Olivine/melt transition metal partitioning, melt composition, and melt structure - Melt polymerization and Q(n)-speciation in alkaline earth silicate systems

The two most abundant network-modifying cations in magmatic liquids are Ca2+ and Mg2+. To evaluate the influence of melt structure on exchange of Ca2+ and Mg2+ with other geochemically important divalent cations (in-cations) between coexisting minerals and melts, high-temperature (1470-1650 degrees C), ambient-pressure (0.1 MPa) forsterite/melt partitioning experiments were carried out in the system Mg2SiO4-CaMgSi2O6-SiO2 with <= 1 wt% m-cations (Mn2+, CO2+, and Ni2+) substituting for Ca2+ and Mg2+. The bulk melt NBO/Si-range (NBOISi: nonbridging oxygen per silicon) of melt in equilibrium with forsterite was between 1.89 and 2.74. In this NBO/Si-range, the NBO/Si(Ca) (fraction of nonbridging oxygens, NBO, that form bonds with Ca2+, Ca2+-NBO) is linearly related to NBOISi, whereas fraction of Mg2+-NBO bonds is essentially independent of NBO/Si. For individual m-cations, rate of change of KD(m-mg) with NBO/Si(Ca) for the exchange equilibrium, librium, m(melt) + mg(otivine) reversible arrow m(olivine) + Mg-melt, is linear. KD(m-mg) decreases as an exponential function of increasing ionic potential, Z/r(2) (Z: formal electrical charge, r: ionic radius-here calculated with oxygen in sixfold coordination around the divalent cations) of the m-cation. The enthalpy change of the exchange equilibrium, Delta H, decreases linearly with increasing Z/r(2) [Delta H = 261(9)-81(3)-Z/r(2) (angstrom(-2))]. From existing information on (Ca,Mg)O-SiO2 melt structure at ambient pressure, these relationships are understood by considering the exchange of divalent cations that form bonds with nonbridging oxygen in individual Q(n)-species in the melts. The negative partial derivative KD(m-mg)/partial derivative(Z/r(2)) and partial derivative(Delta H)/partial derivative(Z/r(2)) is because increasing Z/r(2) is because the cations forming bonds with nonbridging oxygen in increasingly depolymerized Q(n)-species where steric hindrance is decreasingly important. In other words, principles of ionic size/site mismatch commonly observed for trace and minor elements in crystals, also govern their solubility behavior in silicate melts. (C) 2008 Elsevier Ltd. All rights reserved.