Search Results (4)

The paper reports the first findings of a series of alkali carbonate, chloride, and sulfate minerals among the usual groundmass kimberlite minerals, such as olivine, phlogopite, monticellite, calcite, spinel-group minerals, perovskite, ilmenite, rutile, and apatite. The sample was collected from an unserpentinized coherent kimberlite dyke that crosscuts earlier volcaniclastic kimberlite in the central part of the Udachnaya-East pipe. This rock can be described as primary/original kimberlite that did not interact with external/internal hydrothermal fluids either during its formation or after its crystallization. At least three alkali-rich carbonates have been found, a previously unknown (and perhaps, a new one) Na-, Ca-, K-, and S-rich carbonate with the calculated empirical formula (Na,K)₆Ca₄(CO₃,SO₄)₇, shortite Na₂Ca₂(CO₃)₃, and nyerereite (Na,K)₂Ca(CO₃)₂. Chlorides in this kimberlite are halite NaCl and sylvite KCl, and the sulfate is aphthitalite K₃Na(SO₄)₂. The content of the Na-Ca-K-S-rich carbonate in the rock is ~15 vol %, that of shortite and halite is ≤5 vol % each, and those of sylvite and aphthitalite are ≤1 vol %. All alkali-rich minerals are of late magmatic origin. This follows from that (i) the studied kimberlite does not contain any secondary water-rich minerals of hydrothermal transformation of the rocks, such as serpentine, chlorite or iowaite; and (ii) crystalline inclusions of such usual kimberlite minerals as olivine, phlogopite, monticellite, calcite, spinel, perovskite, and apatite were found within Na-Ca-K-S-rich carbonate and halite. This publication expands the list of minerals of magmatic origin identified in the groundmass of worldwide kimberlites by at least three minerals: Na-Ca-K-S-rich (new?) carbonate, sylvite, and aphthitalite. It is important to note that all alkali carbonates, chlorides, and sulfates are unstable during secondary hydrothermal alterations of kimberlites, and hence, these minerals cannot be found in serpentinized rocks. Full article

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—TiO_2; rim3—FeO and TiO₂; rim4—Cr₂O₃; and rim5—CaO, suggesting the multistage injection of different components by mantle fluid. Full article

More than forty mineral species of epigenetic origin have been identified in an orthopyroxenite from the Udachnaya-East kimberlite pipe, Daldyn kimberlite field, Siberian platform. Epigenetic phases occur as: (1) Mineral inclusions in the rock-forming enstatite, (2) daughter minerals within large (up to 2 mm) crystallized melt inclusions (CMI) in the rock-forming enstatite, and (3) individual grains and intergrowths in the intergranular space of the xenolith. The studied minerals include silicates (olivine, clinopyroxene, phlogopite, tetraferriphlogopite, amphibole-supergroup minerals, serpentine-group minerals, talc), oxides (several generations of ilmenite and spinel, rutile, perovskite, rare titanates of the crichtonite, magnetoplumbite and hollandite groups), carbonates (calcite, dolomite), sulfides (pentlandite, djerfisherite, pyrrhotite), sulfate (barite), phosphates (apatite and phosphate with a suggested crystal-chemical formula Na₂BaMg[PO₄]₂), oxyhydroxide (goethite), and hydroxyhalides (kuliginite, iowaite). The examined epigenetic minerals are interpreted to have crystallized at different time spans after the formation of the host rock. The genesis of minerals is ascribed to a series of processes metasomatically superimposed onto the orthopyroxenite, i.e., deep-seated mantle metasomatism, infiltration of a kimberlite-related melt and late post-emplacement hydrothermal alterations. The reaction of orthopyroxene with the kimberlite-related melt has led to orthopyroxene dissolution and formation of the CMI, the latter being surrounded by complex reaction zones and containing zoned olivine grains with extremely high-Mg# (up to 99) cores. This report highlights the utility of minerals present in minor volume proportions in deciphering the evolution and modification of mantle fragments sampled by kimberlitic and other deep-sourced magmas. The obtained results further imply that the whole-rock geochemical analyses of mantle-derived samples should be treated with care due to possible drastic contaminations from “hiding” minor phases of epigenetic origin. Full article

The large compositional variations in spinels from extremely depleted, megacrystalline harzburgite–dunites in the Udachnaya-East kimberlite pipe, Yakutia, apparently reflect multistage metasomatism. Changes in the redox regime are reflected in the compositions of different parts of mineral grains. From most reduced to most oxidized, spinel compositions divide into: (1) primary (rock-forming) Cr-spinel and spinel from the central parts of sulfide grains, (2) spinel from microcracks in olivine, (3) spinel in kelyphitic rim around garnet between garnet and olivine (Rim1 and Rim2 spinel), and (4) spinel in transformed kelyphitic rim around garnet between garnet and kimberlite (Rim3 spinel). P-T conditions for the vast majority of samples, calculated using the composition of primary Cr-spinel, fall in the diamond stability field. A change in the composition of spinels of different generations occurs along the sides of the classical triangle of spinel compositions Al–Cr–Fe³⁺: (1) Rim2 to Rim1 spinel—Al–Cr trend, (2) primary Cr-spinel to magnetite rim in the edge—Cr–Fe³⁺ (kimberlite) trend, (3) replacing Rim1 to Rim3—Al–Fe³⁺ trend. Full article