Olivine samples (Fa 11) have been oxidized in air (fO2 = 0.2 atm) at temperatures ranging from 350–700 °C and examined by Mössbauer spectroscopy, transmission electron microscopy, X-ray powder diffraction and thermomagnetic analysis. Oxidation of olivine was found to result in ferriolivine, magnesioferrite (major oxide phase) and magnetite (minor oxide phase) formation. Ferriolivine forms planar (001) precipitates, 0.6 nm in thickness, in the olivine host; the composition is likely to be Mg0.5v0.5(Fe3+)1.0SiO4. Magnesioferrite MgFe2O4 exsolves as fine-grained precipitates (5–6 nm in size) filling interstices between the ferriolivine planar precipitates. Oxidation kinetic data at 700 °C show two stages of oxidation corresponding to formation of ferriolivine in the first stage and magnesioferrite in the second stage. The linear rate law with a rate constant kFol = 1.23 · 10-3 s-1 was found for the first stage whereas a parabolic rate-law with a constant of koxi = 3.28 · 10-3 s-1 was determined for the second stage of oxidation. It was found that ferriolivine is not an intermediate metastable phase in the oxidation process, terminated by magnesioferrite formation. The ferriolivine and magnesioferrite are considered to have formed by independent reactions which do not necessarily proceed simultaneously. 相似文献
Paleomagnetic research in the Siberian area provides new data to derive models of relative horizontal movements and the process of amalgamation or consolidation of the Siberian plate. Comparison of the Apparent Polar Wander (APW) curves for different regions of Siberia show that there were several stages of plate consolidation, the most important of which belonged to Baikalian time (c. 520–800 Ma). The consolidation was complicated by rifting processes. There were six megaterrains in pre-Vendian times, which underwent differential horizontal displacements. The joining of the Altai-Sayan fold area and the North-East microplates and fold belts to the Siberian plate, and the separation of the Taimyr peninsula, were the main post-Baikalian events. The most complete geologic sections were studied in conjunction with radiometric age dating and related to the geochronological Precambrian time scale of the USSR. This has led to the discovery of a stratigraphic polarity bias in the early Riphean-Vendian interval. There were five polarity stages in this interval 200–240 Ma each. 相似文献
The kinetics of metamorphism of the Staroe Boriskino C2 chondrite heated at 450°C in an inert atmosphere of helium flow was investigated. After being heated at 450°C during 160 minutes one specimen was moreover heated for 10 minutes at 500°C. The phase distribution was determined by means of Mössbauer spectroscopy, X-ray diffraction analysis, and electron probe microanalysis.The material changes rapidly (1–2 minutes). As a result of dehydration, the iron of the phyllosilicate is oxidized, the charge compensation being realized through the removal of iron and magnesium cations with the formation of magnetite and forsterite. Upon 10 minutes additional heating at 500°C iron appears in the olivine structure, the degree of iron oxidation declines, and magnetite disappears. Possible trends of change of C2 chondrite material are:
Collections of Early Triassic rocks sampled from three outcrops on the SW slope of the Pai-Khoi Ridge are studied. The magnetic polarity succession r-n-r is established from the prefolding characteristic component of natural remanence in sedimentary rocks and a basaltic nappe. According to stratigraphic data and isotopic dating of the basaltic nappe, the studied parts of the lower subformation of the Lestanshorskaya Formation correlate with the central part of magnetic zone R1T1 of the European Russia magnetostratigraphic time scale. The angles of rotation in the horizontal plane are estimated for the studied Early Triassic Pai-Khoi structures with respect to the East European platform. 相似文献
Oxygen isotope equilibrium fractionation constants (β18O-factors) of cassiterite were evaluated on the basis of heat capacity and X-ray resonant (Mössbauer spectroscopy and X-ray inelastic scattering) data.The low-temperature heat capacity of cassiterite was measured in the range from 13 to 340 K using an adiabatic calorimeter. Results of measurements of two samples agree very closely but deviate more than 5% from previous heat capacity data used for calculation of thermodynamic functions. The temperature dependence of heat capacity was treated using the modern version of the Thirring expansion, and the appropriate temperature dependence of the vibrational kinetic energy was found.Measurements of temperature-dependent Mössbauer parameters of cassiterite were conducted in the range from 300 to 900 K. The