Paragenetic Trends of Oxide Minerals in Carbonate-rich Kimberlites, with New Analyses from the Benfontein Sill, South Africa

The relationship among kimberlites, carbonate-rich bodies associatedwith them, and the carbonatites associated with alkalis rockcomplexes are reviewed. Particular attention is paid to theparageneses of oxide minerals in six carbonate-kimberlites:Peuyuk, Tunraq, Wesselton, Liqhobong, De Beers, and Benfontein.New analyses of spinel, limonite, and perovskite from the lowerBenfontein Sill, are consistent with previous reports and canbe divided into (1) early macrocrysts and cores of grains, and(2) late rims and groundmass grains. The evolution of a carbonate-richresiduum with progressive crystallization appears to be typicalof carbonate-rich kimberlite magmas, and is texturally relatedto the two stages of oxide precipitation in these carbonate-kimberlites.Thus, early Mg-ilmenite and Cr-rich spinel are separated byreaction textures and carbonate from later Mg-Al-titanomagnetite,perovskite, and accessory utile and apatite. The spinels spana large range in composition from Mg-Al-chromite to Mg-Al-titanomagnetite,with an intermediate gap. This simplified paragenetic scheme,and in particular the spinel trend, is repeated in the fiveother carbonate-kimberlites reviewed. It may be representativeof the hypabyssal kimberlites in general, and others where fluidizationprocesses did not completely disrupt the crystallization sequence.     

Ilmenite as a recorder of kimberlite history from mantle to surface: examples from Indian kimberlites

Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvöspinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM).

     

Groundmass oxide minerals in the Koidu kimberlite dikes,Sierra Leone,West Africa

Petrology and oxide mineral chemistry are presented on 5 kimberlite dikes that are classified into three groups: (1) one dike is highly carbonated and highly oxidized (> MH) and is characterised by chlorite+Mn-titanomagnetite+rutile+hematite (after chlorite)+maghemite (after titanomagnetite), with ilmenite and perovskite being absent; (2) three dikes are typified by atoll-textured spinels +phlogopite+euhedral Mn-picroilmenite, of intermediate oxidation state (WM-FMQ) with coexisting deuterically serpentinized olivine+Ni-Fe alloys and magnetite; (3) the remaining dike records an early crystallization event under very low ( WM) redox conditions that precipitated anionic-deficient spinels and a Mg-Ti-Cr-wüstite-type phase, followed by late stage more oxidizing (=FMQ) Mnpicroilmenite.Spinels are complexly zoned and crystallization trends among the dikes are diverse, underscoring the fact that no single compositional trend, or evolutionary sequence is typical of kimberlites. Ilmenites are euhedral, and criteria for groundmass crystallization are established. Extraordinarily high MnO (max 17 wt%) contents and high geikielite (62 mole%) concentrations expand the ilmenite field typically assigned to that of kimberlites. Zirconium, Nb and Cr are present in concentrations of 0.5–3 wt% (as oxides) in ilmenite. These highly incompatible elements, along with Mn, are concentrated in late stage melt fractions. The high pyrophanite contents, which are more typical of silicic alkali suites, are accompanied by phlogopite in the Koidu dikes.Objective evaluations of kimberlite-carbonatite relations, as outlined in the literature, cannot be made based on the oxide mineral group. Much of the compositional data for oxides in kimberlites are on mantle-derived xenolith suites and are not from oxides derived from the crystallization of kimberlitic melts.Assessments of the fO₂'s of kimberlites have considerable potential in evaluating diamond survival through redox reactions. Manganese-rich (+Nb, Zr, Cr) ilmenites are typical of many kimberlites and should be considered in the suite of index minerals employed in prospecting.     

