Abstract
Constituent grain-size fractions and whole-rock samples of “middle” and “upper” anchizone slates from the north-eastern Rheinisches Schiefergebirge (Federal Republic of Germany) were analyzed with 40 Ar 39 Ar incremental heating techniques. Comparison of the total-gas ages and previously determined KAr ages indicates variable recoil-loss of 39Ar, reflecting differences in grain size and/or grain-edge morphology. Grains with poorly defined edges characterize the middle anchizone size fractions. They have relatively large surface/volume ratios and are more strongly affected by recoil-loss of 39Ar than grains in the upper anchizone size fractions which have clearer, more well-defined edges. The strongly discordant age and K Ca spectra reflect differences in diffusive characteristics of constituent mineralogical phases, including: relatively non-retentive chlorite, white mica, and relatively refractory albite. White mica records anomalously old apparent ages as a result of recoil-loss of 39Ar. Gas fractions evolved from chlorite and albite record anomalously young ages because of recoil-gain of 39Ar. These effects are more prominent in the middle anchizone size fractions.
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