Trace element partitioning in basaltic systems as a function of oxygen fugacity
<p>Along with temperature, pressure and melt chemistry, magmatic oxygen fugacity (<em>f</em>O<sub>2</sub>) has an important influence on liquid and solid differentiation trends and melt structure. To explore the effect of redox conditions on mineral stability and minera...
| Main Authors: | , , |
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| Format: | Journal article |
| Language: | English |
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Springer
2023
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| _version_ | 1797112118472867840 |
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| author | Leuthold, J Blundy, J Ulmer, P |
| author_facet | Leuthold, J Blundy, J Ulmer, P |
| author_sort | Leuthold, J |
| collection | OXFORD |
| description | <p>Along with temperature, pressure and melt chemistry, magmatic oxygen fugacity (<em>f</em>O<sub>2</sub>) has an important influence on liquid and solid differentiation trends and melt structure. To explore the effect of redox conditions on mineral stability and mineral-melt partitioning in basaltic systems we performed equilibrium, one-atmosphere experiments on a picrite at 1200–1110 °C with <em>f</em>O<sub>2</sub> ranging from NNO-4 log units to air. Clinopyroxene crystallizes from 1180 °C to near-solidus, along with plagioclase, olivine and spinel. Olivine Mg# increases with increasing <em>f</em>O<sub>2</sub>, eventually reacting to pigeonite. Spinel is absent under strongly reducing conditions. Mineral-melt partition coefficients (<em>D</em>) of redox-sensitive elements (Cr, Eu, V, Fe) vary systematically with <em>f</em>O<sub>2</sub> and, in some cases, temperature (e.g. <em>D</em><sub>Cr</sub> in clinopyroxene). Clinopyroxene sector zoning is common; sectors along a- and b-axes have higher Al<sup>IV</sup>, Al<sup>VI</sup>, Cr and Ti and lower Mg than c-axis sectors. In terms of coupled substitutions, clinopyroxene CaTs (MgSi = Al<sup>VI</sup>Al<sup>IV</sup>) prevails under oxidized conditions (≥ NNO), where Fe<sup>3+</sup> balances the charge, but is limited under reduced conditions. Overall, Al<sup>IV</sup> is maximised under high temperature, oxidizing conditions and in slowly grown (a–b) sectors. High Al<sup>IV</sup> facilitates incorporation of REE (REEAl<sup>IV</sup> = CaSi), but <em>D</em><sub><em>REE</em></sub> (except <em>D</em><sub>Eu</sub>) show no systematic dependence on <em>f</em>O<sub>2</sub> across the experimental suite. In sector zoned clinopyroxenes enrichment in REE<sup>3+</sup> in Al-rich sectors is quantitatively consistent with the greater availability of suitably-charged M2 lattice sites and the electrostatic energy penalty required to insert REE<sup>3+</sup> onto unsuitably-charged M2 sites. By combining our experimental results with published data, we explore the potential for trace element oxybarometry. We show that olivine-melt <em>D</em><sub>V</sub>, clinopyroxene-melt <em>D</em><sub>V</sub>/<em>D</em><sub>Sc</sub> and plagioclase-melt <em>D</em><sub>Eu</sub>/<em>D</em><sub>Sr</sub> all have potential as oxybarometers and we present expressions for these as a function of <em>f</em>O<sub>2</sub> relative to NNO. The crystal chemical sensitivity of heterovalent cation incorporation into clinopyroxene and the melt compositional sensitivity of the Eu<sup>2+</sup>–Eu<sup>3+</sup> redox potential limit the use of clinopyroxene-melt and plagioclase-melt, however, olivine-melt <em>D</em><sub>V</sub> affords considerable precision and accuracy as an oxybarometer that is independent of temperature, and crystal and melt composition. Variation of <em>D</em><sub>V</sub> and <em>D</em><sub>V</sub>/<em>D</em><sub>Sc</sub> with <em>f</em>O<sub>2</sub> for olivine and clinopyroxene contains information on redox speciation of V in coexisting melt. By comparing the redox speciation constraints from partitioning to data from Fe-free synthetic systems and XANES spectroscopy of quenched glasses, we show that homogenous equilibria involving Fe and V species modify V speciation on quench, leading to a net overall reduction in the average vanadium valence. Mineral-melt partitioning of polyvalent species can be a useful probe of redox speciation in Fe-bearing systems that is unaffected by quench effects.</p> |
| first_indexed | 2024-03-07T08:19:42Z |
| format | Journal article |
| id | oxford-uuid:1e3a460f-b863-4393-941b-b8ec1cfbc50d |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T08:19:42Z |
| publishDate | 2023 |
| publisher | Springer |
| record_format | dspace |
