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Knowledge of the kinetic behavior of He in apatite and other U- and Th-bearing minerals comes largely from detailed step-heating experiments, yet such experiments are time consuming and are rarely performed during routine thermochronological studies using the U-Th/He method. We propose a new analytical method for measuring both the bulk 4He abundance and the kinetics of He release in apatite. Using this method He is extracted from samples by continuous heating using a ramped temperature schedule under static vacuum conditions, and the evolved He is measured periodically as it accumulates in the extraction system. Continuous ramped heating (CRH) experiments can be conducted using instrumentation available in most noble-gas ther- mochronology labs but require particular attention to temperature control, measurement linearity and dynamic range, and suppression of active gases co-evolved with He. CRH experiments require little more time than conventional single-step heating measurements but yield a detailed record of He release not provided by con- ventional methods. Kinetic parameters for He diffusion in Durango apatite derived from continuous heating data agree well with those obtained from published step-heating studies. The continuous record of He release ob- tained from CRH experiments also provides important information about the siting of He and the presence of multiple He components in apatite, some of which may be responsible for anomalous U-Th/He ages and high age dispersion. As such the CRH method shows promise as a useful sample screening tool for apatite U-Th/He thermochronology.
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Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility
Abstract. (U–Th) ∕ He thermochronometry relies on the accurate andprecise quantification of individual grain volume and surface area, whichare used to calculate mass, alpha ejection (FT) correction, equivalentsphere radius (ESR), and ultimately isotope concentrations and age. The vastmajority of studies use 2-D or 3-D microscope dimension measurements and anidealized grain shape to calculate these parameters, and a long-standingquestion is how much uncertainty these assumptions contribute to observedintra-sample age dispersion and accuracy. Here we compare the results forvolume, surface area, grain mass, ESR, and FT correction derived from2-D microscope and 3-D X-ray computed tomography (CT) length and width datafor > 100 apatite grains. We analyzed apatite grains from twosamples that exhibited a variety of crystal habits, some with inclusions. Wealso present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium(eU). The data illustrate that the 2-D approach systematically overestimatesgrain volumes and surface areas by 20 %–25 %, impacting the estimates formass, eU, and ESR – important parameters with implications for interpretingage scatter and inverse modeling. FT factors calculated from 2-D and 3-Dmeasurements differ by ∼2 %. This variation, however, haseffectively no impact on reducing intra-sample age reproducibility, even onsmall aliquot samples (e.g., four grains). We also present a grain-mountingprocedure for X-ray CT scanning that can allow hundreds of grains to be scannedin a single session and new software capabilities for 3-D FT andFT-based ESR calculations that are robust for relatively low-resolutionCT data, which together enable efficient and cost-effective CT-basedcharacterization.
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- Award ID(s):
- 1762458
- PAR ID:
- 10164875
- Date Published:
- Journal Name:
- Geochronology
- Volume:
- 1
- Issue:
- 1
- ISSN:
- 2628-3719
- Page Range / eLocation ID:
- 17 to 41
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/gchron-1-17-2019