The alleged kimberlite-carbonatite relationship: evidence from ilmenite and spinel from Premier and Wesselton mines and the Benfontein sill,South Africa

Carbonate-rich, SiO₂-poor residua are developed in some kimberlites solidifying as ocelli, layers, or discrete dikes which satisfy petrographic definitions of carbonatite. Arguments that these rocks have mineralogies, antecedents, and comagmatic rocks differing from those of the carbonatites in alkaline rock complexes, including the specific observation that kimberlites and carbonatites contain ilmenites and spinels of different composition, have been used to refute the alleged kimberlite-carbonatite relationship. New microprobe analyses of ilmenites and spinels from carbonate-rich rocks associated with kimberlites in three South African localities correspond to spinels and ilmenites of carbonatites from alkalic complexes, or have characteristics intermediate between those of carbonatites and kimberlites. The ilmenites are distinguished from kimberlite ilmenites by higher MnO, FeTiO₃, and Nb₂O₅, and by negligible Cr₂O₃. The spinels are distinguished from kimberlite spinels by their Al₂O₃ and Cr₂O₃ contents. There is clearly a genetic relationship between the kimberlites and the carbonate-rich rocks, despite the observation that their ilmenites and spinels are distinctly different, which indicates that the same observation is not a valid argument against a petrogenetic relationship between kimberlites and carbonatites. These rocks are among the diverse products from mantle processes influenced by CO₂, and we believe that the petrogenetic links among them are forged in the upper mantle. We see insufficient justification to deny the name carbonatite to carbonate-rich rocks associated with kimberlites if they satisfy the petrographic definition in terms of major mineralogy.     

Highly evolved hypabyssal kimberlite sills from Wemindji,Quebec, Canada: insights into the process of flow differentiation in kimberlite magmas

Kimberlite sills emplaced in granite located near the town of Wemindji (Quebec, Canada) range from 2 cm to 1.2 m in thickness. The sills exhibit a wide variation in macroscopic appearance from fine-grained aphanitic dolomitic hypabyssal kimberlite to ilmenite/garnet macrocrystal hypabyssal kimberlite. Diatreme or crater facies rocks are not present. Multiple intrusions are present within the sills, and graded bedding and erosional features such as cross-bedding are common. The sills exhibit a wide range in their modal mineralogy with respect to the abundances of spinel, apatite, phlogopite and dolomite. Olivine is the dominant macrocryst, with an average composition of Fo₉₀. Garnet macrocrysts are low chrome (2–3 wt. %) pyrope (G1/G9 garnet). Ilmenite occurs as rounded macrocrysts (7–13 wt. % MgO). Phlogopite microphenocrysts are Ti-poor and represent a solid solution between phlogopite and kinoshitalite end members. Spinel compositions mainly represent the Cr-poor members of the qandilite–ulvöspinel–magnetite series. The principle carbonate comprising the groundmass is dolomite, with lesser later-forming calcite. Accessory minerals include apatite, Sr-rich calcite, Nb-rich rutile, baddeleyite, monazite-(Ce) and barite. While some of these accessory minerals are atypical of kimberlites in general, it is expected that differentiation products of an evolved carbonate-rich kimberlite magma will crystallize these phases. The Wemindji kimberlites offer insight into the process of crystal fractionation and differentiation in evolved kimberlite magmas. The macroscopic textural features observed in the Wemindji sills are interpreted to represent flow differentiation of a mantle-derived, very fluid, low viscosity carbonate-rich kimberlite. The diverse modes and textural features result entirely from flow differentiation and multiple intrusions of different batches of genetically related kimberlite magma. The mineralogy of the Wemindji kimberlites has some similarities to that of the Wesselton and Benfontein calcite kimberlite sills but differs in detail with respect to dominant carbonate (i.e. dolomite versus calcite), and the character of the rare earth-bearing accessory minerals (i.e. monazite-(Ce) versus rare earth fluorocarbonates).     

Geochemical features of ilmenites from the alkaline complexes of the Ukrainian Shield: LA-ICP MS data

