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Stable isotopes provide deep insights into processes across a wide range of scales, from micron- to cosmic-size systems. Here, we review how continued advances in mass-spectrometry have enabled the analysis of ever-smaller samples and brought the field of heavy stable isotope geochemistry to its next frontier: the single-crystal scale. Accessing this record can be as enlightening as it is challenging. Drawing on novel systematics at different stages of development (from well-established to nascent), we discuss how the isotopes of heavy elements, such as magnesium, iron, zirconium, or uranium, can be used at the single-crystal and subcrystal scales to reconstruct magma thermal histories, crystal growth timescales, or, possibly, magma redox conditions. 

The isotopic variability of the elements in our planet and Solar System is the end result of a complex mixture of processes, including variable production of isotopes in stars, ingrowth of daughter nuclides due to decay of radioactive parents, and selective incorporation of isotopes into solids, liquids, or gases as a function of their mass and/or nuclear volume. Interpreting the isotopic imprints that planetary formation and evolution have left in the rock and mineral record requires not only precise and accurate measurements but also an understanding of the drivers behind isotopic variability. Here, we introduce fundamental concepts needed to “read” the isotopic code, with particular emphasis on heavy stable isotope systems. 

Zirconium (Zr) plays a key role in the development of phases like zircon (ZrSiO 4 ) and baddeleyite (ZrO 2 ) 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 μ 94/90 Zr 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 Zr 4+ 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 μ 94/90 Zr, or ±7 ppm per amu, at 95% confidence. 

Zirconium is a commonly used elemental tracer of silicate differentiation, yet its stable isotope systematics remain poorly known. Accessory phases rich in Zr 4+ such as zircon and baddeleyite may preserve a unique record of Zr isotope behavior in magmatic environments, acting both as potential drivers of isotopic fractionation and recorders of melt compositional evolution. To test this potential, we measured the stable Zr isotope composition of 70 single zircon and baddeleyite crystals from a well-characterized gabbroic igneous cumulate. We show that (i) closed-system magmatic crystallization can fractionate Zr stable isotopes at the >0.5% level, and (ii) zircon and baddeleyite are isotopically heavy relative to the melt from which they crystallize, thus driving chemically differentiated liquids toward isotopically light compositions. Because these effects are contrary to first-order expectations based on mineral-melt bonding environment differences, Zr stable isotope fractionation during zircon crystallization may not solely be a result of closed-system thermodynamic equilibrium.