Home – Home – Jornals – Russian Geology and Geophysics 2008 number 10

Brownmillerite, Fe-perovskite, and two intermediate phases (Ca₃Ti(Fe,Al)₂ O₈ and Ca₅Ti(Fe,Al)₄ O₁₃) were found in combustion metamorphic high-temperature larnite rocks of the Hatrurim Basin, Israel. This is the first finding evidencing the existence of the pseudobinary perovskite-brownmillerite series in nature. Fe-perovskite contains the Ca₂ (Fe,Al)₂ O₅ end-member (up to 18 mole %), whereas brownmillerite Ca₂ (Fe_1-x Al _x )₂O₅ with x = 0.15-0.55 is enriched in the CaTiO₃ end-member (up to 15 mole %). The intermediate phases are Ca₃Ti(Fe,Al)₂ O₈ (phase X) and Ca₅Ti(Fe,Al)₄ O₁₃ (phase Y), which have synthetic analogs. In some larnite rocks, the phase X is the only member of the perovskite-brownmillerite series. It is clustered into two compositional groups, Ca₃Ti(Fe _1.3Al_0.7)O₈ and Ca₃Ti(Fe _1.6 Al_0.4)O₈. The phase Y is extremely rare and has the composition Ca₅Ti(Fe _0.76 Al_0.24)₄O₁₃. The presence of minor Si, Cr, Sr, Zr, and LREE is common for all minerals of the series. The Fe-perovskite + brownmillerite paragenesis was found in none of the studied rocks, which is consistent with the CaTiO₃-Ca₂Fe₂O₃ phase diagram. The rocks bear either one phase or two or three phases (Fe-perovskite + phase X; brownmillerite + phase X ± phase Y). According to the CaTiO₃-Ca₂Fe₂O₃ diagram, the minimum formation temperature for the Hatrurim larnite rocks estimated from the Fe-perovskite + phase X paragenesis is 1170-1200

Humid lithogenesis is characterized by the accumulation and postsedimentary transformation of eluvial products within terrigenous and terrigenous-carbonate (with calcite) formations containing mainly dioctahedral clay minerals. Platform fresh-water sediments of early-catagenesis zone accumulate 1 : 1 and 2 : 1 varieties, with a predominance of expanding layers in the latter. In sand-silty rocks authigenic kaolinite also appears. Trioctahedral chlorite of generation I is developed from the top of this zone in both clayey and sand-silty marine deposits. Also, mixed-layer minerals are synthesized: montmorillonite-glauconite at the top of the zone and montmorillonite-hydromica at the bottom.
Arid lithogenesis is controlled by the accumulation of sediments of terrigenous-carbonate (with dolomite), carbonate, and halogen formations. In contrast to the terrigenous and aclimatic volcanogenic formations, these ones are dominated by trioctahedral clay minerals: chlorite, sepiolite, and mixed-layer chlorite-based minerals. In early-catagenesis zone, sepiolite, palygorskite (di-analog of sepiolite), and halloysite quickly transform into talc- and chlorite-saponite and kaolinite, respectively. In hypergenesis zone climate is the main factor, whereas in early-catagenesis zone, especially in arid environments, the chemical composition of the medium plays a significant role. In late-catagenesis zone, degradation products of mica and montmorillonite transform first into varieties with less than 40% and somewhat more than 40% expanding layers, respectively, whereas trioctahedral minerals preserve up to 50% such layers. This process is accompanied by the recrystallization of both allothigenic and authigenic minerals with the transition of scaly montmorillonite into bladed hydromica, with the authigenic minerals of halogen-formation sediments characterized by the most perfect crystal habits and regular structures. In metagenesis zone, di-minerals of orogenic-area sediments transform into sericite, and trioctahedral minerals, into more magnesian chlorite of generation II. This indicates that in late-catagenesis and metagenesis zones, the PT-conditions of the environment are of crucial importance.

