Characterization of the rhyolite of Bodie Hills and 40Ar/39Ar intercalibration with Ar mineral standards

Robert J. Fleck; Andrew T. Calvert; Matthew Coble; Joseph L. Wooden; Kip V. Hodges; Leslie A. Hayden; Matthijs C. van Soest; Edward A. du Bray; David A. John

doi:10.1016/j.chemgeo.2019.07.022

Characterization of the rhyolite of Bodie Hills and 40Ar/39Ar intercalibration with Ar mineral standards

Chemical Geology

By: Robert J. Fleck, Andrew T. Calvert, Matthew Coble, Joseph L. Wooden, Kip V. Hodges, Leslie A. Hayden, Matthijs C. van Soest, Edward A. du Bray, and David A. John

https://doi.org/10.1016/j.chemgeo.2019.07.022

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Abstract

The rhyolite of Bodie Hills (California) is characterized compositionally and the geochronology of selected phases is studied. Sanidine (BHs) from the rhyolite is well suited as a ⁴⁰Ar/³⁹Ar reference material with high K/Ca and radiogenic yield. Intercalibration with GA1550 biotite from the Dromedary igneous complex (New South Wales, Australia) yields an age of 9.7946 ± 0.0031 Ma for BHs relative to an age of 98.79 ± 0.54 Ma for GA1550 and a calibration factor (R_BHs/GA1550) of 0.096719 ± 0.000032. BHs is also intercalibrated with sanidines of the Taylor Creek Rhyolite (TCs; R_TCs/BHs of 2.90874 ± 0.00067), Fish Canyon Tuff (FCs; R_FCs/BHs of 2.88339 ± 0.00088), and rhyolite of Alder Creek (ACs; R_ACs/BHs of 0.12028 ± 0.00024). These calibration factors yield ages of 28.344 ± 0.011 Ma, 28.099 ± 0.013 Ma, and 1.1809 ± 0.0024 Ma for TCs, FCs, and ACs, respectively, relative to GA1550. Full propagation of errors increases these uncertainties and that of BHs to ±0.9% of their ages. Calibration of BHs using the astronomically tuned age of FCs determined by Kuiper et al. (2008) yields an age of 9.8295 ± 0.0036 Ma. Stepwise heating of BHs reveals the same small, progressive increase in age across the age spectrum reported for FCs, ACs, and other potassium feldspars (e.g., Foland and Xu, 1990; Phillips et al., 2017). This increase is consistent with mass fractionation of argon during step heating and favors use of single-step fusion ages of all sanidines for monitor and calibration purposes.

Additional publication details
Publication type	Article
Publication Subtype	Journal Article
Title	Characterization of the rhyolite of Bodie Hills and 40Ar/39Ar intercalibration with Ar mineral standards
Series title	Chemical Geology
DOI	10.1016/j.chemgeo.2019.07.022
Volume	525
Year Published	2019
Language	English
Publisher	Elsevier
Contributing office(s)	Central Mineral and Environmental Resources Science Center, Geology, Minerals, Energy, and Geophysics Science Center
Description	21 p.
First page	282
Last page	302
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