LA-ICP MS data are presented for ilmenites from different rocks of the alkaline complexes of the Ukrainian Shield (Chernigovka carbonatite, Oktyabr’skii, Malaya Tersa, and Southern Kal’chinskii gabbrosyenite massifs). Ilmenites from the early intrusive phases (alkaline pyroxenites, gabbroids, and ultramafic rocks) have the elevated contents of Cr, Co, Ni, and V, while ilmenites from later alkaline and nepheline syenites, monzonites, and carbonatites are significantly enriched in Nb and Ta, which is caused by change in the alkalinity of the mineral-forming medium. Zr shows the more intrinsic behavior: its content is higher in the ilmenites from basic and ultrabasic rocks than in those from the nepheline syenites and carbonatites. This is mainly caused by temperature conditions of the formation of differentiated alkaline complexes. The carbonatites contain magnesian ilmenite (up to 22 mol % MgTiO₃). Variations of Mg contents in ilmentes are correlated with Mg number of mafic minerals and depend also on the iron oxidation state (amount of magnetite) in the carbonatites. In the alkaline massifs of the Ukrainian Shield, ilmenites usually have the low contents of hematite end member (3–7 mol %). Ilmenite serves as a sensitive indicator of temperature, oxygen fugacity, and alkalinity of the mineral-forming medium during crystallization.     

Carbonate- and silicate-rich globules in the kimberlitic rocks of northwestern Tarim large igneous province,NW China: Evidence for carbonated mantle source

We report carbonate- and silicate-rich globules and andradite from the Wajilitage kimberlitic rocks in the northwestern Tarim large igneous province, NW China. The carbonate-rich globules vary in size from 1 to 3 mm, and most have ellipsoidal or round shape, and are composed of nearly pure calcite. The silicate-rich globules are elliptical to round in shape and are typically larger than the carbonate-rich globules ranging from 2 to several centimeters in diameter. They are characterized by clear reaction rims and contain several silicate minerals such as garnet, diopside and phlogopite. The silicate-rich globules, reported here for the first time, are suggested to be related to the origin of andradite within the kimberlitic rocks. Our results show that calcite in the carbonate-rich globules has a high X_Ca (>0.97) and is characterized by extremely high concentrations of the total rare earth elements (up to 1500 ppm), enrichment in Sr (8521–10,645 ppm) and LREE, and remarkable depletion in Nd, Ta, Zr, Hf and Ti. The calcite in the silicate-rich globules is geochemically similar to those in the carbonate-rich globules except the lower trace element contents. Garnet is dominantly andradite (And_{59.56–92.32}Grs_5.67–36.03Pyr_0.36–4.61Spe_0–0.33) and is enriched in light rare earth elements (LREEs) and relatively depleted in Rb, Ba, Th, Pb, Sr, Zr and Hf. Phlogopite in the silicate-rich globules has a high Mg^# ranging from 0.93 to 0.97. The composition of the diopside is Wo_{45.82–51.39}En_{39.81–49.09}Fs_0.88–0.95 with a high Mg^# ranging from 0.88 to 0.95. Diopside in the silicate-rich globules has low total rare earth element (REE) contents (14–31 ppm) and shows middle REE- (Eu to Gd), slight light REE- and heavy REE-enrichment with elevated Zr, Hf and Sr contents and a negative Nb anomaly in the normalized diagram. The matrix of the kimberlitic rocks are silica undersaturated (27.92–29.31 wt.% SiO₂) with low Al₂O₃ (4.51–5.15 wt.%) and high CaO (17.29–17.77 wt.%) contents. The samples are characterized by incompatible element enrichment with high (La/Yb)_N values (41–58) and remarkable negative anomalies in HFSEs (e.g. Ta, Zr, Hf). Our new data suggest that the carbonate-rich globule most likely crystallized at high-temperature and does not represent immiscible liquids, whereas the silicate-rich globules are related to carbonate-rich deuteric hydrothermal fluids during the later-stage of melt evolution. The fluids reacted with the surrounding silicate melts resulting in the formation of skarn minerals such as phlogopite, diopside and andradite. The presence of the carbonate-bearing globules indicates that the Wajilitage kimberlitic rocks are carbonate-rich and most likely derived from an enriched mantle with abundant carbonate. We correlate the carbonated mantle to metasomatism by the migration of deep-seated fluids (carbonate-rich) in response to the impingement of the early Permian mantle plume.     

