Amphibole: A major carrier of helium isotopes in crustal rocks

Abstract

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 4He and 3He 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 2666Ma old alkaline granites from Ponoy massif (Kola Peninsula), which underwent metamorphism 1802Ma ago.In the amphiboles 3He, 4He and 40Ar* 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 3He/4He ratio indistinguishable from that found by total extraction. The fraction of trapped 40Ar* amounts to ≈40%; both these fractions presumably occupy fluid inclusions and show rather low 4He/40Ar*≈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).3He has been better preserved in amphiboles compared with 4He: the retention parameter (measured amount of He/totally produced amount) for 3He (≈0.4) exceeds that for 4He (≈0.15).He extraction by fast and slow linear heating of amphiboles resulted in different release patterns. The fast heating (within 12 to 40°Cmin−1) revealed a superposition of two peaks. When heating with slower heating rate (below 8°Cmin−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 but not only those from the Ponoy massif; (2) amount of He liberated from the “disappearing site” is variable and generally much less than the total amount of He in the sample; (3) analysis of the powder produced in the crushing experiments never reveals the “disappearing site”; the temperature of He release from the powder is lower than that from the mm grain size sample by ≈50°C. Possible explanations of the nature of the “disappearing site” are discussed. However, independently on nature of this effect, repeated gas extractions by heating at different rates would give additional information about structure and its transformation during heating of amphiboles.The simplest explanation of the observed abundances of noble gas isotopes in the amphibole separates from Ponoy granites suggests local production, redistribution and partial loss of noble gases during evolution of the massif.