39Ar age of a low-potassium tholeiitic basalt in the Lassen region of NE California

Quaternary Research 68 (2007) 96 – 110 www.elsevier.com/locate/yqres

40

Robust 24 ± 6 ka

Ar/ 39 Ar age of a low-potassium tholeiitic basalt in the Lassen region of NE California

Brent D. Turrin a , L.J. Patrick Muffler b,⁎, Michael A. Clynne b , Duane E. Champion b a

b

Rutgers University, Department of Geological Sciences, Piscataway, NJ 08854, USA Volcano Hazards Team, U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA Received 6 January 2006 Available online 11 May 2007

Abstract 40 Ar/39Ar ages on the Hat Creek Basalt (HCB) and stratigraphically related lava flows show that latest Pleistocene tholeiitic basalt with very low K2O can be dated reliably. The HCB underlies ∼ 15 ka glacial gravel and overlies four andesite and basaltic andesite lava flows that yield 40Ar/39Ar ages of 38 ± 7 ka (Cinder Butte; 1.65% K2O), 46 ± 7 ka (Sugarloaf Peak; 1.85% K2O), 67 ± 4 ka (Little Potato Butte; 1.42% K2O) and 77 ± 11 ka (Potato Butte; 1.62% K2O). Given these firm age brackets, we then dated the HCB directly. One sample (0.19% K2O) clearly failed the criteria for plateau-age interpretation, but the inverse isochron age of 26 ± 6 ka is seductively appealing. A second sample (0.17% K2O) yielded concordant plateau, integrated (total fusion), and inverse isochron ages of 26 ± 18, 30 ± 20 and 24 ± 6 ka, all within the time bracket determined by stratigraphic relations; the inverse isochron age of 24 ± 6 ka is preferred. As with all isotopically determined ages, confidence in the results is significantly enhanced when additional constraints imposed by other isotopic ages within a stratigraphic context are taken into account. © 2007 University of Washington. All rights reserved.

Keywords:

40

Ar/39Ar; Basalt; Tholeiite; Low-potassium; Plateau; Isochron; Lassen; Hat Creek; Late Pleistocene

Introduction Obtaining accurate and precise absolute ages on pertinent geologic units is paramount in determining the rates of geologic processes. Simply put: “no dates, no rates”. In this paper, we present 40Ar/39Ar ages on the Hat Creek Basalt, a late Pleistocene low-potassium tholeiite, and on a series of stratigraphically older andesites and basaltic andesites. The Hat Creek Basalt is overlain by gravel deposits that are related to the last major glaciation, which ended ∼15 ka ago (Gerstel, 1989; Gerstel and Clynne, 1989). These stratigraphic relationships provide an external control on the accuracy of the measured 40Ar/39Ar ages. This unusual external control gives us great confidence that the 40Ar/39Ar age measured directly on the Hat Creek Basalt (a tholeiite with only 0.2–0.3% K2O) is meaningful and that the 40Ar/39Ar method is thus indeed

⁎ Corresponding author. Fax: +1 650 329 5203. E-mail address: [email protected] (L.J.P. Muffler).

applicable to dating low-potassium rocks only a few tens of ka old. The Hat Creek Basalt is a late Pleistocene lava flow that floors much of the Hat Creek Valley, north of the Lassen volcanic highland in northeastern California (Muffler et al., 1994). The flow erupted from numerous vents along a fissure trending N10°W located 0.8–3.5 km south of Old Station (Fig. 1). The basalt flowed north nearly 30 km, mainly through tubes (Anderson, 1940). The Hat Creek Basalt is a low-potassium (high-alumina) olivine tholeiite (Anderson and Gottfried, 1971) with 0–4% phenocrysts of forsteritic olivine and 0–2% plagioclase in a conspicuously diktytaxitic groundmass of plagioclase, clinopyroxene, olivine and titanomagnetite (Table 1). Chemical analyses (Table 2) show that the basalt is very low in K2O, averaging only 0.27%, with only 4 out of 20 analyzed samples exceeding 0.3%. The higher-K2O samples show various degrees of enrichment in residual liquid, now glass (Anderson and Gottfried, 1971). The mean of the remaining 16 analyzed samples is 0.21% K2O.

0033-5894/$ - see front matter © 2007 University of Washington. All rights reserved. doi:10.1016/j.yqres.2007.02.004

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

Figure 1. Geologic map of the Hat Creek Basalt (adapted from Muffler et al., 1994).

97

98

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

Table 1 Petrographic descriptions of dated samples from the andesites and basaltic andesites of Cinder Butte and the Sugarloaf chain (Sugarloaf Peak, Little Potato Butte and Potato Butte) and from the Hat Creek Basalt LM88-1579—basaltic andesite of Cinder Butte: Sparsely porphyritic olivine basaltic andesite. Phenocrysts: 2–3% olivine, 0.5–2 mm, rarely to 3 mm; 1% weakly zoned plagioclase 0.25–1 mm. Many phenocrysts in small glomerophyric clots of olivine and plagioclase. Pilotaxitic, microvesicular groundmass composed of pyroxene, plagioclase, and titanomagnetite microlites plus cryptocrystalline material. LC92-1757—andesite of Sugarloaf Peak: Sparsely porphyritic augitehypersthene andesite. Phenocrysts: 1% hypersthene 0.25–0.5 mm, few larger; < 1% augite 0.25–0.5 mm; < 1% plagioclase 0.25–1 mm. Many phenocrysts in small glomerophyric clots of pyroxene and plagioclase. Pilotaxitic, microvesicular groundmass composed of pyroxene, plagioclase, and titanomagnetite microlites plus dark-brown glass and cryptocrystalline material. LC88-1411—basaltic andesite of Little Potato Butte: Porphyritic olivine-augite basaltic andesite. Phenocrysts: 6% augite 0.25–1 mm; 4% olivine 0.25– 1 mm; 5% plagioclase 0.25–1 mm. Most phenocrysts in glomerophyric clots up to 1 cm of olivine, augite, and plagioclase. Abundantly microvesicular, hyalopilitic groundmass composed of pyroxene, plagioclase, and titanomagnetite microlites plus brownish glass. LC83-329—andesite of Potato Butte: Porphyritic hypersthene-augite-olivine andesite. Phenocrysts: 3% olivine 0.25–2 mm; 1–2% augite 0.25–1 mm; < 1% hypersthene 0.25–1 mm; 5% oscillatory-zoned plagioclase 0.25–1 mm. Many phenocrysts in glomerophyric clots of olivine, pyroxene, and plagioclase up to 5 mm. Abundantly microvesicular groundmass composed of pyroxene, plagioclase, and titanomagnetite microlites plus dark, oxidecharged glass. LM79-477—Hat Creek Basalt: Nearly aphyric olivine basalt. Phenocrysts: 1% olivine, 0.25–1 mm; < 1% weakly zoned plagioclase, 0.25–1 mm. Finegrained, diktytaxitic groundmass composed of olivine, augite, titanomagnetite, and plagioclase with a small amount of glass as selvages on groundmass microlites and vesicle walls. LC88-1402—Hat Creek Basalt: Sparsely porphyritic olivine basalt. Phenocrysts: 1% olivine, 0.25–1 mm, rarely to 2 mm; 2% weakly zoned plagioclase 0.25– 1 mm, rarely to 2 mm. Most phenocrysts in small glomerophyric clots of olivine and plagioclase. Fine-grained, holocrystalline, weakly diktytaxitic groundmass composed of olivine, augite, titanomagnetite, and plagioclase.

