Phenanthrene biomarkers in the organic matter of Precambrian and Phanerozoic deposits and in the oils of the Siberian Platform

The composition and distribution of phenanthrenes (polyaromatic compounds) have been studied in chloroform extracts from dispersed organic matter (OM) of clayey, siliceous, carbonate, and terrigenous rocks of different ages and facies and from some oils of the Siberian Platform. Phenanthrenes have been analyzed by gas chromatography-mass spectrometry. High contents of 1,7,8-trimethylphenanthrene and 1,1,7,8-tetramethyl-1,2,3,4-tetrahydrophenanthrene are present in the OM of Vendian and Cambrian carbonate-shale deposits and in ancient oils of the Nepa-Botuobiya and Anabar anteclises. The OM of Permian continental deposits and oils of the Vilyui syneclise is dominated by 1-methyl-7-isopropylphenanthrene (retene). A triangular diagram for identification of the types of original OM of rocks and classification of genetically related oils has been constructed based on the assessment of phenanthrene biomarker distribution. Putative pathways of the formation of phenanthrene biomarkers are discussed.     

Petrology and Rare Earth Elements Mineral Chemistry of Chadegan Metabasites (Sanandaj-Sirjan Zone,Iran): Evidence for Eclogite-Facies Metamorphism during Neotethyan Subduction

The metabasites of Chadegan, including eclogite, garnet amphibolite and amphibolite, are forming a part of Sanandaj–Sirjan Zone. These rocks have formed during the subduction of the Neo–Tethys ocean crust under Iranian plate. This subduction resulted in a subduction metamorphism under high pressuremedium temperature of eclogite and amphibolites facies condition. Then the metamorphic rocks were exhumed during the continental collision between the Afro–Arabian continent and the Iranian microcontinent. In the metabasite rocks, with typical MORB composition, garnet preserved a compositional zoning occurred during metamorphism. The magnesium (X_Mg) gradually increases from core to rim of garnets, while the manganese (X_Mn) decreases towards the rim. Chondrite–normalized Rare Earth Element patterns for these garnets exhibit core–to–rim increases in Light Rare Earth Elements. The chondrite–normalized REE patterns of garnets, amphiboles and pyroxenes display positive trend from LREEs to Heavy Rare Earth Elements (especially in garnet), which suggests the role of these minerals as the major controller of HREE distribution. The geochemical features show that the studied eclogite and associated rocks have a MORB origin, and probably formed in a deep–seated subduction channel environment. The geothermometry estimation yields average pressure of ~22 kbar and temperature of 470–520°C for eclogite fomation. The thermobarometry results gave T = 650–700°C and P ≈ 10–11 kbar for amphibolite facies.     

Some Problems of Clastogenic Mica Transformation during Anchimetamorphism (Metagenesis)

The transformation of clastogenic muscovite and biotite from the Tyumen superdeep well was studied with the SEM and microprobe techniques. It was shown that under conditions of anchimetamorphism (metagenesis), the white mica becomes unstable and may be replaced by several newly formed minerals. In particular, the brammallite, a sodic analogue of illite, is formed at the expense of sodic component of muscovite. The chloritization of mica provokes the removal of excess silica, which is present in the anchizone as a readily soluble in water modification rather than quartz. The release of excess silica as a result of clastogenic mica destruction may be a source of rock silification. As follows from the review of experimental data on silica solubility, the replacement of many silicate minerals leads to the formation of excess silica. This process can be highly extensive during the breakdown of clastogenic minerals.     

Kerogen from the Cambrian deposits of the Kuonamka Formation (northeastern Siberian Platform)

Kerogens from the Lower and Middle Cambrian deposits of the Kuonamka Formation in the northeastern Siberian Platform have been analyzed by modern methods. We have determined the pyrolytic characteristics of insoluble organic matter and the contents of C, H, S, N, O, and the stable C isotope. The type and catagenesis of organic matter have been estimated, as well as the generating potential of oil source rocks. It has been found that the composition of kerogens taken from the Molodo River outcrops was affected by supergene processes and that the degree of their alteration is related to the organic content of rocks and their structure.     

