Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required?

Phillips, David; Matchan, Erin

doi:10.1016/j.gca.2013.07.003

Cited by 144 publications

(100 citation statements)

References 32 publications

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“…To maximize the temporal resolu- 40 Ar/ 39 Ar step-heating spectra for two aliquots of grains from Fish Canyon Tuff sanidine from AG10-02 (B and C) from the distal quarry site, compared to sample FC3 sanidine from the classic site (from Phillips and Matchan, 2013). These three release spectra are essentially identical.…”

Section: Resultsmentioning

confidence: 99%

“…Approximately 60 grains of AG10-02 were co-irradiated in the Cd-shielded CLICIT facility of the Oregon State University TRIGA reactor (can UM#52), together with Fish Canyon sanidine grains from the classic site. Fusion and step-heating analyses were carried out using a Thermo Fisher Scientific ARGUS VI multi-collector noble gas mass spectrometer (see detailed descriptions by Matchan, 2013 andPhillips, 2014). Sample heating was carried out using a homogenised 55 W Fusions 10.6 CO 2 laser system to achieve uniform heating over the sanidine crystals.…”

Section: Ar/ 39 Ar Analysismentioning

confidence: 99%

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The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

Gleadow

Harrison

Kohn

et al. 2015

Earth and Planetary Science Letters

155

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The Fish Canyon Tuff (FCT) has served as an important source for geochronology standards, particularly for fission track, K-Ar and ( 40 Ar/ 39 Ar) dating, even though efforts to establish precise ages for its constituent minerals have proved to be unexpectedly complex. To evaluate the suitability of FCT apatite as a standard for apatite (U-Th-Sm)/He (AHe) thermochronometry, and to test underlying assumptions about its suitability for apatite fission track (AFT) thermochronometry, we analysed samples from a series of sites throughout the vertical and lateral extent of the host ignimbrite sheet. Samples were collected from the relatively lithic-rich, classic sampling location in the lower part of the thick proximal ignimbrite and a ∼330 m vertical section of FCT immediately above it. Average weighted mean AHe ages from multiple analyses at five sites in this profile range from 20.8 ± 0.4 Ma from the classic site at the base, to an average of 28.4 ± 0.2 Ma (all ±1σ ) in the upper part of the section. The AHe age at the classic site is substantially reduced at 20.8 ± 0.4 Ma relative to a reference age for ignimbrite emplacement of 28.2 Ma. Corresponding zircon (U-Th)/He (ZHe) ages for these samples are all concordant at 28.3 ± 0.4 Ma. By contrast, apatite fission track (AFT) ages from the same vertical section are all concordant at 27.4 ± 0.7 Ma with the central age of 28.8 ± 0.8 Ma at the classic site, except for the uppermost sample (23.2 ± 1.7 Ma) for which clear evidence for local, probably fire, disturbance is seen in the track length distribution. The AHe data at the classic site thus provide evidence for substantial post-eruptive Early Miocene cooling of the tuff consistent with its position at the bottom of a deeply incised valley with ∼800 m of local relief and probably >1000 m of removed section above it. The AFT age of the classic sampling site, however, is indistinguishable from ignimbrite emplacement and thus continues to be a useful standard for AFT (but not for AHe) geochronology. Apatites from this site have the highest measured Cl concentrations (0.82 wt%) of any of the FCT apatites analysed, contributing to this suitability. AFT and AHe ages at three distal localities, some 35-45 km to the East of the classic site, where the thickness of the FCT is reduced to <100 m, all yielded concordant ages with a weighted mean of 28.5 ± 0.11 Ma with no evidence for post-emplacement thermal disturbance. One of the distal sites -a quarry in the upper part of the FCT -appears to be an ideal locality for a standard reference material that would be suitable for both AFT and AHe low-temperature thermochronometers. U-Pb ages for zircons and 40 Ar/ 39 Ar step heating age spectra for sanidine at this new distal site are essentially identical to those found at the classic site.

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Section: Resultsmentioning

confidence: 99%

Section: Ar/ 39 Ar Analysismentioning

confidence: 99%

The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

Gleadow

Harrison

Kohn

et al. 2015

Earth and Planetary Science Letters

155

View full text Add to dashboard Cite

show abstract

“…Even these samples seem to be inhomogeneous when measured with the new multicollector mass spectrometers (Phillips and Matchan, 2013). Leaving this point aside, with the used instrumentation it is not possible to measure confidently the age of samples younger than approximately 30 Ma, so all these reference materials are too recent for our purposes.…”

Section: Reference Materialsmentioning

confidence: 99%

“…The proper detection of 36 Ar, a desiderata not obtained in this work, will be feasible, and the method will work for samples with trapped argon of arbitrary 40 Ar/ 36 Ar ratio, using an isochron approach. Based on the reported analytical performance of the new multicollector instruments (Mark et al, 2009;Coble et al, 2011;Phillips and Matchan, 2013), a determination of 40 Ar* argon with b1.0% uncertainty might be easily obtained. For extraterrestrial missions it is mandatory to use a much smaller mass spectrometer; an issue that will not be discussed in this work.…”

Section: Noble Gas Mass Spectrometer and Extraction Linementioning

confidence: 99%

“…It has been observed for years that in situ analysis by laser ablation reveals age gradients on minerals that whole-sample methods reveal as homogeneous (e.g., zircons, micas). There are few reference samples for potassium-argon that can withstand a detailed analysis by the Ar-Ar laser microprobe, without showing any internal gradient (e.g., Phillips and Matchan, 2013). Some of these young reference feldspars (e.g.…”

Section: Reference Materialsmentioning

confidence: 99%

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In situ determination of K–Ar ages from minerals and rocks using simultaneous laser-induced plasma spectroscopy and noble gas mass spectrometry

Solé¹

2014

Chemical Geology

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Radial fast‐neutron fluence gradients during rotating⁴⁰Ar/³⁹Ar sample irradiation recorded with metallic fluence monitors and geological age standards

Rutte

Pfänder

Koleška

et al. 2015

Geochem Geophys Geosyst

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Characterizing the neutron-irradiation parameter J is one of the major uncertainties in 40 Ar/ 39 Ar dating. The associated uncertainty of the individual J-value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast-neutron fluence distribution in the reactor, and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J-values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three-dimensionally distributed metallic fast (Ni) and thermal-(Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd shielding in the LVR-15 reactor in Re z, Czech Republic. The 58 Ni(n f ,p) 58 Co activation reaction and c-spectrometry of the 811 keV peak associated with the subsequent decay of 58 Co to 58 Fe allow one to calculate the fast-neutron fluence. The fast-neutron fluences at known positions in the irradiation container correlate with the J-values determined by massspectrometric 40 Ar/ 39 Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR-15; the corresponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not always suffice to meet the needs of highprecision dating and gradient monitoring can be crucial.

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Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required?

Cited by 144 publications

References 32 publications

The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

In situ determination of K–Ar ages from minerals and rocks using simultaneous laser-induced plasma spectroscopy and noble gas mass spectrometry

Radial fast‐neutron fluence gradients during rotating⁴⁰Ar/³⁹Ar sample irradiation recorded with metallic fluence monitors and geological age standards

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Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required?

Cited by 144 publications

References 32 publications

The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

In situ determination of K–Ar ages from minerals and rocks using simultaneous laser-induced plasma spectroscopy and noble gas mass spectrometry

Radial fast‐neutron fluence gradients during rotating40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards

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Radial fast‐neutron fluence gradients during rotating⁴⁰Ar/³⁹Ar sample irradiation recorded with metallic fluence monitors and geological age standards