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Physico-chemical constraints for perovskite crystallization

in melilite-bearing and melilite-free rocks of alkaline-ultrabasic-carbonatite complexes

Rass I.T.*, Plechov P.Yu. **

* Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow, Russia; rass@igem.ru

** Petrology Department, Geological Faculty, Moscow State University, Moscow, Russia

Data on the composition and zoning of perovskite and coexisting with it magnetite and apatite that occur in equilibrium in melilite-bearing and melilite-free rocks in alkaline-ultrabasic-carbonatite complexes and materials obtained on inclusions in perovskite from an olivine-melilite rock (kugdite) from the Guli Complex are used to evaluate and compare the crystallization parameters of perovskite in rocks that crystallized from primitive magmas of various composition

It has been established that silicate rocks of alkaline-ultrabasic-carbonatite complexes belong to two differentiated series that compose the single ring complexes (Kravchenko and Rass, 1985). Melilite-bearing rocks (which are fairly often contained in these complexes) and more widely spread melilite-free rocks are derivatives of mantle magmas that contained variable proportions of alkalis and CaO (Kravchenko et al., 1992) and were derived under various physicochemical conditions. Theoretical and experimental studies of phase equilibria in silica-undersaturated magmas have demonstrated that Ca-rich melts can occur under mantle parameters (Kogarko, Green, 1998). We (Rass, Plechov, 2000) studied melt inclusions in olivine from kugdite (olivine-melilite rock) from the Guli Complex in the Maymecha-Kotui province, Polar Siberia, and determined their concentrations of alkalis, Ca, and Al, which are remarkably higher than in melt inclusions in olivine of similar Mg# from meymechite from the same Guli Complex (Sobolev et al., 1991). The composition of the latter is thought to be close to the composition of the parental magmas of all rocks, including the melilite-bearing ones. Later studies of melt inclusions in minerals from the melilite-bearing rocks of the alkaline-ultrabasic complexes Kaiserstuhl in Germany and Belaya Zima and Krestovskii in Russia (Solovova et al., 2005; Andreeva et al., 2007; Panina et al., 2001) have confirmed much higher CaO/SiO₂ ratios of the parental magmas of the melilite-bearing rocks compared to those of the parental magmas of melilite-free rocks.

Parental magmas of different major- and trace-element composition derived at different depths from differently metasomatized heterogeneous mantle sources underwent different evolution and differentiation in the crust and gave way to melilite-bearing or melilite-free rocks upon crystallization. The CaO-SiO₂-MgO phase diagram and the diagram for phase equilibria in the pseudoternary titanite-nepheline-diopside system (Veksler, Teptelev, 1990) show that the crystallization sequences of the minerals can differ, i.e., either perovskite or magnetite, perovskite or pyroxene, or melilite or pyroxene can be the first to crystallize.

Differences in the concentrations and mineral-melt partition coefficients of trace elements are controlled not only by the differences between these concentrations in the parental melts, conditions under which they were derived from the mantle, and the crystallization sequence of minerals in the course of melt differentiation but also by the conditions of crustal crystallization, such as P,T, f(O₂), P(CO₂), and a(SiO₂).

These parameters can be estimated from the composition, zoning, and inclusions in equilibrium accessory minerals, first of all, perovskite, a mineral contained in all of the melilite-free and melilite-bearing rocks. It was experimentally proved that perovskite is the second (after olivine) phase to occur on the liquidus during the crystallization of melted natural Ca-rich rocks and is formed, along with olivine and glass, at temperatures of 1150-1100^оC (Rass et al., 1996). Perovskite crystallization from melt is controlled by the perovskite-titanite equilibrium (Veksler and Teptelev, 1990), which is dependent on a(SiO₂). The Mg and Si concentrations of magnetite and apatite in equilibrium with perovskite in, respectively, melilite-bearing and melilite-free rocks testify that the melilite-bearing rocks crystallized at shallower depths, at lower P(СО₂) and higher f(О₂) (Rass, 2008). Perovskite in melilite-bearing rocks is more ferrous than in melilite-free rocks, and the Fe concentration in this mineral in melilite-bearing rocks decreases from the cores of its crystals to their margins, whereas magnetite in equilibrium with the perovskite shows a simultaneous increase in its Ti concentration from cores to margins (Rass, 2008). The reasons for these trends were not only the earlier crystallization of perovskite or magnetite (Chakhmouradian and Mitchell, 1997) but also a higher f(О₂) during perovskite crystallization in melilite-bearing rocks (Canil, Bellis, 2007; Speidel, 1970).

We have examined perovskite from the same kugdite from the Guli Massif for which the composition of parental melt was inferred from data on melt inclusions in olivine (Rass, Plechov, 2000) and recognized three successively crystallizing perovskite populations. The oldest one (Prv-I) comprises grains with pronounced growth zoning. This zonal perovskite is cut across by perovskite of the second population (Prv-II), which is, in turn, overgrown by perovskite of the third population (Prv-III). The zoning of Prv-I involves a notable decrease in the concentrations of Fe₂O₃ from cores to margins (from 0.98 to 0.65 wt %) Nb₂O₅ (from 0.26 to 0.14 wt %), and REE (Ce₂O₃ from 0.98 to 0.10 wt %), which suggests that the perovskite likely started to crystallized earlier than magnetite and other minerals and was then the only concentrator of trace elements. Prv-II has a composition analogous to that of Prv-I margins, and the concentrations of these elements in Prv-III are remarkably higher than in Prv-II.

Perovskite crystals contain solid olivine, clinopyroxene, and phlogopite inclusions, which are concentrated mostly in Prv-II, or located to the boundary between Prv-I and Prv-II. All of the examined inclusions are monomineralic and thus could not result from the recrystallization of melt inclusions. Along with the crystalline inclusions, perovskite of the second population sometimes bears cavities. Qualitative EDS microprobe analysis indicates that some of them contain amphibole and Na-Ca carbonates. Conceivably, these could be silicate-salt inclusions. No melt inclusions were found in any of the 700 examined perovskite grains.

The preferable occurrence of the solid inclusions in Prv-II of very little varying composition led us to recognize the Ol-CPx-Phl-Prv mineral assemblage, whose crystallization parameters can now be evaluated. The olivine-clinopyroxene geothermometer (Loucks, 1996) indicates that these phases could be in equilibrium with magmatic melt at a temperature of about 870-880°C and a pressure of 0.8-0.9 GPa, according to (Nimis, 1999). Temperature values of 1230°C determined in melt inclusions in perovskite from the Krestovskaya intrusion (Panina et al., 2001) likely pertain to the early perovskite of the first population.

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