Are Zr stable isotopes tracking magmatic differentiation in the zoned La Posta pluton?

Non-traditional stable isotopes provide invaluable information about magmatic systems. Recently, with the proliferation of the MC-ICPMS and refinements to the double-spike method, both the number of studies performed and isotopes investigated have increased to include investigations of Zr. Previous studies examined Zr isotope variations in zircon [1, 2] and in bulk-rock samples [3] and reported different fractionation factors. At the bulk-rock scale, Inglis et al. (2019) found a positive correlation between Zr stable isotope values and whole rock SiO₂ for rocks above zircon saturation at Hekla volcano (Iceland) indicating that Zr stable isotopes may track magmatic differentiation. However, at the mineral scale, Ibañez-Mejia and Tissot (2019) studied 42 zircons from the Duluth complex anorthosite (MN) and found a >5 ‰ variation in δ^94/90Zr which they attributed to a Rayleigh-like removal of isotopically heavy zircon. Furthermore, Tompkins et al. (2020) found no resolvable intra-crystalline variability when studying a zircon megacryst from the Mud Tank carbonatite, which was interpreted to indicate that no isotopic fractionation took place during zircon crystallization. Interestingly, these three studies suggest conflicting values for zircon-melt fractionation factors (Ibañez-Meija and Tissot (2019): Δ_zircon-melt ~= +1 ‰; Inglis et al. (2019): Δ_zircon-melt ~= -0.5 ‰; Tompkins et al. (2020): Δ_zircon-melt ~= 0 ‰), indicating a need for further study.

In order to understand Zr stable isotope variations at the whole rock, intergrain, and intragrain scales, we geochemically investigate zircons and bulk-rock samples from the zoned La Posta pluton (CA). Six La Posta samples ranging in composition from hornblende-biotite tonalite (68.63 wt% SiO₂) to muscovite-biotite granodiorite (74.02 wt% SiO₂) and a sample of the Cuyamaca gabbro cumulate (41.5 wt% SiO₂) were analyzed. Zr isotopic and trace element measurements of these samples by SIMS and DS-MC-ICPMS allow us to test the relationship between petrogenetic processes and Zr stable isotope variations and will allow us to gain a greater understanding of Zr stable isotope systematics.

[1] Ibanez-Meija and Tissot (2019) Sci. Adv., 5, eaax8648

[2] Tompkins et al. (2020) J. Anal. At. Spectrom., 35, 1167-1186

[3] Inglis et al. (2019) Geochim Cosmochim Acta 250, 311