A. L. Ragozin

Merwinite in diamond from São Luiz, Brazil: A new mineral of the Ca-rich mantle environment

Abstract Diamonds from Juina province, Brazil, and some others localities reveal the existence of a deep, Ca-rich carbonate-silicate source different from ultramafic and eclogite compositions. In this study, we describe the first observation of merwinite (Ca2.85Mg0.96Fe0.11Si2.04O8) in a diamond; it occurs as an inclusion in the central growth domain of a diamond from the São Luiz river alluvial deposits (Juina, Brazil). In addition, the diamond contains inclusions of walstromite-structured CaSiO3 in the core and (Mg0.86Fe0.14)2SiO4 olivine in the rim. According to available experimental data, under mantle conditions, merwinite can only be formed in a specific Ca-rich and Mg- and Si-depleted enviroment that differs from any known mantle lithology (peridotitic or eclogitic). We suggest that such chemical conditions can occur during the interaction of subduction-derived calcium carbonatite melt with peridotitic mantle. The partial reduction of the melt could cause the simultaneous crystallization of Ca-rich silicates (CaSiO3 and merwinite) and diamond at an early stage, and (Mg0.86Fe0.14)2SiO4 olivine and diamond at a later stage, after the Ca-Mg exchange between carbonatite melt and peridotite has ceased. This scenario is supported by the presence of calcite microinclusions within merwinite.

Evidence for evolution of diamond crystallization medium in eclogite xenolith from the Udachnaya kimberlite pipe, Yakutia

The recently obtained data on the superimposed character of diamonds in eclogite xenoliths have compelled us to consider the problem of diamond formation in the upper mantle from a new standpoint [1]. In [2], we reported the first results of the study of a unique diamond-bearing xenolith (length18 cm, weight 6900 g) from the Udachnaya kimberlite pipe (Yakutia). The results testify to the superimposed character of diamonds (particularly, the chemistry of minerals) and suggest that diamond crystallized as a product of interaction between the high-K and C-bearing fluid with the ultramafic substrate [2]. The study of diamond-hosted inclusions from xenoliths and the comparison of their chemical composition with minerals of the xenolith groundmass would be of special interest for reconstructing conditions of formation, composition, and evolution of diamond. In this communication, we present results of the study of inclusions along with defects, impurities, and the internal structure of diamond from this xenolith. We selected from the xenolith 44 diamond crystals 1‐5 mm in size. The crystals with crystalline inclusions were polished along axes (110) and (100) until the exposure of inclusions. These inclusions were examined with a Camebax-micro electron microprobe. The internal structure of diamond crystals was investigated with cathode luminescence on a LEO 1430VP scanning electron microscope. The IR absorption spectra were recorded with a Bruker VERTEX 70 IR Fourier spectrometer to study the impurities and to estimate concentration of structural defects in diamonds from this xenolith. The diamonds in xenoliths occur largely in the intergranular space and as inclusions in pyroxene and mica. They have the shape of differently distorted octahedra. Cathode luminescent images distinctly show two zones (core and shell). As a rule, the core is characterized by a weak luminescence, while the shell reveals a more intense luminescence (Fig. 1). The nonuniform zoning and distorted growth layers confirm the previous conclusion on the growth of the xenolith-hosted diamonds in a confined space. According to the results of IR Fourier spectroscopy, diamonds from this xenoliths pertain to the IaA/B type, widespread among natural diamonds; i.e., they contain impurity centers A (a pair of nitrogen atoms that isomorphically replace carbon), B1 (four nitrogen atoms around a vacancy), and in some cases B2 (sheetlike defects oriented parallel to (001)) with linear dimensions varying from a few nanometers to a few micrometers [3]. The total content of nitrogen admixture in the crystals ranges from 40 to 800 ppm. Besides the absorption bands related to the main nitrogen impurity centers, additional lines 3310, 3237, 3150, 3107, 1405,

