Age measurements of potassium-bearing sulfide minerals by the 40Ar/ 39Ar technique

Abstract

K-Ar ages have been determined for sulfide minerals for the first time. The occurrence of adequate amounts of potassium-bearing sulfides with ideal compositions K 3Fe 10S 14 (∼10 wt.% K) and KFe 2S 3 (∼16 wt.% K) in samples from a mafic alkalic diatreme at Coyote Peak, California, prompted an attempt to date these materials. K 3Fe 10S 14, a massive mineral with conchoidal fracture, gives an age of 29.4 ± 0.5m.y.( 40Ar/ 39Ar) , indistinguishable from the 28.3 ± 0.4m.y.( 40Ar/ 39Ar) and 30.2 ± 1.0m.y.8 (conventional K-Ar) ages obtained for associated phlogopite (8.7 wt.% K). KFe 2S 3, a bladed, fibrous sulfide, gives a younger age, 26.5 ± 0.5m.y.( 40Ar/ 39Ar) , presumably owing to Ar loss.