A predictive thermodynamic model of garnet-melt trace element partitioning
We have developed a predictive model for the partitioning of magnesium and a range of trivalent trace elements (rare earth elements, Y, In and Sc) between garnet and anhydrous silicate melt as a function of pressure, temperature and bulk composition. The model for the magnesium partition coefficient...
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| Format: | Journal article |
| Language: | English |
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2001
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| _version_ | 1826292226597060608 |
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| author | van Westrenen, W Wood, B Blundy, J |
| author_facet | van Westrenen, W Wood, B Blundy, J |
| author_sort | van Westrenen, W |
| collection | OXFORD |
| description | We have developed a predictive model for the partitioning of magnesium and a range of trivalent trace elements (rare earth elements, Y, In and Sc) between garnet and anhydrous silicate melt as a function of pressure, temperature and bulk composition. The model for the magnesium partition coefficient, DMg, is based on a thermodynamic description of the pyrope (Mg3Al2-Si3O12) melting reaction between garnet and melt. Simple activity-composition relations, which take explicit account of garnet non-ideality, link DMg to the free energy of fusion (ΔGf) of pure pyrope without the need to invoke non-ideality in the liquid phase. The resulting predictive equation, based on the compositions of a large set (n = 160) of published garnet-melt pairs, produces values of DMg that are within 20% of measured values at temperatures between 1,450 and 1,930 °C, and pressures between 2.5 and 7.5 GPa. The model for trivalent (3+) trace elements is based on the lattice strain approach to partitioning, which describes mineral-melt partition coefficients in terms of three parameters; the effective radius, ro(3+), of the site on which partitioning takes place (in this case, the garnet X-site); the apparent site Young's modulus Ex(3+); and the partition coefficient Do(3+) for a fictive trivalent element J3+, with radius ro(3+), that does not strain the crystal lattice when entering the garnet X-site. Analogous to the model for DMg, simple activity-composition relations link Do(3+) to ΔGf of a hypothetical garnet component incorporating a hypothetical rare earth element J3+ through a YAG-type charge-balancing mechanism (J3+Mg2Al3Si2O12). Through analysis of existing garnet-melt rare earth element partitioning data (n = 18 garnet-melt pairs), an expression is derived relating Do(3+) to pressure, temperature and DMg. Predicted DREE/Y/Sc values agree to within 5-50% of experimental measurements for all elements except La and Ce, which are liable to large experimental errors, spanning pressures between 2.5 and 5.0 GPa and temperatures between 1,430 and 1,640 °C. In conjunction with our new parameterisation for DMg, and previously published equations linking ro(3+) and Ex(3+) to garnet major element composition, this model gives a description of trivalent REE, Y, In and Sc partitioning between garnets and anhydrous melts over a range of pressures, temperatures and compositions relevant to melting of garnet-bearing sources in the Earth's upper mantle. |
| first_indexed | 2024-03-07T03:11:25Z |
| format | Journal article |
| id | oxford-uuid:b4568abd-0ee6-48fb-b959-a4fcdfc710c7 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T03:11:25Z |
| publishDate | 2001 |
| record_format | dspace |
| spelling | oxford-uuid:b4568abd-0ee6-48fb-b959-a4fcdfc710c72022-03-27T04:25:16ZA predictive thermodynamic model of garnet-melt trace element partitioningJournal articlehttp://purl.org/coar/resource_type/c_dcae04bcuuid:b4568abd-0ee6-48fb-b959-a4fcdfc710c7EnglishSymplectic Elements at Oxford2001van Westrenen, WWood, BBlundy, JWe have developed a predictive model for the partitioning of magnesium and a range of trivalent trace elements (rare earth elements, Y, In and Sc) between garnet and anhydrous silicate melt as a function of pressure, temperature and bulk composition. The model for the magnesium partition coefficient, DMg, is based on a thermodynamic description of the pyrope (Mg3Al2-Si3O12) melting reaction between garnet and melt. Simple activity-composition relations, which take explicit account of garnet non-ideality, link DMg to the free energy of fusion (ΔGf) of pure pyrope without the need to invoke non-ideality in the liquid phase. The resulting predictive equation, based on the compositions of a large set (n = 160) of published garnet-melt pairs, produces values of DMg that are within 20% of measured values at temperatures between 1,450 and 1,930 °C, and pressures between 2.5 and 7.5 GPa. The model for trivalent (3+) trace elements is based on the lattice strain approach to partitioning, which describes mineral-melt partition coefficients in terms of three parameters; the effective radius, ro(3+), of the site on which partitioning takes place (in this case, the garnet X-site); the apparent site Young's modulus Ex(3+); and the partition coefficient Do(3+) for a fictive trivalent element J3+, with radius ro(3+), that does not strain the crystal lattice when entering the garnet X-site. Analogous to the model for DMg, simple activity-composition relations link Do(3+) to ΔGf of a hypothetical garnet component incorporating a hypothetical rare earth element J3+ through a YAG-type charge-balancing mechanism (J3+Mg2Al3Si2O12). Through analysis of existing garnet-melt rare earth element partitioning data (n = 18 garnet-melt pairs), an expression is derived relating Do(3+) to pressure, temperature and DMg. Predicted DREE/Y/Sc values agree to within 5-50% of experimental measurements for all elements except La and Ce, which are liable to large experimental errors, spanning pressures between 2.5 and 5.0 GPa and temperatures between 1,430 and 1,640 °C. In conjunction with our new parameterisation for DMg, and previously published equations linking ro(3+) and Ex(3+) to garnet major element composition, this model gives a description of trivalent REE, Y, In and Sc partitioning between garnets and anhydrous melts over a range of pressures, temperatures and compositions relevant to melting of garnet-bearing sources in the Earth's upper mantle. |
| spellingShingle | van Westrenen, W Wood, B Blundy, J A predictive thermodynamic model of garnet-melt trace element partitioning |
| title | A predictive thermodynamic model of garnet-melt trace element partitioning |
| title_full | A predictive thermodynamic model of garnet-melt trace element partitioning |
| title_fullStr | A predictive thermodynamic model of garnet-melt trace element partitioning |
| title_full_unstemmed | A predictive thermodynamic model of garnet-melt trace element partitioning |
| title_short | A predictive thermodynamic model of garnet-melt trace element partitioning |
| title_sort | predictive thermodynamic model of garnet melt trace element partitioning |
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