Zirconium Isotopes Research
Above: Determining the concentration of 91Zr and 96Zr single spikes by the ‘standard addition’ method prior to mixing of an isotopic double-spike
Zirconium is a commonly used elemental tracer of silicate differentiation, yet its stable isotope systematics remain poorly known. Along with collaborator François Tissot (Caltech’s Isotoparium) we have developed analytical routines to determine the Zr stable isotope composition of single minerals and bulk-rock samples using a new 91Zr-96Zr double-spike prepared at the UR-LIG. These capabilities are being used to explore the Zr isotopic systematics of various geochemical reservoirs on Earth, their variability, and the petrologic processes responsible for the observed fractionations.
This research is supported by NSF-EAR grants 1823748 (to Ibañez-Mejia) and 1824002 (to Tissot). Duration: 09/2018 to 09/2021.
Highlights
Analytical methods for Zr isotopic analysis of zircon
We have developed an analytical routine for the precise and accurate determination of mass-dependent Zr stable isotope variations in zircon (also applicable to other Zr-rich accessory phases). Single zircon crystals (or fragments) are first processed using the ‘chemical abrasion’ routine to remove open-system material, and the remaining solids are digested, spiked with a new 91Zr-96Zr tracer, and chemically purified prior to measurement.
A manuscript describing these methods in detail, led by graduate student Hannah Tompkins, has been published in the Journal of Analytical Atomic Spectrometry (JAAS).
Zr isotopic fractionation during closed-system magmatic fractional crystallization
A detailed study of the Zr isotopic composition of single zircon and baddeleyite crystals from the FC-1 gabbro of the Duluth Complex has revealed that Zr-rich phases can exhibit 𝛿94/90Zr variability in excess of 5‰! This variability, the largest known to date for a transition metal in a high-temperature environment, is recorded by zircon and baddeleyite that arguably formed during closed-system magmatic fractional crystallization, and tracks the chemical evolution of the magma as concurrent isotope fractionation and zircon crystallization took place. These observations suggest that Zr stable isotopes in zircon may provide a powerful new tracer of magmatic differentiation and a means to probe the history of Zr removal from magmatic liquids during fractional crystallization.
Results of this study (Ibañez-Mejia and Tissot, 2019) are published in the journal Science Advances.
NEGLIGIBLE EQUILIBRIUM Zr isotopic fractionation AT magmatic CONDITIONS
New results from ab initio calculations indicate that equilibrium Zr isotope effects can only yield minute zircon-melt Zr isotope fractionation at magmatic temperatures; i.e., Δ94/90Zr < ~0.05 ‰ between zircon and melt at >700 °C. Therefore, equilibrium effects alone cannot explain the variability observed in zircon from several magmatic systems and particularly in the Duluth Gabbro. On the other hand, kinetic isotopic fractionation effects within the diffusive boundary layers that develop in crystallizing magmas have the potential to induce mass-dependent Zr isotope fractionation, and can explain the preferential formation of isotopically heavy zircon inferred from the FC-1 zircon and baddeleyite 𝛿94/90Zr data.
Results of this study (Meheut et al., 2021) are published in the journal Geochimica Et Cosmochimica Acta.
Publications
[1]. Ibañez-Mejia, M., & Tissot, F. L. H. (2019). Extreme Zr stable isotope fractionation during magmatic fractional crystallization. Science Advances, 5(12), eaax8648. http://doi.org/10.1126/sciadv.aax8648
[2]. Tompkins, H.G.D., Zieman L.J., Ibañez-Mejia, M., Tissot, F.L.H. (2020) Zirconium stable isotope analysis of zircon by MC-ICP-MS: Methods and application to evaluating intra-crystalline zonation in a zircon megacryst. Journal of Analytical Atomic Spectrometry. https://doi.org/10.1039/C9JA00315K
[3]. Méheut, M., Ibañez-Mejia, M., Tissot, F.L.H. (2021) Drivers of zirconium isotope fractionation in Zr-bearing phases and melts: the roles of vibrational, nuclear field shift and diffusive effects. Accepted in Geochimica Et Cosmochimica Acta