The mineralogy of Ca-rich inclusions in sublithospheric diamonds

This paper discusses mineralogy of Ca-rich inclusions in ultra-deep (sublithospheric) diamonds. It was shown that most of the Ca-rich majoritic garnets are of metabasic (eclogitic) affinity. The observed variation in major and trace element composition is consistent with variations in the composition of the protolith and the degree of enrichment or depletion during interaction with melts. Major and trace element compositions of the inclusions of Ca minerals in ultra-deep diamonds indicate that they crystallized from Ca-carbonatite melts that were derived from partial melting of eclogite bodies in deeply subducted oceanic crust in the transition zone or even the lower mantle. The occurrence of merwinite or CAS inclusions in ultra-deep diamonds can serve as mineralogical indicators of the interaction of metaperidotitic and metabasic mantle lithologies with alkaline carbonatite melts. The discovery of the inclusions of carbonates in association with ultra-deep Ca minerals can not only provide additional support for their role in the diamond formation process but also help to define additional mantle reservoirs involved in global carbon cycle.

Chemical Heterogeneity in the Diamondiferous Eclogite Xenolith from the Udachnaya Kimberlite Pipe

The first studies of diamond-hosted inclusions in xenoliths have demonstrated that the compositions of inclusions and minerals in xenoliths may differ substantially [1]. It was noted that a higher K content in clinopyroxene as compared with that in the matrix is among the most important differences [2]. The Mir Pipe yielded diamonds with numerous garnet and pyroxene inclusions with highly variable compositions [3, 4]. Some authors explain these compositional variations in inclusions by fractionation from the silicate melt [3].

Design Issues of RASTA SDL–92 Translator

Complete standard-conforming translator for SDL-92 is under development at the Institute for Systems Programming, Moscow. Goals of the project include an intermediate form for SDL-92 as a common layer for various front—end and back-end tools, application programmers interface to the intermediate form, C++ as a target language and, finally, automatic generation of code for SDL abstract data types based on term-rewriting systems. The implementation of the translator is performed using state—of—the—art compiler—compiler technology (Karlsruhe Compiler Toolkit). This paper provides an introduction into the RASTA project and comparison with related work.

Evidence of neoproterosoic continental subduction in the Baikal-Muya fold belt

The results of isotope-geochemical studies of eclogits and host rocks of the North-Muya block are presented. The studies showed broad variations both in the character of distribution of incompatible elements and in the Nd and Sr isotope composition of eclogits from the North-Muya block. The Nd isotope composition of eclogits is characterized by broad variations, which is reflected in the value of ɛNd(T), which has both positive (from +0.3 to +6.9) and negative values (from −0.5 to −16.8). The isotope characteristics for the both samples of eclogits (Mu 12-11, 12-12) with the lowest values of ɛNd(T) clearly indicate protolith contamination by an ancient source of Meso- or Paleoarchean age. Consequently, the melts of the protoliths of the eclogites intruded into the continental crust, and the eclogite-gneiss complex of the North-Muya block may be considered as a paleozone of the continental subduction.

