Juvenile helium in ancient rocks: I. 3He excess in amphiboles from 2.8 Ga charnockite series—Crust-mantle fluid in intracrustal magmatic processes

Abstract

The 3He 4He in materials derived from the mantle (⩾10 × 10 −6) is decreasing in the course of time and approaches the radiogenic value (~10 −8) during ≅1 Ga. Some minerals, however, trapped volatiles, including He, and preserve them well during a much longer time; correspondingly, the isotope composition of trapped He in the minerals may shed light on the relation between crustal and mantle He in ancient fluids. Concentrations of U, Th, and K and noble gas (He and Ar) isotopes were measured in crustal charnockite series rocks and rock-forming minerals from the Vezha Tundra Complex, Kola Peninsula. The U-Pb zircon age of the Complex is 2.8 Ga. Amphibole was found to be the principal concentrator of He: 4He concentrations attain 670 × 10 −6 cm 3/ g, greater than that expected from U and Th decay, implying a significant contribution from a trapped He component. The measured 3He 4He ratios (≅0.2 × 10 −6) are considerably higher than the radiogenic value, and the 3He concentrations are up to 25 times higher than those which could be produced from nuclear processes. Therefore, mixing of crustal and mantle volatiles appears to be responsible for the observed 3He 4 He ratios. Partial degassing of the melt in the course of crystallization differentiation was revealed by the increase of 4He 40Ar RAD from ≅2.5 in hypersthene to ⪢ 10 in amphiboles, formed close to the end of the process. A similarity of U-Pb and K-Ar ages suggests that the rocks have not been altered by relatively recent metamorphic processes and have conserved He, trapped since crystallization. Distribution of U, Th, 4He, and 3H implies a considerable release of He RAD by the rocks and minerals whereas He TRAP appears to be preserved.