Alder Creek sanidine (ACs-2): A Quaternary 40Ar/39Ar dating standard tied to the Cobb Mountain geomagnetic event
Introduction
The 40Ar/39Ar dating method is increasingly used to date rocks as young as 50 to 300 ka by analyzing single crystals of K-rich minerals such as sanidine and leucite (e.g., Karner and Renne, 1998, Trauth et al., 2003), but also K-poor rocks such as tholeiitic basalts (Sharp et al., 1996). Under favorable circumstances, it is even possible to date Quaternary geological phenomena as young as 2 ka (e.g. Renne et al., 1997, Rose et al., 1999, Singer et al., 2000, Pederson et al., 2002).
The principal advantages of the 40Ar/39Ar method compared to the conventional 40K/40Ar technique, which has also been used to date Quaternary rocks, include (1) measurements of potassium and argon from the same material; (2) avoidance of the need to determine the absolute abundance of potassium; (3) incomplete degassing can still yield accurate ages; (4) potential identification of non-radiogenic 40Ar via isochrons; (5) the need for orders of magnitude less material than the K/Ar method. Nevertheless, the ultimate accuracy of the 40Ar/39Ar method depends on well dated homogeneous standards (neutron fluence monitors). The need for an accurate monitor for Quaternary material is also underscored by the fact that the recent geological timescales for the Neogene period rely solely on astronomical dating (e.g. Lourens et al., 2005). It is crucial to obtain ages via 40Ar/39Ar dating and other radioisotopic methods that can be directly compared with these new geological timescales. In the last decade several studies (e.g. Baksi et al., 1996, Renne et al., 1998, Dazé et al., 2003, Spell and McDougall, 2003) have substantially improved the precision and accuracy of several 40Ar/39Ar standards using intercalibration between primary (age determined by 40K/40Ar and other methods) and secondary standards (based on 40Ar/39Ar intercalibration). Nevertheless, while intercalibration among widely used standards is typically better than 0.2% on an intralaboratory basis, interlaboratory variations are much larger as shown by the ∼1% difference in RFCsGA1550 reported by Spell and McDougall (2003) compared with Renne et al. (1998). R is defined as the ratio between the F values of two standards [F value is the ratio between 40Ar*(radiogenic argon) and 39Ar derived from potassium; see Karner and Renne, 1998 for more detail]. Moreover, the ages of 40Ar/39Ar standards remain problematic at the 2% level when compared to other radioisotopic systems, such as Pb/U (e.g., Min et al., 2000). For all 40Ar/39Ar studies, it is desirable to match the standard's age to unknowns by using a monitor with an age similar to that of the unknown (e.g., Renne et al., 1997) in order to minimize the range of isotopic ratios to be measured in order to restrict range of measured 40Ar/39Ar values.
Sanidine from the Rhyolite of Alder Creek (ACR) was proposed as a Quaternary dating standard by Turrin et al. (1994). The ACR is part of the Geyser–Cobb Mountain system located in Sonoma County (NW California) southwest of the Cascade volcanic arc (Fig. 1). The 2.1–0.01 Ma Cobb Mountain volcanic field area constitutes the southernmost part of the Clear Lake volcanic field (Schmitt et al., 2003, Donnely-Nolan et al., 1981). The ACR is the oldest unit of the Cobb Mountain area (Schmitt et al., 2003). It has a transitional paleomagnetic direction (Mankinen et al., 1978) and constitutes the type occurrence of the Cobb Mountain event (CM; Fig. 1). The CM event although short in duration (∼5 to 25 ka2) is recognized worldwide (e.g., Clement and Martinson, 1992, Channell et al., 2002, Horng et al., 2002), and can therefore be easily correlated to the independent astronomical and magnetostratigraphic time scales (e.g., Shackleton et al., 1990, Channell et al., 2002, Horng et al., 2002). Sanidines from the ACR are homogeneous in age and display an excellent grain to grain reproducibility (Turrin et al., 1994). Renne et al. (1998) proposed an age of 1.194±0.014 Ma based on intercalibration with the primary monitor GA1550 (Mac-Dougall and Rosksandic, 1974) with an age of 98.79±1.08 Ma (considering systematic errors). ACs has been also intercalibrated at the 0.3% level with the secondary Fish Canyon sanidine monitor (FCs; 28.02±0.32 Ma; Renne et al., 1998). Unfortunately, although well calibrated, the ACs is not widely used due to its limited availability.
