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Vol 31, No 2 (2023)
Articles
Compositions of Kimberlite Melts: A Review of Melt Inclusions in Kimberlite Minerals
Abstract
This work is a comprehensive review of existing data on melt inclusions entrapped in minerals of kimberlite rocks emplaced in different cratonic settings at different times. The crystallized melt inclusions represent snapshots of kimberlite melts at different stages of their evolution. All inclusions are composed of daughter minerals and shrinkage bubbles, but no aqueous fluids and quenched silicate glasses have been so far found. More than 60 mineral species were identified among the daughter phases in the inclusions, however, such diverse phase assemblages are typical of all kimberlites studied to date. Daughter minerals are represented by various Na-K-Ca-, Na-Ca-, Na-Mg-, K-Ca-, Ca-Mg-, Ca-, Mg-, Na-carbonates, Na-Mg- and Na-carbonates with additional anions Сl–, \({\text{SO}}_{4}^{{2 - }},\) \({\text{PO}}_{4}^{{3 - }},\) alkali- sulfates, chlorides, phosphates, sulfides, oxides and silicates. Alkali carbonates, sulfates, and chlorides are usually absent among groundmass minerals the majority of kimberlites in the world, except the Udachnaya-East kimberlite in Siberia. On the other hand, this paragenesis in association with traditional kimberlite minerals, such as olivine, micas, monticellite, spinel group minerals, perovskite, rutile, ilmenite, calcite, and dolomite, is common in the crystallized melt inclusions in all studied kimberlites. Carbonates (~30 to 85 vol. %) always dominate over silicates (only up to 18 vol. %) within inclusions. All inclusions also contain variable (2 to 55 vol. %.) amounts of chlorides (halite and sylvite). When relatively low abundances of carbonate minerals (30–50 vol. %) are observed in the inclusions, chlorides (18–55 vol. %) appear to take over other minerals, including silicates that are traditionally considered as main components of “ultramafic” kimberlite parental melts. The published results on melt inclusions in the kimberlite minerals strongly imply that parental kimberlite melts were generated and further evolved within the Na2O-K2O-CaO-MgO-CO2-Cl system, that is, they were alkali-rich carbonatite/carbonatite-chloride liquids. According to various estimates, the SiO2 content in kimberlite melts at different stages of their evolution could have varied from the first to 19 wt. %. Obviously, during and after of а kimberlite bodies formation, interaction with external waters leads to serpentinization of kimberlite olivine and dissolution of a significant part of kimberlite igneous minerals, such as alkaline carbonates, sulfates, and chlorides. In the traditional approach to studying kimberlites, the role of components such as Na2O, CO2, Cl, and to a lesser extent K2O and S, F in the petrogenesis of kimberlite magmas and rocks have been largely underestimated, while olivine- and serpentine-forming components, such as of SiO2, MgO and H2O are still overestimated in contemporaneous literature.
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Carbonatization of Serpentinites of the Mid-Atlantic Ridge: 1. Geochemical Trends and Mineral Assemblages
Abstract
Abyssal peridotite outcrops compose vast areas of the ocean floor in the Abyssal peridotite outcrops compose vast areas of the ocean floor in the Atlantic, Indian, and Arctic Oceans, where they are an indispensable part of the structure of the oceanic crust section formed in low-velocity oceanic spreading centers. The final stage in the evolution of abyssal peridotites in the oceanic crust is their carbonatization, which they undergo on the surface of the ocean floor or near it. The main goal of this study was reconstruction of the geochemical trends accompanying the carbonatization of abyssal peridotites using the example of MAR ultramafic rocks and to identify the main factors that determine their geochemical and mineralogical differences. It is shown that variations in the composition of rock-forming minerals and their characteristic assemblages indicate that the initial stages of carbonatization of abyssal peridotites occur in intra-crustal conditions simultaneously with the serpentinization of these rocks. The final stage in the crustal evolution of abyssal peridotites is their exposure to the surface of the ocean floor, to which they are transported along the detachment faults. Here, abyssal peridotites in close association with gabbro form oceanic core complexes, and the degree of their carbonatization sharply increases with the duration of their exposure on the surface of the ocean floor. The presented data made it possible to qualitatively reconstruct the sequence of events that determined the mineralogical and geochemical features of carbonatized abyssal peridotites of the MAR.
