Vesicularity, bubble formation and noble gas fractionation during MORB degassing

摘要

The objective of this study is to use molecular dynamics simulation (MD) toevaluate the vesicularity and noble gas fractionation, and to shed light onbubble formation during MORB degassing. A previous simulation study (Guillotand Sator (2011) GCA 75, 1829-1857) has shown that the solubility of CO2 inbasaltic melts increases steadily with the pressure and deviates significantlyfrom Henry's law at high pressures (e.g. 9.5 wt% CO2 at 50 kbar as comparedwith 2.5 wt% from Henry's law). From the CO2 solubility curve and the equationsof state of the two coexisting phases (silicate melt and supercritical CO2),deduced from the MD simulation, we have evaluated the evolution of thevesicularity of a MORB melt at depth as function of its initial CO2 contents.An excellent agreement is obtained between calculations and data on MORBsamples collected at oceanic ridges. Moreover, by implementing the testparticle method (Guillot and Sator (2012) GCA 80, 51-69), the solubility ofnoble gases in the two coexisting phases (supercritical CO2 and CO2-saturatedmelt), the partitioning and the fractionation of noble gases between melt andvesicles have been evaluated as function of the pressure. We show that themelt/CO2 partition coefficients of noble gases increase significantly with thepressure whereas the large distribution of the 4He/40Ar* ratio reported in theliterature is explained if the magma experiences a suite of vesiculation andvesicle loss during ascent. By applying a pressure drop to a volatile bearingmelt, the MD simulation reveals the main steps of bubble formation and noblegas transfer at the nanometric scale. A key result is that the transfer ofnoble gases is found to be driven by CO2 bubble nucleation, a finding whichsuggests that the diffusivity difference between He and Ar in the degassingmelt has virtually no effect on the 4He/40Ar* ratio measured in the vesicles.