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