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Zirconium stable isotope analysis of zircon by MC-ICP-MS: Methods and application to evaluating intra-crystalline zonation in a zircon megacryst

Tompkins, Hannah G. D. and Zieman, Lisa J. and Ibanez-Mejia, Mauricio and Tissot, François 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, 35 (6). pp. 1167-1186. ISSN 0267-9477. doi:10.1039/c9ja00315k.

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Zirconium (Zr) plays a key role in the development of phases like zircon (ZrSiO₄) and baddeleyite (ZrO₂) in magmatic systems. These minerals are crucial for the study of geologic time and crustal evolution, and their high resistivity to weathering and erosion results in their preservation on timescales of billions of years. Although zircon and baddeleyite may also preserve a robust record of Zr isotope behavior in high-temperature terrestrial environments, little is known about the factors that control Zr isotope partitioning in magmatic systems, the petrogenetic significance of fractionated compositions, or how these variations are recorded in Zr-rich accessory phases. Here, we describe a new analytical protocol for accurately determining the Zr stable isotope composition of zircon by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), using the double-spike method to correct for procedural and instrumental mass bias. We apply this technique to test whether zircon crystallization in carbonatite magmatic systems is a driver of Zr isotope fractionation by interrogating the internal zonation of a zircon megacryst from the Mud Tank carbonatite (MTUR1). We find the MTUR1 megacryst to lack internal zoning within analytical uncertainties with a mean μ⁹⁴/⁹⁰Zr_(NIST) = −55 ± 28 ppm (2 SD, n = 151), which suggests that zircon crystallization is not a driver of Zr isotope fractionation in carbonatite magmas. This observation is in stark contrast with those made in silicate magmatic systems, raising the possibility that the bonding environment of Zr⁴⁺ ions may be fundamentally different in carbonatite vs. silicate melts. Because of its remarkable homogeneity, the MTUR1 megacryst is an ideal natural reference material for Zr isotopic analysis of zircon using both solution and spatially resolved methods. The reproducibility of a pure Zr solution and our chemically purified zircon fractions indicate that the external reproducibility of our method is on the order of ±28 ppm for μ⁹⁴/⁹⁰Zr, or ±7 ppm per amu, at 95% confidence.

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Ibanez-Mejia, Mauricio0000-0002-7839-2425
Tissot, François L. H.0000-0001-6622-2907
Additional Information:© 2020 The Royal Society of Chemistry. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Submitted 17 Sep 2019; Accepted 05 May 2020; First published 07 May 2020. David McGee is thanked for access to the MIT Nu Plasma II, and Joel DesOrmeau for the careful and time-consuming SEM-CL imaging of MTUR1 at U. Nevada in Reno. Matthew Bown, Chaofeng Li and an anonymous reviewer are thanked for constructive comments that helped improve the final version of this manuscript. This research was supported by a Stephen Pollock Undergraduate Research Grant awarded by the NE section of the Geological Society of America (to HT), NSF-EAR grants 1823748 (to MIM) and 1824002 (to FT) and start-up funds to MIM provided by University of Rochester and to FT provided by Caltech. There are no conflicts to declare.
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Geological Society of AmericaUNSPECIFIED
University of RochesterUNSPECIFIED
Issue or Number:6
Record Number:CaltechAUTHORS:20200519-100855433
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Official Citation:Zirconium stable isotope analysis of zircon by MC-ICP-MS: Methods and application to evaluating intra-crystalline zonation in a zircon megacryst. J. Anal. At. Spectrom., 2020, 35, 1167-1186; doi: 10.1039/c9ja00315k
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103310
Deposited By: Tony Diaz
Deposited On:19 May 2020 17:18
Last Modified:16 Nov 2021 18:20

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