U-Pb age of rutile from the eclogite xenolith of the Udachnaya kimberlite pipe

861 The Udachnaya kimberlite pipe in the Daldyn kimberlite field (Daldyn–Alakit region) of the central part of the Yakutian diamond province is the largest diamond deposit in Russia and one of the largest in the world. The pipe consists of two conjugate bodies: Udachnaya East and Udachnaya West. As is evident from the U–Pb dating of perovskites from kimberlites, the ages of the Udachnaya pipe formation are 367 ± 3 and 367 ± 5 Ma (Udachnaya East); 361 ± 4 and 353 ± 5 Ma (Udachnaya West) [1]. Deep seated xenoliths of mantle rocks in kimber lite pipes are represented by a wide spectrum of ultra basic rocks, as well as pyroxenites and eclogites. Eclogites are coarse granular rocks mostly composed of garnet and clinopyroxene with accessory rutile. In addition to bimineral eclogites, the most abundant among xenoliths of the basic composition, there are corundum and kyanite eclogites [2]. Numerous stud ies of eclogite xenoliths from the Udachnaya kimber lite pipe have provided evidence for the influence of mantle metasomatism on the upper mantle rocks and have distinguished some signs of metasomatic origin of diamonds [3, 4]. However, the age of these processes has been not determined to date. Because of this, we have performed the dating of rutiles from the eclogite xenolith (Sample UD 208 05) entering the composi tion of the secondary mineral association. Eclogite xenolith from the Udachnaya pipe (Sam ple UD 208 05) is bimineral eclogite composed of dark green xenomorphic clinopyroxene (3–6 mm) and round garnet (2–5 mm) grains reaching ~55 and ~44 vol %, respectively. The xenolith contains the areas of partial melting including veins crossing rock forming omphacite and garnet, which consist of amor phous material (silicate glass) and products of its replacement (chlorite and quartz). These areas are surrounded by replaced omphacite with the typical “sponged textures” (symplectites) [3]. Among the accessory minerals are rutile, ilmenite, and pyrrhotite. The chemical composition of minerals was analyzed by energy dispersive spectrometry on a Tescan MYRA 3 LMU electron microscope with an EDS detector (Oxford Instruments) and using a JEOL JXA 8100 X ray microanalyzer at the Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences.

Local variations in carbon isotopes and nitrogen contents in diamonds from placers of the northeastern portion of the Siberian Platform

Variations in the isotopic composition of carbon and the nitrogen contents in diamonds from placers of the northeastern portion of the Siberian Platform have been examined. The results obtained indicate that there is no correlation between local variations in the isotopic composition of carbon and the nitrogen content and the degree of nitrogen aggregation. It has been demonstrated that changes in oxygen fugacity in the course of the growth of diamonds of the V variety do not affect the isotopic composition of carbon. The nature of the variations in the isotopic composition of carbon and the nitrogen contents indicate that there were at least two sources of carbon. The sources of lightweight and weighted compositions of carbon in the diamonds could be organic carbon and carbon in marine carbonates of the subducted crust of the Earth, respectively. Mantle carbon was involved in the diamond formation process at the final stages of the growth.

Micro-Raman spectra of ugrandite garnet

The natural garnets from chromite ores associated with pegmatoid pyroxenites of Sangalyk area (Uchaly ore district, southern Urals, Russia) were studied by means of micro-Raman spectroscopy. The compositions of these garnets were close to ugrandite, an isomorphous intermediate group of uvarovite-grossularite-andradite, X(3)Y(2)(SiO(4))(3), X = Ca(2+), Y = Al(3+), Fe(3+), Cr(3+), according to Raman spectra and X-ray microprobe analyses. An assignment of most of the observed bands in visible and near infrared Raman spectra is reported.

Isotope–Geochemical Evidence for the Nature of Protolite Eclogite of the Kokchetav Massif (Kazakhstan)

In the present paper, the results of our isotope–geochemical studies on eclogites of the ultrahighpressure metamorphic complex of the Kokchetav massif are reported. The fact that the distribution of nonmobile elements in most of the samples was close to that of E-type MORB basalts is shown by using geochemical multielement diagrams normalized to N-MORB. Six samples were found to have a negative anomaly over niobium that may have resulted from contamination with crustal material. For eclogites of the Kokchetav massif, the 147Sm/144Nd ratio was found to range widely from 0.143 to 0.367. The εNd-values calculated for the age of the highly barometric stage of metamorphism (530 million years) varied from–10.3 to +8.1. Eclogites show a dispersion of model ages from 1.95 billion years to 670 million years. On the graphs in the εNd(T)–87Sr/86Sr and εNd(T)–T coordinates, eclogites were shown to form trends that can be interpreted as a result of contamination of the eclogite protolith by the host rocks. Based on the data obtained, it is proposed that the basalts of rift zones that may have geochemical characteristics of N-MORB basalts and at the same time may be contaminated by the continental crust may have served as proxies for eclogite protoliths of the Kokchetav massif.