Partitioning of Volatile Components (Cl, F, and CO2) in Water-Saturated Fluid–Magma Systems of Various Composition

Аннотация

The paper presents results of experimental studies of the behavior of volatile components (Cl, F, CO₂, and H₂O) in fluid–magmatic systems. The maximum Cl content in magmatic melts depends mainly on the composition of the melt and less on pressure (10–300 MPa) and temperature (800–1000°C). The Cl content in the melt increases from 0.2–0.3 to 3–5 wt % with increasing Ca content during the transition from polymerized granitoid to depolymerized basaltic melts. The pressure dependence of the solubility has a maximum at a pressure of about 100–200 MPa. The Cl and F contents in the melt tend to increase and decrease, respectively, at the transition from acid and alkaline to basalt melts. The maximum Cl content in the melt significantly increases from rhyolite (no more than 0.25 wt %) to phonolite (no more than 0.85 wt %), and dacite (no more than 1.2 wt %) melts at temperatures of 1000–1200°C and a pressure of 200 MPa. The addition of CO₂ to the system leads to an increase in the Cl content in the melt by 20–25 relative %, which is likely explained by an increase in Cl activity in the fluid. Thereby the H₂O content in the melt decreases by ~0.5–1.0 wt %. Hydrolysis is demonstrated to strongly affect interaction between alumina-rich granitic melt and ~0.5–1 N chloride fluid. This effect shows that the fluid is acidic (pH after the experiment is ~1–1.5) at hypabyssal magmatic conditions (P = 100 MPa, T = 750°C) and is characterized by a high dissolving power. The experiments show that interaction between aqueous Na–K–Ca–chloride fluid of variable composition and granodioritic and granitic melts in the pressure range of ~100–200 MPa, temperatures of 820–1000°C, and an increasing total salt content leads to that Na and K substitute Ca in the silicate melt, with Ca simultaneously passing into the fluid. The latter is enriched in CaCl₂ and is depleted in NaCl. Experimental results on the coupled Cl and F partitioning provide a quantitative basis for understanding degassing processes in the course of the evolution of alkaline and basaltic magmas. They are important for assessing the extent of Cl and F removal into the Earth’s atmosphere during volcanic activity and the effects of this removal on climate changes.