Amphibole: A major carrier of helium isotopes in crustal rocks

• Published in: Chemical Geology
• Main Authors: Tolstikhin, I.N., Verchovsky, A.B., Kamensky, I.L., Skiba, V.I., Gannibal, M.A., Vetrin, V.R., Tarakanov, S.V.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2016
• Subjects: Kola Peninsula; Ponoy; kola peninsula
• Online Access: https://oro.open.ac.uk/53877/; https://oro.open.ac.uk/53877/1/53877.pdf; https://doi.org/10.1016/j.chemgeo.2016.10.020

The first evidence for a specific role of amphiboles in He isotope balance of crustal rocks was presented in early contributions by Gerling et al. (1971, 1976). Since then it was shown that 4 He and 3 He concentrations in amphiboles generally exceed those in the host rock samples. Recently amphibole was considered as an important carrier of noble gases and other volatiles components in the course of their subduction into the mantle. This paper presents new data on the balance and mobility of noble gas isotopes and major gas constituents in amphibole separates in order to understand sources and evolution of volatile components of 2666 Ma old alkaline granites from Ponoy massif (Kola Peninsula), which underwent metamorphism 1802 Ma ago. In the amphiboles 3 He, 4 He and 40 Ar* were dominantly produced in situ due to radioactive decay of the parent isotopes and associated nuclear reactions. A small fraction of He (≈ 3% of the total) is liberated by crushing and shows 3 He/ 4 He ratio indistinguishable from that found by total extraction. The fraction of trapped 40 Ar* amounts to ≈ 40%; both these fractions presumably occupy fluid inclusions and show rather low 4 He/ 40 Ar* ≈ 0.1, a factor of ≈ 150 below the production ratio (calculated assuming no loss / gain of the species has happened since the time of metamorphism). 3 He has been better preserved in amphiboles compared with 4 He: the retention parameter (measured amount of He / totally produced amount) for 3 He (≈ 0.4) exceeds that for 4 He (≈ 0.15). He extraction by fast and slow linear heating of amphiboles resulted in different release patterns. The fast heating (within 12 to 40 °C min − 1 ) revealed a superposition of two peaks. When heating with slower heating rate (below 8 °C min − 1 ) was applied, the high-temperature peak disappeared (the “disappearing site”). Extractions of He atoms from grain and powder samples at different heating rates have shown that: (1) the “disappearing site” is revealed by the fast heating analyses of different amphibole samples ...