Duluth Complex FC1 Apatite and Zircon: Reference Materials for (U‐Th)/He Dating?

Abstract

The accuracy and validation of geo‐ and thermochronological dating hinges on the availability of well‐characterised age reference materials. The Mesoproterozoic gabbroic anorthosite FC1 from the Duluth Complex, Minnesota is a reference material for zircon U‐Pb and a suggested reference material for apatite fission‐track dating. We evaluate FC1 as (U‐Th)/He reference material, and determine its apatite U‐Pb, and zircon and apatite (U‐Th)/He age. Our dating results constrain the thermal history of FC1, showing that fast cooling occurred between ~ 1099 and 1040 Ma from ≥ 600 °C to ~ 200 °C. The zircon (U‐Th)/He data from air‐abraded grains give a robust isochron age of 1037 ± 25 Ma (2s) without overdispersion. The within‐grain homogeneity of U and Th, the availability of FC1 zircon, and the absence of radiation‐damage effects on the (U‐Th)/He age support its use as reference material. Unabraded zircon grains give lower and more dispersed ages, highlighting the usefulness of air abrasion to control for α‐ejection in (U‐Th)/He dating. Our apatite (U‐Th‐Sm)/He single‐grain ages vary between 180 and 300 Ma. Their wide dispersion argues against the use of FC1 apatite as (U‐Th‐Sm)/He reference material and makes the interpretation of their low‐temperature thermal history complicated.

The daughter-parent plot: a tool for analyzing thermochronological data

“…However, defining a parent concentration is difficult, because several α-emitting nuclides -238 U, 235 U, 232 Th (and 147 Sm) -have to be considered. One solution is to express the parents as an effective uranium concentration (eU) -the sum of the parent concentrations weighted by their relative He production rate (e.g., Härtel et al, 2023), and thereby reduce the number of parents to one:…”

“…The coefficients in Eq. ( 2) give accurate results for samples at ages <1000 Ma (Härtel et al, 2023), but may be modified if the expected ages for a set of samples are consistently higher or constrained well-enough to calculate them more accurately. The coefficients for eU are derived in Eq.…”