Implications of liquid-liquid distribution coefficients to mineral-liquid partitioning |
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| Authors: | FJ Ryerson PC Hess |
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| Institution: | Department of Geological Sciences, Brown University, Providence, RI 02912 USA |
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| Abstract: | The effect of silicate liquid structure upon mineral-liquid partitioning has been investigated by determining element partitioning data for coexisting immiscible granitic and ferrobasaltic magmas. The resulting elemental distribution patterns may be interpreted in terms of the relative states of polymerization of the coexisting magmas. Highly charged cations (REE, Ti, Fe, Mn, etc.) are enriched in the ferrobasaltic melt. The ferrobasaltic melt is relatively depolymerized due to its low ratio. This allows highly charged cations to obtain stable coordination polyhedra of oxygen within the ferrobasaltic melt. The granitic melt is a highly polymerized network structure in which Al can occupy tetrahedral sites in copolymerization with Si. The substitution of Al+3 for Si+4 produces a local charge imbalance in the granitic melt which is satisfied by a coupled substitution of alkalis, thus explaining the enrichment of low charge density cations, the alkalis, in the granitic melt. P2O5 increases the width of the solvus and, therefore, the values of the distribution coefficients of the trace elements. This effect is attributed to complexing of metal cations with PO4?3 groups in the ferrobasaltic melt.The values of ferrobasalt-granite liquid distribution coefficients are reflected in distribution coefficients for a mineral and melts of different compositions. The mineral-liquid distribution coefficient for a highly charged cation is greater for a mineral coexisting with a highly polymerized melt (granite) than it is for that same mineral and a depolymerized melt (ferrobasalt). The opposite is true for low charge density cations. Mineralliquid and liquid-liquid distribution coefficients determined for the REE's indicate that fractionated REE patterns are due to mineral selectivity and not the state of polymerization of the melt. |
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