Partitioning of volatile components (Cl, F, CO2) in water-saturated fluid-magmatic systems of various composition

Abstract

The results of experimental studies of the behavior of volatile components (Cl, F, CO2 and H2O) in fluid-magmatic systems are presented. The maximum chlorine content in magmatic melts mainly depends on the composition of the melt and, to a lesser extent, on pressure (10300 MPa) and temperature (8001000C). The Cl content in the melt increases from 0.20.3 to 35 wt.% with increasing in the Ca content during the transition from polymerized granitoid to depolymerized basaltic melts. The pressure dependence of solubility has a maximum at a pressure of about 100200 MPa. A tendency of increasing in the Cl content and decreasing in the F content in the melt during the transition from acidic and alkaline to basaltic melts has been established. The maximum Cl content in the melt significantly increases from rhyolitic (up to 0.25 wt.%) to phonolitic (up to 0.85 wt.%) and dacitic (up to 1.2 wt.%) at temperatures of 10001200C and pressure of 200 MPa. The addition of CO2 to the system causes an increase in the Cl content in the melt by 2025 relative %, which is apparently associated with an increase in the Cl activity in the fluid. In this case, the H2O content in the melt decreases by ~ 0.51.0 wt.%. A strong effect of hydrolysis was shown in the interaction of an alumina-rich granitic melt with ~ 0.51N chloride fluid. This effect shows that at hypabyssal magmatic conditions (P = 100 MPa, T = 750C), the fluid is acidic (the pH after the experiment is ~ 11.5) and it is characterized by high dissolving power. It was established experimentally that as a result of the interaction of aqueous Na-K-Ca-chloride fluid of variable composition with granodioritic and granitic melts in the pressure range of ~ 100200 MPa and temperatures of 8201000C and with increasing in the total salt content, the Na and K replace Ca in the silicate melt, displacing the latter into the fluid, that is enriched in CaCl2 and is depleted in NaCl. Experimental results on the joint partitioning of Cl and F provide a quantitative basis for understanding the degassing processes in the course of the evolution of alkaline and basaltic magmas. They are important for assessing the extent of the removal of Cl and F into the earths atmosphere during volcanic activity and the effects of this removal on climate change.