Polygenetic sources of kimberlites, magma composition, and diamond potential exemplified by the East European and Siberian cratons

The petrological and geochemical characteristics of kimberlites from two Russian provinces of the northern East European craton (EEP) and the Siberian craton (SC) (especially the Yakutian diamondiferous province, YDP), and aphanitic kimberlites from the Jericho pipe (Canada) were compared for the elucidation of some aspects of the genesis of these rocks. The comparison of the EEP and YDP showed that they comprise identical rock associations with some variations in kimberlite composition between particular fields and regions, which are clearly manifested in the TiO₂-K₂O, TiO₂-(Y, Zr, HREE), SiO₂-MgO, SiO₂-Al₂O₃, MgO-Ni, MgO-CO₂, and MgO-H₂O diagrams and in variations in light element ratios (Li/Yb, Be/Nd, and B/Nb). The compositions of YDP kimberlites are confined mainly to quadrant III; i.e., their source was mainly the depleted mantle, whereas the compositions of EEP kimberlites fall within all four quadrants in the fields of both enriched and slightly depleted mantle reservoirs. The initial (¹⁴³Nd/¹⁴⁴Nd) _i ratio of kimberlites from the Yakutian collection is 0.5121–0.5126. The lead isotopic characteristics of the EEP and YDP kimberlites are similar to mantle values: ²⁰⁶Pb/²⁰⁴Pb of 16.19–19.14, ²⁰⁷Pb/²⁰⁴Pb of 15.44–15.61, and ²⁰⁸Pb/²⁰⁴Pb of 34.99–38.55. In the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram, part of the kimberlites, including those from the Botuobiya pipe, fall within the lower part of the field of group I kimberlites from southern Africa near the Pb isotopic composition of the depleted mantle. It was shown that the chemical compositions of the aphanitic kimberlites of the Jericho pipe (supposedly approaching the composition of primary magmas) are similar to those of some individual kimberlite samples from the YDP and EEP. It was supposed that the initial kimberlite melt arrived from the asthenosphere and was enriched in water and other volatile components (especially CO₂). During its ascent to the surface, the melt assimilated mantle components, primarily MgO; as a result, it acquired the compositional characteristics observed in kimberlites. Subsequent compositional modifications were related to diverse factors, including the type of mantle metasomatism, degree of melting, etc. We emphasized the importance of petrological and geochemical criteria (low contents of HREE and Ti in the rocks and a kimberlite source similar to BSE or EMI) for the estimation of the diamond potential of rocks.     

Mesoproterozoic orangeites (Kimberlites II) of West Karelia: Mineralogy,geochemistry, and Sr-Nd isotope composition

Mineralogical and petrological-geochemical features of the Mesoproterozoic (1.23–1.20 Ga) alkaline ultrabasic rocks from the Kostomuksha-Taloveis (Russia) and Lentiira-Kuhmo (Finland) areas, West Karelia, have been studied. In terms of mineralogy and geochemistry, these rocks more resemble group II kimberlites of South Africa (orangeites) than olivine lamproites or ultramafic lamprophyres. On the basis of phenocryst composition, the studied orangeites are divided into three types: Cpx-Phl-Ol, Phl-Ol, and Phl-Carb orangeites. The Cpx-Phl-Ol orangeites from the Kostomuksha cluster clearly differ from analogous rocks from the Lentiira cluster. The composition of Phl-Ol orangeites is indicative of derivation by intense fractional crystallization; Cpx-Phl-Ol orangeites from the Kostomuksha area display evidence of intense lithosphere assimilation. The Phl-Carb orangeites from the Taloveis cluster and Cpx-Ol orangeites from the Lentiira cluster most closely approximate primary melts. The Kostomuksha orangeites are characterized by lowto moderate-radiogenic (⁸⁷Sr/⁸⁶Sr)1220 ratio varying from 0.7038 to 0.7067. The Phl-Carb orangeites of Taloveis have less radiogenic Nd isotope composition (?_Nd from ?11 to ?12) as compared to the Cpx-Phl-Ol and Phl-Ol orangeites of Kostomuksha (?_Nd from ?6.9 to ?9.4). The Cpx-Phl-Ol orangeites from Lentiira contain fresh olivine. By morphology and composition, there are three olivine generations: (1) large rounded, usually zoned crystals with Fo ₉₂ core, 0.33–0.37 wt % NiO, and 0.03–0.04 wt% CaO, which are interpreted as xenocrysts from depleted peridotites; (2) anhedral rounded zoned olivines of intermediate size with Fo _82–83 cores, 0.03–0.05 wt % CaO, 0.12–0.17 wt % NiO, and up to 0.40 wt % MnO. These olivines were entrapped by orangeite melt and presumably represent a cumulate of basaltic melts or were derived from metasomatized peridotites; (3) fine euhedral olivines and xenocryst rims corresponding to Fo _88–89 with 0.10–0.42 wt % CaO, 0.14–0.35 wt % NiO, and up to 0.07–0.21 wt % MnO; their origin was presumably related to the crystallization from kimberlite melt. The calculation of $f_{O_2 }$ of kimberlite melt during crystallization of perovskites using Nb-Fe perovskite oxyba-rometer showed that Cpx-Phl-Ol orangeites of Kostomuksha and orangeites of Lentiira crystallized at similar oxygen fugacities corresponding to ΔNNO from ?3.3 to ?1.1 and from ?3.3 to ?0.9, respectively. The Sm-Nd and Rb-Sr isotope study provided evidence for the contribution from ancient enriched source in the genesis of the orangeites. It was proposed that their mantle source was formed in two stages: (1) metasomatic reworking of previously depleted lithospheric source at the Karelian Craton base during Paleoproterozoic orogenic events 2.1–2.0 Ga ago; (2) extension-related generation of orangeite melts 1.27–1.20 Ga ago.     