Prior to the present investigation, the age of the Hat Creek Basalt had not been determined isotopically. Anderson (1940) noted the flow's youthful appearance and suggested that it erupted fewer than 2 ka ago. Stratigraphic relations described in Muffler et al. (1994, p. 198), however, showed that the Hat Creek Basalt is overlain by gravel related to the last major glaciation, which ended about 15 ka ago (Gerstel, 1989; Gerstel and Clynne, 1989; Turrin et al., 1998). Despite intensive efforts, we have been unable to find charcoal under the Hat Creek Basalt for 14 C dating. Prior to this study, the low K2O concentration and the extreme youth of the Hat Creek Basalt were thought to preclude its dating by the 40Ar/39Ar method. Our geologic mapping shows that the Hat Creek Basalt overlies a number of closely related calc-alkaline andesite and basaltic andesite lava flows that constitute the Sugarloaf chain (Clynne and Muffler, in press), the vents of which form a prominent N10°W lineament west of Old Station (Fig. 1). The Hat Creek Basalt also overlies the calc-alkaline basaltic andesite lava flow from Cinder Butte. These units all contain substantially more K2O than the Hat Creek Basalt (Table 2) and thus are more amenable to 40Ar/39Ar analysis. Therefore,

we analyzed these adjacent flows in order to constrain the lower age limit of the Hat Creek Basalt, and then we attempted to analyze the Hat Creek Basalt itself to see if direct 40Ar/39Ar age determinations of such a low-K2O basalt were meaningful. Methods and interpretation of data The 40Ar/39Ar analyses were performed at the Berkeley Geochronology Center, Berkeley, California using the methods of Turrin et al. (1998, p. 940). The J-value for the sample irradiation was determined using an age of 1.186 Ma for the sanidine from the rhyolite of Alder Creek, California (Turrin et al., 1994). For all of the ages reported here, uncertainties are expressed as 1σ unless otherwise specified, and we use the following symbols and constants: Ar* = radiogenic argon; 39 ArK = 39Ar produced from 39K; λε = 5.81 × 10− 11 y− 1; λβ = 4.962 × 10− 10 y− 1; 40K/Ktota l= 1.167 × 10− 4 (Steiger and Jäger, 1977); 36 ArCa/ 37 ArCa = (2.59 ± 0.06) × 10 − 4 ; 39 ArCa/ 37 ArCa = (6.73 ± 0.3) × 10 − 4 ; 40 ArK/ 39 ArK = (8.6 ± 0.7) × 10 − 3 ; 38 ArK/ 39 ArK = (1.1 ± 0.7) × 10− 2. Each whole-rock sample was crushed and sieved to 0.15– 0.5 mm size fraction and washed with distilled water in an ultrasonic bath. No other sample-preparation methods were used. In any given 40Ar/39Ar experiment, four sources of Ar are determined and accounted for in the calculated age(s) of the sample: (1) Radiogenic Ar derived from the decay of 40K, (2) Ar absorbed onto the inside surfaces of the extraction systems, commonly referred to as the “system blank”, (3) atmospheric Ar absorbed onto the surfaces of the sample grains, and (4) “initial” Ar retained in the sample as it cooled below the Ar blocking temperature. Most of the Ar of sources 2 and 3 is removed by lowtemperature (∼ 150–200 °C) baking of the extraction system and the sample and by the first few low-temperature steps in the step-heating experiment. The remaining “initial” Ar of source 4 in igneous rocks is composed of atmospheric Ar, “inherited” Ar from contamination by older mineral grains, and “excess” Ar, defined by Dalrymple and Lanphere (1969) as “Ar40 that somehow is incorporated into rocks and minerals by processes (for example, diffusion) other than in situ radioactive decay of K40”. Much of the argon in young volcanic rocks is from the atmospheric component of initial Ar. Accounting for this large amount of atmospheric Ar requires an exceptionally firm knowledge of mass discrimination for the specific mass spectrometer at the exact time of analysis. Accordingly, to determine mass discrimination before and after each experiment, we used three to six air pipettes each delivering approximately 7 × 10− 14 moles of atmospheric Ar. The mass discrimination thus determined was used in the data reduction for that specific experiment. To minimize possible dependence of mass discrimination on beam current, we attempted to keep the beam

Table 2 Chemical analyses of the andesites and basaltic andesites of Cinder Butte and the Sugarloaf chain (Sugarloaf Peak, Potato Butte and Little Potato Butte) and of the Hat Creek Basalt Field number

SiO2

Al2O3

Fe2O3

FeO

FeO ⁎

MgO

CaO

Na2O

K2 O

TiO2

P2O5

MnO

LOI

FeO ⁎/MgO

Mg#

Location description

Latitude

Longitude

Degrees-Decimal Minutes 51.21 54.98 55.01 58.25

17.88 17.69 17.66 17.67

1.92 1.58 1.58 1.44

6.91 5.70 5.70 5.19

8.64 7.13 7.13 6.49

6.60 4.97 4.87 3.58

8.20 7.71 7.70 7.07

3.80 3.89 3.91 3.61

1.20 1.64 1.65 1.83

1.65 1.29 1.31 0.92

0.49 0.43 0.47 0.32

0.15 0.12 0.13 0.11

−0.07 0.37 0.06 0.80

1.31 1.44 1.47 1.81

63.0 60.8 60.3 55.2

Flow field N side Flow field south side Flow field south side Flow field east side

40 54.11′ 40 50.45′ 40 50.46′ 40 51.13′

121 28.41′ 121 31.45′ 121 28.13′ 121 34.15′

Sugarloaf chain LC83-356 LC83-357 LC92-1757 ⁎ LC88-1411 ⁎ LC90-1552 LC90-1555 LC83-329 ⁎

60.62 59.34 60.69 54.78 56.02 58.45 57.89

17.42 17.54 17.28 17.45 17.35 17.12 17.84

1.22 1.32 1.23 1.68 1.54 1.41 1.41

4.39 4.75 4.41 6.04 5.56 5.06 5.09

5.49 5.94 5.52 7.55 6.95 6.33 6.36

3.41 3.78 3.25 5.34 4.62 4.43 3.80

6.46 6.72 6.44 8.03 7.78 7.00 7.14

3.56 3.62 3.73 3.62 3.68 3.63 3.88

1.84 1.75 1.85 1.42 1.45 1.66 1.62

0.71 0.79 0.73 1.12 1.46 0.83 0.89

0.25 0.30 0.28 0.39 0.37 0.30 0.33

0.10 0.10 0.10 0.12 0.12 0.11 0.11

0.86 0.72 0.86 0.43 0.32 0.60 0.60

1.61 1.57 1.70 1.41 1.50 1.43 1.67

58.1 58.7 56.8 61.2 59.7 60.9 57.1

Sugarloaf Peak Sugarloaf Peak Sugarloaf Peak Little Potato Butte Potato Butte Potato Butte Potato Butte

40 40 40 40 40 40 40

41.40′ 41.31′ 42.36′ 38.88′ 38.87′ 39.19′ 38.95′

121 25.44′ 121 25.36′ 121 25.11′ 121 25.85′ 121 26.98′ 121 24.81′ 121 23.86′

Hat Creek Basalt (tholeiite) LM81-932 48.22 18.03 LM81-937 50.84 17.68 LM88-1576 50.07 17.89 LM88-1580 48.24 17.74 LC85-760 48.25 17.57 LM88-1587 48.24 17.38 LC86-810 49.53 18.35 LC88-1401 48.31 17.64 LC88-1402 ⁎ 48.10 17.90 LC88-1403 48.13 17.74 LC88-1447 48.15 17.79 LC88-1450 48.23 17.78 LC88-1451 48.41 17.88 LM90-1885 49.01 17.73 LM90-1891A 48.75 17.82 LM90-1891B 48.70 17.46 LM90-1891C 48.38 17.42 LM88-1586 48.46 17.22 LM90-1896 49.43 18.21 LM79-477 ⁎ 48.51 17.91

1.52 1.39 1.39 1.49 1.61 1.63 1.37 1.59 1.58 1.58 1.58 1.54 1.51 1.46 1.50 1.54 1.60 1.63 1.41 1.51

7.77 7.09 7.09 7.61 8.20 8.33 6.97 8.13 8.03 8.08 8.07 7.84 7.68 7.45 7.65 7.86 8.15 8.30 7.19 7.70

9.14 8.35 8.35 8.95 9.65 9.80 8.19 9.56 9.45 9.51 9.50 9.22 9.04 8.77 9.00 9.25 9.59 9.76 8.46 9.06

8.97 7.67 8.07 9.42 8.53 8.80 7.52 8.80 8.75 8.59 8.69 8.89 8.81 8.88 8.92 8.78 8.85 8.80 8.63 8.93

11.36 9.92 10.49 11.56 11.32 11.38 11.83 11.33 11.40 11.46 11.46 11.35 11.15 10.96 11.21 11.38 11.35 11.24 10.91 11.37

2.73 3.15 3.06 2.54 2.94 2.69 3.01 2.70 2.72 2.84 2.71 2.71 2.79 2.84 2.70 2.75 2.72 2.76 2.78 2.62

0.19 0.74 0.53 0.21 0.19 0.18 0.22 0.20 0.17 0.19 0.15 0.29 0.38 0.34 0.25 0.24 0.21 0.22 0.28 0.19

0.95 1.11 1.07 0.90 1.10 1.08 0.94 1.03 1.08 1.10 1.10 1.04 1.07 1.03 0.93 1.00 1.05 1.08 0.89 0.95

0.10 0.26 0.19 0.11 0.11 0.12 0.10 0.11 0.10 0.12 0.12 0.14 0.16 0.14 0.11 0.11 0.11 0.12 0.13 0.15

