Alder Creek sanidine (ACs-2): A Quaternary 40Ar/39Ar dating standard tied to the Cobb Mountain geomagnetic event

Accurate 40Ar/39Ar dating of Quaternary and Holocene samples requires a standard (neutron fluence monitor) of appropriate age. This standard should be isotopically homogenous at the scale of analysis, and should be well intercalibrated with other standards. Several such standards have been proposed, including sanidine from the ∼1.19 Ma Alder Creek Rhyolite (ACs;Turrin et al., 1994 [Turrin, B.D., Donnely-Nolan, J.M., Hearn, B.C. Jr., 1994. 40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain Normal-Polarity Subchron revisited. Geology 22, 251-254]). This sanidine is satisfactorily homogeneous, well intercalibrated with Fish Canyon sanidine (FCs; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117-152]), and is linked to the astronomically calibrated magnetic polarity time scale, since the Alder Creek Rhyolite lava is the type occurrence of the Cobb Mountain event (CM). We collected 408 kg of this rock, from which we anticipate being able to extract more than 8 kg of >425 μm sanidine crystals. Analyses of sanidine phenocryts from the ACs-2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1.190±0.004 to 1.194±0.003 Ma1 with a weighted mean age of 1.193±0.001 Ma (MSWD=0.74, n=225; based on FCs=28.02 Ma; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117-152]). The value of the intercalibration factor (Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117-152]) is RFCsACs-2=0.04229±0.00012 (n=8), identical with that of the original ACs measurements (0.04229±0.00012; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117-152]). The age of ACs-2 is in agreement with the recent astronomical calibration of the CM event (1.215 to 1.190 Ma; [Channell, J.E.T., Mazaud, A., Sullivan, P., Turner, S., Raymo, M.E., 2002. Geomagnetic excursions and paleointensities in the Matuyama Chron at Ocean Drilling Program Sites 983 and 984 (Iceland Basin). J. Geophys. Res. 107 (6), 10.1029/2001JB000491.]). The apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements. Aliquots from various grain sizes will be available upon request. Zircon is also potentially available, which may be of interest to (U-Th)/He researchers.