Geochemistry of metasomatised spinel peridotite xenoliths from the Dariganga Plateau, South-eastern Mongolia

Summary One fresh (green), one altered (black) and one composite (green/black) peridotite xenolith from the Neogene-Quaternary basalts of the Dariganga Plateau, SE Mongolia, were studied by electron microprobe, X-ray fluorescence, wet chemical and instrumental neutron activation analysis. The history of the upper mantle underneath the Dariganga Plateau has been complex and is characterised by elemental depletions and enrichments processes. The rocks investigated appear to have been processed in several steps, have been moderately depleted (relative to the primitive upper mantle composition) in incompatible elements and subsequently metasomatically enriched in alkalis, Fe, Ca, LREE, Th and U. As a result, most peridotites are moderately depleted in Si, Cr, Ti, HREE and Hf, are slightly enriched in LREE and have elevated Th and U abundances. The minerals in all rocks are out of chemical equilibrium. In the green peridotites disequilibrium is modest but it is severe in the blackened lherzolites. The latter have experienced strong Fe metasomatism accompanied by strong oxidation. As a result, Mg-rich olivines formed by oxidation and precipitation of Fe oxides in the primary olivines (blackening) and Fe-rich olivines formed in the Fe metasomatic event. The latter could only have taken place after the oxidising event, otherwise the Fe-rich olivines would also have been affected by it. Three of the four rocks show negative anomalies (relative to the Ce abundance) of Hf and Ti, one is enriched in these elements, which is considered an indication of the action of carbonatitic melts/fluids in the upper mantle. Enrichment of U over Th in some of our samples seems also to indicate the presence of water in the fluid phase, however, the lack of (OH)-bearing minerals in the Dariganga xenoliths suggests a low activity of water in these fluids. The latest of the metasomatic events probably took place shortly before entrapment of the rocks by the basaltic lava that carried them to the earth’s surface. The composite sample consisting of a green harzburgite and a black lherzolite suggests that blackening took place at the original location of the rock rather than in the basaltic tuff because the latter should have altered the whole xenolith. It also demonstrates that metasomatic processes in the upper mantle can be confined to rather restricted locations with sharp boundaries towards the wall rocks. Blackening as well as the metasomatic events apparently took place because of a better permeability in one part of the rock as compared to the other, probably the result of tectonisation. Received May 28, 1999; revised version accepted February 24, 2001     

