The Effect of Variable Na/K on the CO2 Content of Slab-Derived Rhyolitic Melts

Abstract

We conducted high pressure, high temperature experiments to investigate the effect of variable alkali ratio on the CO2 -rich fluid solubility in hydrous rhyolitic melts at sub-arc depths. Experiments were performed at 3.0 and 1.5 GPa, 1300 °C on rhyolitic compositions similar to low-degree partial melts of subducted slab lithologies, with fixed total alkalis (Na2 O+K2 O ~11.5 wt.%, volatile-free), but Na# (molar Na2 O/[Na2 O+K2 O]) varying from 0.15 to 0.88. In the experimental glasses, total dissolved CO2 (CO2 tot. ) ranged from 2.14 ± 0.07 to 3.20 ± 0.07 wt.% at 3.0 GPa, and from 0.70 ± 0.02 to 1.19 ± 0.02 wt.% at 1.5 GPa. Experiments showed a general positive correlation between Na# and CO2 tot., with the exception of the highest Na# experiment at 1.5 GPa. Carbon was dissolved as molecular CO2 (CO2 mol. ) and carbonate (CO3 2- ). As Na# increased, CO2 mol. /CO2 tot. decreased from 0.94 to ~0.00 in the 1.5 GPa experiments and from 0.65 to 0.05 in the 3.0 GPa experiments. Variability in CO2 concentration is larger and more clearly correlated with Na# at 3.0 GPa, indicating that this effect is pressure dependent. Our results show that compositional variability in silicic melts must be considered to accurately place constraints on the limit of CO2 transfer in subduction zones.