0.17 0.15 0.15 0.17 0.18 0.18 0.16 0.17 0.17 0.17 0.17 0.17 0.16 0.16 0.16 0.17 0.18 0.18 0.15 0.17

< 0.01

1.02 1.09 1.03 0.95 1.13 1.11 1.09 1.09 1.08 1.11 1.09 1.04 1.03 0.99 1.01 1.05 1.08 1.11 0.98 1.01

67.3 65.8 67.0 68.8 64.9 65.3 65.8 65.9 66.0 65.5 65.7 66.9 67.1 68.0 67.5 66.6 65.9 65.4 68.1 67.4

Honn Campground 1 mi S of Bidwell Ranch 2 mi S of Bidwell Ranch N side of Bidwell Ranch South end of vent fissure W. of Wilcox Ranch gravel pit Intersection Hwys 89 and 44 Hwy 89 at Bridge Campground Old Station Northernmost vent Vent fissure, central area Hwy 89, 150 m N of Doty Road Hwy 89, 3 mi N of Hat Creek, CA Wilcox Ranch gravel pit E of Rocky Campground E of Rocky Campground E of Rocky Campground Wilcox Ranch gravel pit W of Lost Creek Canyon USFS 22 N of Bidwell Ranch

40 46.63′ 40 48.07′ 40 47.15′ 40 50.17′ 40 38.93′ 40 44.61′ 40 41.51′ 40 43.85′ 40 40.55′ 40 38.52′ 40 39.61′ 40 48.46′ 40 52.10′ 40 46.50′ 40 43.67′ 40 43.67′ 40 43.67′ 40 44.51′ 40 45.76′ 40 49.86′

121 30.20′ 121 26.87′ 121 26.23′ 121 28.04′ 121 25.16′ 121 26.71′ 121 25.10′ 121 26.13′ 121 26.07′ 121 25.59′ 121 26.29′ 121 30.74′ 121 32.85′ 121 25.61′ 121 24.71′ 121 24.71′ 121 24.71′ 121 26.84′ 121 24.95′ 121 27.59′

0.15 < 0.01 < 0.01 < 0.01 < 0.01 < 0.02 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 0.05 0.03 < 0.01 < 0.01 < 0.01 0.16 −0.06

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

Cinder Butte LM03-3997 LM81-921 LM88-1579 ⁎ LM81-938

Analyst: David Siems, USGS. Analyses recalculated to 100% anhydrous with Fe2O3 = 0.15 total Fe as Fe2O3 for the Hat Creek Basalt and Fe2O3 = 0.2 total Fe as Fe2O3 for the others. Mg# = 100(Fe+ 2 + Mg)/Mg where Fe+ 2 and Mg are molar. ⁎ 40Ar/39Ar samples.

99

100

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

intensity of each step in the sample-extraction experiments within a factor of 10 of the beam intensity during measurement of the air pipettes. The furnace environment must also be evaluated carefully to ensure that gas contributions from the furnace and previously run samples are accounted for and therefore do not bias the results of the current experiment. Accordingly, the following preparatory process was performed before each step-heating experiment. The furnace crucible was held at ∼ 1600 °C for 20 min and then allowed to cool to ambient temperature while being pumped under high vacuum. This “burn-out” process was repeated two to three times. After the burn-out process, successive empty-furnace blanks (“hot blanks”) were extracted and measured. Initially, when using a new molybdenum crucible, this process was repeated at 100 °C intervals from ambient temperature to 1500 °C, thus defining the gas contribution from the furnace and establishing a profile of the quantity of this Ar gas as a function of temperature (Fig. 2). The process was repeated, shifting the starting temperature back and

Figure 2. Typical hot-blank release patterns. The naturally occurring Ar isotopes (40Ar, 38Ar, and 36Ar) all have hot-blank release patterns similar to those shown in panel a. Moreover, the isotopic ratios are atmospheric within analytical uncertainty; hence the absolute signals scale accordingly. When a hot-blank release pattern becomes repeatable and predictable in the temperature range of the samples measured, or small relative to the size of the signal (e.g., hot blanks 2 and 3 of panel a), it generally is fit with line segments for the temperature range of interest. As indicated in Figure 3–5, typically 85–90% of the 39ArK is released between 600° and 1000 °C. The Ar isotopes induced by neutron irradiation, 39Ar and 37Ar, have profiles similar to the 39Ar patterns shown in panel b. Typically, the absolute signal for 37Ar is 10 times that of 39Ar. The repeatable and predictable hot-blank release patterns for these two isotopes are fit with a single line.

forth by 50 °C, until the yield of Ar was low and the profiles repeatable and predictable. Curves were fit to these data to determine the necessary hot-blank correction at any given temperature. It should be noted, however that once a crucible has been ”broken-in”, the hot-blank release pattern can be determined in five steps, as shown in Figure 2. Typically the signal when analyzing samples is 10 to 100 times greater than the hot blank. Even if the furnace is fully prepared as described above, in some situations slag remaining in the furnace from previous experiments can react at high temperatures with the fused material of the sample being measured, potentially contributing extraneous 39 ArK. This phenomenon is particularly important when a young, multi-component sample (e.g., a whole-rock basalt) is fused in the furnace after a high-K, single-component sample (e.g., sanidine) has been run. This problem was easily avoided by running all the mafic samples for this study before running any high-K material from other localities. Evaluation of the results follows the outline of Turrin et al. (1998, p. 940), which in turn is based on Fleck et al. (1977) and Dalrymple and Lanphere (1969, 1974). Ideally, a 40Ar/39Ar age determination should meet five criteria: (1) The step-heating release pattern should form a plateau, defined as that part of the age spectrum composed of at least three contiguous gas fractions that together represent > 50% of the total 39 ArK released from the sample and for which no difference in age can be detected between any two fractions at the 95% confidence level critical value test (see Dalrymple and Lanphere, 1969). The plateau ages and their pooled errors are calculated following Dalrymple and Lanphere (1974), using the varianceweighted mean of the plateau steps (see Taylor, 1997, for a complete discussion on the variance-weighted mean and pooled errors). (2) The inverse isotope correlation diagram (henceforth termed simply “inverse isochron diagram”) should form a linear array that yields a mean square of weighted deviates (MSWD) ≤ 2.5 (Brooks et al., 1972). In a true isochron, analytical errors express most or all of the error on the age, and MSWD = 1 (Dickin, 1997, p. 35). Nearly all experiments, however, also display “geological” scatter (McIntyre et al., 1966). Requiring all experiments to have a MSWD near 1 would therefore be unrealistic. Accordingly, Brooks et al. (1972) and Wendt and Carl (1991) both recommended rejecting data sets in which there is a > 95% certainty of excess scatter over analytical error. This 95% confidence limit represents a MSWD of approximately 1.61 to 4.96, depending on the number of samples and the number of duplicates (Dickin, 1997, Table 2.1; simplified from Brooks et al., 1972, Appendix 3). A similar conclusion was reached by Wendt and Carl (1991) using the formulation MSWD-limit = 1 + 2 (2/[n−2])1/2, where n = the number of points regressed (the abscissa of Appendix 3 of the Brooks et al., 1972). A precise treatment of MSWD is probably not justified,

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

101

Table 3 Ar/39Ar data for the andesites and basaltic andesites of Cinder Butte and the Sugarloaf chain (Sugarloaf Peak. Potato Butte and Little Potato Butte) and for the Hat Creek Basalt

40

Sample

40

Ar/39Ar

37

Ar/39Ar

36

Ar/39Ar

40

Ar*/39Ar

38

Ar/39Ar

−0.92146

0.99675

40

Ar* Age (%) (ka)

39

Temp 39Ar J Ar − 15 (Moles × 10 ) (°C) (%) (×10− 4)

− 579.3

152.1

0.12

670

43.18874 0.75037 0.14732 −0.29478 0.98260 −0.7 − 185.3 33.32689 1.67688 0.11312 0.02087 0.99223 0.1 13.1 22.02426 1.86793 0.07486 0.03967 0.99103 0.2 24.9 10.69357 1.87357 0.03646 0.05608 0.96618 0.5 35.2 4.02851 1.81948 0.01386 0.06340 0.98871 1.6 39.8 4.14111 1.54556 0.01415 0.06950 0.99574 1.7 43.7 9.64345 1.90255 0.03302 0.02402 0.99480 0.2 15.1 21.62137 1.13691 0.07333 0.03218 0.99702 0.1 20.2 25.91462 19.67318 0.09247 0.09029 0.99592 0.3 56.7 Plateau age: 38 ± 7 ka; integrated total-fusion age = 9 ± 12 ka; inverse isochron age (n = 8) = 44 ± 9 ka

