The importance of melt composition in controlling trace-element behaviour: an experimental study of Mn and Zn partitioning between forsterite and silicate melts

Abstract

Partition coefficients for Mn 2+ and Zn 2+ between forsterite and melts in the system Fo-Ab-An at temperatures of 1150–1400°C and 1-bar pressure have been determined experimentally. For a given temperature the values of the Nernst partition coefficients ( D Mn and D Zn) depend on the composition, and hence structure, of the melt phase, however, for a given degree of melt polymerization (expressed as the number of non-bridging oxygens per tetrahedral cation, NBO/T), the temperature dependence of the partition coefficients is less than the error in measurement. In these systems, melt composition is therefore more important than temperature in controlling trace-element behaviour. The exchange partition coefficients K D( Mn Mg ) and K D( Zn Mg ) are also more dependent on NBO/T than on temperature. Previous attempts to extract the ΔS and ΔH for the exchange reaction by regressing ln K D vs. 1 T are not valid as they ignore the changes in K D controlled by melt composition. Our data imply that for this system ΔH is small and ΔS varies with melt composition. In deploymerized melts the Nernst partition coefficients are ∼0.8 for Zn 2+ and ∼0.6 for Mn 2+ whereas in highly polymerized melts, values up to 4.5 for Zn 2+ and 3.0 for Mn 2+ have been found. In these highly polymerized systems small changes in the degree of polymerization are accompanied by large changes in the partition coefficients. This implies that fixed values of mineral/melt partition coefficients are unlikely to be adequate in trace-element modelling studies of highly differentiated igneous rocks.