The Monticellite-bearing Rocks of the Krestovskaya Intrusion: Genesis according to Melt Inclusion Study

The investigation of monticellitolites and olivine–monticellite rocks from the Krestovskaya Intrusion shows that the major minerals (olivine and monticellite) have higher MgO content than the same minerals in olivinites and kugdites of the intrusion. In the studied rocks olivine contains 90–93 mol % Fo and monticellite has 41.6–42.3 mol % Fo, whereas olivine and monticellite in olivinites and kugdites contain 86–87 and 37.2–41.2 mol % Fo, respectively. Melt inclusion study in minerals of monticellite rocks demonstrates that the monticellite rocks of the Krestovskaya Intrusion were formed by mixing of volatile-rich melts of different composition: K-rich high-iron low-alumina kamafugitic melt and Na-rich high-magnesium high-alumina picritic melt. Minerals crystallized at high temperatures in the following sequence: perovskite I (1250–1230°C) → perovskite II (≥1200°C) ↔ olivine (>1200°C) → monticellite (>1150°C). Perovskite I in monticellite rocks, as well as olivine in olivinites, crystallized from K-rich high-iron (Mg# = MgO/(MgO + FeO) = 0.37), low-alumina kamafugitic melt. During crystallization of late perovskite II in the monticellite rocks, the melt became more enriched in MgO (Mg# = 0.41) and richer in Na₂O and Al₂O₃, which is intermediate in composition between kamafugite and alkali picrite. Olivine in the monticellite rocks crystallized from melts similar in composition to melilitite, having a K-rich composition with Mg# = 0.39, whereas monticellite formed from a heterogeneous high-Mg Si-undersaturated melt, which is highly enriched with volatile components (including H₂O) and salts. The crystallization of minerals was accompanied by subsequent accumulation of volatile components in mixing melts, silicate–carbonate liquid immiscibility under 1250–1190°C, and polyphase carbonate–salt immiscibility under below 1190°C. In the latter event, the exsolved carbonate melt began to split into simpler immiscible fractions: alkali–sulfate–carbonate, alkali–phosphate–carbonate, and calcio–carbonate.