EVIDENCE FROM OLIVINE/MELT ELEMENT PARTITIONING THAT NONBRIDGING OXYGEN IN SILICATE MELTS ARE NOT EQUIVALENT

Abstract

Partitioning of Ca, Mn, Mg, and Fe2+ between olivine and melt has been used to examine the influence of energetically nonequivalent nonbridging oxygen in silicate melts. Partitioning experiments were conducted at ambient pressure in air and 1400°C with melts in equilibrium with forsterite-rich olivine (Fo >95 mol%). The main compositional variables of the melts were NBO/T and Na/(Na+Ca). In all melts, the main structural units were of Q4, Q3, and Q2 type with nonbridging oxygen, therefore, in the Q3 and Q2 units.For melts with high Q3/Q2-abundance ratio (corresponding to NBO/T near 1), increasing Na/(Na+Ca) [and Na/(Na+Ca+Mn+Mg+Fe2+)] results in a systematic decrease of the partition coefficients, KCaol/melt, KMnol/melt, KMgol/melt, and KFe2+ol/melt, because of ordering of the network-modifying Ca, Mn, Mg, and Fe2+ among nonbridging oxygen in Q3 and Q2 structural units. This decrease is more pronounced the smaller the ionic radius of the cation. With decreasing Q3/Q2 abundance ratio (less-polymerized melts) this effect becomes less pronounced.Activity-composition relations among network-modifying cations in silicate melts are, therefore, governed by availability of energetically nonequivalent nonbridging oxygen in individual Qn-species in the melt. As a result, any composition change that enhances abundance of highly depolymerized Qn-species will cause partition coefficients to decrease.