An experimental study of liquid-phase separation in the systems Fe2SiO4-Fe3O4-KAlSi2O6-SiO2-H2O, Fe3O4-KAlSi2O6-SiO2-H2O and Fe3O4-Fe2O3-KAlSi2O6-SiO2-H2O with or without P, S, F, Cl or Ca0.5Na0.5Al1.5Si2.5O8: Implications for immiscibility in volatile-rich natural magmas

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Lester, Gregory W.
immiscibility in iron-rich magams , volatiles in immiscible magmas , trace element partitioning
Abstract Isobaric (200 MPa) experiments have been performed to investigate the effects of H2O alone or in combination with P, S, F or Cl on the phase relations and elemental and oxygen isotopic partitioning between immiscible silicate melts in the systems Fe2SiO4-Fe3O4-KAlSi2O6-SiO2, Fe3O4-KAlSi2O6-SiO2 and Fe3O4-Fe2O3-KAlSi2O6-SiO2 +/- plagioclase (An50). Experiments were heated in a newly-designed rapid-quench internally-heated pressure vessel at 1075, 1150 or 1200 oC for 2 hours. Water alone or in combination with P, S, or F significantly increases the temperature and composition range of two-liquid fields at fO2= NNO and MH buffers. Water-induced suppression of liquidus temperatures, considered with the effects of pressure on two-liquid fields stability in silicate melts, suggests that liquid phase separation may occur in some volatile-rich silicate magmas at pressures up to 2GPa. Two-liquid partition coefficients for Fe, Si, P and S correlate well with the degree of polymerization of the SiO2-rich conjugate melts and the data can be applied to assess the involvement of liquid-phase separation in the genesis of coexisting volatile-rich magmas. The partitioning of trace concentrations of selected HFSE, REE and transition elements between immiscible experimental volatile-rich melts at 1200 oC, 200 MPa has been determined at QFM, NNO and MH oxygen buffers. Water generally increases the partitioning of HFSE, REE and transition elements into the Fe-rich melt. Water alone, or combined with P or S, produces nearly parallel partitioning trends for HFSE and REE. Absolute partitioning values of transition elements are strongly dependent on the network-modifier composition of the melt. 18O in experimental immiscible melts with H2O or H2O and P or S partitions preferentially into the felsic conjugate melt (δ18O felsic melt- δ18O mafic melt values range from 0.4 to 0.8 permil) consistent with observations in anhydrous immiscible silicate melts. The expansion of the P-T-X-fO2 stability ranges of two- or three-liquid fields observed in the experimental melts demonstrates that liquid-immiscibility may be an important process in the evolution of some volatile-rich natural magmas. The results support an immiscible petrogenetic origin for some iron-oxide dominated, Kiruna-type, ore-deposits.
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