Vol 56, No 11 (2018)

Generalization of available literature and new original data showed that the Early Cretaceous high-Mg alkaline magmatism is confined to the ancient Lambert rift zone. Alkaline ultramafic rocks developed in the areas of this zone (Jetty Oasis, western flank of the Beaver and Radok lakes, Fisher and Meredith massifs) were derived through melting of metasomatized continental mantle at ~1270°С and at depths of 130–140 km. Variations of major and trace-element composition and the wide range in olivine composition (Fo₉₁–Fo₈₀) are consistent with its change through intrachamber crystallization. The average values of initial isotope composition of ultra-alkaline high-Mg basalts are as follows: ¹⁴³Nd/¹⁴⁴Nd—0.512485, ⁸⁷Sr/⁸⁶Sr—0.70637, ²⁰⁷Pb/²⁰⁴Pb—15.671, ²⁰⁶Pb/²⁰⁴Pb—18.391, ²⁰⁸Pb/²⁰⁴Pb—38.409. They are close to the model ЕМII source and can be arbitrarily taken as the preliminary assessment of isotope composition of a source of Mesozoic melts. Based on lithophile element and isotope compositions, the alkaline high-Mg basaltic magmatism is thought to be related to the thermal impact of the Kerguelen plume on the lithospheric mantle of East Gondwana 120–110 Ma. Similar ancient deep-seated metasomatized eastern Gondwanan mantle, which contains carbonates and biotite and has an age of 2.4 Ga, was found in southern East Antarctica as well as in the north within eastern India.

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, ³He, ⁴Не, and ⁴⁰Ar* 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% ³Не and only approximately 16% ⁴Не 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 high ³Не/⁴He ratios (up to ≈1 × 10^–6) in the amphibole were caused by the high Li concentrations and better preservation of ³Не. The ³He/⁴He ratios in helium extracted from the samples by melting and crushing are indistinguishable, while the ⁴He/⁴⁰Ar* 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.