EXPERIMENTAL-DETERMINATION OF MN-MG MIXING PROPERTIES IN GARNET, OLIVINE AND OXIDE

We have measured the mixing properties of Mn-Mg olivine and Mn-Mg garn et at 1300-degrees-C from a combination of interphase partitioning exp eriments involving these phases, Pt-Mn alloys and Mn-Mg oxide solid so lutions. Activity coefficients of Mn dilute in Pt-Mn alloys at 1300-de grees-C/1 atm were measured by equilibrating the alloy with MnO at kno wn f(O2). Assuming that the log f(O2) of the Mn-MnO equilibrium under these conditions is -17.80 (Robie et al. 1978), we obtain for gamma(Mn ): log gamma(Mn) = -5.25 + 3.67 X(Mn) + 11.41 X(Mn)2 (X(Mn) = atom fra ction) Mixing properties of (Mn,Mg)O were determined by reversing the Mn contents of the alloys in equilibrium with oxide at known f(O2). Ad ditional constraints were obtained by measuring the maximum extent of immiscibility in (Mn,Mg)O at 800 and 750-degrees-C. The data are adequ ately described by an asymmetric (Mn,Mg)O solution with the following upper and lower limits on nonideality: (a) W(Mn) = 19.9 kJ/Mol; W(Mg) = 13.7 kJ/mol (b) W(Mn) = 19.2 kJ/Mol; W(Mg) = 8.2 kJ/mol Olivine-oxid e partitioning was tightly bracketed at 1300-degrees-C and oxide prope rties used to obtain activity-composition relations for Mn-Mg olivine. Despite strong M2 ordering of Mn in olivine, the macroscopic properti es are adequately described by a symmetric model with: W(ol) = 5.5 +/- 2.1 kJ/mol (1-site basis) Using these values for olivine, garnet-oliv ine partitioning at 27 kbar/1300-degrees-C leads to an Mn-Mg interacti on parameter in garnet given by: W(gt) = 1.5 +/- 2.5 kJ/mol (1-site pe r formula unit) Garnet-olivine partitioning at 9 kbar/1000-degrees-C i s consistent with the same extent of garnet nonideality and the appare nt absence of excess volume on the pyrope-spessartine join indicates t hat any pressure-dependence of W(Gt) must be small. If olivine and gar net properties are both treated as unknown and the garnet-olivine part itioning data alone used to derive W(Ol) and W(Gt), by multiple linear regression, best-fit values of 6.16 and 1.44 kJ/mol. are obtained. Th ese are in excellent agreement with the values derived from metal-oxid e, oxide-olivine and olivine-garnet equilibria.