Macrocryst Fe - Ti oxides in olivine melilitites from Namaqualand-Bushmanland South Africa

Ilmenite macrocrysts in olivine melilitites from Namaqualand-Bushmanland, South Africa, have decomposed by subsolidus reduction to form oriented Mg-titanomagnetite along {0001} ilmenite planes. Residual ilmenite contains 10–11 wt% MgO, 1 wt% MnO, and 0.1 wt% Cr₂O₃. This macrocryst assemblage is mantled by an annulus of Mg-titanomagnetite, followed by an overgrowth of radiating magnetite + perovskite. Terminal compositions of these magnetites are similar to groundmass spinels, and to the outermost margins of magnetite macrocrysts that have very high Fe³⁺ core contents. The assemblages are remarkably similar to oxide intergrowths in kimberlites and an upper mantle derivation is proposed for ilmenite macrocrysts in these melilitites. Oxidation states in the source regions are also very similar, whether on-or off-craton, being slightly above FMQ (NNO), but reduced to FMQWM with the onset of decompression, volatile loss, and carbonate immiscibility. In the case of the melilitites, late stage, low pressure crystallization above NNO precipated abundant magnetite + perovskite. The oxide fO₂ data are consistent with, and refine the fO₂ estimates obtained previously for the behavior of Fe/Mg and Ni contents in olivine from the same suite of samples.     

我国金伯利岩型金刚石矿床的若干地质特征及其找矿标志

金伯利岩型金刚石矿床是世界上重要的金刚石矿床类型:也是我国迄今所发现的唯一的金刚石原生矿床类型。本文阐述了我国金伯利岩分布的构造部位、时代、产状、与暗色岩类的关系、围岩及其蚀变特征、岩管的岩相特征、岩石类型及岩石化学特征,并指出了该类矿床的找矿标志。     

钙钛矿族矿物的晶体化学分类和地球化学演化

钙钛矿族矿物是硅不饱和岩石 (如金伯利岩、黄长岩、似长岩、辉闪苦橄岩、单斜辉石岩和碳酸盐岩等 )中普遍存在的副矿物。它们的一般化学式为ABX3 ,其中A代表Na ,Ca ,Sr等 ,B代表Ti,Nb ,Fe3 +,Th等 ,X代表O ,F。钙钛矿结构是理想的等轴钙钛矿结构发生畸变而衍生出的正交和四方结构。自然界中产出的钙钛矿族矿物的化学成分可以用 7端员化合物来表示(即“钙钛矿空间”) ,但大部分落在斜方钠铌矿钠铈钛矿锶钛矿钙钛矿四元体系内 ,并明显受地质或热力学条件的控制。钙钛矿族矿物的一般成分演化趋势为 :狭义的钙钛矿→钠铈钛矿→铈铌钙钛矿→钙铌、铌钙和铌钠铈钛矿→ (等轴或斜方 )钠铌矿。此外 ,钙钛矿族矿物是碱性岩中主要的稀土载体矿物之一 ,它们表现出强烈的轻稀土富集。虽然在许多岩浆组合中钙钛矿是液相线上的稳定相 ,但在亚固相再平衡过程、交代作用和次生蚀变作用中 ,会被其它含钛矿物(如榍石、锐钛矿等 )所交代 ,形成特殊的组合结构     

Diamond potential versus oxygen regime of carbonatites

Physicochemical conditions of graphite and diamond formation in the carbonate-rich melts were estimated. A large body of analytical data was obtained for compositions of coexisting minerals in the studied objects (Chernigovka Massif, Ukraine, and Chagatai carbonatite complex, Uzbekistan). The carbon isotopic composition of the coexisting carbonates and graphite from these carbonatites was analyzed. New thermodynamic methods were proposed to estimate the oxygen potential in graphite- and diamond-bearing carbonatites. Oxygen fugacity in the graphite-bearing carbonatites is slightly below the quartz-fayalite-magnetite buffer. It was proved that diamond is generated in the course of reduction of carbonate components arriving from plume material into the lower subcontinental lithosphere rather than owing to partial oxidation of methane fluids. As follows from the study of olivine and nominally anhydrous minerals in kimberlites, the limited role of methane in deep mantle is determined by low water activity. Methane is generated in mantle under special conditions such as extremely low oxygen fugacity (for instance, at the base of continental lithosphere) and elevated water activity. These conditions may occur during crystallization differentiation in deep-level chambers of kimberlite and proto-kimberlite magmas.     