114.9 69.1 39.1 22.9 10.0 11.7 22.7 40.4 68.3

0.33 0.63 0.86 1.02 1.32 0.98 0.50 0.28 0.21

710 750 790 830 870 910 960 1020 1250

5.3 10.1 13.8 16.3 21.1 15.7 7.9 4.5 3.4

49.5 9.5 11.4 9.2 19.1 11.8 73.2 83.7

0.11 1.68 1.78 1.91 1.47 0.96 0.10 0.11

680 725 750 780 810 870 925 1600

1.3 3.48 ± 0.06 20.7 22.0 23.5 18.1 11.8 1.2 1.4

− 51.6

58.3

0.28

700

4.4 3.48 ± 0.06

14.48012 1.26551 0.04888 0.12594 0.02328 0.9 79.2 7.87656 0.65417 0.02649 0.09184 0.01919 1.2 57.7 5.56068 0.86744 0.01867 0.10259 0.01806 1.8 64.5 4.10931 1.12036 0.01384 0.09651 0.01710 2.4 60.7 3.61987 1.83097 0.01234 0.10664 0.01651 2.9 67.0 3.38794 2.81448 0.01174 0.12801 0.01620 3.8 80.5 3.59390 3.96149 0.01273 0.12926 0.01595 3.6 81.2 4.75027 5.80247 0.01684 0.21013 0.01671 4.4 132.1 5.35474 7.11446 0.01951 0.12808 0.01762 2.4 80.5 43.85189 9.56223 0.14772 0.93237 0.04641 2.1 585.9 14.07793 27.79772 0.05369 0.34086 0.03143 2.4 214.2 Plateau age: 67 ± 4 ka; integrated total-fusion age = 75 ± 6 ka; inverse isochron age (n = 8) = 73 ± 9 ka

25.1 14.3 12.9 7.7 9.4 11.3 15.8 18.0 24.4 116.5 50.7

0.79 0.79 0.89 1.16 0.83 0.60 0.38 0.22 0.25 0.07 0.19

715 730 745 760 775 790 805 820 870 1000 1500

30.42378 0.21663 0.10399 −0.29794 0.03390 −1.0 − 180.3 18.58602 0.28885 0.06202 0.27186 0.02626 1.5 164.5 16.92044 0.40802 0.05724 0.02912 0.02533 0.2 17.6 16.00325 0.89267 0.05390 0.13598 0.02487 0.8 82.3 13.72811 2.09772 0.04642 0.16290 0.02342 1.2 98.6 19.29185 4.67453 0.06600 0.13775 0.02668 0.7 83.4 34.89725 7.34531 0.12028 −0.09191 0.03696 −0.3 − 55.6 64.16973 8.72185 0.22153 −0.63821 0.05741 −1.0 − 386.3 Plateau age: 77 ± 11 ka; integrated total-fusion age = 79 ± 13 ka; inverse isochron age (n = 4) = 130 ± 80 ka.

25.3 21.3 26.0 35.6 18.6 18.9 53.9 134.3

44.45 208.65 192.62 352.33 333.56 72.09 20.47 8.56

600 650 700 750 800 825 850 875

3.6 3.36 ± 0.06 16.9 15.6 28.6 27.1 5.8 1.7 0.7

53.8 1604.0 100.3 1387.3 −289.6 199.2 73.8 168.5 114.7 238.8 21.6 73.3 164.7 161.8 42.8 84.4 223.1 145.1 290.3 99.6 12.8 126.1 142.6 579.4 2357.4 10,922.3

1.64 11.78 5.80 8.11 15.07 7.63 14.49 5.89 6.66 5.70 5.12 4.15 3.37

700 750 775 800 825 850 875 900 950 1000 1100 1200 1500

1.7 1.28 ± 0.003 12.2 6.0 8.4 15.6 7.9 15.0 6.1 6.9 5.9 5.3 4.3 4.7

Basaltic andesite of Cinder Butte LM88–1579 Experiment 6801

51.96293

0.00000 0.17894

−1.8

±1σ (ka)

Andesite of Sugarloaf Peak LC92-1757 Experiment 6804

13.02762 5.08965 0.04507 0.09283 0.02254 0.7 58.3 7.50733 0.86911 0.02549 0.03518 0.01913 0.5 22.1 6.00897 0.79394 0.02044 0.02344 0.01857 0.4 14.7 5.48102 1.50721 0.01863 0.08256 0.01823 1.5 51.9 5.43760 2.33292 0.01863 0.10419 0.01831 1.9 65.5 7.56372 3.06808 0.02621 0.04606 0.02192 0.6 29.0 17.87968 8.64006 0.06100 0.51121 0.02949 2.8 321.3 29.77550 22.84223 0.10353 0.93800 0.03733 3.1 589.5 Plateau age: 46 ± 7 ka; integrated total-fusion age = 52 ± 6 ka; inverse isochron age (n = 6) = 60 ± 20 ka

Basaltic andesite of Little Potato Butte LC88-1411 Experiment 6805

27.75003

1.49750 0.09455

−0.08214

0.03136

−0.3

Andesite of Potato Butte LC83-329 Experiment 6817

Andesite of Potato Butte LC83-329 Experiment 8197

704.72588 149.39758 165.46030 103.68352 87.65627 83.31053 84.32374 88.78438 91.20331 103.01813 112.66375 179.23397 792.16392

0.26811 0.14109 0.25657 0.37776 0.58870 0.59808 0.96322 1.72484 2.04101 2.92299 4.78712 6.23208 6.08336

2.38414 0.50417 0.56424 0.34990 0.29512 0.28178 0.28321 0.30030 0.30593 0.34517 0.38237 0.60615 2.64803

0.23288 0.43399 −1.25253 0.31917 0.49632 0.09339 0.71264 0.18529 0.96504 1.25579 0.05535 0.61703 10.20390

0.45137 0.21951 0.11807 0.07918 0.06888 0.06451 0.06583 0.06946 0.06861 0.07677 0.08529 0.12481 0.51053

0.0 0.3 −0.8 0.3 0.6 0.1 0.8 0.2 1.1 1.2 0.0 0.3 1.3

1.9 3.48 ± 0.06

12.3 12.3 13.8 18.0 12.9 9.2 5.9 3.4 3.8 1.1 3.0

(continued on next page)

102

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

Table 3 (continued) Sample

40

Ar/39Ar

37

Ar/39Ar

36

Ar/39Ar

40

Ar*/39Ar

38

Ar/39Ar

40

Ar* Age (%) (ka)

±1σ (ka)

Plateau age: 83 ± 39 ka; integrated total-fusion age = 200 ± 500 ka; inverse isochron age (n = 13) = 200 ± 60 ka. Hat Creek Basalt 79.43 29.2 0.2701 1.89 0.06346 2.31 1000.0 370.0 LM79-477 56.47 16.58 0.19286 0.76 0.04976 1.32 400.0 250.0 Experiment 6802 40.225 16.55 0.13887 0.45 0.03932 1.11 240.0 180.0 35.699 17.65 0.12349 0.56 0.03684 1.54 300.0 160.0 31.198 20.45 0.10897 0.57 0.03334 1.78 300.0 140.0 29.593 86.5 0.10509 5.59 0.03277 17.41 2950.0 160.0 32.765 25.62 0.11619 0.39 0.03507 1.17 210.0 160.0 50.91 27.19 0.17923 0.03 0.0474 0.05 10.0 230.0 81.73 26.7 0.28179 0.51 0.06815 0.61 270.0 370.0 102.31 28.2 0.34941 1.24 0.07989 1.19 660.0 450.0 112.64 43.3 0.3881 1.31 0.08712 1.12 690.0 510.0 102.21 111 0.3687 1.95 0.08193 1.72 1030.0 530.0 80.95 113.4 0.2981 1.72 0.06577 1.91 910.0 430.0 68.47 168.6 0.2639 3.99 0.05811 4.95 2,110.0 500.0 192.4 93 0.6818 −2.2 0.1326 −1.03 1,100.0 1,300.0 Plateau age: undefined; integrated total-fusion age = 600 ± 200 ka; inverse isochron age (n = 14) = 26 ± 6 ka Hat Creek Basalt LC88-1402 Experiment 6813

18.00764 17.46026 6.06143 20.47469 3.23280 21.77861 2.70718 16.97128 5.18434 10.08005 5.64386 7.47096 6.96598 8.59277 10.62730 12.42816 24.30578 81.87856 66.99824 124.42048

