Further Evaluation of Penglai Zircon Megacrysts as a Reference Material for (U‐Th)/He Dating

Abstract

The α‐ejection‐corrected (U‐Th)/He dates of the widely used reference materials (e.g., Fish Canyon Tuff zircon) can be inaccurate for several reasons (e.g., inclusions, U‐Th zonation). There is a need for large, widely available, compositionally homogeneous mineral reference materials with accurately and precisely known ages. In this work, four individual Penglai zircon megacrysts are evaluated via interlaboratory (U‐Th)/He dating comparison, trace element analysis, U‐Pb dating and a range of characterisation methods. The (U‐Th)/He ages and 206Pb/238U ages are variable between Penglai megacrysts – 4.12 ± 0.10 Ma (1s) and 4.29 ± 0.05 Ma (1s) (PL1), 3.07 ± 0.25 Ma (1s) and 3.95 ± 0.05 Ma (1s) (PL2), 3.04 ± 0.24 Ma (1s) and 3.06 ± 0.07 Ma (1s) (PL4), and 4.49 ± 0.07 Ma (1s) (PL5; (U‐Th)/He age only), respectively. A single, cm‐sized megacryst (PL1) qualifies as a new (U‐Th)/He dating reference material, as it is structurally and chemically homogeneous, and yielded a highly reproducible and statistically indistinguishable (U‐Th)/He age of 4.12 ± 0.10 Ma from repeated analysis in multiple laboratories (slightly younger than the 206Pb/238U age of 4.29 ± 0.05 Ma). These results confirm Penglai zircon as a suitable reference material for (U‐Th)/He dating, on the proviso that some care needs to be taken to characterise individual megacrysts for trace element zonation, and to verify (U‐Th)/He (and U‐Pb ages, if double dating) prior to routine use. The 206Pb/238U ages and (U‐Th)/He ages obtained in this work provide constraints on the timing of Penglai megacryst crystallisation and eruption, and confirm at least two eruptive episodes during the Neogene in the Leiqiong area.