Intercalibration of 40Ar39Ar dating standards

A number of standards used for KAr and 40Ar39Ar dating have been intercalibrated. Multiple splits (∼ 15–40 mg each) of MMhb-1, SB-3 Biotite, LP-6 Biotite 40–60#, GA1550 Biotite, Fish Canyon Tuff-3 Biotite, Taylor Creek Rhyolite Sanidine, Bern 4 Muscovite, Bern 4 Biotite and Pennsylvania State University Orthoclase-1A were irradiated in capsules, fused and analyzed. In replicate analyses, most of these standards proved to be homogeneous in 40Ar★39ArK ratio (age) K, Ca, and 36Ar contents. MMhb-1 is inhomogeneous in age at the ∼ 15-mg level, indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle; the orthoclase sample is also inhomogeneous at the ∼ 30-mg level. Quoted relative to an age of 162.9 Ma for the SB-3 Biotite standard, total fusion ages are: iotite 40–60# = 128.1 Ma; GA1550 Biotite = 97.8 Ma; Fish Canyon Tuff-3 Biotite = 27.95 Ma; Taylor Creek Rhyolite Sanidine = 28.0 Ma; Bern 4 Muscovite = 18.51 Ma; Bern 4 Biotite = 17.21 Ma. These ages are in good agreement with the (absolute) KAr ages of these minerals. Different preparations of biotite and sanidine standards from a welded tuff such as the Fish Canyon Tuff may not be of identical ages. on submilligram splits reveals that SB-3 Biotite, GA1550 Biotite, Fish Canyon Tuff-3 Biotite and Taylor Creek Rhyolite Sanidine are suitable for use as monitors in laser work; the sanidine specimen appears to be the most suitable for use as the primary monitor for 40Ar39Ar dating.