0.06530 0.02560 0.01642 0.01334 0.01996 0.02081 0.02580 0.03964 0.10316 0.25845

0.04093 0.05729 0.03817 0.05720 0.04937 0.05733 −0.00837 −0.14512 0.08216 0.15011

however, since the analytical errors assigned to the ages are only estimates of error (Dickin, 1997, p. 35). Hence, we follow the Brooks et al. (1972) “rule of thumb” that rejects data arrays with MSWD > 2.5. For the experiments in this paper, the Wendt and Carl (1991) formulation results in the same inclusion/exclusion as the Brooks et al. (1972) “rule of thumb”. (3) The isochron age should be analytically indistinguishable from the plateau age at the 95% confidence level. (4) The measured initial 40Ar/36Ar ratio should be analytically indistinguishable from the accepted atmospheric ratio of 295.5 ± 2 at the 95% confidence level. (5) The integrated (total-fusion) age should be analytically indistinguishable from the plateau and isochron ages at the 95% confidence level. In cases where the integrated (total-fusion), plateau, and/or isochron ages are analytically indistinguishable at the 95% confidence level, we select the age with the highest precision. This preference is based on the assumption that reported error(s) are representative of the quality of the respective measurements. 40

Ar/39Ar dating results 40

Ar/39Ar dating results are presented in Table 3 and depicted in Figures 3–5. The data symbols used on the inverse isochron diagrams (Figs. 3b, d, f, 4b, d, 5b and d) are error

0.02753 0.01906 0.01606 0.01533 0.01650 0.01639 0.01847 0.02193 0.03065 0.05771

0.2 0.9 1.2 2.1 0.9 1.0 −0.1 −1.4 0.3 0.2

25.2 35.3 23.5 35.2 30.4 35.3 −5.2 −89.3 50.6 92.4

58.8 45.1 53.0 72.1 30.0 47.0 113.0 88.6 144.2 441.3

39

Ar Temp 39Ar J −15 (Moles × 10 ) (°C) (%) (×10− 4) 0.04 0.09 0.15 0.12 0.11 0.09 0.07 0.07 0.06 0.05 0.05 0.03 0.06 0.01 0.00

710 750 800 825 850 875 900 950 1000 1050 1100 1150 1200 1265 1400

4.2 2.93 ± 0.01 9.1 15.1 11.6 11.0 8.6 6.9 7.2 5.6 5.4 4.4 3.0 6.2 1.3 0.3

0.11 0.18 0.17 0.13 0.12 0.07 0.05 0.06 0.09

725 750 775 800 850 900 950 1000 1100 1500

11.0 3.41 ± 0.06 17.8 16.7 13.1 11.5 6.9 5.1 5.8 8.9 3.2

ellipses and indicate the uncertainties in both 36Ar/40Ar and 39 Ar/40Ar ratios. Generally, the uncertainty in the 39Ar/40Ar ratio is significantly smaller than the uncertainty in the 36 Ar/40Ar ratio. Thus, most error ellipses look like single vertical lines, with the conspicuous exception of Figure 4d. The temperatures and error ellipses indicated in black are used to calculate the inverse isochron age. The horizontal arrow on each inverse isochron diagram designates the atmospheric 36Ar/40Ar ratio of 3.384 × 10− 3 (40Ar/36Ar ratio = 295.5). Basaltic andesite of Cinder Butte Cinder Butte is a rugged edifice of block lava flows that range in composition from calc-alkaline basalt (51.21% SiO2 and 1.20% K2O) to andesite (58.25% SiO2 and 1.83% K2O). The Hat Creek Basalt flowed directly north until it was diverted to the north-northwest around the edifice of Cinder Butte. The basaltic andesite of Cinder Butte unequivocally underlies the Hat Creek Basalt in excellent exposures along the Hat Creek fault (Muffler et al., 1994). The Cinder Butte sample dated by 40Ar/39Ar (LM88-1579; experiment 6801) is a basaltic andesite containing 55.01% SiO2 and 1.65% K2O (Table 2). This sample yielded a concordant step-heating spectrum defined by eight steps from 750° to 1250 °C (in bold in Table 3) that comprise over 92% of the total 39ArK released. These eight steps yield a plateau age of 38 ± 7 ka (Fig. 3a). This plateau age is not concordant

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

103

Figure 3. 40Ar/39Ar age spectra and inverse isochron diagrams for three volcanic units underlying the Hat Creek Basalt. In Figures 3–5, the small arrow on the Ar/40Ar (×10−3) axis of the inverse isochron diagrams indicates the atmospheric ratio. Dashed lines graphically represent the 95% confidence error envelope about the isotopic mixing line. (a) Step-heating spectrum for the basaltic andesite of Cinder Butte (experiment 6801). (b) Inverse isochron diagram for the basaltic andesite of Cinder Butte (experiment 6801). (c) Step-heating spectrum for the andesite of Sugarloaf Peak (experiment 6804). (d) Inverse isochron diagram for the andesite of Sugarloaf Peak (experiment 6804). (e) Step-heating spectrum for the basaltic andesite of Little Potato Butte (experiment 6805). (f) Inverse isochron diagram for the basaltic andesite of Little Potato Butte (experiment 6805).

36

with the integrated (total fusion) age of 9 ± 12 ka at the 95% confidence level. The Ar isotopic data, when cast on an inverse isochron diagram (Fig. 3b), indicate that the first two steps are enriched

in 36Ar and thus yield negative ages. These first two steps are most likely influenced by low-temperature alteration of the sample. This sample provides a good example of the additional information that is provided by the 40Ar/39Ar step-

104

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

Figure 4. 40Ar/39Ar age spectra and inverse isochron diagrams for the andesite of Potato Butte. (a) Step-heating spectrum of experiment 6817. (b) Inverse isochron diagram of experiment 6817. (c) Step-heating spectrum of experiment 8197. (d) Inverse isochron diagram of experiment 8197.

heating method on the internal distribution of K relative to Ar within a mineral or whole-rock system (e.g., Turrin et al., 1998). These first two low-temperature steps are excluded from the inverse isochron calculations because they appear to lie on a different mixing line than the remaining eight hightemperature steps (Fig. 3b). The remaining eight steps lie along a well-defined mixing line with 40Ar/36Arinital of 295.2 ± 0.5, an excellent MSWD of 0.25 and an age of 44 ± 9 ka. If one chooses to include the first two steps, however, the results (age = 53 ± 10 ka, 40Ar/36Arinital = 294.3 ± 0.7, and MSWD = 2.0) are still analytically indistinguishable from the plateau at the 95% confidence level. The 38 ± 7 ka plateau age of Cinder Butte has the greater precision and therefore is preferred over the inverse isochron age. Andesite of Sugarloaf Peak Andesite flows from Sugarloaf Peak also underlie the Hat Creek Basalt, and the 40Ar/39Ar age on a sample from these lava flows provides a similar lower limit to the age of the Hat Creek Basalt. The Sugarloaf Peak sample dated by 40Ar/39Ar (LC92-1757; experiment 6804) is an andesite containing 60.69% SiO2 and

1.85% K2O (Tables 1 and 2). Three steps of the concordant stepheating spectrum (780, 810 and 870 °C) define a plateau age of 46 ± 7 ka (Fig. 3c). The plateau comprises greater than 53% of the total 39ArK released. The ages of the first two steps (680 and 725 °C) are also concordant with the ages of the plateau steps at the 95% confidence level. However, the third step (750 °C) is not and thus breaks the concordant consecutive age requirement for the definition of a plateau age (Fleck et al., 1977). The last two steps (925 and 1600 °C) are anomalously old, but they account for only 3% of the total 39ArK released. When combined, all eight steps produce an integrated age of 52 ± 6 ka, which is concordant with the plateau age at the 95% confidence level. On an inverse isochron diagram (Fig. 3d), the 39Ar/40Ar ratio increases and the 36 Ar/ 40 Ar ratio decreases with increasing temperature for the first five steps (680–810 °C). The 870 °C step moves back up along this array and plots near the 725 °C step. These first six steps define a mixing line between the initial 40Ar/36Ar ratio of 294 ± 3 and a 39Ar/40Ar ratio corresponding to an age of 60 ± 20 ka. The initial 40 Ar/36Ar ratio of 294 ± 3 is similar to the atmospheric 40 Ar/36Ar ratio of 295.5. A MSWD of 3.6 for the isochron suggests either that the errors are slightly underestimated or

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

105

Figure 5. 40Ar/39Ar age spectra and inverse isochron diagrams for the Hat Creek Basalt. (a) Step-heating spectrum of experiment 6802. (b) Inverse isochron diagram of experiment 6802. (c) Step-heating spectrum of experiment 6813. (d) Inverse isochron diagram of experiment 6813.

