Investigation of excess argon in ultramafic rocks from the Kola Peninsula by the 40Ar/ 39Ar method

Abstract

In several xenolithic ultramafic rocks from the Kola Peninsula, including a magnetic separate, abnormally high 40Ar/ 39Ar ratios persisted at low and high temperatures. The lowest 40Ar/ 39Ar ratio was consistently observed at intermediate temperatures (900–1100°C), indicating an apparent age of 2.8–3.1 b.y.; however, this may not indicate the formation age. The quantity of excess 40Ar was estimated at each temperature fraction, adopting ages inferred from published Rb-Sr ages or the minimum 40Ar/ 39Ar age. Excess 40Ar is abundantly trapped both in mineral lattices and nonretentive trapping sites, but the trapping sites are different from those of in-situ radiogenic 40Ar. The high temperature component of excess 40Ar is considered to represent Ar dissolved during mineral formation in the upper mantle or the lower crust. A correlation between the amount of high temperature excess 40Ar and 36Ar exists for some samples. The 40Ar excess/ 36Ar ratios of the rocks of probable upper mantle or lower crust origin vary from about 10 000 to 35 000, which may suggest large fluctuations of this ratio in the deep interior of the earth. The high value implies that most 36Ar was already degassed from the earth's interior at least 2 or 3 b.y. ago.