Groundmass Chromospinellides from Kimberlites of Khompu-May Kimberlite Field

The current paper presents the results of studying chromites of kimberlite mesostasis forming the Manchary, Aprelskaya, Erel, Turakhskaya, and Artemova pipes within the Khompu-May kimberlite field (central Yakutia). Despite shared texture and structural characteristics and mineral composition of the kimberlites, chromospinellide composition is distinctive in each pipe. Groundmass chromium spinel of the Aprelskaya and Erel kimberlite pipes is characterized by the highest aluminum oxide content (>10 wt. %). Chromites from the Erel and Turakhskaya pipes as well as a fraction of grains from the Manchary pipe with titanium oxide (<4 wt. %) form a field of common composition by Cr2O3 and TiO2 content. The Aprelskaya and Artemova pipes show up to 17 wt. % TiO2 in chromites. Such a difference in titanium content correlates with perovskite content in kimberlite groundmass of the Khompu-May field. The results of the study revealed two trends in evolution of chromospinellide microcrystals (R. Mitchell, 1986) – ulvöspinel associated with typical kimberlites and titanomagnetite characteristic of micaceous kimberlites. Chromospinellides of the Aprelskaya pipe demonstrate the ulvöspinel trend only, suggesting earlier spinel crystallization relative to groundmass mica. Spinellides from the Erel and Artemova pipes follow the titanomagnetite trend only, being crystallized after formation of mesostasis mica. Spinellides from the Manchary and Turakhskaya pipes meet the ulvöspinel and titanomagnetite trend, indicating two stages of mineral crystallization relative to phlogopite.

Kelyphite Rims on Garnets of Contrast Parageneses in Mantle Xenoliths from the Udachnaya-East Kimberlite Pipe (Yakutia)

A new classification of kelyphitic rims on garnets from xenoliths of peridotitic and eclogitic parageneses of the mantle section under the Udachnaya-East kimberlite pipe (Yakutia) is presented. Five types of rims are identified: Rim1 develops between garnet and olivine/pyroxene (or rim2) and is composed of high-alumina pyroxenes, spinel, phlogopite; rim2, the coarse grain part of rim1, is located between rim1 and olivine/pyroxene, and mainly consists of phlogopite and less aluminous larger pyroxenes and spinel; rim3 develops between garnet and kimberlite, and presents with phlogopite and Fe-Ti spinel; rim4 sometimes presents instead of rim1/rim2 and consists of zoned high-Cr phlogopite with rare fine grains of chromium spinel; rim5, a “pocket” between garnet and rim1, is represented by microcrystalline aggregates of clinopyroxene, mica, spinel, calcite, and feldspar in different variations. Rims 1, 2, and 3 are typical for garnets of all studied parageneses. Rims 4 and 5 develop on high-Cr subcalcic garnets of the most depleted peridotites. Reactions of the formation of all types of rims are given in the article. Each type of kelyphite demonstrates a clear enrichment with a certain component: Rim1—MgO and alkalis; rim2—TiO2; rim3—FeO and TiO2; rim4—Cr2O3; and rim5—СаО, suggesting the multistage injection of different components by mantle fluid.

Compositional Variations of Spinels from Ultramafic Lamprophyres of the Chadobets Complex (Siberian Craton, Russia)

Ultramafic lamprophyres (UMLs) are mantle rocks that provide important information about the composition of specific carbonate–silicate alkaline melts in the mantle as well as the processes contributing to their origin. Minerals of the spinel group typically occur in UMLs and have a unique “genetic memory.” Investigations of the spinel minerals from the UMLs of the Chadobets complex show the physicochemical and thermodynamic features of the alkaline rocks’ crystallization. The spinels of these UMLs have four stages of crystallization. The first spinel xenocrysts were found only in damtjernite pipes, formed from mantle peridotite, and were captured during the rising of the primary melt to the surface. The next stages of the spinel composition evolution are related to the high-chromium spinel crystallization, which changed to a high-alumina composition. The composition then changed to magnesian ulvöspinel–magnetites with strong decreases in the Al and Cr amounts caused by the release of carbon dioxide, rapid temperature changes, and crystallization of the main primary groundmass minerals such as phlogopite and carbonates. Melt inclusion analyses showed the predominance of aluminosilicate (phlogopite, clinopyroxene, and/or albite) and carbonate (calcite and dolomite) daughter phases in the inclusions that are consistent with the chemical evolution of the Cr-spinel trend. The further evolution of the spinels from magnesian ulvöspinel–magnetite to Ti-magnetite is accompanied by the formation of atoll structures caused by resorption of the spinel minerals.