the 750° step is beyond the expected errors. The last two hightemperature steps, 925 °C and 1600 °C, plot below the mixing line, back toward the 36Ar/40Ar axis and probably represent contributions from a low-potassium refractory mineral phase. These last two steps are not included in the isochron calculations. The concordant total fusion, plateau and isochron data indicate that the K-Ar system for the andesite of Sugarloaf Peak has remained a closed system with respect to K and Ar and that non-atmospheric 40Ar contamination is minimal. Thus the preferred age for the andesite of Sugarloaf Peak is the plateau age (46 ± 7 ka). Basaltic andesite of Little Potato Butte Basaltic andesite flows from Little Potato Butte also underlie the Hat Creek Basalt. Specimen LC88-1411 (experiment 6805), which contains 54.78% SiO2 and 1.42% K2O, gave a robust plateau age of 67 ± 4 ka (Fig. 3e) by a twelve-step incrementalheating experiment. The plateau age is defined by seven 15 °C incremental steps from 715 to 805 °C, inclusively, and these steps comprise over 84% of the total 39ArK released. The 820 °C step yielded an anomalously old age of 132 ± 18 ka. The age of the following 870 °C step (81 ± 24 ka), however, fell back to the

plateau, suggesting that the 820 °C step possibly was affected by decrepitation of fluid inclusions in this crystal-rich rock (Table 1). When cast on an inverse isochron diagram (Fig. 3f), the Ar isotopic data for Little Potato Butte define a mixing line. Starting with the 715 °C step, the isotopic data plot down and to the right until the 790 °C step. With the next highertemperature step (805 °C), the 39Ar/40Ar ratios start to decrease whereas the 36Ar/40Ar ratios increase, thus reversing direction. Consequently, each successive step then moves up and to the left along the previously defined mixing line. The anomalously old 820 °C step falls off the mixing array, but the 870 °C step plots on the array. The mixing line defines a 40 Ar/36Arinitial ratio of 295.1 ± 0.9 and an age of 73 ± 9 ka. The MSWD of 1.2 suggests that the dispersion about the mixing line is what would be expected given the analytical errors. The integrated total fusion age (75 ± 6 ka) is concordant with both the plateau and isochron age at the 95% confidence level. This requires that the K-Ar system has remained closed since thermal closure (eruption) and that the initial 36Ar/40Ar ratio is analytically indistinguishable from that of atmosphere. The preferred age for the andesite of Little Potato Butte is the plateau age (67 ± 4 ka).

106

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

Andesite of Potato Butte Andesite flows from Potato Butte underlie the Hat Creek Basalt and the andesite of Little Potato Butte. Two step-heating experiments were carried out on sample LC83-329, which contains 57.89% SiO2 and 1.62% K2O. In experiment 6817 (Fig. 4a), four concordant steps, from 700 to 825 °C, comprising about 77% of the total 39ArK released, define a plateau age of 77 ± 11 ka. The plateau age is concordant with the total fusion age of 79 ± 13 ka at the 95% confidence level. The inverse isochron diagram (Fig. 4b) indicates an age of 130 ± 80 ka with an initial 40Ar/36Ar ratio of 294 ± 4 and an MSWD of 3.1. Experiment 8197 gives a ten-step plateau age of 83 ± 39 ka (Fig. 4c) comprising 89% of the argon released. The plateau age is concordant with the total-fusion age of 200 ± 500 ka at the 95% confidence level. The inverse isochron diagram for 13 steps (Fig. 4d) indicates an age of 200 ± 60 ka with an initial ratio of 294 ± 2 and an MSWD of 0.9. The results of both experiments meet the criterion that the initial 40Ar/36Ar ratio should be 295.5 ± 2, but the MSWD of experiment 6817 does not meet the criterion that the MSWD should be ≤ 2.5. Furthermore, the isochron ages for both experiments have large uncertainties because of the narrow spread in the 40Ar/39Ar ratios. The plateau ages of both experiments and the total fusion age of experiment 6817 are essentially identical but differ strikingly from the total-fusion age of experiment 8197. Considering all these factors, we choose to discount the results of experiment 8197. We suggest that the age of the andesite of Potato Butte is best expressed by the plateau age of 77 ± 11 ka for experiment 6817, although the data are far from robust, and the precision is almost certainly overstated. Hat Creek Basalt Given the firm lower age constraints for the Hat Creek Basalt supplied by the above experiments, we carried out 40Ar/39Ar experiments on two samples of the Hat Creek Basalt itself (Fig. 5 and Table 3). A fourteen-step 40 Ar/ 39 Ar step-heating experiment on sample LM79-477 (experiment 6802), which contains 48.51% SiO2 and 0.19% K2O (Table 2), did not yield a plateau age (Fig. 5a). The plateau is saddle-shaped, suggesting the system is affected by excess 40Ar. In addition, the 875 °C step yields an excessively old apparent age. The inverse isochron diagram, however, does form a linear array (except for the 875 °C step) and yields an isochron age of 26 ± 6 ka with an initial 40Ar/36Ar ratio of 299 ± 1.6 and a MSWD of 1.6 (Fig. 5b). The MSWD indicates that the scatter about the isochron is what would be expected given the analytical errors. The mixing line defined by the step-heating data consists of fourteen steps. The first five steps (710–850 °C) plot to the right along a linear array (Fig. 5b). The 875 °C step falls off this array and has an anomalous K/Ca ratio; it may represent the temperature at which the eutectic point is encountered and the

system goes from diffusion degassing of solid phases to diffusion degassing of both solid and liquid phases. In addition, fluid inclusions in a phenocryst phase may have decrepitated at this temperature and released Ar with an anomalous isotopic composition. The next five steps (900–1100 °C) re-establish K/Ca ratios similar to the first five steps and reverse direction on the inverse isochron diagram, plotting to the left along the original linear array. The 1150, 1200 and 1265 °C steps reverse direction again and plot to the right along the original array. At 1400 °C the sample is nearly completely degassed. Although the hot-blank-corrected 1400 °C step contains little 39 Ar, it is critical in determining the 39Ar/40Ar (X-axis) intercept on the inverse isochron diagram. The 1400 °C step thus has a profound impact on the calculated age. Without the 1400 °C step, the 39Ar/40Ar (X-axis) intercept corresponds to an unrealistic age of − 68 ± 18 ka with an initial 40Ar/36Ar ratio of 300 ± 2 and an MSWD of 1.3. With the 1400 °C step, the 39 Ar/40Ar intercept corresponds to an age of 26 ± 6 ka with an initial 40Ar/36Ar ratio of 299 ± 2 and an MSWD of 1.6. Sample LM79-477 thus fails the first, fourth (the initial 40 Ar/36Ar ratio of the inverse isochron diagram is not 295.5 ± 2) and fifth (the integrated age should be the same as the plateau and isochron ages at the 95% confidence level) of the five criteria specified above for an age to be acceptable. Moreover, the highprecision result is very dependent on a single high-temperature step. If this result were a single determination, it therefore would not be considered reliable, and we would caution readers from being enticed into accepting this result. However, because the sample is in stratigraphic context with older dated flows and is constrained by ∼ 15 ka glacial deposits, we can perhaps have some confidence in this inverse isochron age. The other sample of Hat Creek Basalt (LC88-1402 [experiment 8613], which contains 48.10% SiO2 and 0.17% K2O), does give a robust 40Ar/39Ar age. The bulk K/Ca ratio (as determined from the 37Ar/39Ar) for sample LC88-1402 is 0.020, 50% higher than of LM79-477 (0.013). Accordingly, all ten steps lie on a plateau (Fig. 5c) and indicate an age of 26 ±18 ka. The integrated total-fusion age for all steps is 30 ± 20 ka. When the Ar isotopic data are cast on an inverse isochron diagram (Fig. 5d), the first four steps (725–800 °C) plot down and to the right, defining a mixing line. The 850 °C step through the 1500 °C step plot up and to the left along the same line defined by the lower-temperature steps. An age of 24 ± 6 ka with a 40Ar/36Arinital ratio of 295 ± 0.7 is indicated by the inverse isochron interpretation. A MSWD of 1.9 indicates that the dispersion of the points along the mixing line is what can be expected given the analytical errors of the isotopic measurements. The reported error (± 6 ka) for the inverse isochron age is similar to the errors for other samples with mixing-line arrays of similar length: Hat Creek Basalt 39 Ar/40Ar values range from 0.0149 to 0.369, Cinder Butte (±9 ka) 39Ar/40Ar values range from 0.019 to 0.248, and Sugarloaf Peak (± 20 ka) 39Ar/40Ar values range from 0.033 to 0.183. The longer the mixing line array, the better the 39 Ar/40Ar and 40Ar/36Arinital intercepts are defined, and the

