Oxygen Isotopic Fractionation in a Silicate–Carbonate System during the Formation of Rocks of the Kovdor Pluton (Kola Peninsula)

Abstract

The δ18О values of carbonates and silicates (Ol, Phl, Mt) of calcite carbonatites, phoscorites, ijolites, and turjaites of the Kovdor pluton have been studied for the first time to elucidate the role of processes of contamination, retrograde exchange, and high-temperature fractionation in a magmatic system. It is established that no processes of crustal contamination are registered in the isotopic characteristics of silicates and that the δ18О values of olivine, phlogopite, and magnetite are lower relative to the compositions typical of equilibrium with mantle peridotite. At the same time, the carbonates show a complementary enrichment in the 18О isotope relative to the composition of derivative mantle melts. The estimated closure temperature of the O isotopic composition of olivine and the cooling rates of rocks do not support a hypothesis on retrograde isotope exchange. A redistribution of O isotopes between silicate and carbonate constituents of melts in the magmatic system before the origination of the Kovdor pluton and its cooling is suggested. According to the O isotope data, the carbonate constituent at that moment could strongly be dominant (in 1.5–4.5 times) over the silicate one.