K. S. Ivanov

Inorganic Geochemistry of the Oil of West Siberia: First ICP-MS Data

The evolution in innovation technologies and analytical devices has raised geochemistry to a higher level. As is known, modern petroleum geochemistry is mainly composed of organic geochemistry. The trace element composition of oils was studied usually in combustion products, whereas works on direct determination of the trace element composition of hydrocarbons are scanty and generally related to a very small number of elements and/or heavy fractions [1]. The rapid progress in the method of mass spectrometry with inductively coupled plasma (ICP-MS) has made it possible to study the trace element composition of not only rocks, but also complex organic compounds, for instance, oil and its derivatives. A series of works has recently been reported on the geochemistry of asphaltenes and bitumens, i.e., solid components of oil [2, 3]. We analyzed for the first time the trace element composition of crude oil from West Siberia by the ICP-MS method. More than 50 trace, rare earth, and other elements were determined in oils from the Shaim and Srednii Ob fields (see [4‐6] for geological data). The analyses were conducted using an ELEMENT2 highresolution mass spectrometer following the technique developed at the Laboratory of Radiogeology (Zavaritskii Institute of Geology and Geochemistry) [7, 8]. The measurement results are shown in the table. Oils from West Siberia are characterized by extremely low contents of the majority of elements. The PM-normalized trace element contents are equal to ~0.1 u for the most depleted ultramafic rocks and ~0.001 for oils. It should be noted that the PM-normalized contents of trace elements in Triassic basalts of The table shows that crude oils of the Shaim and Srednii Ob fields have relatively high contents (>1 g/t) of major (Mg, Al, Fe, Na, Ti) and transition elements (Cr, V, Ni, Cu, Zn). In the Ni‐Cu‐Cr diagram [3] for the resinous‐asphaltene fractions, these oils belong to the chromium type. Their data points fall at the continuation of the trend outlined by the authors of [3] for the West Siberian province. The contents of other trace elements in these crude oils are lower (<1 g/t), but often higher than in some rocks. The Cs, Rb, Sr, and Zr contents in the oils are comparable with those in ultramafic rocks. The U content in oils is similar to that in the basaltoids and significantly higher than that in ultramafic rocks, chondrites, and intermediate rocks [9]. Such a high U content in oils is presumably related to the reducing conditions and, correspondingly, to the presence of a geochemical barrier. These conditions could also promote the high Pb content (up to 0.3 ppm). The Ag content is also relatively high (up to 0.1 ppm), while the Au content is low (<0.002 ppm).

Nature and Age of Metamorphic Rocks from the Basement of the West Siberian Megabasin (according to U-Pb Isotopic Dates)

The SHRIMP-II zircon U-Pb dates for metamorphic rocks from the West Siberian basement are determined for the first time. It is established that the major protolith of the metamorphic strata from the Shaimsk-Kuznetsovsk meganticlinorium is composed of sedimentary Late- and Middle-Devonian rocks (395–398 Ma). It is likely that the greywackes, whose strata were mainly formed under erosion of ophiolitic rocks, served as a substrate for the metamorphic rocks. The metamorphic transformations of the rocks occurred under conditions of greenschist and occasionally lower amphibolite facies of metamorphism during the Late Carboniferous-Early Permian period.

Chemical Microprobe Th-U-Pb Age Dating of Monazite and Uraninite Grains from Granites of the Yamal Crystalline Basement

994 Nowadays, the study of Arctic geology is of partic� ular importance, especially in connection with the oil and gas potential of this territory. Accordingly, study the basement of sedimentary basins in the Arctic is one of the priorities. The Yamal is the main gas province of our country and one of the few regions where the crys� talline basement is exposed for visual study. It is also important that gas condensate inflows at the fields on the Yamal Peninsula (Novoportovskoe, etc.) origi� nated from the complexes of the Paleozoic folded basement. By now, on the Yamal Peninsula the basement rocks have been exposed in about 100 wells drilled. Intrusive complexes were found at the Bovanenk� ovskaya, Novoportovskaya, Verkhnerechenskaya, and

Zircon geochronology of the Klyuchevskoi gabbro-ultramafic massif and the problem of the age of the Mohorovicic paleoboundary in the Central Urals

The Klyuveskoi gabbro-ultramafic massif is the most representative ophiolite complex on the eastern portion of the Uralian paleoisland arc part. The massif is composed of dunite-harzburgite (tectonized mantle peridotites) and dunite-wehrlite-clinopyroxenite-gabbro (layered part of the ophiolite section) rock associations. The U-Pb age was obtained for the accessory zircons from the latter association using a SHRIMP-II ion microprobe at the Center for Isotopic Research at the Karpinskii Russian Geological Research Institute. The euhedral zircon crystals with thin rhythmic zoning from dunites are 441.4 ± 5.0 Ma in age. Zircons from olivine clinopyroxenite show three age clusters with sharply prevalent grains 449.0 ± 6.8 Ma in age. Two points give 1.7 Ga, which is probably related to the age of the mantle generating the layered complex. One value corresponds to 280 Ma, which possibly reflects exhumation of ultramafic rocks in the upper crust during the collision of the Uralian foldbelt. Thus, dunites and olivine pyroxenites from the Klyuchevskoi massif are similar in age at 441–449 Ma. The bottom of the layered part of the ophiolite section corresponds to the M paleoboundary and, consequently, the age of the Mohorovicic discontinuity conforms with the Ordovician-Silurian boundary in this part of the Urals.