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

smaller the error. Given the consistency of the first six steps, which comprise 77% of the total 39ArK released, and the high quality of the isochron data, the best age for the Hat Creek Basalt is the inverse isochron age (24 ± 6 ka). This age is compatible with the younger constraint of ∼15 ka provided by the overlying glacial deposits and the older constraint provided by the Cinder Butte age (38 ± 7 ka). It may be useful to speculate why sample LC88-1402 produced acceptable results whereas LM79-477 did not. LM79-477 has a texture typical of low-potassium olivine tholeiites. Its groundmass is diktytaxitic with augite, titanomagnetite, and plagioclase microlites projecting into irregular microvesicles. The rock contains a few percent of glass as selvages on groundmass crystals (primarily plagioclase) and as linings of the larger round vesicles where it was squeezed by vesiculation of the groundmass. The texture of LC88-1402, on the other hand, is not typical of low-potassium olivine tholeiites. The sample came from the core of the flow and is only vaguely diktytaxitic. The rock lacks large round vesicles, is holocrystalline and completely lacks glass as selvages on plagioclase.

107

The lower bulk K/Ca of sample LM79-477 suggests that it lost some K either by migration of K-rich residual glass during final solidification of the rock or during sample preparation. Conversely, sample LC88-1402 retained more of its K by incorporation into the outer zones of plagioclase microlites during final solidification. The combination of higher effective K content and lack of glass, which correlate with the weakly diktytaxitic texture, seems to contribute to better performance of LC88-1402 for 40Ar/39Ar dating. Paleomagnetic constraints In our studies of western US volcanoes, we routinely include paleomagnetic data as an aid in determining how long these eruptive episodes lasted and, sometimes, even when they have occurred (Kuntz et al., 1986; Donnelly-Nolan et al., 1990; Turrin et al., 1998). Geomagnetic secular variation causes the local magnetic field to change its orientation at rates of 4° to 6° each century at temperate latitudes (Vestine et al., 1959). These rates are high enough to allow us to determine whether individual flows of a lava field have different

Table 4 Paleomagnetic data for the andesites and basaltic andesites of Cinder Butte and the Sugarloaf chain (Sugarloaf Peak and Potato Butte) and for the Hat Creek Basalt Unit name

Site

N Lat.

α95

E Long.

N/No

Basaltic andesite of Cinder Butte (38 ± 7 ka Ar/ Ar) Murken Bench 890B2 40.841° N margin of flow, N of Figure 1 287B3 40.902°

238.532° 238.526°

14/15 Pl 52.9° 323.3° 1.8° 556 (two unrelated subsite directions; will not clean)

Andesite of Sugarloaf Peak (46 ± 7 ka 40Ar/39Ar) E of Bridge Campground 451B3 40.729°

238.568°

6/9 (9/9

Andesite of Potato Butte (77 ± 11 ka 40Ar/39Ar) Near vent B8392 Distal flow to E B5326 Distal flow to E B5338 Distal flow to E B5350

40.632° 40.636° 40.645° 40.652°

238.561° 238.588° 238.600° 238.597°

(scattered directions; no consensus) 12/12 20 43.6° 354.4° 3.0° 211 11.94781 74.1° (shallow 30° slightly east direction possible from planes vector analysis) 8/12 Pl 54.6° 349.1° 3.9° 227 .– 79.8° (9/12 Pl 55.9° 354.5° 5.0° 108 .– 84.0°

Hat Creek Basalt (24 ± 6 ka 40Ar/39Ar) Vent area B8318 Vent area B8889 Old Station B8306 Near Cave Campground HtCC Bridge Picnic Area B8294 Wilcox Quarry B8279 Hwy 89/Doty Rd. NW of Figure 1 B8919 Distal on Hwy 89 NW of Figure 1 B8931

40.642° 40.660° 40.676° 40.688° 40.731° 40.742° 40.808° 40.868°

238.574° 238.562° 238.565° 238.580° 238.565° 238.553° 238.488° 238.452°

5+ 10 5 20 5+ 10 10 10

2B133

40.830°

238.542°

12/12 11/12 11/12 12/12 12/12 15/15 12/12 12/12 8/8 12/12

2B127

40.831°

238.539°

6/6

20

40

Along young fault near USFS 22 Tilted block near USFS 22

Exp.

I

D

k

R

Plat.

Plong.

.–

60.1°

146.2°

76.0° 80.5°

118.9° 105.6°)

39

30

20

52.2° 53.4°

345.5° 351.6°

63.8° 65.9° 63.4° 65.1° 64.0° 63.0° 65.5° 63.0° 64.3° 60.4°

353.3° 2.3° 2.0° 4.0° 10.0° 5.6° 0.6° 350.5° 1.0° 359.5°

46.2° 70.8°

5.8° 14.6°

5.0° 5.5°

2.0° 2.4° 1.1° 1.4° 1.3° 1.3° 1.3° 2.0° 2.1° 1.6°

178 87

465 377 1753 956 1120 934 1111 350 720 771

5.97195 8.90824

77.5° 119.3° 106.0°)

11.97634 10.97349 10.99430 11.98849 11.99018 14.98502 11.99010 11.97764 7.99028 11.98572

83.2° 82.4° 85.5° 83.0° 81.2° 84.5° 83.1° 82.1°

195.1° 250.1° 257.1° 261.5° 290.9° 284.7° 241.9° 179.0°

89.4°

202.9°

2.4° 601 5.99168 (corrected for tilt of block)

75.9°

37.0°

Notes. Samples were collected, processed, analyzed and interpreted using the methods established by McElhinny (1973) as modified by Kuntz et al. (1986, p. 581). Site is an alphanumeric identifier; N Lat. is site latitude in degrees north; E Long. is site longitude in degrees east; N/No is the number of cores used compared with the number originally taken at the site; Exp. is the strength of the peak AF cleaning field in mT; Pl indicates that a planes vector analysis was used to define the mean remanent direction; I is the remanent inclination in degrees; D is the remanent declination in degrees; α95 is the radius of the 95% confidence limit about the mean direction; k is the estimate of the Fisherian precision parameter; R is the length of the resultant vector; Plat. and Plong. give the location in degrees north and east of the virtual geomagnetic pole (VGP) calculated from the mean direction of the site. Data indicated in bold below the horizontal lines are the means of the values for the 8 sites of the Hat Creek Basalt.

108

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

remanent magnetic directions, (implying eruption over a protracted period of time) or whether the directions are very similar (implying that the flows were erupted within a few decades). We compare paleomagnetic results from sampled lava flows with the record of geomagnetic secular variation derived from the sediments of Mono Lake (Lund et al., 1988). This record, which spans the timeframe from 13 ka to > 60 ka, was used by us previously as an aid in the 40Ar/39Ar age assignment of the dacite of Lassen Peak (Turrin et al., 1998; that paper includes an extensive discussion of methodology and uncertainties). Comparison of the 1-σ range of 40Ar/39Ar ages with the Mono Lake record allows us to eliminate those times when the magnetic field does not match the Mono Lake record and thus focus on the times when the two data sets are coincident. The eight paleomagnetic sites for the Hat Creek Basalt were distributed throughout the areal extent of the flow (Fig. 1 and Table 4). Within the error of the measurements, paleomagnetic directions for the Hat Creek Basalt (Table 4 and Fig. 6) are uniform with a mean inclination of 64.3° and a mean declination of 1.0°, indicating that the eruption was brief. The inclination and declination of the Hat Creek Basalt simultaneously match the Mono Lake record (Lund et al., 1988) at seven times (Fig. 7, light-gray circles) and thus provide no definitive refinement to the 40Ar/39Ar age of the Hat Creek Basalt. Paleomagnetic data for Cinder Butte, Sugarloaf Peak and Potato Butte flows (Table 4 and Fig. 6) generally exhibit statistically different directions, implying different emplacement ages for these units. The one site suitable for paleomagnetic measurements on the rugged and blocky basaltic andesite of Cinder Butte, however, records an unusual westerly declination that is compatible with the Mono Lake record only at 34 ka (Fig. 7, dark-gray circles), which is within the error envelope of Cinder Butte's 38 ± 7 ka 40Ar/39Ar age. This correspondence suggests the true age of the basaltic

Figure 7. Remanent magnetization directions of Hat Creek Basalt and basaltic andesite of Cinder Butte compared to paleomagnetic secular variation data of Mono Lake sediments, eastern California (Lund et al., 1988). Times when both the inclination and declination match the secular-variation curve are shown by light-gray circles for the Hat Creek Basalt and dark-gray circles for the basaltic andesite of Cinder Butte.

andesite of Cinder Butte might be ∼ 34 ka, but with only one stable paleomagnetic site in the unit, this inference is not robust. Vertical displacement along the Hat Creek fault

Figure 6. Part of an equal-area stereographic projection (lower hemisphere) showing mean directions and ellipses of 95% confidence of paleomagnetic directions measured on the Hat Creek Basalt and related units.

