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

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• 作者：Nomade ; S. ; Renne ; P.R. ; Vogel ; N. ; Deino ; A.L. ; Sharp ; W.D. ; Becker ; T.A. ; Jaouni ; A.R. ; Mundil ; R.
• 关键词：Quaternary ; Rhyolite ; Ar-40/Ar-39 ; Sanidine ; Flux ; Monitor ; Standard
• 刊名：Chemical Geology
• 出版年：2005
• 出版时间：May 25, 2005
• 年：2005
• 卷：218
• 期：3-4
• 页码：315-338
• 全文大小：451 K

文摘

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