|Title||Screening apatites for (U-Th)/He thermochronometry via continuous ramped heating: He age components and implications for age dispersion|
|Author||McDannell, K T;
Zeitler, P K; Janes, D G; Idleman, B D; Fayon, A K|
|Source||Geochimica et Cosmochimica Acta vol. 223, 2017 p. 90-106, https://doi.org/10.1016/j.gca.2017.11.031|
|Alt Series||Natural Resources Canada, Contribution Series 20180055|
|Media||paper; on-line; digital|
|File format||pdf (Adobe® Reader®); html; xlsx (Microsoft® Excel®); docx (Microsoft® Word®)|
|Subjects||geochronology; radiometric dating; apatite; Methodology|
|Illustrations||graphs; models; tables; plots; spectra|
Geo-mapping for Energy and Minerals TransGEM|
|Released||2017 12 05|
|Abstract||Old slowly-cooled apatites often yield dispersed (U-Th)/He ages for a variety of reasons, some well understood and some not. Analytical protocols like careful grain selection can reduce the impact of
this dispersion but add costs in time and resources and too often have proven insufficient. We assess a new analytical protocol that utilizes static-gas measurement during continuous ramped heating (CRH) as a means to rapidly screen apatite samples.
In about the time required for a conventional total-gas analysis, this method can discriminate between samples showing expected volume-diffusion behavior and those showing anomalous release patterns inconsistent with their direct use in
thermochronologic applications. This method also appears able to discriminate between the radiogenic and extraneous 4He fractions released by a sample, potentially allowing ages to be corrected.|
Well-behaved examples such as the Durango standard
and other apatites with good age reproducibility show the expected smooth, sigmoidal gas-release curves predicted for volume diffusion using typical apatite kinetics, with complete exhaustion by ~900 ºC for linear heating at 20 ºC/min. Secondary
factors such as U and Th zoning and alpha-loss distribution have a relatively minor impact on such profiles. In contrast, samples having greater age dispersion show significant He release in the form of outgassing spikes and He release deferred to
higher temperatures. Screening results for a range of samples permit us to assess the degree to which CRH screening can identify misbehaving grains, give insight into the source of extraneous He, and suggest that in some cases it may be possible to
correct ages for the presence of such components.
|Summary||(Plain Language Summary, not published)|
This article describes an analytical technique for (U-Th)/He dating that greatly improves sample throughput by decreasing analysis times and yielding
more kinetic information about an apatite grain during precise, controlled ramped heating and monitoring of the evolved radiogenic 4He released from the grain. This method has implications for rocks in slowly-cooled settings and for addressing high
He age dispersion commonly associated with complicated thermal histories.