INTERCALIBRATION OF AR-40 AR-39 DATING STANDARDS/

A number of standards used for K-Ar and Ar-40/Ar-39 dating have been i ntercalibrated. Multiple splits (similar to 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, Fern 4 Muscovite, Bern 4 Biot ite and Pennsylvania State University Orthoclase-1A were irradiated in capsules, fused and analyzed. In replicate analyses, most of these st andards proved to be homogeneous in Ar-40/Ar-39(K) ratio (age) K, Ca, and Ar-36 contents. MMhb-1 is inhomogeneous in age at the similar to 15-mg level, indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle; the orthoclas e sample is also inhomogeneous at the similar to 30-mg level. Quoted r elative ro an age of 162.9 Ma for the SB-3 Biotite standard, total fus ion ages are: LP-6 Biotite 40-60# = 128.1 Ma; GA1550 Biotite = 97.8 Ma ; Fish Canyon Tuff-3 Biotite = 27.95 Ma; Taylor Creek Rhyolite Sanidin e = 28.0 Ma; Bern 4 Muscovite = 18.51 Ma; Bern 4 Biotite = 17.21 Ma. T hese ages are in good agreement with the (absolute) K-Ar ages of these minerals. Different preparations of biotite and sanidine standards fr om a welded tuff such as the Fish Canyon Tuff may not be of identical ages. Tests on submilligram splits reveals that SB-3 Biotite, GA1550 B iotite, 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 Ar- 40/Ar-39 dating.