Long-term air temperature variability on the Crimean Peninsula

A comparison is made of the calculated values of solar radiation incident on the upper atmospheric boundary with the measured values of surface temperature on the territory of the Crimean Peninsula. It is shown that the long-term temperature regime on the territory of the Crimean Peninsula is characterized by a stability. It is determined that the stability of the long-term regime of mean annual surface air temperatures is associated with the characteristics of the latitudinal distribution of solar radiation incident on the upper atmospheric boundary. The incident solar radiation increases in the regions of heat sources and decreases in the regions of heat sink. Stability of long-term mean annual values of surface air temperature is associated with the location of the Crimea on the boundary of the regions of heat sources and sinks. The study revealed the chronological structure of long-term changes in surface air temperature. The anomaly in the long-term surface air temperature variability is characterized by short-duration variations. An analysis is made of the chronological structure of interannual variability in surface air temperature on the territory of the peninsula. The dominant interannual and 2–3-year periodicities in the temperature regime variations are correlated with variations in incident solar radiation. In 62.7% of cases, the sign of interannual variability in surface air temperature corresponds to the sign of interannual variability in incident solar radiation. Thus it is shown that a small tendency in the long-term surface air temperature variability on the territory of the Crimean Peninsula, and the characteristics of its variations are determined largely by the specific character of the input and distribution of solar radiation incident on the upper atmospheric boundary.     

Conodont-based stratigraphy of the Mosolovian Regional Stage (Eifelian,Middle Devonian) of the Voronezh Anteclise

Conodonts from the deposits of the Mosolovian Regional Stage (Middle Devonian) of the Voronezh Anteclise are studied on the basis of the data from six boreholes. Three assemblages are recognized. The index species of the lowermost assemblage is Icriodus formosus Naz. The index species of the middle assemblage are I. formosus Naz. and Pseudobipennatus ziegleri Kon. et Kim, as well as I. arkonensis Stauff. and I. jejunus Naz. This assemblage is the most diverse and is recognized in all boreholes. The deposits containing this assemblage correspond to the maximum of the Nara transgression. The upper assemblage is also recognized in all the studied boreholes. Its index species is Ps. ziegleri Kon. et Kim. The Mosolovian Regional Stage is assigned to the kockelianus Zone. The distribution diagrams of species in the most diverse middle assemblage support the hypothesis that, in the southeastern regions of the Voronezh anteclise, the depth was greater than in its western part. The characteristic conodont species are shown in the plates.     

Kimberlites of the Daldyn-Alakit region (Yakutia): Spatial distribution of the rocks with different chemical characteristics

New petrogeochemical data on a collection of 138 samples taken from 101 kimberlite bodies of the Alakit region of Yakutia have been interpreted. It was concluded that all studied kimberlites are homogenous in geochemical composition and comparable with Group I kimberlites of South Africa. Based on cluster analysis, kimberlites of the region are subdivided into six clusters. From the first to sixth clusters, kimberlites show a decrease in carbonate material and increase in magnesian component. The spatial distribution of clusters allowed us to distinguish zoned areas with central parts consisting of kimberlites with elevated CaO, CO₂, Rb, Sr, Ba, and lowered contents of SiO₂, TiO₂, Fe₂O₃, FeO, MgO, V, Cr, and Ni. From the center outward, the values of δ_Nd and (⁸⁷Sr/⁸⁶Sr)i decrease, which indicate increasing contribution of the lithospheric source. The formation of magnesian kimberlites at the periphery was related to the intense interaction of protokimberlite melt with lithospheric mantle, which was accompanied by metasomatic reworking of mantle rocks with formation of minerals of megacryst assemblage and assimilation of mantle material. Economically viable diamondiferous kimberlites are confined to the peripheral parts of distinguished zones, i.e., to the kimberlites of 5–6 clusters.     

Effect of temperature on contents of essential highly unsaturated fatty acids in freshwater zooplankton

In 11 lakes from cold and warm regions we studied the content of highly unsaturated fatty acids (HUFA) in seston (<130 μm) and crustacean zooplankton using gas chromatography–mass spectrometry. An increase of temperature correlated with a decrease of HUFA content in zooplankton. A multivariate canonical correlation analyses revealed, that the decrease of HUFA content was related with a decrease of per cent of copepods in zooplankton communities, which are known to have higher HUFA levels in their biomass, than cladocerans. This means that temperature primarily affected the HUFA levels indirectly, via changing of taxonomic structure of zooplankton community, while the homeoviscous adaptation of zooplankton individuals had comparatively lower importance. As found, water temperature was better predictor of HUFA contents of zooplankton, than the fatty acid composition of seston. Thus, it can be predicted, that a probable climate warming will decrease the content of the essential HUFA in freshwater zooplankton with possible negative consequences for animals of higher trophic levels.