Trachytes,comendites, and pantellerites of the Late Paleozoic bimodal rift association of the Noen and Tost ranges,southern Mongolia: Differentiation and contamination of peralkaline salic melts

The bimodal association of the Noen and Tost ranges is ascribed to the Gobi-Tien Shan rift zone and was formed 318 Ma ago at the continental margin of the North Asian paleocontinent. It is made up of volcanic series of alternating basalts and peralkaline rhyolites with subordinate trachytes, dike belts, and massifs of peralkaline granites. The association also includes a coeval massif of biotite granites. Based on Al₂O₃ and FeO_tot contents, the peralkaline rhyolites are subdivided into comendites (FeO_tot 1.5–5.7 wt %, Al₂O₃ 10.5–15.4 wt %) and pantellerites (FeO_tot 5.2–7.5 wt %, Al₂O₃ 9.1–10.2 wt %). The peralkaline salic rocks of the bimodal association were formed by the crystallization differentiation of rift basaltic magmas combined with crustal assimilation. The comendites, pantellerites, and peralkaline granites inherited negative Nb and Ta and positive K and Pb anomalies from basalts. They are also similar to basalts in Nd isotope composition (?_Nd(T) = 5.5–7.4) and have nearly mantle oxygen isotope composition (δ¹⁸O = 5.9–7.3‰). The most differentiated and least contaminated rocks of the bimodal series of the Noen and Tost ranges are pantellerites. Calculations indicate that the fraction of the residual pantellerite melt was 8% or less of the parental basaltic magma. The comendites were derived from peralkaline salic melts by the assimilation of anatectic crustal melts compositionally similar to biotite granites. The formation of the latter within the Noen and Tost ranges is explained by the specific geodynamic position of the Gobi-Tien Shan rift zone, which was formed near a paleocontinental margin that evolved in an active margin regime shortly before the beginning of rifting.     

Heating rate dependency of petroleum-forming reactions: implications for compositional kinetic predictions

The generation of bulk petroleum, liquid and gaseous hydrocarbons from the Duvernay Formation was simulated by heating immature kerogens in a closed system (MSSV pyrolysis) at four different heating rates (0.013, 0.1, 0.7 and 5.0 K/min). Using the established parallel reaction kinetic model, temperature and compositional predictions were tested to be suitable for geological conditions by comparing the laboratory results with natural changes in source bitumens and reservoir oil maturity sequences from the Duvernay Formation. In the case of bulk liquid and gaseous hydrocarbons, the above kinetic calculations can be considered valid because their maximum yields are independent of laboratory heating rates. In contrast, the contents of paraffins, aromatics and sulfur compounds show a pronounced heating rate dependence. Extrapolated to geological heating rates, the compositional predictions are consistent with the bulk composition of natural products in the Duvernay-petroleum system showing an increase of paraffinicity and hydrogen content. In contrast to that, the “hump” decreases with decreasing heating rate, a trend which is confirmed by the low amounts of unresolved compounds in natural high maturity products. Because of these heating-rate dependent compositional changes, geological predictions of natural molecular composition by the commonly used kinetic models are not suitable.     

Interannual variability of tropical cyclone activity along the Pacific coast of North America

The interannual variability of near-coastal eastern North Pacific tropical cyclones is described using a data set of cyclone tracks constructed from U.S. and Mexican oceanic and atmospheric reports for the period 1951-2006. Near-coastal cyclone counts are enumerated monthly, allowing us to distinguish interannual variability during different phases of the May-November tropical cyclone season. In these data more tropical cyclones affect the Pacific coast in May-July, the early months of the tropical cyclone season, during La Niña years, when equatorial Pacific sea surface temperatures are anomalously cool, than during El Niño years. The difference in early season cyclone counts between La Niña and El Niño years was particularly pronounced during the mid-twentieth century epoch when cool equatorial temperatures were enhanced as described by an index of the Pacific Decadal Oscillation. Composite maps from years with high and low near-coastal cyclone counts show that the atmospheric circulation anomalies associated with cool sea surface temperatures in the eastern equatorial Pacific are consistent with preferential steering of tropical cyclones northeastward toward the west coast of Mexico.     

Structural and Morphological Features of the Participation of Microorganisms in the Formation of Nb–REE–Rich Ores of the Tomtor Field (Russia)

Doklady Earth Sciences - Data indicating the important role of microorganisms in the redistribution of REEs in the weathering crust and the decisive role in the concentration of REEs during the...     

The Structure and Formation Conditions of the Bedenekir Formation Deposits (Thithonian Stage) of Mountainous Crimea

The composition and origin of the Bedenekir Formation deposits of Mountainous Crimea are detailed on the basis of our work results and analysis of both published and unpublished data.     

Estimation of possible climatic changes of river runoff in the Northern Dvina River basin in the 21st century

Presented is a method of studying possible climate-driven changes of river runoff characteristics, according to which the atmosphere-ocean general circulation models are used for computing the climate system impact, whereas the response of a watershed to given disturbance is estimated using the model of runoff formation in a river basin. The study is carried out for the Northern Dvina River basin. Estimates of possible changes (relative to the reference period) by the middle and late 21st century of such water regime characteristics as mean annual and maximum water discharge, as well as mean discharge values for the spring, summer-fall and winter seasons, are presented.     

Thermally driven hydromagnetic instabilities in a non-constantly stratified layer

Summary The convection in a rapidly rotating electrically conducting, fluid horizontal layer of non-constant stratification, permeated by an inhomogeneous magnetic field, is studied. In this connection, a temperature model of the layer is constructed, which creates a structure such that part of the layer is unstably and a part stably stratified. The results obtained are applied to the conditions in the fluid Earth's core.

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Cooling of the mesosphere and lower thermosphere due to doubling of CO2

A new parameterization of infrared radiative transfer in the 15-m CO₂ band has been incorporated into the Spectral mesosphere/lower thermosphere model (SMLTM). The parameterization is applicable to calculations of heating rates above approximately 15 km for arbitrary vertical profiles of the CO₂ concentration corresponding to the surface mixing ratio in the range 150–720 ppm. The sensitivity of the mesosphere and lower thermosphere (MLT) to doubling of CO₂ has been studied. The thermal response in the MLT is mostly negative (cooling) and much stronger than in the lower atmosphere. An average cooling at the stratopause is about 14 K. It gradually decreases to approximately 8 K in the upper mesosphere and again increases to about 40–50 K in the thermosphere. The cooling and associated thermal shrinking result in a substantial density reduction in the MLT that reaches 40–45% in the thermosphere. Various radiative, chemical, and dynamical feedbacks potentially important for the thermal response in the MLT are discussed. It is noted that the results of simulations are strikingly similar to observations of long-term trends in the MLT. This suggests that during the last 3–4 decades the thermal structure in the real upper atmosphere has undergone substantial changes driven by forcing comparable with that due to doubling of CO₂.     

Experimental Studies of the Dynamic Interaction between Model Channel Forms and Flow

Interaction between water flow and ridge bedforms has been studied. The results of experimental studies of velocity distribution over ridge bedforms in a hydraulic laboratory flume are given. The distribution of pressure along the contour of a rigid model ridge, determined in an aerodynamic canal, is also given.