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Variation in Trace Element and Isotope Composition of Neoarchean Mafic Granulites of the Southwest Siberian Craton: A Consequence of Various Mantle Sources or Crustal Contamination
Abstract
The paper presents geochemical and isotopic characteristics of Neoarchean (2.7–2.66 Ga) mafic granulites of the Sharyzhalgay uplift in the southwestern Siberian craton. Mafic and predominant felsic granulites compose fragments of the metamorphic complex among the Neoarchean and Paleoproterozoic granitoids. Mafic granulites are characterized by the mineral association Cpx + Pl ± Hbl ± Opx ± Qz and include two types with different major and immobile trace element contents. The dominant rocks of the first type have a wide range of Mg# and concetrations of TiO2 and immobile trace elements (REE, Zr, Nb, and most positive εNd(Т) va-lues. The first type of mafic granulites show elevated (La/Sm)n and enrichment in Th and LREE relative Nb which is typical of basalts of subduction origin or crustal contaminated basalts. The absence of negative correlation between (La/Sm)n and εNd(Т) and a clear positive correlation of TiO2 with Nb testify against the effect of crustal contamination on the composition of the mafic granulites. The magmatic protoliths of first type of mafic granulites are suggested to form by the melting of depleted peridotites of the subcontinental mantle which metasomatized by melts formed from basalts or terrigenous sediments of the subducting plate. Mafic granulites of the second type have a narrower range of Mg#, TiO2 content, positive εNd(Т), flat rare earth patterns and no subduction signatures, which indicates an asthenospheric depleted mantle source. Ma-fic granulites contaminated by the Paleoarchean crust are characterized by increased (La/Sm)n, depletion of Nb relative to Th and LREE, and negative εNd(Т) values. Post magmatic influence of granitoids lead to the enrichment of mafic granulites in biotite and apatite, an increased in concentrations of K2O, P2O5, a signi-ficant enrichment of Zr, Nb, Th, LREE, and negative εNd(Т) values. The difference between mafic granulites of the first and second types is not resulted from crustal contamination, but is due to the melting of two types of sources: asthenospheric and subcontinental lithospheric mantle. The subcontinental lithospheric mantle of the Irkut block was isotopically depleted for the Neoarchean time (∼2.7 Ga), and its enrichment in incompatible trace elements, presumably by felsic melts generated from the rocks of subducting plate, immediately preceded mafic magmatism.
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Native Iron in Siberian Traps
Abstract
The results of study of intrusive traps with a large-scale occurrence of native iron allowed us to identify general patterns of their composition and origin. Intrusive bodies are weakly differentiated; they feature a similar structure and mineralogical, petrochemical and geochemical composition. Two associations of rock-forming minerals were found in all the studied bodies, i.e. early deep (pre-chamber) and intra-chamber. Native iron forms nodular segregation, with a subordinate amount of cohenite, troilite and magnetite-wustite. Natural reduced iron can concentrate many elements, such as Ni, Co, Au and PGE. Their content in metal increases by hundreds or even thousands of times compared to the hosting silicate part. The formation of native iron is based on the fluid-magmatic interaction between magma substance and reducing components of the fluid, primarily of methane-hydrogen composition. As a result, dispersion of a primarily homogeneous basalt liquid into silicate and metallic components occurs. In the process of transfer, finely dispersed phases of iron form droplet-liquid segregations with a monomolecular layer of gas on their surface that prevents enlargement of metallic droplets. In the hypabyssal chamber, magma degassing occurs, including degassing from metallic spherules. The processes of droplet fusion and formation of native phase segregations begin.
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