V032-0004
Are Zr stable isotopes tracking magmatic differentiation in the zoned La Posta pluton?
Monday, 14 December 2020
Poster
Heather Kirkpatrick1, T. Mark Harrison2, Elizabeth A Bell3, Ming-chang Liu4, Francois Tissot5, Scott Angus MacLennan6 and Mauricio Ibanez-Mejia6, (1)University of California Los Angeles, Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, (2)University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, United States, (3)UCLA, Los Angeles, CA, United States, (4)University of California Los Angeles, Earth, Planetary and Space Sciences, Los Angeles, CA, United States, (5)California Institute of Technology, Pasadena, CA, United States, (6)University of Rochester, Rochester, NY, United States
Abstract:
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 SiO
2 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
