Sites and Origin of Noble Gases in Minerals: A Case Study of Amphibole from Alkaline Granitoids of the Kola Peninsula

The paper presents newly acquired data on the budget and mobility of isotopes of noble gases in samples of amphibole, a mineral commonly characterized by relatively high He concentrations compared to other rock-forming minerals of Earth’s crust. In the amphibole samples from alkaline granites of the Ponoy Massif, Kola Peninsula, 3 He, 4 Не, and 40 Ar* isotopes were mostly radiogenic, generated by radioactive decay and nuclear reactions. Retention ability of two helium isotopes was found to be different: since the time of metamorphic event approximately 38% 3 Не and only approximately 16% 4 Не have been preserved. A small He fraction (≈3% of its total concentration) and a larger part of radiogenic 40 Ar* (≈35%) are hosted in fluid inclusions. Relatively high 3 Не/ 4 He ratios (up to ≈1 × 10 –6 ) in the amphibole were caused by the high Li concentrations and better preservation of 3 Не. The 3 He/ 4 He ratios in helium extracted from the samples by melting and crushing are indistinguishable, while the 4 He/ 40 Ar* ratios in the fluid inclusions (opened by sample crushing) (~0.6) are notably lower than those in the bulk samples (melting) (5) and those calculated from the U, Th, and K concentrations (15). Two He release peaks were observed on the curves of rapid (12–40°C min –1 ) liner heating of the samples. At a lower heating rate (~5°C min –1 ), the high-temperature peak disappeared. Further investigation of this peculiarity of He migration has shown that (1) it is also typical of some other amphibole samples (not only those from Ponoy granites); (2) He amount in the “disappearing” peak varies from sample to sample and also depends on the heating rate and grain size (powdered samples with an average grain size of ≤50 µm did not yield the second peak, regardless of the heating rate); (3) the temperature of He release from powdered samples is approximately 50°C lower than that for mineral grains; and (4) preliminary long-term heating of the sample to 400°C (a temperature at which He starts to diffuse from the amphibole structure) results in a significant decrease in the low-temperature peak and a shift of the “disappearing” peak toward lower temperatures. The probable reasons for this phenomenon are discussed.