To facilitate the use of this excellent monitor in the 40Ar/39Ar geochronological community, we collected a total of 408 kg of ACR (Fig. 1; ACs-2) in 2002 (N38°48′6.1″; W122°45′05.4″; Fig. 1). We anticipate being able to extract several kilograms of pure sanidine crystals from this collection. Sample locations for other studies of ACR [CM0002 and KA3154; Schmitt et al., 2003 (original ACs): Turrin et al., 1994 and Renne et al., 1998 are also presented in Fig. 1; all these localities are within the same flow (Donnely-Nolan et al., 1981)]. In this paper we report analyses from the recollected Alder Creek sanidine monitor (ACs-2), separated from 12 kg of the ACR, as well as from sanidine of the original ACs. Both single and multiple grain analyses from two grain size fractions (1400–850 μm; 850–425 μm) and three irradiations were analyzed. The goal of this work is to confirm the homogeneity of the new ACs-2 at all scales, to compare the age of ACs-2 with that previously proposed by Renne et al. (1998) for the original ACs, as well as to further intercalibrate ACs-2 and FCs. Finally, we intend to provide free access to ACs-2 splits to the 40Ar/39Ar geochronological community.
Section snippets
Why does the 40Ar/39Ar community need a Quaternary monitor?
For all 40Ar/39Ar studies, but especially those of Quaternary samples, it is desirable to use a monitor with an age similar to that of the unknown (e.g., Renne et al., 1997) in order to minimize the range of isotopic ratios to be measured. 40Ar/39Ar ages are functions of an “R-value”, the ratio of radiogenic 40Ar to 39Ar produced from potassium (40Ar*/39ArK) in the standard to that of the unknown (Karner and Renne, 1998, Renne et al., 1998). Where standards and unknowns differ dramatically in
Review of ACR and CM normal polarity subchron ages
Table 1 and Fig. 3 summarize ages using various methods, for ACR and the CM event. Uncertainties shown exclude systematic contributions to the age uncertainties from the standard and decay constants (external errors); e.g., Min et al. (2000). We emphasize that including these uncertainties will engender an additional 1–2% total age uncertainty (Min et al., 2000, Begemann et al., 2001).
Mineral separation
Twelve kilograms of ACR was crushed and sieved. Two of the coarser fractions (1400–850 μm; 850–425 μm) were repeatedly washed ultrasonically in distilled water. After drying, magnetic minerals or crystals with magnetic inclusions were separated from the feldspars after several passes (0.2 to 1.7A) through a Frantz Isodynamic magnetic separator. In the 850–425 μm fraction, sanidine crystals were separated from plagioclase using a heavy liquid (d=2.62). Adhering groundmass was removed with
Results
40Ar/39Ar isotopic data for individual analyses are reported in the Appendix A. A total of 272 total-fusion analyses of ACs-2 have been acquired. Note that an irradiation of 2.5 h in the TRIGA reactor corresponds to the optimal 40Ar/39Ar ratio of ∼1 (see Appendix A). Depending on grain size, irradiation time and level of purification, single- or multiple-grain fusion were analyzed. All results are shown as age probability diagrams in Fig. 5. Each probability diagram also shows the recent
Intercalibration factor “R” between ACs-2 and FCs
The R value between ACs-2 and FCs was obtained by calculating the mean and standard deviation of F-values (40Ar*/39ArK) for ACs-2 in a given irradiation position. The best estimate for the F-value for FCs at the same position than the ACs-2 was usually calculated using the mean and standard deviation of the three FCs pits bracketing ACs-2. For example for ACs-2 irradiation 298PR-A (Lab. No. 33379) we used the arithmetic mean and standard deviation of F (σF) for FCs [Lab Nos. 33378 (1a), 33380
Availability
We have processed only 12 kg, i.e. ∼3% of the 408 kg of ACR collected, which yielded about 250 g of sanidine phenocrysts (2% of the total weight of the processed rock). Based on this initial separation we anticipate an overall yield of 8 kg of sanidine in the >425 μm fractions. We expect to complete the separation within a year (a funding request for this purpose is pending). We will then be able to provide 5 g aliquots of three grain sizes [425–600 μm (mesh 40–30); 600–850 μm (mesh 30–20) and
Conclusions
Major conclusions about the new ACs-2 Quaternary Sanidine monitor are summarized below:
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Excluding <7% of sanidine crystals with low 40Ar* due to melt inclusions, ACs-2 is age-homogeneous at the grain to grain scale.
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The absence of xenocrysts indicates that multi-grain analyses are feasible allowing researchers to tailor the amount of standard material analyzed according to experimental requirements.
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The weighted-mean age of all ACs-2 analyses based on FCs (28.02 Ma) is 1.193±0.001 Ma (MSWD=0.74, n
Acknowledgments
We thank Axel Schmitt, Marty Grove, Zuzana Fekiakova, and geologists of CALPINE for their assistance in collecting ACs-2, and Brent Turrin for originally suggesting the ACR as a standard. Constructive reviews by Brad Singer and Mike Villeneuve are appreciated. [D.R.]
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