Experiments on liquid immiscibility in silicate melts with H2O, P, S, F and Cl: Implications for natural magmas

Isobaric (200 MPa) experiments have been performed to investigate the effects of H2O alone or in combination with P, S, F or Cl on liquid-phase separation in melts in the systems Fe2SiO4–Fe3O4–KAlSi2O6–SiO2, Fe3O4–KAlSi2O6–SiO2 and Fe3O4–Fe2O3–KAlSi2O6–SiO2 with or without plagioclase (An50). Experiments were heated in a rapid-quench internally heated pressure vessel at 1,075, 1,150 or 1,200 °C for 2 h. Experimental fO2 was maintained at QFM, NNO or MH oxygen buffers. H2O alone or in combination with P, S or F increases the temperature and composition range of two-liquid fields at fO2 = NNO and MH buffers. P, S, F and Cl partition preferentially into the Fe-rich immiscible liquid. Two-liquid partition coefficients for Fe, Si, P and S correlate well with the degree of polymerization of the SiO2-rich liquid and plot on similar but distinct power-law curves compared with equivalent anhydrous or basaltic melts. The addition of 2 wt% S to the system Fe3O4–Fe2O3–KAlSi2O6–SiO2 stabilizes three immiscible melts with Fe-, FeS- and Si-rich compositions. H2O-induced suppression of liquidus temperatures in the experimental systems, considered with the effects of pressure on the temperature and composition ranges of two-liquid fields in silicate melts, suggests that liquid-phase separation may be stable in some H2O-rich silicate magmas at pressures in excess of 200 MPa.