The structure of the tectonosphere of the Urals and West Siberian Platform by electromagnetic data

Advances in magnetotelluric sounding (MTS) have provided the possibility to study deep parts of the planet and obtaining information on electroconductivity of the Earth’s tectonosphere, which is very sensitive to the lithology of rocks in the upper part of the crust, as well as to the phase state of substance, the temperature, concentration, and mineralization of fluids. Introduction of modern broadband digital measuring and computation equipment, as well as programs of computational modeling allowed us to realize more completely the possibilities of geoelectric methods. The technical approach including the integration of geoelectric methods, the processing of experimental materials, and interpretation of data with construction of geoelectric models of the medium was considered in detail by the example of geotraverses through the Northern and Southern Urals [1, 2]. The material on the study of fields of natural sources of geomagnetic variations of the ionospheric and magnetospheric origin (AMT‐MTS‐DMTS methods) and the induction electromagnetic sounding (IEMS) method with a controlled source along a traverse about 1000 km long from the Settlement of Askino (Bashkortostan) in the west to Tyukalinsk (Omsk oblast) in the east with an a posteriori test of the previously obtained results [3] was generalized for the first time in this work; the geotraverse partially coincides with the Sverdlovsk profile of deep seismic sounding (DSS). In this case, major faults and zones of higher fracturing in the crust, as well as relationships between electroconductivity and the tectonic structure of regions, were studied. The originality of the studies performed lies in the fact that peculiarities of tectonosphere stratification were traced by electric parameters within a depth from 10 m to 600 km, and traces of events that proceeded there over a large time interval were revealed. The results are presented in Fig. 1. According to the distribution of total longitudinal conductance of the lithosphere ( S ), the territory under consideration can be subdivided into three sectors, the boundaries of which mark zones of abnormally high conductivity, which

The U-Pb SIMS zircon age and geodynamic conditions of formation of granitoides of the Verkhisetsk batholith, the eastern slope of the Middle Urals

The U-Pb SIMS age dating of zircons from different-age granitoid assemblages varying in composition of the Verkhisetsk batholith shows that it comprises rocks of three age groups, formed at different stages of the Ural Mobile Belt. The first age group is represented by quartz diorites (396 ± 5 Ma) with insignificant distribution in area. Their formation was synchronous to island arc volcanism, manifested in the area of study from the second half of the Emsian to late Givetian-early Frasnian. According to this, we could consider these granitoides as comagmatic to island arc volcanites. The second age group includes tonalites and trondhjemites (367 ± 4 Ma), comprising the western part of the batholith. On the basis of similarity between these rocks and granitoides of modern active continental margins in material composition, it is assumed that they formed throughout the island arc-continental stage of development of the Ural Mobile Belt. Granitoides of the third age group, dominating in the Verkhisetsk batholith, formed as a result of several homodrome rhythms of granodiorite-granite intrusions of moderate-potassium composition during a short period of time (315–300 Ma). Their formation is related to the initial stage of the collision stage of development of the region, lasting from the early Bashkirian to Late Permian in the Middle Urals, which is fixed by deposition of flysch and molasse sediments in the Ural Foredeep. The data obtained change our understanding significantly of the character of evolution of granitoid magmatism and the place of rock assemblages studied in the geological history of the Urals.

The First Zircon U-Pb Dating (SHRIMP II) for the Silurian Ophiolites in the Urals

