of volatile fluxes to the atmosphere, which are an important control on long-term climate. Additionally, by providing noble gas concentrations in individual bubbles across their size and growth history, we provide a framework for interpreting individual bubble composition analyses (Colin et al., 2015).