Low-temperature thermochronology of the northern Rocky Mountains, western U.S.A.

Abstract

We dated 86 borehole and surface samples from basement-cored Laramide uplifts of the northern Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges) using the apatite (U-Th)/He system, and eleven samples using the apatite fission-track system (Wind River and Bighorn Ranges). Apatite (U-Th)/He ages generally decrease with increasing subsurface depth (decreasing elevation), and typically range from ϳ100 to 50 Ma (Cretaceous to Eocene) within ϳ1 km of the surface, to ϳ20 Ma (Miocene) and younger ages at depths greater than ϳ2 to 2.5 km. Most samples display (U-Th)/He age dispersion ranging from tens to hundreds of Ma, and for some samples we find ages that are older than corresponding fission-track ages. At least one sample per range shows a correlation between apatite (U-Th)/He age and effective U concentration (eU ‫؍‬ [U] ؉ 0.235[Th]) of the crystal, indicating that radiation damage has affected He diffusivity, and hence (U-Th)/He age. Forward modeling of simple Laramide-type thermal histories using a radiation damage diffusion model predicts: 1) fossil apatite fission-track partial annealing and apatite (U-Th)/He partial retention zones over similar elevation ranges, 2) (U-Th)/He age dispersion within a fossil partial retention zone up to hundreds of Ma, and 3) (U-Th)/He ages older than fission-track ages within a fossil partial retention zone if eU տ 20 ppm. We observe these features in our data from the Bighorn and Laramie Ranges. Most of our samples, however, do not show the correlation between (U-Th)/He age and eU predicted by radiation damage diffusion models. The age dispersion of these samples could be due to the influence of both grain size and eU content, or alternatively due to high U or Th secondary rims around the apatite crystals. (U-Th)/He ages that are older than fission-track ages from Gannett Peak and Fremont Peak in the Wind River Range, and some samples from the Beartooth Range, are most likely the result of He implantation from high eU secondary rims. Best-fit time-temperature paths from inverse modeling of (U-Th)/He age-eU pairs, when extrapolated to other elevations to create model age-elevation plots, reproduce the general distribution and dispersion of (U-Th)/He ages from the Bighorn, Beartooth and Wind River Ranges and suggest that rapid exhumation within the Laramide province likely began earlier in the Bighorn Range (before ϳ71 Ma) than the Beartooth Range (before ϳ58 Ma). Inverse modeling of borehole data at the northern end of the Laramie Range suggests that the well penetrated a fault sliver at depth. The amount and timing of post-Laramide burial and exhumation cannot be determined from these data.

Effects of U–Th-rich grain boundary phases on apatite helium ages

“…10a; Orme, 2011;Peyton et al, 2012), modern river sediments in the Appalachian Mountains ( Fig. 10b; McKeon, personal comm.…”

“…Peyton et al (2012) had previously discounted U-Thrich inclusions in these grains as a possible source of dispersion by processing euhedral apatite grains using zircon dissolution methods. Apatite grains from the Henry Mountains were picked with careful attention to the presence and characteristics of any GBPs or discoloration in the sample separates and on individual grains picked for He dating.…”

“…10a), as would be expected of grains that have partially retained some of their [eU]-rich GBP coatings prior to dating. Using the RDAAM-predicted age-[eU] trend for these samples from Peyton et al (2012) and earlyformed and lost GBP model predictions from this study, the full range of age trends can be matched with a reasonable range of GBP compositions (Fig. 11a).…”

“…In some cases, we used the youngest apatite He age in the dataset as the "correct" age for this conversion. In others it is clear that a RDAAMpredicted age-[eU] trend should be used as the "correct" age (Peyton et al, 2012). To generate these trends, we used the RDAAM (Flowers et al, 2009) in the software HeFTy (Version 1.7.4; Ketcham, 2005) to forward model the age-[eU] patterns given a likely thermal history and the range of apatite grain size and [eU].…”

“…Despite its widespread use, many studies have documented significant age dispersion among single-grain He ages from individual (non-detrital) samples (Fitzgerald et al, 2006;Spiegel et al, 2009;Peyton et al, 2012) that can make geological interpretations difficult.…”