The effect of high temperature on sulfide inclusions in diamonds was studied. The object of study was sulfide-bearing diamond crystals from Yakutian kimberlite pipes. The experiments were carried out on a multiple anvil apparatus BARS with a solid-phase refractory oxide cell at 5.0 GPa and 1000-1600

Large gold-brannerite aggregates (nuggets up to 50 g in weight) were found in placers of the Ozernoe ore cluster, western Transbaikalia. Brannerite occurs as large crystals and crystal aggregates and has the near-stoichiometric composition UTi₂O₆. It lacks REE, contains little, if any, Th, and has admixtures of FeO (2.0-3.7 wt.%), CaO (up to 2.7 wt.%), and SiO₂ (up to 2.14 wt.%). It also bears fine (<1 μm) inclusions of Au-Ag- and Bi-containing tellurium minerals and bismuth minerals. Gold occurs in cracks of brannerite or aggregates with it on coprecipitation from solutions. Gold was also found as fine (1-3 μm) segregations in muscovite. Accessory minerals are hematite, barite, rutile (containing 0.36-3.61 wt.% FeO and up to 7.5 wt.% WO₃), dolomite, siderite, and muscovite. High contents of Te and Bi were detected in Fe, Ti, and U hydroxides. A specific relationship between gold and rutile has been revealed both in large aggregates and in gold microinclusions (<1 μm) in rutile grains. In composition and paragenesis the studied gold-brannerite association is similar to ores of the El'kon type (Central Aldan) related to the Mesozoic magmatism.

In the western Baikal area, the structural position, composition, and age of volcanic rocks in the section of the Riphean margin of the Siberian craton were studied. The age of these rocks, earlier assigned to the Khotskaya Formation, is estimated at 274±3 Ma (concordia constructed over 11 zircon grains, SHRIMP-II). The geochemical and isotope compositions of volcanics evidence that they resulted from the melting of mantle source of EM-I type contaminated by crustal material. The intrusion of volcanics into the upper crustal horizons might have been caused by the evolution of the Permian active margin of the Siberian continent, which took place on the background of the closure of the Mongolo-Okhotsk ocean. Based on the results of studies, a new subvolcanic complex of Early Permian age has been recognized in the region, which includes the above-mentioned volcanics and earlier described porphyrite dikes of close age in the Sharyzhalgai uplift. The data obtained disprove the concept that the studied volcanics are of Riphean age; therefore, the available stratigraphic charts of the Siberian Precambrian must be revised.

A Jurassic accretionary prism in the Sikhote-Alin-Amur area is an assembly of terranes, which are tectonic-sedimentary complexes consisting of multiple strongly deformed fragments of an oceanic plate. The stack of tectonic-stratigraphic units (complexes) in the prism section records its geodynamic history, each unit being signature of a geologic event on the Paleoasian eastern margin. The succession of accretion events brought together fragments of a Paleozoic oceanic plateau in the Early Jurassic and abyssal plain fragments of different ages in Middle and Late Jurassic time.

The lithospheric structure in Asia has been analyzed in terms of sizes of blocks and mobile zones between them on local (northeastern Olkhon area in western Baikal region), regional (Baikal region), and global (Central and Eastern Asia) scales. A single approach to reference mapping was used to update Sadovskii's hierarchy of blocks and to rank the mobile zones. According to the updated model, the 1ithospheric structure of Central and Eastern Asia includes eleven hierarchic levels.

We suggest a new technique for processing head-wave data with a multistage Wiener filter, which provides conversion and stacking of head waves from a receiver array into a user-specified point. Different conversion algorithms are compared and shown to be a combination of parallel and serial Wiener filters.
The Wiener filter accuracy for a conversion iteration depends on the in-line fold and on the coherence spectrum. The latter describes the signal/noise ratio dependence where head waves are the signal and the noise includes all other components and microseisms. Equations for the accuracy of head-wave traces have been derived from a corresponding equation for a single conversion iteration using the multistage Wiener filter schemes and proceeding from the theory of coupling stochastic systems. The obtained equations are applicable to different field data processing algorithms.
The discussed conversion algorithms differ in noise regularization and are fit for data of any quality.

We suggest new inversion techniques for IP-affected TDEM data. The method implies joint inversion of inductive and galvanic measurements, the galvanic array being configured in a way to separate the polarization and induction components of transient responses from a layered polarizable earth. The IP effects turned out to be reducible at a certain receiver configuration, which was predicted theoretically and proved valid in field tests.