Abstract
The behavior of argon in H2O-CO2 bearing basaltic melts was experimentally investigated in the pressure range 1-5 GPa and at temperatures between 1350 and 1600 degrees C. Our experimental data simulate the partitioning of argon between an Ar-H2O-CO2 bearing fluid and a silicate melt occurring during magma ascent and degassing.
The experimental results show several features: - At a pressure of 1 GPa the variation of H2O from 035 to about 2 wt.% (at constant CO2 content) does not induce any systematic variation of argon dissolved in basaltic melt. For higher water contents (>2 wt.%) we observe a positive effect of water on argon solubility. - In the range of 3-5 GPa, we did not observe any systematic variation of the argon content in the basalt melt for water concentration from 0.35 to 5.3 wt.% and at CO2 content 0.5 wt.%, argon concentration in basalt melt decreases from about 3800 to 2400 ppm when the CO2 content increases from 0.5 to 0.8 wt.%. - At all pressures investigated in the present study, a negligible effect of CO2 for concentration < 5000 ppm on argon content in the silicate melt is observed. - More importantly (despite all these variations) it seems that the effect of pressure in the range of 1 to 5 GPa is the dominant parameter on argon solubility in basaltic melt.
Pressure has a positive effect on argon incorporation in the H2O-CO2 bearing basaltic melt reaching at 3 GPa a concentration of similar to 038 wt.%. This maximum of similar to 038 wt.% corresponds to 6.8 x 10(-5) cm(3) g(-1) bar(-1) at standard temperature and pressure, a value of the same order of magnitude as that derived from volatiles free basaltic melts equilibrated with argon.
The experimental data can be well described by a thermodynamic model assuming mixing of volatile species and oxygen in the silicate melt. The results can be applied for a better understanding of the fractionation noble gas/noble gas and noble gas/CO2 occurring in degassing processes during magma ascent. (C) 2016 Published by Elsevier B.V.