40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals

• 40Ar/39Ar dating co-magmatic lithic clasts from the Rotoiti eruption, New Zealand.
• Step heating and total fusion experiments on single K-feldspar and biotite crystals.
• Step-heating shows some crystals contain excess 40Ar.
• A younger crystal population indicates an eruption age of 47.4 ± 1.5 ka (1σ).

Co-magmatic granitoid clasts erupted as part of the Rotoiti Ignimbrite (Rotoehu Tephra) contain euhedral K-feldspar and biotite crystals that protrude into miarolytic cavities and show textural evidence for growth in super-cooled conditions and are thus interpreted as growing during eruption. 40Ar/39Ar stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess 40Ar, preferentially released at lower temperature steps (most likely from fluid/melt inclusions), which cannot reliably be characterised by, or corrected for using isotope correlation diagrams due to mixing between three reservoirs of 40Ar (radiogenic, atmospheric and excess). This excess 40Ar component is common, but not ubiquitous, and an age population unmixing algorithm applied to single-crystal fusion data identifies a younger group of K-feldspar and biotite crystals that appear to be largely unaffected by excess 40Ar. This population gives a statistically robust weighted mean age of 47.4 ± 1.5 ka (1σ, n = 13) and an indistinguishable inverse isochron age of 50 ± 3 ka for this historically difficult to date eruption. The weighted mean age is significantly younger than previous age estimates of the Rotoiti eruption obtained by K/Ar and 40Ar/39Ar dating of bracketing lavas, but is indistinguishable from recent 14C and (U–Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by 14C ages.