Alexander V. Golovin

Melt inclusions in olivine phenocrysts in unaltered kimberlites from the Udachnaya-East pipe, Yakutia: Some aspects of kimberlite magma evolution during late crystallization stages

The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO2, H2O, F, Cl, and S) at lower concentrations of SiO2, MgO, Al2O3, Cr2O3, and TiO2. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.

Carbonate-chloride enrichment in fresh kimberlites of the Udachnaya-East pipe, Siberia: A clue to physical properties of kimberlite magmas?

Kimberlites, the deepest terrestrial magmas and the principal source of diamonds, must have low viscosity and high buoyancy, which govern their exceptionally fast transport from mantle depths to the surface. Appreciation of the rheological properties of kimberlite magmas relies on research into their temperatures and compositions. Understanding of the alkali and volatile element budget is central to these studies, but is hampered by contaminated and altered compositions of kimberlites worldwide. Kimberlites of the diamondiferous Udachnaya-East pipe (Siberia) are exceptionally fresh, with low H2O (<0.5 wt%), but high CO2 (up to 14 wt%), Cl (up to 6 wt%), and alkalies ( up to 6 wt% Na2O and 2.0 wt% K2O). After crystallization of olivine the kimberlite melt evolved towards essentially carbonate-chloride compositions. The groundmass assemblage and compositions of the Udachnaya-East kimberlite resemble modern halogen-rich natrocarbonatite lavas from the Oldoinyo Lengai volcano. Rheological measurements on the Oldoinyo Lengai lavas can be used to constrain properties of the kimberlite magma.

Djerfisherite in the Udachnaya-East pipe kimberlites (Sakha-Yakutia, Russia): paragenesis, composition and origin

Djerfisherite, an unusual potassium- and chlorine-bearing sulphide K6Na(Fe,Ni,Cu)(24)S26Cl, is found in remarkably fresh rocks of the Udachnaya-East kimberlite pipe, including several varieties of kimberlite and a kimberlite-hosted phlogopite-spinel lherzolite xenolith. In both kimberlite breccia and monticellite kimberlite djerfisherite is a common groundmass mineral. Djerfisherite is also present as a daughter phase in olivine-hosted inclusions of trapped carbonate-chloride melt and sulphide melt. The mineral is present as irregular or rounded grains (up to 80-100 mu m) in association with magnetite and pyrrhotite in the kimberlite groundmass, and together with carbonates, Na-K-chlorides, silicates, magnetite, sulphates and Fe-Ni-sulphides in melt inclusions. Djerfisherite in the lherzolite xenolith is mainly interstitial (up to 100 mu m) and commonly rims primary mantle sulphides that show clear signs of replacement. Broad compositional variations in Fe, Ni and Cu are common in djerfisherite from different occurrences of the Udachnaya-East pipe. Textural relations, heating stage experiments with melt inclusions and compositional data, suggest a late magmatic origin of djerfisherite in the Udachnaya-East kimberlite groundmass, at shallow depths and at T <= 800 degrees C. In contrast, djerfisherite in the lherzolite xenolith appears to be a product of direct precipitation from evolved kimberlite magma infiltrating into lithospheric xenoliths or reactions of evolved kimberlite fluids/melts with primary minerals in xenoliths.

Melting of kimberlite of the Udachnaya-East pipe: Experimental study at 3–6.5 GPa and 900–1500°C

200 Kimberlites are the products of crystallization of the deepest magmas generated in the Earth’s mantle (below 150 km) and are the main sources of natural diamonds. In spite of significant progress in under� standing kimberlite petrogenesis, many problems, such as reconstruction of the compositions of primary kimberlite melts and their evolution during intrusion, are still debatable. The main problem is that the com� position of kimberlites does not correspond to the composition of parental melts. First, kimberlites are contaminated by a significant portion of xenogenic material represented by xenoliths and their fragments (xenocrysts). Second, most kimberlites worldwide underwent postmagmatic alterations to various

First finding of burkeite in melt inclusions in olivine from sheared lherzolite xenoliths.