The rocks of the ophiolite association are wide� spread within the Ural mobile belt. For this reason dating of their formation is of great interest due to its tectonic significance, allowing interpretation of the history of geological development of the region. For a long time, the geochronological dating of ophiolite rocks has been limited to the search for faunal remains in interlayers of sedimentary rocks among the volca� nites of the upper part of the ophiolite section. Over the last 20 years, in connection with enhancement of the analytical possibilities, a set of isotope data for rocks of the ophiolite association has been obtained (mainly from the Polar segment of the Urals) [1–5 etc.]. The age data available indicate that the forma� tion of the Uralian ophiolites is connected mainly with two separate time intervals on the geochronological age scale. Older rocks (604–490 Ma) formed during the Late Vendian–Cambrian; the younger rocks (410–370 Ma) formed during the Devonian. Single intermediate dating (Ordovician and Silurian), dated by the Ar–Ar method [6, 7], is not regarded as conclu� sive evidence, so it lies in a very wide time interval (70–90 Ma for the same geological object, dated by the Ar–Ar method). At the same time, according to the present geological evidence, the predominant part of ophiolites on the eastern slope of the Urals formed between the Late Ordovician and the Silurian. Based on the zircon U–Pb age for ophiolite gabbro from the eastern zone of the Middle Urals using the SHRIMP II ion microprobe (Center for Isotope Research, VSEGEI), good evidence of the occurrence of Sil� urian ophiolites in the Urals was obtained for the first time. The Eastern zone of the Urals is known as a zone made of essentially Middle Paleozoic volcanogenic formations and comagmatic intrusive bodies, which extends along the eastern border of the Trans Ural region, eastward of the zone of granite batholiths (the main granite axis of the Urals) [8 etc.]. The ophiolite rocks, which are widespread within the zone, are rep� resented by dunite–harzburgite massifs, layered ultra� basite–gabbro complexes, and parallel dolerite dykes

The first U-Pb age data of zircons from relict spreading zones in the Middle Urals

Zones of paleospreading and crustal extension inthe Urals, as well as around the world, are clearlymarked sheeted dyke complexes. In the Middle Uralssuch a complex was first described by S.N. Ivanov andhis colleagues in 1973 [1] as a relic fragment of theoceanic spreading crust. Later, complexes of paralleldikes were distinguished and described in detail alongthe Urals fold belt, from th e Polar Urals in the north toWestern Mugodzhary Mountains in the south [2–8,etc.]. However, there are still no reliable isotope agedata for these sheeted dyke complexes. As a rule, theage of formation of dikes was determined on the basisof age data of the surrounding rocks [2, 4, etc.] or wereconsidered to be the same as the age of gabbro andbasalts from the ophiolite sections studied.We have made an attempt to obtain isotope age datafor accessory zircons from dolerite dykes of the representative ophiolite complex of Mt. Azov (MiddleUrals) (Fig. 1). Within this complex, fragments of thesheeted dyke complex, extending with breaks over60 km in the eastern framework of the Revda massif,are part of the Uralian Platinum Belt [6, etc.]. Gabbros and pyroxenites of the Revda massif and surrounding basalts were intruded by dolerite dykes. Contacts between dykes and surrounding rocks are oftencomplicated by numerous tectonic faults. The sheeteddyke complex is well exposed among basaltic andandesitebasaltic pillow lavas in the upper part of Mt.Azov (Fig. 1), located 5 km west of the town ofPolevskoi. Parallel dolerite dykes of northeasternstrike and steep southeastern or northwestern dippingare mainly 0.5–2 m thick. Dykes form swarms anddikeindike structures. In total, the volume of dykesexceeds the volume of pillow lavas approximately twofold. As noted in [1, 6, 7], in the contact zone betweendolerites and pillow lavas, hardening zones with athickness of a few centimeters and nonhardened contacts with rugged edges occur. It follows that somedykes first intruded into the hot pillow lava strata; subsequent portions of the magma intruded into thealready cooled strata. The dykes are made of finegrained gabbrodolerites and porphyritic doleriteswith plagioclase and hornblende phenocrysts, as wellas less common aphyric and fine porphyritic varieties.Dolerites were metamorphosed under the conditionsof lower greenschist facies. Microscopically, they usually consist of a amphibole–sossurite aggregate withchlorite, quartz, pumpellyite, and an ore mineral.The dyke complex and pillow lavas are presented bylowK basalts and andesite–basalts of normal alkalinity [7, etc.]. Dolerites are characterized by a lowerTi content (TiO

The first Lu–Hf zircon isotope data for gabbro–diorite–tonalite associations of the Urals

The Lu–Hf isotope systematics of zircon from the gabbro–plagiogranite association (gabbro, diorite, tonalite, and plagiogranite), which is one of the most typical associations of igneous rocks in the Urals, was studied for the first time. The isotope study yielded a unified age limit of 433 Ma, which corresponds to the time of formation of this rock association. The younger “rejuvenated” ages characterize superimposed thermal impact events, induced by the volcanic arc activity, as well as collisional and postcollisional processes. Here, the initial 176Hf/177Hf(t) ratio in the studied zircon from gabbro and plagiogranite corresponds in fact to a highly LILE-depleted (DM) mantle.

The Sr, Nd, and Hf isotopic geochemistry of rocks of the gabbro–diorite–tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

According to isotopic analysis of rocks of the Reft gabbro–diorite–tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in εNd(T) = 8.6–9.7 and εHf(T) = 15.9–17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: εNd(T) = 3.7–6.0, εHf(T) = 11.1–12.7, and TDM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary 87Sr/86Sr ratio in rocks of both groups is highly variable (0.70348–0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro–diorite–tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.