中国金伯利岩中的钛铁矿

本文研究了金伯利岩中,作为巨晶和粗晶,基质相矿物,与金云母、镁铝榴石、铬尖晶石等矿物的连生体,金刚石中包裹体矿物及金伯利岩地幔岩包裹体矿物产出的钛铁矿的大小,形态、皮壳及化学成分、端元组分、环带及成分变异趋势。并与其他岩类中的钛铁矿作了对比。探讨了不同产状、共生组合类型的钛铁矿的成因。指出了与金刚石紧密伴生的钛铁矿的标型特征及找矿意义。     

Mechanisms of retrograde changes in oxide minerals from the Proterozoic Serrote da Laje deposit,northeastern Brazil

Retrograde textural and chemical changes in oxide minerals from the Proterozoic Serrote da Laje deposit, northeastern Brazil, have been investigated. The deposit is situated in a mafic-ultramafic layered sill. Oxidation and cooling leading to successively decreasing diffusion rates resulted in disequilibrium on the microscale. Pleonaste in particular shows a rapid change in composition between (a) coarse grains in a granoblastic magnetite host, indicating metamorphic peak conditions, (b) coarse lamellae in magnetite, indicating commencement of exsolution, and (c) composite pleonaste — ilmenite lamellae in magnetite, which indicate oxidation exsolution. Barren rock layers cooled under more oxidized conditions compared with oxide-rich layers. Formation of pleonaste- and ilmenite lamellae in magnetite and ilmenite — hematite relations are discussed.     

Origin of complex zoning in olivine from diverse,diamondiferous kimberlites and tectonic settings: Ekati (Canada), Alto Paranaiba (Brazil) and Kaalvallei (South Africa)

Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg# = 78–95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism.

     

Trace element partitioning between carbonatitic melts and mantle transition zone minerals: Implications for the source of carbonatites

The occurrence of CO₂-rich lavas (carbonatites, kimberlites) and carbonate-rich xenoliths provide evidence for the existence of carbonatitic melts in the mantle. To model the chemical composition of such melts in the deep mantle, we experimentally determined partition coefficients for 23 trace elements (including REE, U-Th, HFSE, LILE) between deep mantle minerals and carbonatite liquids at 20 and 25 GPa and 1600 °C. Under these conditions, majoritic garnet and CaSiO₃ perovskite are the main reservoirs for trace elements. This study used both femtosecond LA-ICP-MS and SIMS techniques to measure reliable trace element concentrations. Comparison of the two techniques shows a general agreement, except for Sc and Ba. Our experimentally determined partition coefficients are consistent with the lattice strain model. The data suggest an effect of melt structure on partition coefficients in this pressure range. For instance, strain-free partition coefficient (D₀) for majorite-carbonatite melts do not follow the order of cation valence, , observed for majorite-CO₂-free silicate melts. The newly determined partition coefficients were combined with trace element composition of majoritic garnets found as inclusions in diamond to model trace element patterns of deep-seated carbonatites. The result compares favorably with natural carbonatites. This suggests that carbonatites can originate from the mantle transition zone.     

Melt inclusions from the deep Slave lithosphere: implications for the origin and evolution of mantle-derived carbonatite and kimberlite

Melt inclusions in clinopyroxenes from lherzolitic xenoliths from the deep lithospheric mantle beneath the Slave Craton (Lac de Gras area, Canada) reveal multiple origins for carbonatitic melts. One type of inclusions consists of a series of silicate–carbonate–silicate concentric layers, interpreted to have unmixed under disequilibrium conditions during rapid ascent to the surface. Bulk major- and trace-element compositions are typical of Group 1 kimberlites and quantitative nuclear microprobe imaging of the globules reveals fractionation of related elements (e.g. F–Br, Nb–Ta) between the silicate and carbonate components. The globules probably formed by partial melting of carbonated peridotite, consistent with results of melting experiments and some models for the generation of kimberlite magmas. They provide evidence for a genetic relationship between some carbonate-rich magmas and ultramafic silicate magmas, and for the possibility of unmixing processes of these melts during their evolution.

