Leah E. Morgan and Michael A. Cosca
U.S. Geological Survey, Denver, Colorado
Neutron flux monitors
The 40Ar/39Ar method requires a priori knowledge of a mineral standard, or neutron flux monitor, which is co-irradiated with samples of interest. The age of a mineral standard can be determined in several ways, including first-principles measurements (Lanphere and Dalrymple, 1966; Lanphere and Dalrymple, 2000; McDougall and Roksandic, 1974; McDougall and Wellman, 2011), intercalibration with primary standards (Dazé et al., 2003; Jourdan and Renne, 2007; Renne et al., 1998; Spell and McDougall, 2003), astronomical calibrations (e.g. Kuiper et al., 2008), and optimizations involving intercalibration with the U-Pb system (Renne et al., 2011; Renne et al., 2010).
The most direct of these options is through first principles measurements, which require accurately calibrated laboratory equipment to make concentration measurements of both 40K and 40Ar*. Given the difficulty in quantitatively extracting all 40Ar* from highly viscous K-feldspar melts, these concentration measurements have proven most reliable when applied to phases such as biotite and hornblende. Thus many commonly used neutron flux monitors, such as Fish Canyon sanidine, are considered secondary standards, in that they have been intercalibrated with primary standards that have reliable first principles data.
Fish Canyon Tuff sanidine
Among the most commonly used mineral standards in 40Ar/39Ar geochronology is Fish Canyon sanidine (FCs). FCs has been separated from the Fish Canyon Tuff (FCT), which erupted from the La Garita Caldera in the San Juan Mountains of southern Colorado. Minerals from FCT were first dated by K-Ar by Steven et al. (1967). Within the uncertainties attainable at the time (ca. ±1-3 Ma), results indicated that ages for all dated minerals (sanidine, biotite, hornblende, plagioclase) from FCT were indistinguishable. The sample measured by Steven et al. (1967) was collected at the summit of Agua Ramon Mountain, north of South Fork, Colorado.
Further measurements were made by Hurford and Hammerschmidt (1985) on a sample collected along Hwy. 160 about 9 km southwest of South Fork, Colorado by Naeser et al. (1981). This locality is near what is now the Fun Valley Family Resort. A number of other early K-Ar measurements on FCT phases are summarized by McDougall and Harrison (1999).
In what became the most used calibration for a decade, Renne et al. (1998) published an age of 28.02 ± 0.16 Ma (1σ) for FCs, based on decay constants tabulated in Steiger and Jäger (Steiger and Jäger, 1977), new isotope dilution K measurements of primary biotite standard GA-1550, previous Ar concentration measurements of GA-1550 (McDougall and Roksandic, 1974), and extensive intercalibration measurements between GA-1550 and FCs. Subsequent characterization and calibrations of the Fish Canyon sanidine ranged from an age of ca. 27.5 Ma (Lanphere and Baadsgaard, 2001) to an age of ca. 28.5 Ma (Schmitz and Bowring, 2001). More congruent results included 27.98 ± 0.08 Ma (1σ) (Villeneuve et al., 2000) and 28. 10 ± 0.04 Ma (1σ) (Spell and McDougall, 2003).
Recent developments in the ages of FCs and other standards
In 2008, Kuiper et al. published an astronomical calibration of the age of FCs. This was accomplished using tephra from the astronomically-tuned Messâdit section in the Melilla-Nador Basin of Morocco. The astronomical ages of tephra horizons allowed for these tephra to be used as 40Ar/39Ar standards when they were co-irradiated with FCs. The age for FCs determined in this way is 28.201 ± 0.023 Ma (1σ), which is based on (and must be used with) decay constants as compiled and calculated by Min et al. (2000), which have significantly larger (and more reasonable) uncertainties than those tabulated by Steiger and Jäger (1977). The youngest U-Pb zircon age from Wotzlaw et al. (2013) is indistinguishable from the Kuiper age, at 28.196 ± 0.019 Ma.
More recently, a statistical optimization model (Renne et al., 2011; Renne et al., 2010) allowed for the simultaneous determination of an age for FCs and the 40K decay constants. The model utilizes several existing constraints on the 40Ar/39Ar system, including 40Ar/40K values for FCs, activity data for 40K decay, and results from “data pairs,” where the same samples were dated with both the 40Ar/39Ar and the 238U-206Pb systems. The model yields most likely values (and uncertainties) for 40K decay constants and the 40Ar*/40K ratio for FCs; combined, these indicate an age for FCs of 28.294 ± 0.036 Ma (1σ) (Renne et al., 2011).
How does a revised age of Fish Canyon sanidine affect previous age calculations?
Figure 1 graphically displays the effect of using different calibrations over much of geological history. There are three combinations of FCs age and decay constants shown, relative to the reference calibration of FCs = Renne et al. (1998) and λ = Steiger and Jäger (1977): 1. FCs= Renne et al. (1998), λ=Min et al. (2000); 2. FCs = Kuiper et al. (2008), λ=Min et al. (2000); 3. FCs = Renne et al. (2011), λ= Renne et al. (2011). Over the last 50 Ma, the calculated difference in age between the four calibrations (including the reference calibration) is always <1%. For example, at 30 Ma, the Renne et al. (2011) calibration differs from the reference by ca. 0.3 Ma, and the Kuiper et al. (2008) calibration by ca. 0.2 Ma.
Most geochronologists now use either the Kuiper et al. (2008) or Renne et al. (2011) age for FCs, and the associated decay constants (Min et al. (2000), and Renne et al. (2011), respectively). Recalculating previously determined ages to use the Kuiper et al. age is relatively straightforward, but the Renne et al. (2011) calibration requires the incorporation of error correlations.
Given the dwindling supply of high purity FCs, Morgan et al. (2014) published data from a new sample, taken off County Road 433, south of South Fork.
Although it is no doubt frustrating for other geologists to have continually updated parameters, updates to standard ages and decay constants will continue. One possibility is an iteration of the statistical optimization model (Renne et al., 2011; Renne et al., 2010), with updated input parameters. Towards this, work is in progress in a determination of 40Ar concentrations in primary mineral standards (Morgan and Davidheiser-Kroll, 2015; Morgan et al., 2011). Future primary measurements of decay constants will also be integral to further improvements of the 40Ar/39Ar geochronometer.
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