Argon 40 in cogenetic feldspar-mica mineral assemblages

Abstract

K and Ar analyses on cogenetic feldspars and micas from four plutonic bodies, two pegmatites, and one metamorphic terrain indicate that (1) albite and plagioclase have equal or greater argon retentivity than their cogenetic micas, (2) some potash feldspars of Tertiary age may quantitatively retain argon, and (3) intense metamorphic processes can thoroughly degas both feldspars and micas. The K-Ar ages of the micas and known chronostratigraphic relations indicate that pegmatitic albite, Tertiary plutonic plagioclase, and possibly Tertiary plutonic potash feldspar contain Ar40 in excess of that generated by the decay of their contained K. This excess component is believed to have been occluded within these minerals at the time of their crystallization from the liquid magma. The excess Ar40 contents derived from these measurements is 0.15 to 2.71 × 10−10 mole/gram in albites and plagioclases of Paleozoic and Tertiary age. The amount of excess Ar40 most likely reflects the partial pressure of Ar40 in the lithospheric gases. K-Ar ages of plagioclase from two Precambrian quartz monzonites are very nearly the same as the K-Ar ages of cogenetic biotite and muscovite. These results are concordant with Rb-Sr whole-rock isochron and mica ages of 1420 m.y. and demonstrate that plagioclase from Precambrian acidic plutonic rocks can be as precise a chronometer as mica minerals.