Stable isotopes in the Khibiny and Lovozero massifs: Magma sources and conditions of the postmagmatic alterations

The oxygen isotopic composition was determined in whole-rock samples and mo nomineral separates from the major alkaline rocks of the Khibiny (70 determ inations) and Lovozero (50 determinations) massifs. Carbon, oxygen, and sul fide sulfur isotopic compositions were analyzed in the carbonatites of the Khibiny Massif. Various nepheline syenites from the Khibiny and Lovozero ma ssifs show delta O-18 = 6.0 0.4 parts per thousand, which is similar to tho se of MORB and other mantle rocks. No disturbance of the thermodynamic equi librium was found between coexisting minerals in the unaltered rocks. The a lbitized rocks at both massifs are restricted to relatively narrow zones. H owever, the albitized rocks of the Khibiny Massif are depleted in O-18 (to delta O-18 = 2.3), while those of the Lovozero massif are enriched in O-18 (to delta O-18 = 8.7), thus indicating a strong contrast in conditions of t heir postmagmatic alterations, which occur in both massifs within relativel y narrow zones. Unlike the alkaline rocks, carbonatites of the Khibiny Mass if could not be considered pure derivatives of a mantle magma. They demonst rate significant variations in delta O-18 (from 6.6 to 12.4 parts per thous and), delta C-13 (from -6.8 to -2.3 parts per thousand), and delta S-34 (fr om -7.6 to -0.1 parts per thousand), which may be interpreted as reflecting a contribution of water interacting with the host rocks.