High precision intercalibration of 40Ar-39Ar standards

Intercalibration of one intralaboratory and three interlaboratory standards used in ⁴⁰Ar-³⁹Ar dating has been carried out. In order to provide homogeneous values for ${}^{40}\text{Ar}{}^1{}^{40}\text{K}$ the standards were prepared by careful handpicking. To control the neutron fluence in the Herald Reactor (A.W.R.E.) 16 aliquots of the standards were arranged along 0.6 × 60 cm of a single silica tube. The corrections for all known interferences from K, Ca, Cl were carefully assessed. Two of the hornblende standards, Hb3gr and MMHb-1 appear homogeneous at the 0.1% level while the other two standards, LP-6 and FY12a are not completely homogeneous. The mean values of ${}^{40}\text{Ar}{}^1{}^{40}\text{K}$ when referenced to the previously determined value for Hb3gr (turner et al., 1971) are:

Hb3gr	hbld	$\text{.08504 ± .05\%}$(±lσ)	1072. m.y.
MMHb-1	hbld	$\text{.03493 ± 05\%}$	518.9 m.y.
LP-6	biot	$\text{.007735 ± .13\%}$	128.5 m.y.
FY12a	hbld	$\text{.02858 ± .25\%}$	435.0 m.y.
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