Three-isotope plots of δ(⁹⁰Zr/⁹⁴Zr), δ(⁹¹Zr/⁹⁴Zr), and δ(⁹²Zr/⁹⁴Zr) vs. δ(⁹⁶Zr/⁹⁴Zr). The grains (data sources in Section 2, 2σ uncertainty) are compared to Three-zone (left, open symbols) and Zone-II (right, filled symbols) AGB model predictions for a 2 M_☉, 0.5 Z_☉ AGB star with a range of ¹³C strengths. The entire evolution of the AGB envelope composition is shown, but symbols are plotted only when C > O. Dotted lines represent solar zirconium isotope ratios.