For the first time burkeite was observed as a daughter phase in the melt inclusions in olivine by Raman spectroscopy. The olivine comes from sheared lherzolite xenoliths from the Udachnaya-East kimberlite pipe (Yakutia, Russia). This anhydrous sulfate-carbonate mineral (Na(6)(CO(3))(SO(4))(2)) is generally considered to be a characteristic mineral in saline soils or in continental lacustrine evaporite deposits. Recently, however, this mineral was identified in hydrothermal fluids. Our observations indicate that burkeite can also be formed from a mantle-derived melt.

Eitelite in sheared peridotite xenoliths from Udachnaya-East kimberlite pipe (Russia) – a new locality and host rock type

For the first time eitelite Na 2 Mg(CO 3 ) 2 was observed as a daughter phase in the melt inclusions in olivine from one of the deepest known mantle rocks sampled by kimberlite magma – sheared peridotite xenoliths (190 – 230 km), taken from the Devonian (~370 Ma) Udachnaya-East kimberlite pipe (Siberian craton, Russia). Eitelite was identified by confocal Raman spectroscopy and confirmed by energy-dispersive X-ray spectroscopy. Raman spectra of eitelite in the melt inclusions are characterized by a very strong band at 1105 cm −1 attributed to CO 3 2− symmetric stretching, and weaker bands at 207–208 and 260–263 cm −1 due to lattice vibration. Our findings of eitelite in the melt inclusions entrapped by olivine of mantle xenoliths indicate that this rare carbonate can crystallise from primitive mantle-derived alkaline carbonatite melt.

Geochemical evolution of rocks at the base of the lithospheric mantle: Evidence from study of xenoliths of deformed peridotites from kimberlite of the Udachnaya pipe

This paper presents the results of study of the geochemical and mineralogical compositions of deformed peridotite xenoliths from the Eastern Udachnaya kimberlite pipe. The performed investiga� tions were mainly aimed at obtaining information on the composition and evolution of the lower layers of the lithospheric mantle in the central part of the Sibe� rian Craton. Xenoliths of deformed peridotites are present in kimberlite pipes of all old cratons and are the deepest of all mantle nodules as is evident from the PTcondi� tions of equilibrium. These rocks provide the main information on the composition and evolution of the lower layers of the lithospheric mantle; because of this, their study is of significant interest (1-7). Because of the small sizes and serpentinization (partial or com� plete) of the deformed peridotites, most of the previ� ously performed investigations concerned only the composition of individual rockforming minerals of xenoliths. There is a lack of data on the bulk composi� tion of these rocks at present, and they were obtained for partially serpentinized xenoliths (7-9). Representative collection of xenoliths of deformed peridotites from the eastern body of the Late Devo� nian Udachnaya pipe related to the Daldyn kimberlite field of the Yakutian kimberlite province, which is the largest diamond deposit in Russia, was selected for geochemical study. This body consists of two com� bined kimberlite bodies: Western Udachnaya and Eastern Udachnaya; the eastern body is unique in rela� tion to the amount deepseated rock xenoliths, their diversity, and preservation. Xenoliths are presented by a large set of facial groups and varieties, among which deformed peridotites occupy a significant place (2, 3). Samples of deepseated rocks selected for the study are uniquely fresh. These xenoliths contain no signs of secondary alterations, and the L.O.I. value in the stud� ied rocks is close to zero. The weight of the samples varies from 500 g to several kilograms. Only the central parts of xenoliths, without surfaces in the contact with kimberlite, were taken for analysis of the bulk compo� sition. Most of the xenoliths from this collection are typi� cal lherzolites composed of olivine, orthopyroxene, clinopyroxene, and garnet in various proportions. According to the content of clinopyroxene (2-4 wt %), five samples may be formally attributed to garnet harzburgites. In all samples, large (up to 1 cm) grains of rockforming minerals and the matrix composed of finegranular olivine form porphyroclastic or mosaic� porphyroclastic textures. Olivine in the studied xeno� liths is the prevailing mineral; its content varies from 60 to 85%. The concentration of the forsterite mole� cule in olivines from nodules ranges from 86.4 to 91.3 mol %. Orthopyroxene, as well as olivine, is pre� sented by relatively large grains (porphyroclasts) with a size from 2 to 10 mm and small grains (neoblasts) with a size of <0.5 mm. The amount of orthopyroxene in the studied samples varies from 5 to 18%. The study of the orthopyroxene composition demonstrates that it relates to highmagnesium varieties (En = 88.2- 92.9 mol %), however being more ironrich in compar� ison with orthopyroxenes from granular garnet peridot� ites. Orthopyroxenes contain admixtures of Al 2O3 (0.42- 0.68 wt %), Cr2O3 (0.11-0.52 wt %), and TiO2 (0.21- 0.6 wt %). The clinopyroxene content varies from 2 to 14% of rock volume. This mineral is characterized by a low concentration of the diopside component (Ca/(Ca + Mg) = 35.9-43.3 mol %) that results from high temperatures of the equilibrium of characterized associations. The concentration of garnet in rocks var� ies from 4 to 15%. Garnet grains are practically not deformed, have an isometric shape, and often form linearly elongated chains. The chemical composition of garnets corresponds to the lherzolite paragenesis

