Olivine/melt transition metal partitioning, melt composition, and melt structure—Influence of Al for Si substitution in the tetrahedral network of silicate melts |
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| Authors: | Bjorn O Mysen |
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| Institution: | Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, USA |
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| Abstract: | The influence on olivine/melt transition metal (Mn, Co, Ni) partitioning of  substitution in the tetrahedral network of silicate melt structure has been examined at ambient pressure in the 1450-1550 °C temperature range. Experiments were conducted in the systems NaAlSiO4-Mg2SiO4- SiO2 and CaAl2Si2O8-Mg2SiO4-SiO2 with about 1 wt% each of MnO, CoO, and NiO added. These compositions were used to evaluate how, in silicate melts,  substitution and ionization potential of charge-balancing cations affect activity-composition relations in silicate melts and mineral/melt partitioning.The exchange equilibrium coefficient,  , is a positive and linear function of melt Al/(Al + Si) at constant degree of melt polymerization, NBO/T. The  is negatively correlated with the ionic radius, r, of the M-cation and also with the ionization potential (Z/r2, Z = electrical charge) of the cation that serves to charge-balance Al3+ in tetrahedral coordination in the melts. The activity coefficient ratio, (γM/γMg)melt, is therefore similarly correlated.These melt composition  relationships are governed by the distribution of Al3+ among coexisting Q-species in the peralkaline (depolymerized) melts coexisting with olivine. This distribution controls Q-speciation abundance, which, in turn, controls (γM/γMg)melt and  . The relations between melt structure and olivine/melt partitioning behavior lead to the suggestion that in natural magmatic systems mineral/melt partition coefficients are more dependent on melt composition and, therefore, melt structure the more alkali-rich and the more felsic the melt. Moreover, mineral/melt partition coefficients are more sensitive to melt composition the more highly charged or the smaller the ionic radius of the cation of interest. |
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