Olivine from Alkali Ultramafic Rocks of the Izhmozero Pipe: Multistage Melt Evolution from Kimberlite to Melanephelinite

This paper reports the petrographic and geochemical characteristics of olivine from the ultramafic alkaline rocks of the Izhmozero pipe, Arkhangelsk Diamond Province, which are used to reconstruct the evolution of alkali ultramafic melts of the province from Fe–Ti-rich kimberlite to Mg–Al-rich rocks approaching melanephelinite. Based on the morphology and composition of olivine cores, three types of macrocrysts were identified: (1) Ol-I with the least magnesian cores (Mg# 0.904–0.913), high Ni (2856–3135 ppm) and Ti (84.2–120 ppm), moderate Ca (417–607 ppm) and Mn (829–986 ppm), V/Sc of 1.7–1.8, and diffusive transitional zones; (2) Ol-II with moderately magnesian cores (Mg# 0.923–0.927), high Ni (3041–3177 ppm), moderate Mn (875–1035 ppm) and Ca (557–872 ppm), low Ti (6.01–17.9 ppm), V/Sc = 1.16, and reaction zones along the core–rim boundary; and (3) Ol-III with highly magnesian cores (Mg# 0.935–0.948), high Ni (2692–3234 ppm), Mn (973–1451 ppm), and Ca (1461–2266 ppm), moderate Ti (39.3–70.1 ppm) and Al (175–227 ppm), and low V/Sc (0.17–0.24). Olivine macrocrysts of the Ol-I and Ol-III types have oscillatory rim zones, and outer reaction margins are characteristic of all olivine types. Ol-I macrocrysts are interpreted as products of interaction between a silicate–carbonate alkali-ultramafic melt (kimberlitic or aillikitic) and the lithospheric mantle at an early stage of melt ascent or as early phases crystallizing from such melts. Ol-II macrocrysts are rare and were derived from the depleted lithospheric mantle of the Arkhangelsk Diamond Province, but their compositions were modified by the alkali-ultramafic melts transporting them. After the formation of Ol-I, the ascending melt evolved toward a more magnesian composition owing to fractionation of Fe–Ti phases under changing P–T– f_O₂ conditions. This process increased the Mg# of melt and resulted in the development of transitional zones on Ol-I cores owing to diffusive reequilibration and subsequent crystallization of high-Mg# Ol-III. After Ol-III formation, the melts that produced the Izhmozero rocks experienced contamination with ancient crustal material, which shifted their composition toward melanephelinite. These observations suggest that the formation of Mg–Al-rich rocks of the Izhmozero type within the Arkhangelsk Diamond Province was controlled not only by source characteristics, but also by hybridization processes at a lower crustal level.