MANTLE FLUID IN PROTEROZOIC GRANITOIDS: HE AND AR ISOTOPES IN ROCKS AND MINERALS OF THE LITZA-ARAGUBA DIORITE-GRANITE COMPLEX, KOLA PENINSULA

Abstract

The isotopic composition of He and Ar was analyzed in rocks and minerals from the Early Protero-zoic Litza-Araguba diorite-granite complex in the northwestern part of the Kola Peninsula. The initial 4He/3He ratio of the fluid captured by the granitoids is (3-5) × 105, which is significantly lower than this ratio in continental crustal rocks (∼ 1 × 108) but higher than the ratio of the upper mantle (8.9 × 104), the least variable Earth s reservoir. The He isotopic composition of the fluid was interpreted within the guidelines of the mixing model of He from the continental crest and mantle. Based on the results obtained on deep-seated xenoliths, the source of the latter was assumed to have been the enriched mantle. The differences between the concentrations and isotopic compositions of He in minerals and rocks of the "dioritic" and "granitic" associations are explained proceeding from the variable degassing degrees of the parental melts and the later in-situ generation of the radiogenic 4He isotope. It is demonstrated that the Ar extracted from the granitoids and their minerals contains an atmogenic component.