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

Abstract

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 <0 to ∼150 °C. These domains may correspond to crystallites in the specimen, which are observed to range from smaller than a few nm to a few μm. ^4He/^3He age spectra acquired from the same experiments define a distinctive sigmoidal shape with step ages increasing monotonically from 0 to ∼210 Ma. This pattern clearly records the presence of the multiple domains as the hematite acquired its radiogenic helium. Coupling of the ^4He/^3He spectra and the diffusion data allow modeling of permissible time–temperature paths experienced by the hematite. These paths can be compared with independent thermal history reconstructions for eastern Grand Canyon characterized by ∼170 m.yr. of sedimentary burial followed by regional unroofing and canyon incision beginning at ∼80 Ma. The unroofing portion of the path inferred from the hematite is in good agreement with apatite (U–Th)/He and fission-track data. The results permit rapid cooling in Late Cretaceous–early Tertiary time, suggest a subsequent period of slow cooling through temperatures of ∼50–70 °C, with accelerated cooling ensuing after 20 Ma. In contrast, the hematite data demand temperatures in some part(s) of the Mesozoic that are at least 100 °C hotter than expected based on burial depth estimates. This may indicate that the sample experienced a reheating event associated with the passage of fluids similar to those observed at nearby mineralized breccia pipes. Overall these data provide motivation for further work to understand He and Ne behavior in hematite and their potential use in thermochronometry.

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