Experimental calibration of vanadium partitioning and stable isotope fractionation between hydrous granitic melt and magnetite at 800°C and 0.5GPa

Experimental calibration of vanadium partitioning and stable isotope fractionation between... Vanadium has multiple oxidation states in silicate melts and minerals, a property that also promotes fractionation of its isotopes. As a result, vanadium isotopes vary during magmatic differentiation, and can be powerful indicators of redox processes at high temperatures if their partitioning behaviour can be determined. To quantify the partitioning and isotope fractionation factor of V between magnetite and melt, piston cylinder experiments were performed in which magnetite and a hydrous, haplogranitic melt were equilibrated at 800 °C and 0.5 GPa over a range of oxygen fugacities ( $${f_{{{\text{O}}_{\text{2}}}}}$$ f O 2 ), bracketing those of terrestrial magmas. Magnetite is isotopically light with respect to the coexisting melt, a tendency ascribed to the VI-fold V3+ and V4+ in magnetite, and a mixture of IV- and VI-fold V5+ and V4+ in the melt. The magnitude of the fractionation factor systematically increases with increasing log $${f_{{{\text{O}}_{\text{2}}}}}$$ f O 2 relative to the Fayalite–Magnetite–Quartz buffer (FMQ), from ∆51Vmag-gl = − 0.63 ± 0.09‰ at FMQ − 1 to − 0.92 ± 0.11‰ (SD) at ≈ FMQ + 5, reflecting constant V3+/V4+ in magnetite but increasing V5+/V4+ in the melt with increasing log $${f_{{{\text{O}}_{\text{2}}}}}$$ f O 2 . These first mineral-melt measurements of V isotope fractionation factors underline the importance of both oxidation state and co-ordination environment in controlling isotopic fractionation. The fractionation factors determined experimentally are in excellent agreement with those needed to explain natural isotope variations in magmatic suites. Furthermore, these experiments provide a useful framework in which to interpret vanadium isotope variations in natural rocks and magnetites, and may be used as a potential fingerprint the redox state of the magma from which they crystallise. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Contributions to Mineralogy and Petrology Springer Journals

Experimental calibration of vanadium partitioning and stable isotope fractionation between hydrous granitic melt and magnetite at 800°C and 0.5GPa

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Publisher
Springer Journals
Copyright
Copyright © 2018 by Springer-Verlag GmbH Germany, part of Springer Nature
Subject
Earth Sciences; Geology; Mineral Resources; Mineralogy
ISSN
0010-7999
eISSN
1432-0967
D.O.I.
10.1007/s00410-018-1451-8
Publisher site
See Article on Publisher Site

Abstract

Vanadium has multiple oxidation states in silicate melts and minerals, a property that also promotes fractionation of its isotopes. As a result, vanadium isotopes vary during magmatic differentiation, and can be powerful indicators of redox processes at high temperatures if their partitioning behaviour can be determined. To quantify the partitioning and isotope fractionation factor of V between magnetite and melt, piston cylinder experiments were performed in which magnetite and a hydrous, haplogranitic melt were equilibrated at 800 °C and 0.5 GPa over a range of oxygen fugacities ( $${f_{{{\text{O}}_{\text{2}}}}}$$ f O 2 ), bracketing those of terrestrial magmas. Magnetite is isotopically light with respect to the coexisting melt, a tendency ascribed to the VI-fold V3+ and V4+ in magnetite, and a mixture of IV- and VI-fold V5+ and V4+ in the melt. The magnitude of the fractionation factor systematically increases with increasing log $${f_{{{\text{O}}_{\text{2}}}}}$$ f O 2 relative to the Fayalite–Magnetite–Quartz buffer (FMQ), from ∆51Vmag-gl = − 0.63 ± 0.09‰ at FMQ − 1 to − 0.92 ± 0.11‰ (SD) at ≈ FMQ + 5, reflecting constant V3+/V4+ in magnetite but increasing V5+/V4+ in the melt with increasing log $${f_{{{\text{O}}_{\text{2}}}}}$$ f O 2 . These first mineral-melt measurements of V isotope fractionation factors underline the importance of both oxidation state and co-ordination environment in controlling isotopic fractionation. The fractionation factors determined experimentally are in excellent agreement with those needed to explain natural isotope variations in magmatic suites. Furthermore, these experiments provide a useful framework in which to interpret vanadium isotope variations in natural rocks and magnetites, and may be used as a potential fingerprint the redox state of the magma from which they crystallise.

Journal

Contributions to Mineralogy and PetrologySpringer Journals

Published: Mar 12, 2018

References

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