Effect of temperature and pressure on the composition of oils from the Bazhenovskaya Formation in the latitudinal segment of the Ob River area

Detailed data obtained on surface oil samples from the Bazhenovskaya Formation in the latitudinal segment of the Ob River area indicate that the composition of the oil is largely controlled by the P-T conditions in the reservoir. It was determined that the generation of hydrocarbon fluids starts at higher temperatures (T _f > 100°C) if the fluid reflux is hindered and the formation pressures are anomalously high. In systems with the free reflux of hydrocarbon fluids, the processes generating oil and gas are different. The composition of oil in the Bazhenovskaya Formation with the free reflux of hydrocarbon fluids is largely controlled by migration processes, which determine, along with the P-T parameters, the physicochemical characteristics of the oils and their concentrations of hydrocarbon groups and individual hydrocarbons. Original Russian Text ? O.F. Stasova, A.I. Larichev, N.I. Larichkina, 2006, published in Geokhimiya, 2006, No. 9, pp. 996–1005.     

On two types of metalliferous carbonaceous rocks

Two types of metalliferous carbonaceous rocks that differ in associated metals are considered. One of them is characterized by gold occurring in finely dispersed arsenopyrite and arsenic-bearing pyrite, and the other, by finely impregnated native metals in carbonaceous matter. Rocks of the first type also contain elements of the platinum group, while those of the second type bear rhenium, silver, iron, molybdenum, and other metals. Graphitic mineralization is usually localized in the shear zone associated with alkaline basalts.     

Alkaline magmatism and enriched mantle reservoirs: Mechanisms, time, and depth of formation

Alkaline magmatism has occurred since 2.5–2.7 Ga and its abundance has continuously increased throughout the Earth’s history. Alkaline rocks appeared on the Earth with changes in the geodynamic regime of our planet, i.e., when plume tectonics was supplemented by plate tectonics. Global-scale development of plate tectonics at the Archean—Proterozoic boundary initiated subduction of already significantly oxidized oceanic crust enriched in volatiles and large-scale mantle metasomatism caused the formation of enriched reservoirs as sources of alkaline and carbonatite magmatism. Study of metasomatized mantle material showed the occurrence of traces of primary carbonatite melts, which are strongly enriched in rare elements, according to ion-microprobe analyses. The results obtained allowed us to propose a new two-stage genetic model for Ca-rich carbonatites including (1) metasomatic wehrlitization and carbonatization of mantle material and (2) partial melting of wehrlitized mantle with formation of carbonate-rich melts or three immiscible liquids (at high alkali contents), i.e., silicate, carbonatitic, and sulfide (at high sulfur activity). Original Russian Text L.N. Kogarko, 2006, published in Geokhimiya, 2006, No. 1, pp. 5–13.     

Computer microtomography of zircon: A new approach to the selection of targets for U-Pb geochronologic studies

The paper reports results obtained by the complex studying of zircon crystals from rocks of various genesis. Zircon is one of the minerals most often used as geochronometers. It also provides genetic information on superimposed processes that is “recorded” in the external and internal morphology of its crystals. This mineral is thoroughly examined to select its unaltered crystals for U-Pb dating by the single-grain method. Zircon grains are selected with the use of optical and electron microscopy and cathodoluminescence. This publication presents the first results obtained by examining zircon by computer microtomography (μCT) and the results of the studying of the external and internal structure by conventional techniques ([optical microscopy and SEM (SE and CL)]. The paper demonstrates the advantages of the application of the μCT techniques to the selection of targets for U-Pb zircon dating: there is no need for the special preparation of the samples and no material is destructed. However, this technique may be not informative enough if the zircon contains inherited core whose density does not differ from the density of the surrounding mineral.     

Energy transfer among Pb, Ce and Mn in fluorescent calcite from Kuerle, Xinjiang, China

Natural calcite from Kuerle, Xinjiang, China, shows orange-red fluorescence when exposed to short-wave ultraviolet (UV) light (Hg 253.7 nm). Photoluminescence (PL) emission and excitation spectra of the calcite are observed at room temperature in detail. The PL emission spectrum under 208 nm excitation consists of three bands: two UV bands at 325 and 355 nm and an orange-red band at 620 nm. The three bands are ascribed to Pb²⁺, Ce³⁺ and Mn²⁺, respectively, as activators. The Pb²⁺ excitation band is observed at 243 nm, and the Ce³⁺ excitation band at 295 nm. The Pb²⁺ excitation band is also observed by monitoring the Ce³⁺ fluorescence, and the Pb²⁺ and Ce³⁺ excitation bands, in addition to six Mn²⁺ excitation bands, are also observed by monitoring the Mn²⁺ fluorescence. These indicate that four types of the energy transfer can occur in calcite through the following processes: (1) Pb²⁺ → Ce³⁺, (2) Pb²⁺ → Mn²⁺, (3) Ce³⁺ → Mn²⁺ and (4) Pb²⁺ → Ce³⁺ → Mn²⁺.