OXYGEN ISOTOPE FRACTIONATION IN THE SILICATE-CARBONATE SYSTEM DURING THE ROCK FORMATION OF THE KOVDOR MASSIF (KOLA PENINSULA)

Abstract

For the first time the δ18O values both in carbonate and non-carbonate (Ol, Phl, Mt) components of the rocks (calcite carbonatites, phoscorites, ijolites, turyaites) of the Kovdor massif were studied to clarify the role of contamination processes, retrograde exchange, and high-temperature fractionation in the igneous system. It has been established that the processes of crustal contamination are not recorded in the isotopic characteristics of the silicate part of the rocks, and the δ18О values of olivine, phlogopite, and magnetite are lowered compare to the compositions attributed to equilibrium with mantle peridotite. At the same time, the carbonate component of the rocks shows complementary enrichment in the 18O isotope with respect to the composition of derivatives of mantle melts. Estimates of the δ18О closure temperature of the olivine and the rates of rock cooling do not support the hypothesis of retrograde isotopic exchange. A hypothesis is proposed about the presence of a stage of redistribution of oxygen isotopes between the silicate and carbonate components of melts in the magmatic system, which took place before the formation of the Kovdor massif and its cooling. According to oxygen isotope data, the carbonate component could sharply predominate (1.5–4.5 times) over the silicate component during the equilibration.