| spelling | oxford-uuid:1e3a460f-b863-4393-941b-b8ec1cfbc50d2024-01-25T07:23:08ZTrace element partitioning in basaltic systems as a function of oxygen fugacityJournal articlehttp://purl.org/coar/resource_type/c_dcae04bcuuid:1e3a460f-b863-4393-941b-b8ec1cfbc50dEnglishSymplectic ElementsSpringer2023Leuthold, JBlundy, JUlmer, P<p>Along with temperature, pressure and melt chemistry, magmatic oxygen fugacity (<em>f</em>O<sub>2</sub>) has an important influence on liquid and solid differentiation trends and melt structure. To explore the effect of redox conditions on mineral stability and mineral-melt partitioning in basaltic systems we performed equilibrium, one-atmosphere experiments on a picrite at 1200–1110 °C with <em>f</em>O<sub>2</sub> ranging from NNO-4 log units to air. Clinopyroxene crystallizes from 1180 °C to near-solidus, along with plagioclase, olivine and spinel. Olivine Mg# increases with increasing <em>f</em>O<sub>2</sub>, eventually reacting to pigeonite. Spinel is absent under strongly reducing conditions. Mineral-melt partition coefficients (<em>D</em>) of redox-sensitive elements (Cr, Eu, V, Fe) vary systematically with <em>f</em>O<sub>2</sub> and, in some cases, temperature (e.g. <em>D</em><sub>Cr</sub> in clinopyroxene). Clinopyroxene sector zoning is common; sectors along a- and b-axes have higher Al<sup>IV</sup>, Al<sup>VI</sup>, Cr and Ti and lower Mg than c-axis sectors. In terms of coupled substitutions, clinopyroxene CaTs (MgSi = Al<sup>VI</sup>Al<sup>IV</sup>) prevails under oxidized conditions (≥ NNO), where Fe<sup>3+</sup> balances the charge, but is limited under reduced conditions. Overall, Al<sup>IV</sup> is maximised under high temperature, oxidizing conditions and in slowly grown (a–b) sectors. High Al<sup>IV</sup> facilitates incorporation of REE (REEAl<sup>IV</sup> = CaSi), but <em>D</em><sub><em>REE</em></sub> (except <em>D</em><sub>Eu</sub>) show no systematic dependence on <em>f</em>O<sub>2</sub> across the experimental suite. In sector zoned clinopyroxenes enrichment in REE<sup>3+</sup> in Al-rich sectors is quantitatively consistent with the greater availability of suitably-charged M2 lattice sites and the electrostatic energy penalty required to insert REE<sup>3+</sup> onto unsuitably-charged M2 sites. By combining our experimental results with published data, we explore the potential for trace element oxybarometry. We show that olivine-melt <em>D</em><sub>V</sub>, clinopyroxene-melt <em>D</em><sub>V</sub>/<em>D</em><sub>Sc</sub> and plagioclase-melt <em>D</em><sub>Eu</sub>/<em>D</em><sub>Sr</sub> all have potential as oxybarometers and we present expressions for these as a function of <em>f</em>O<sub>2</sub> relative to NNO. The crystal chemical sensitivity of heterovalent cation incorporation into clinopyroxene and the melt compositional sensitivity of the Eu<sup>2+</sup>–Eu<sup>3+</sup> redox potential limit the use of clinopyroxene-melt and plagioclase-melt, however, olivine-melt <em>D</em><sub>V</sub> affords considerable precision and accuracy as an oxybarometer that is independent of temperature, and crystal and melt composition. Variation of <em>D</em><sub>V</sub> and <em>D</em><sub>V</sub>/<em>D</em><sub>Sc</sub> with <em>f</em>O<sub>2</sub> for olivine and clinopyroxene contains information on redox speciation of V in coexisting melt. By comparing the redox speciation constraints from partitioning to data from Fe-free synthetic systems and XANES spectroscopy of quenched glasses, we show that homogenous equilibria involving Fe and V species modify V speciation on quench, leading to a net overall reduction in the average vanadium valence. Mineral-melt partitioning of polyvalent species can be a useful probe of redox speciation in Fe-bearing systems that is unaffected by quench effects.</p> |
| spellingShingle | Leuthold, J Blundy, J Ulmer, P Trace element partitioning in basaltic systems as a function of oxygen fugacity |
| title | Trace element partitioning in basaltic systems as a function of oxygen fugacity |
| title_full | Trace element partitioning in basaltic systems as a function of oxygen fugacity |
| title_fullStr | Trace element partitioning in basaltic systems as a function of oxygen fugacity |
| title_full_unstemmed | Trace element partitioning in basaltic systems as a function of oxygen fugacity |
| title_short | Trace element partitioning in basaltic systems as a function of oxygen fugacity |
| title_sort | trace element partitioning in basaltic systems as a function of oxygen fugacity |
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