The Hat Creek Basalt is displaced as much as 30 m vertically along the Hat Creek fault (Muffler et al., 1994, p. 198). Using the 40 Ar/39Ar inverse isochron age of 24 ka for the Hat Creek Basalt, we calculate an average vertical movement of ∼ 1.2 mm yr− 1 for the past ∼ 24 ka. This value is consistent with the estimate of ∼1.3 mm yr− 1 on the Hat Creek fault for the past ∼ 15 ka derived from 20-m displacement of the glacial gravels that overlie the Hat Creek Basalt (Muffler et al., 1994, p. 198). By considering errors in the reported 40Ar/39Ar ages, we can place some limits on the average displacement rate. For example, in the unlikely scenario that the Hat Creek Basalt is just a few years older than the overlying ∼15 ka glacial deposits, the average vertical movement would be ∼2 mm yr− 1. On the other hand, if the basaltic andesite of Cinder Butte was erupted at the 1σ upper uncertainty of the 40Ar/39Ar age (45 ka) and the Hat Creek Basalt was erupted immediately thereafter, the average

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

109

Figure 8. Ages and 1σ uncertainties of Hat Creek Basalt and other units that constrain its age.

vertical movement on the Hat Creek fault would be only ∼ 0.7 mm yr− 1. Thus, the data constrain the average displacement rate on the Hat Creek fault during the late Quaternary to 0.7–2 mm yr− 1, with the most likely average displacement rate being 1.2–1.3 mm yr− 1. Conclusions • The Hat Creek Basalt must be older than ∼ 15 ka, since it is overlain by gravel related to the last major glaciation, which ended at that time. • The Hat Creek Basalt must be younger than the basaltic andesite of Cinder Butte, dated at 38 ± 7 ka (Fig. 8). This geochronologic constraint is consistent with the paleomagnetic data and is supported by 40Ar/39Ar ages on the other underlying units (the andesite of Sugarloaf Peak at 46 ± 7 ka, the basaltic andesite of Little Potato Butte at 67 ± 4 ka, and the andesite of Potato Butte at 77 ± 11 ka). • The one good experiment on a sample of the Hat Creek Basalt gives a plateau age of 26 ± 18 ka and an inverse isochron age (preferred) of 24 ± 6 ka. The second, experiment, albeit of low reliability, gives an inverse isochron age of 26 ± 6 ka. Viewed alone, such young ages on a low-K2O tholeiitic basalt should rightly be greeted with skepticism. Given the stratigraphic and geochronologic constraints, however, the dating experiment on the Hat Creek Basalt appears to give a valid age. • Comparison of the two Hat Creek Basalt samples suggests that low-K2O tholeiitic basalts with holocrystalline textures completely lacking glass are more likely to yield useful results than those with diktytaxitic textures. • The Hat Creek Basalt is displaced vertically as much as 30 m by the youngest strand of the Hat Creek fault, giving an average vertical displacement rate of between 2.0 and 0.7 mm yr− 1 for the past 15–45 ka, with the most likely displacement rate being ∼ 1.2–1.3 mm yr− 1 for the past 24 kyr. • Direct 40Ar/39Ar dating of very young low-K2O basalts is thus possible, although the precision will not be good. Individual dates should be treated with caution until

constrained by experiments on multiple samples or by dating of stratigraphically related, more potassium-rich rocks. Acknowledgments We gratefully acknowledge thorough and thoughtful USGS reviews by A.T. Calvert and R.C. Evarts and journal reviews by S.T. Nelson and T.L. Spell. References Anderson, C.A., 1940. Hat Creek lava flow. American Journal of Science 238, 477–492. Anderson, A.T., Gottfried, D., 1971. Contrasting behavior of P, Ti, and Nb in a differentiated high-alumina olivine tholeiite and a calc-alkaline andesitic suite. Geological Society of America Bulletin 82, 1929–1942. Brooks, C., Hart, S.R., Wendt, I., 1972. Realistic use of two-error regression treatments as applied to rubidium–strontium data. Review of Geophysics and Space Physics 10, 551–577. Clynne, M.A., Muffler, L.J.P., in press. Geologic map of Lassen Volcanic National Park and vicinity, California. U.S. Geological Survey Scientific Investigations Map 2899, scale 1:50,000. Dalrymple, G.B., Lanphere, M.A., 1969. Potassium-Argon Dating-Principles, Techniques, and Applications to Geochronology. W.H. Freeman and Company, San Francisco. Dalrymple, G.B., Lanphere, M.A., 1974. 40Ar/39Ar age spectra of some undisturbed terrestrial samples. Geochimica et Cosmochimica Acta 38, 715–738. Dickin, A.P., 1997. Radiogenic Isotope Geology. Cambridge University Press, New York. Donnelly-Nolan, J.M., Champion, D.E., Miller, C.D., Grove, T.L., Trimble, D.A., 1990. Implications of post-11,000 year volcanism at Medicine Lake Volcano, northern California. Journal of Geophysical Research 95 (19), 693–19704. Fleck, R.J., Sutter, J.F., Elliot, D.H., 1977. Interpretation of discordant 40Ar/39Ar age-spectra of Mesozoic tholeiites from Antarctica. Geochimica et Cosmochimica Acta 41, 15–32. Gerstel, W.J., 1989. Glacial chronology and the relationship to volcanic stratigraphy in the Hat and Lost Creek drainages, Lassen Volcanic National Park, California. M.Sc. Thesis, Humboldt State University. Gerstel, W.J., Clynne, M.A., 1989. Glacial stratigraphy in the Lassen Peak area of northern California—Implications for the age of Lassen Peak dacite dome (abs.). Geological Society of America Abstracts with Programs 21–5, 83. Kuntz, M.A., Champion, D.E., Spiker, E.C., Lefebvre, R.H., 1986. Contrasting magma types and steady state, volume-predictable basaltic

110

B.D. Turrin et al. / Quaternary Research 68 (2007) 96–110

volcanism along the Great Rift, Idaho. Geological Society of America Bulletin 97, 579–594. Lund, P.S., Liddicoat, J.C., Lajoie, K.R, Henyey, T.L., Robinson, S.W., 1988. Paleomagnetic evidence for long-term (104 yr) memory and periodic behavior in the Earth's core dynamo process. Geophysical Research Letters 15, 1101–1104. McElhinny, M.W., 1973. Paleomagnetism and Plate Tectonics. Cambridge University Press, Cambridge, England. McIntyre, G.A., Brooks, A.C., Compston, W., Turek, A., 1966. The statistical assessment of Rb–Sr isochrons. Journal of Geophysical Research 71, 5459–5468. Muffler, L.J.P., Clynne, M.A., Champion, D.E., 1994. Late Quaternary normal faulting of the Hat Creek Basalt. Geological Society of America Bulletin 106, 195–200. Steiger, R.H., Jäger, E., 1977. Subcommission on the geochronology convention on the use of decay constants in geo- and cosomochronology. Earth and Planetary Science Letters 36, 359–362.

Taylor, J.R., 1997. An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements, 2nd edition. University Science Books. 327 pp. Turrin, B.D., Donnelly, J.M., Hearn, B.C., 1994. 40Ar/39Ar ages from the rhyolite of Alder Creek, California; age of the Cobb Mountain normal-polarity subchron revisited. Geology 22, 251–254. Turrin, B.D., Christiansen, R.L., Clynne, M.A., Champion, D.E., Gerstel, W.J., Muffler, L.J.P., Trimble, D.A., 1998. Age of Lassen Peak, California, and implications for the ages of late Pleistocene glaciations in the southern Cascade Range. Geological Society of America Bulletin 110, 931–945. Vestine, E.H., Laporte, L., Cooper, C., Lange, I., Hendrix, W.C., 1959. Description of the Earth's Main Magnetic Field and its Secular Change, 1905–1945, vol. 578. Carnegie Institution of Washington Publication. 532 pp. Wendt, I., Carl, C., 1991. The statistical distribution of the mean squared weighted deviation. Chemical Geology (Isotope Geoscience Section) 86, 275–285.