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Sites and origin of noble gases in minerals: A case study of amphibole from alkaline granitoids of the kola peninsula

Geochemistry International, ISSN: 1556-1968, Vol: 56, Issue: 11, Page: 1084-1094
2018
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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,He,He, andAr* 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% 3He and only approximately 16% 4He have been preserved. A small He fraction (≈3% of its total concentration) and a larger part of radiogenic 40Ar* (≈35%) are hosted in fluid inclusions. Relatively highHe/He ratios (up to ≈1 × 10) in the amphibole were caused by the high Li concentrations and better preservation of 3He. The 3He/4He ratios in helium extracted from the samples by melting and crushing are indistinguishable, while theHe/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.

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