Apatite (U-Th)/He (AHe) dating is a widely-applied thermochronological technique used to decipher low-temperature thermal histories. Accurate dates require that the results are corrected for α-ejection because 4He atoms travel ~20 µm during α-decay and a correction is required to account for He lost by this effect. Effective uranium concentrations (eU) are important for accurate AHe data interpretation because radiation damage scales with eU, which affects He retentivity. Both α-ejection correction parameter (Ft) and eU are calculated on the basis of crystal size and assuming an idealized morphology. However, the uncertainty stemming from the calculations’ assumptions depends on how much the real crystal geometry deviates from that assumed, and this uncertainty is typically not included in the propagated uncertainties on AHe data. Our goal for this study was to develop a ‘rule of thumb’ for Ft and eU uncertainties associated with the full range of commonly analyzed apatite geometries by comparing manually measured grain size and actual grain size using nano-computed tomography (nano-CT). Apatite geometry and roughness were characterized using a Grain Evaluation Matrix (GEM). The geometry of each grain was described as: A (prismatic/hexagonal), B (subprismatic), or C (rounded/ellipsoid). Surface roughness was graded from ‘least’ to ‘most’ using values from 1 to 3. The GEM allows for a single parameter (eg. B2) to succinctly classify a grain’s morphology. High resolution nano-CT scans of ~260 grains representative of those usually analyzed for AHe dates were completed and processed using Dragonfly and Blob3D. Initial analysis shows that manual grain measurements systematically overestimate the actual grain size, leading to overestimates in Ft and eU values. One correction exists for A and B grains (hexagonal) and another for C grains (ellipsoid). The correction is controlled primarily by grain size and shape, while the uncertainty on the correction appears to be controlled primarily by surface roughness. Together, this approach provides a simple and practical tool for deriving more accurate Ft and concentration values, and for incorporating this oft neglected geometric uncertainty into AHe dates.

Grappling with systemic discrimination and bias in geosciences can be overwhelming to the point that one may feel powerless to fix them. Despite the sweeping nature of this challenge, faculty, principal investigators, and other scientists with leadership roles have unparalleled power to mitigate harm in environments they oversee. Here, we identify ways that scientists in these roles can immediately address bias in three common spaces --- the lab, field, and classroom. We highlight key actions that can be taken to improve the quality of life of marginalized students and other trainees quickly, while important but comparatively slow institutional changes proceed.

The Colorado Rockies were initially raised during the Laramide Orogeny ca. 70-45 Ma. But consensus exists that the range experienced a second, post-Laramide episode of surface uplift; the timing and cause of that post-Laramide surface uplift event remains enigmatic. Low-temperature thermochronologic studies conducted by us and others using apatite (U-Th)/He (AHe), apatite fission track (AFT), and zircon (U-Th)/He (ZHe) techniques reveal that a dome of kilometer-scale exhumation occurred in Colorado’s Elk and West Elk mountains between ca. 18-6 Ma. We call this feature the “Gothic Dome” because it is centered on Gothic Mountain, near the town of Crested Butte. We suggest the ~100-km-diameter Gothic Dome likely experienced Miocene surface uplift, which triggered the dome-shaped exhumation pattern documented by the low-temperature thermochronometry. The exhumation magnitude exceeds 4 km in the center of the dome (as revealed by a 16 Ma ZHe date on an Oligocene pluton) and diminishes toward its perimeter. This diminution of exhumation magnitude toward the perimeter is revealed by progressively older AHe, AFT, and ZHe dates in all directions away from Gothic Mountain. AHe dates for samples that lie outside the perimeter are Laramide-age or older, further documenting the dome-shaped nature of this Miocene exhumation event and illustrating the low magnitude of Miocene to recent exhumation outside the dome’s perimeter. Outcrops of ca. 11 Ma basalt surround the Gothic Dome to the north, west, and south, requiring that Miocene exhumation outside the dome’s perimeter was minimal. A suite of alkaline, low-volume, felsic plutons and ultramafic lamprophyres intruded the Gothic Dome between ca. 18-12 Ma. This alkalic magmatism began either immediately prior to or contemporaneous with the onset of Gothic Dome exhumation, hinting that the same root cause might be responsible for both. Workers elsewhere, including Tibet, the Altiplano, and California’s Sierra Nevada mountains, have attributed small-volume alkalic magmatism, surface uplift, and exhumation to the activity of lithospheric drips. We offer that Miocene activity of such a drip beneath Colorado’s Elk and West Elk mountains is an appealing mechanism to explain the near simultaneity of those same phenomena here.