Melt evolution during the crystallization of basanites of the Tergesh pipe, northern Minusinsk Depression

In this paper we describe the mineralogy and geochemistry of basanites and melt inclusions in minerals from the Tergesh pipe, northern Minusinsk Depression. The rocks are composed of olivine and clinopyroxene phenocrysts and a groundmass of olivine, clinopyroxene, titanomagnetite, plagioclase, apatite, ilmenite, and glass. Melt inclusions were found only in the olivine and clinopyroxene phenocrysts. Primary melt inclusions in olivine contain glass, rhönite, clinopyroxene, a sulfide globule, and low-density fluid. The phase composition of melt inclusions in clinopyroxene is glass + low-density fluid ± xenogenous magnetite. According to thermometric investigations, the olivine phenocrysts began crystallizing at T = 1280–1320°C and P > 3.5 kbar, whereas groundmass minerals were formed under near-surface conditions at T ≤ 1200°C. The oxygen fugacity gradually changed during basanite crystallization from oxidizing (NNO) to more reducing conditions (QFM). The investigation of glass compositions (heated and unheated inclusions in phenocrysts and groundmass) showed that the evolution of a basanite melt during its crystallization included mainly an increase in SiO2, Al2O3, and alkalis, while a decrease in femic components, and the melt composition moved gradually toward tephriphonolite and trachyandesite. Geochemical evidence suggests that the primary basanite melt was derived from a mantle source affected by differentiation.

Incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2

The incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2 has been first determined in the (3 + 1)-dimensional symmetry group Cmcm(α00)00s with modulation vector q = 0.383a*. Unit-cell values are a = 5.062 (1), b = 8.790 (1), c = 12.744 (1) Å. Three orthorhombic components are related by threefold rotation about [001]. Discontinuous crenel functions are used to describe the occupation modulation of Ca and some CO3 groups. The strong displacive modulation of the O atoms in vertexes of such CO3 groups is described using x-harmonics in crenel intervals. The Na, K atoms occupy mixed sites whose occupation modulation is described in two ways using either complementary harmonic functions or crenels. The nyerereite structure has been compared both with the commensurately modulated structure of K-free Na2Ca(CO3)2 and with the widely known incommensurately modulated structure of γ-Na2CO3.

Tychite in mantle xenoliths from kimberlites: The first find and a new genetic type

Tychite Na6Mg2(CO3)4(SO3) is a rare natural Na and Mg sulfatocarbonate. It is found only as minor mineral in deposits of saline lakes in the United States, Canada, Uganda, and China. In these continental evaporites tychite has sedimentary genesis. In this study, we report the first occurrence of tychite as a crystal phase in the melt inclusions in olivine from mantle xenoliths of the Udachnaya-East kimberlite pipe. This find provides an evidence for the probability of tychite crystallization from melts; i.e., this rare sulfatocarbonate may have a magmatic origin as well.