科研之友
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摘要
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. Ar-40/Ar-39 stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess Ar-40, 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 Ar-40 (radiogenic, atmospheric and excess). This excess Ar-40 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 Ar-40. This population gives a statistically robust weighted mean age of 47.4 +/- 1.5 ka (1 sigma, 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 Ar-40/Ar-39 dating of bracketing lavas, but is indistinguishable from recent C-14 and (U-Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by C-14 ages.
- 出版日期2016-6