The second inclusion type comprises carbonate-rich globules interpreted as samples of Mg-carbonatite melt that quenched on ascent to the surface. Bulk major- and trace-element compositions indicate that the melts were derived from a carbonate-rich source and oxygen, carbon, and strontium isotope data are consistent with the involvement of recycled crustal material and suggest that some mantle-derived carbonatites are unrelated to kimberlites.     

Petrology of the Basistoppen Sill, East Greenland: A Calculated Magma Differentiation Trend

The 660 m thick Basistoppen sill is an Eocene, tholeiitic, layeredintrusion emplaced in the upper part of the Skaergaard complexshortly after solidification of the Skaergaard magma. Despiteits small size, the Basistoppen sill has one of the most extensivedifferentiation sequences known. The ranges of the solid solutionsin olivine, plagioclase, and pyroxene from the Basistoppen arecomparable to those in the Skaergaard and Bushveld intrusions.The rocks of the sill are orthocumulates composed of approximately35% trapped liquid and 65% cumulus minerals and can be dividedinto zones based on changes in the cumulus mineral assemblage.From the base upward those zones are: a Gabbro Picrite Zonecontaining cumulus olivine, Fe-Cr spinel, and minor biotite;a Bronzite Gabbro Zone containing cumulus orthopyroxene, Ca-richclinopyroxene, plagioclase, and minor Fe-Cr spinel; a PigeoniteGabbro Zone containing cumulus plagioclase, Ca-rich clinopyroxene,pigeonite, magnetite, and minor ilmenite; and a Fayalite DioriteZone containing cumulus plagioclase, Ca-rich clinopyroxene,magnetite, ilmenite, apatite, and olivine. The Basistoppen isoverlain by a zoned granophyre sill that was most likely derivedin part from the Basistoppen magma and in part from melted Precambriangneiss. The excellent exposure, uncomplicated structure, goodchilled margin, and lack of strong modal layering facilitatethe calculation of a differentiation trend for the Basistoppensill. During crystallization the Basistoppen magma became progressivelyricher in Fe, P, Na, K, Zn, Rb, Zr, La, Sm, and Th, became progressivelypoorer in Mg, Ca, Al, Cr, and Ni, and remained relatively unchangedin Si, Sc, and Sr through at least the first 90% of crystallization.     

Opaque Minerals in LL3.0-6 Chondrites I: Mineralogy of Ti-oxides and ~(53)Mn-~(53)Cr Systematics of Ilmenite

We conducted a systematic study of oxide minerals in LL3.0-6 chondrites, and found ilmenite, rutile, perovskite and an unknown Al-Ti-Zr-oxide. Ilmenite is low in abundance, but is present in the chondrules and matrix of all the samples that we studied. The MnO content of ilmenite in LL3.0-3.3 is lower than that in LL3.5-6. The low concentration of MnO in the former is due to crystallization from chondrules melts at high temperatures. On the other hand, ilmenite composition in LL3.5-6 reflects thermal metamorphism. Therefore, ilmenite is indicative of petrologic type. We also made the first measurements of the 53Mn-53Cr systematics of ilmenite in ordinary chondrites. The age for ilmenite in Y790256 (LL6) is determined to be about 2 Ma older than angrites. This may represent the metamorphic age of the LL chondrites.     

Geochemistry of radioactive elements in the rocks of the Guli Massif,Polar Siberia

The study of radioactive element distribution in the rocks of the Guli Complex revealed an increase of uranium and thorium contents in the final products of magmatic differentiation. In the carbonatite complex, the radioactive elements are mainly accumulated in the early rocks—phoscorites, while their contents in the late phases, dolomitic carbonatites, decrease. The Th/U ratio increases from near-chondritic values in the weakly differentiated highly-magnesian primary magmas to the late rocks—phoscorites, calcitic carbonatites, and dolomitic carbonatites. The majority of radioactive elements are hosted in rare-metal accessory minerals: perovskite, pyrochlore, calzirtite, and apatite. Rock-forming minerals are characterized by extremely low contents of radioactive elements.     

Superdeep diamonds from the Juina area, Mato Grosso State, Brazil

Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO₂. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr₂O₃<36.5 wt%, and TiO₂>10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87-89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)₂Si₂O₄. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition ('¹³C=-7.8 to -2.5‰), except for the one majorite-bearing diamond ('¹³C=-11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.