Zr Isotope Variations in Zircon as an Indicator of Magmatic Differentiation

Isotopic investigations of igneous minerals and rocks show significant Zr stable isotope variability, suggesting their potential use as tracers of magmatic differentiation. To better understand 𝛿^94/90Zr variability at the intragrain scale, we analyzed grains from zircon reference localities (OG-1, Oracle, FC-1, 91500, R33, Temora, Plešovice, 94-35, Fish Canyon, Mud Tank, and KL1), zircons from ten I-type (Big Bear & Peninsular Ranges batholith (PRB), CA), and one S-type granite (Strathbogie Range, Australia) and eight Jack Hills Hadean grains.

Analyses were performed on the ims1290 at UCLA, which combines high lateral resolution (~8 μm spots), high-sample throughput with only ~4 min analysis time to achieve ±0.1‰ (2σ) on 𝛿 ^94/90Zr values, and small sample consumption (~5 pg). Large (~7‰) inter- and intra-grain variations in several zircons underscore the benefits of using high spatial resolution analysis. Samples showing uniform 𝛿^94/90Zr in multi-grain analyses (e.g., 91500) were also measured using a double spike solution to confirm the accuracy and precision of the SIMS data. FC-1 zircons exhibit 𝛿^94/90Zr variations up to 6.5‰, in excellent agreement with solution data [1]. By contrast, Hadean zircons show a limited range with an average 𝛿^94/90Zr_NIST of -0.4±0.5‰ and Big Bear samples differ by only 1.1‰ from one another. 𝛿^94/90Zr in zircon decreases with increasing whole rock SiO₂ in 4 units of the zoned La Posta Pluton (PRB) confirming magma differentiation as a key source of Zr isotopic variations. These results confirm that early crystallizing zircons are isotopically heavy for Zr and are inconsistent with the suggestion by [1] that the increasing 𝛿^94/90Zr with increasing whole rock SiO₂ observed in the Hekla suite are the product of zircon crystallization. The isotopic uniformity in Hadean zircons is more consistent with their origin in undifferentiated granitoids (e.g., leucogranites) than in a fractionating igneous complex. Once the full range of granitoid magma compositions and their associated zircons have been characterized, Zr isotopes could become a powerful new tool in characterizing protolith composition of detrital igneous zircons.

[1] Inglis et al. (2019) GCA 250, 311