(U–Th)/Ne and multidomain (U–Th)/He systematics of a hydrothermal hematite from eastern Grand Canyon

A hydrothermal hematite from the Redwall Limestone in eastern Grand Canyon (Arizona, USA) was analyzed for (U–Th)/He and (U–Th)/Ne systematics. The dense polycrystalline aggregate has U and Th concentrations of ∼16 ppm each. Neon produced by α particle capture on ^(18)O is abundant in the sample—it constitutes ∼90% of total ^(21)Ne and indicates a (U–Th)/Ne age of 217±5 Ma. This value provides a lower bound for the formation age of the sample but detailed interpretation is not possible given the current absence of neon diffusion data for hematite. The 130±2 Ma (U–Th)/He age is much younger than the Ne age and therefore must be a cooling age. Highly reproducible step-heating He release experiments reveal the existence of a spectrum of diffusion domain sizes spanning about 3 orders of magnitude in radius, corresponding to He closure temperatures ranging from