IMMISCIBLE SILICATE LIQUIDS AT HIGH-PRESSURE - THE INFLUENCE OF MELT STRUCTURE ON ELEMENTAL PARTITIONING

Immiscible silicate melts in the system SiO2-FeO-Al2O3-K2O (+P2O5) hav e been stabilised at 0.5 and 1.0 GPa over a temperature range of 1160- 1240 degrees C. Unmixing in this system produces one melt rich in SiO2 and another rich in FeO, and a minor component of beta-quartz. A stud y of partitioning of elements of geochemical interest: Rb, Pa, Pb, Sr, La, Ce, Sm, Ho, Y, Lu, Th, U, Zr, Hf, Nb and Ta between the immiscibl e silicate melts at trace levels has been made possible by the develop ment of quantitative microbeam PIXE. The trace elements were introduce d at approximately 200 ppm level for all elements except for the REEs, Ba and Ta (600-1200 ppm). Trace element partitioning was found to be a complex function of cation held strength (charge/radius(2)), and in broad terms can be modelled by a third-order polynomial. Although fiel d strength is important in determining the nature and degree of partit ioning, it is clearly only one component of the underlying mechanism f or the way in which elements distribute themselves between two silicat e liquids. Hf partition coefficients were found to approximate unity, and thus its partitioning is nearly independent of the melt compositio n.