An intercalibration study of the Fish Canyon sanidine and biotite 40Ar/ 39Ar standards and some comments on the age of the Fish Canyon Tuff

Abstract

The age of the Fish Canyon Tuff (FCT) has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the K/Ar and U/Pb isotopic systems. Minerals from the FCT (notably sanidine and biotite) are in widespread use as 40Ar/ 39Ar fluence monitors (“standards”) and have been employed as both primary and intercalibrated standards. In attempting to constrain the true age of FCT, the distinction between primary vs. intercalibrated ages is important, as only the primary age determinations are valid. An investigation of both Fish Canyon sanidine (FCs) and Fish Canyon biotite (FCT-3) was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain-to-grain scale. Electron microprobe results show that FCs is chemically homogeneous, verifying previous studies, although there are some very small zones of minor K depletion; FCT-3 biotite commonly contains numerous and small mineral inclusions. 40Ar/ 39Ar experiments further show that FCs is remarkably homogeneous, confirming previous observations, whereas FCT-3 shows much greater isotopic variability due to contamination from the mineral inclusions. By setting the age of FCs equal to 27.98±0.15 Ma (2 σ), as intercalibrated to the U/Pb system by Villeneuve et al. (2000), the resultant intercalibration factor between FCs and FCT-3 is 1.005±0.017 yielding an age of 28.13±0.47 Ma (2 σ) for FCT-3, although the significant intragrain chemical and isotopic variability precludes its use as an effective 40Ar/ 39Ar fluence monitor. The true age of the FCT is problematic; a survey of all primary ages for FCT shows that mean K/Ar ages for plagioclase, hornblende, biotite, and sanidine are consistently younger than mean U/Pb ages from titanite and zircon. In conjunction with recently published petrological results which indicate that the FCT contains material from multiple crystallization events, this significant discrepancy between the K/Ar and U/Pb data sets demonstrates that the Fish Canyon Tuff cannot serve as a golden standard between the K/Ar and U/Pb systems.