Relative contributions of crust and mantle to generation of Oligo-Miocene medium-K calc-alkaline I-type granitoids in a subduction setting—a case study from the Nabar Pluton,central Iran

The Nabar pluton with the age of Oligo-Miocene located northwest of Isfahan, the Urumieh-Dokhtar magmatic belt, is composed of gabbro, gabbro diorite, diorite, quartz diorite, tonalite, and quartz monzonite. These rocks contain plagioclase, quartz, alkali-feldspar, magnesiohornblende, actinolite, tremolite-hornblende, actinolite-hornblende, anthophyllite, biotite, and Na-poor pyroxene. Application of the Al-in-hornblende barometry indicates pressures of 2–2.15 kbar, whereas the clinopyroxene barometry shows a pressure of 5 kbar. The temperature (i.e., 750–800°C) is estimated using the amphibole-clinopyroxene thermometry in a dioritic sample. Magmatic water content was greater than 10% at the time of formation of dioritic rocks in the Nabar pluton. Based on chemistry of mafic minerals and geochemical data, the Nabar plutonic complex comprises medium-K, calc-alkaline, and I-type granitoid. The rocks are characterized by enrichment of lithophile elements (LILEs) and depletion of high-field-strength elements (HFSEs). The Nabar rocks have weak concave-upward rare earth element (REE) patterns, suggesting that amphibole played a significant role in their generation during magma segregation. Low (Al₂O₃/(FeO + MgO + TiO₂) and (Na₂O + K₂O)/(FeO + MgO + TiO₂) ratios, and the patterns of trace and rare earth elements suggest that these rocks formed along a destructive plate margin and were derived from a lower crustal source. The magma probably formed by partial melting of lower crustal protoliths (amphibolites). Lower crust contamination with magma derived from partial melting of the upper mantle has an important role in the formation of this intrusive body, and a fractional crystallization of melts in higher crustal levels generated this spectrum of rock types. Mantle-derived gabbroic magmas emplaced into the lower crust are the most likely heat sources for partial melting.     

Lanostanes in Cambrian organic matter (Southeastern part of the Siberian platform)

Sterane hydrocarbons (HCs) of bitumoids of organic matter (OM) from the Lower Cambrian Sinyaya formation of the northern slope of the Aldan anteclise were studied using the chromatography-mass spectrometry technique. Norlanostanes C29 and lanostanes C30 were first discovered in fossilized OM from Cambrian sedimentary basins. This report considers the features of sedimentation, diagenesis, and maturity of lanosteroid-containing organic matter. Lanostane HCs are recommended for usage as biomarkers of oilproducing rocks of the Sinyaya formation to determine the source of naphthides in the eastern part of the Siberian platform.     

The features of the compositional evolution of felsic rocks in the low-potassium calc-alkaline series of the Zavaritskii volcano,Kurile Arc,Simushir Island

This article is focused on dacitic pumices, which are the felsic members of the basalt–andesite–dacite series. The phenoscrysts of all of the rocks from this series are the same: plagioclase, olivine, clino- and orthopyroxenes, and titanomagnetite. The groundmass of dacitic pumices that contain microlites of the same minerals and felsic glass has been studied in detail. Quartz and K–Na feldspar are absent. The study of that microlite zoning that formed in the upper parts of the channels or at the surface under the most nonequilibrium conditions was one of the most important tasks; it revealed several interesting features. As an example, anorthite plagioclases were found as microlites. The resorption zones are absent in both plagioclase phenocrysts and microlites, which implies the major role of fractionation rather than magma mixing.     

Climate-induced boreal forest change: Predictions versus current observations

For about three decades, there have been many predictions of the potential ecological response in boreal regions to the currently warmer conditions. In essence, a widespread, naturally occurring experiment has been conducted over time. In this paper, we describe previously modeled predictions of ecological change in boreal Alaska, Canada and Russia, and then we investigate potential evidence of current climate-induced change. For instance, ecological models have suggested that warming will induce the northern and upslope migration of the treeline and an alteration in the current mosaic structure of boreal forests. We present evidence of the migration of keystone ecosystems in the upland and lowland treeline of mountainous regions across southern Siberia. Ecological models have also predicted a moisture-stress-related dieback in white spruce trees in Alaska, and current investigations show that as temperatures increase, white spruce tree growth is declining. Additionally, it was suggested that increases in infestation and wildfire disturbance would be catalysts that precipitate the alteration of the current mosaic forest composition. In Siberia, 7 of the last 9 yr have resulted in extreme fire seasons, and extreme fire years have also been more frequent in both Alaska and Canada. In addition, Alaska has experienced extreme and geographically expansive multi-year outbreaks of the spruce beetle, which had been previously limited by the cold, moist environment. We suggest that there is substantial evidence throughout the circumboreal region to conclude that the biosphere within the boreal terrestrial environment has already responded to the transient effects of climate change. Additionally, temperature increases and warming-induced change are progressing faster than had been predicted in some regions, suggesting a potential non-linear rapid response to changes in climate, as opposed to the predicted slow linear response to climate change.