Hybrid Nature of the Platinum Group Element Chromite-Rich Rocks of the Norilsk 1 Intrusion: Genetic Constraints from Cr Spinel and Spinel-Hosted Multiphase Inclusions

The Norilsk 1 intrusion (Russia), renowned for its abundance of sulfide ores, contains an upper contact zone, which hosts sulfide-poor and Cr spinel and platinum group element (PGE)-rich discontinuous reefs with significant economic potential. Located within strongly inhomogeneous contact rocks of various compositions, the origin of these reefs is complex and debated. Enrichment in PGEs in these rocks is distributed heterogeneously, occasionally occurring in extremely dense disseminations of Cr spinel, which are unusual for other rocks of the Norilsk 1 intrusion. The compositions of Cr spinel vary significantly between individual samples, even within the same samples across clusters of several Cr spinel grains and single grains. Chromium spinel grains are broadly characterized by low Mg# (3–50 mol %), moderate to extremely high TiO2 content (1–18 wt %), diverse Fe2+/Fe3+ ratios, and elevated V and Zn. Multiphase silicate inclusions hosted by Cr spinel are dominated by orthopyroxene, alkali-feldspar, clinopyroxene, Na phlogopite, high-Al amphibole, chlorite, and albite, along with minor felsic glass, sulfide, apatite, baddeleyite, titanite, calcite, halite, and cordierite. Heating experiments (1,250°C) on the silicate inclusions failed to produce homogeneous glasses but showed evidence of partial melting and reactions with precursor minerals that crystallized new phases. The experimentally obtained glasses are characterized by compositions that strongly differ from any known igneous rock in the Norilsk region, and the assemblage of phases in these inclusions is not supportive of the entrapment of a homogeneous silicate melt. Trace element patterns of the glasses of the experimentally heated inclusions are compositionally distinct from the Norilsk trap basalts, and instead are closer to the sedimentary rocks of the Norilsk region. We suggest that an in situ interaction between the mafic melt and the sedimentary rocks was responsible for Cr spinel mineralization and the formation of the host rocks. The subsequent subsolidus modification of the initial rocks expanded the Cr spinel compositional range and formed muscovite-albite-chlorite assemblages, which replaced the original silicate minerals.

Experimental study of formation of chromian priderite and yimengite as products of modal mantle metasomatism

The article presents the data of a series of experiments on the synthesis of exotic titanates (priderite and yimengite) as a result of the reproduction of metasomatic conditions of transformation of mantle minerals (chromite, ilmenite). It was first synthesized chromian priderite not containing Ba. The experiments showed the possibility of crystallization of this mineral as a reaction product of high-chromium spinel and rutile with potassium aqueous carbonate fluid (melt) in the upper mantle. In particular, the obtained experimental data allow us to interpret the relationship between K-Cr-priderite and carbonate-silicate inclusions in chromites of Bohemian garnet peridotites. In the experimental study of the reaction of chromite and ilmenite with aqueous potassium-carbonate fluid (melt) obtained both phases of titanates (priderite and yimengite) - indicator minerals, mantle metasomatism, which directly confirms the possibility of formation of yimengite and K-Cr-priderite and other titanates, the result of mantle metasomatism of upper mantle peridotite under conditions of the highest activity of potassium.

Non-isothermal crystallization behavior of spinel crystals in FeO-SiO2-TiO2-V2O3-Cr2O3 slag

The growth behaviour of spinel crystals in vanadium slag with high Cr2O3 content was investigated and clarified by statistical analyses based on the Crystal Size Distribution (CSD) theory. The results indicate that low cooling rate and Cr2O3 content benefit the growth of spinel crystals. The chromium spinel crystals firstly precipitated and then acted as the heterogeneous nuclei of vanadium and titanium spinel crystals. The growth mechanisms of the spinel crystals at the cooling rate of 5 K/min consist two regimes: firstly, nucleation control in the temperature range of 1873 to 1773 K, in which the shapes of CSD curves are asymptotic; secondly, surface and supply control within the temperature range of 1773 to 1473 K, in which the shapes of CSD curves are lognormal. The mean diameter of spinel crystals increases from 3.97 to 52.21 µm with the decrease of temperature from 1873 to 1473 K.