attempt to describe Mössbauer fraction and the second order Doppler (SOD) shift on the basis of the Debye model failed. The first term of the Thirring expansion of the Mössbauer SOD shift agrees with that calculated from the Sn sublattice vibration density of states (VDOS) obtained via synchrotron X-ray scattering. Based on this agreement we calculated the kinetic energy of the cassiterite Sn sublattice from VDOS.From the kinetic energy of the total cassiterite crystalline lattice and its Sn sublattice, β18O-factors of cassiterite were computed in the temperature range 300-1500 K by the method of Polyakov and Mineev (2000). Appropriate polynomials, which are valid at temperatures above 400 K, are the following:
A palaeomagnetic investigation has been carried out of rocks from the eastern part of the Voronezh Massif, which constitutes, together with the Ukrainian Shield, the Sarmatian segment in the southern part of the East European Craton. The samples were collected in a quarry close to the town of Pavlovsk (50.4°N, 40.1°E), where a syenitic-granitic body intrudes Archaean units. U–Pb (zircon) dating has yielded an age of 2080 Ma for the intrusion. Two characteristic magnetic components, A and B, were isolated by thermal and alternating-field demagnetization. Component A was obtained from granites and quartz syenites (11 samples) and has a mean direction of D = 229°, I = 28°, and a pole position at 12°N, 172°E. This pole is close to a contemporary mean pole (9°N, 187°E) for the Ukrainian Shield, which implies that the Voronezh Massif and the Shield constituted a single entity at 2.06 Ga. These poles differ from contemporaneous poles of the Fennoscandian Shield, indicating that the relative positions of the two shields were different from their present configuration about 2100 Myr ago. A component B, isolated only in quartz monzonites (five samples), has a mean direction D = 144°, I = 49°, and a pole position at 4°N, 251°E, which is close to late Sveconorwegian (approximately 900 Ma) poles for Baltica. This suggests that the East European Craton was consolidated some time between 2080 and 900 Ma. Comparison with other palaeomagnetic data permit us to narrow this time span to 1770–1340 Ma. 相似文献
The paleomagnetic study of the Lower Ordovician and Cambrian sedimentary rocks exposed on the Narva River’s right bank revealed a multicomponent composition of natural remanent magnetization. Among four distinguished medium- and high-temperature magnetization components, the bipolar component, which carries the reversal test, is probably the primary component and reflects the geomagnetic field direction and variations during the Late Cambrian and Early Ordovician. The pole positions corresponding to this component have coordinates 22°N, 87°E (dp/dm = 5°/6°) for the Late Cambrian, and 18°N, 55°E (dp/dm = 5°/7°) for the Early Ordovician (Tremadocian and Arenigian). Together with the recently published paleomagnetic poles for the sections of the Early Ordovician in the Leningrad Region and the series of poles obtained when the Ordovician limestones were studied in Sweden, these poles form new key frameworks for the Upper Cambrian-Middle Ordovician segment of the apparent polar-wander path (APWP) for the Baltica. Based on these data, we propose a renewed version of the APWP segment: the model of the Baltica motion as its clockwise turn by 68° around the remote Euler pole. This motion around the great circle describes (with an error of A95 = 10°) both variations in the Baltic position from 500 to 456 Ma ago in paleolatitude and its turn relative to paleomeridians. According to the monopolar components of natural remanent magnetization detected in the Narva rocks, the South Pole positions are 2°S, 351°E (dp/dm = 5°/9°), 39°S, 327°E, (dp/dm = 4°/7°), and 42°S and 311°E (dp/dm = 9°/13°). It is assumed that these components reflect regional remagnetization events in the Silurian, Late Permian, and Triassic. 相似文献
The Maslovsky ore deposit is an interesting but poorly explored geological locality in the Noril’sk ore region. Interpretations
of the morphology and internal structure of the intrusion, which hosts PGE and copper-nickel deposits, are provided in this
study. The complicated morphology of the Maslovsky intrusion enables us to recognize two layered gabbro-dolerite bodies, which
are connected by a feeder dyke. Their internal structure testifies to the similarities and differences in these intrusive
bodies. The results of this study have potential for further petrological and geochemical research. 相似文献
In terms of plate tectonic concepts, horizontal movement are responsible for the formation of fold belts, and modern structures are interpreted by the position of the belt relative to the zones of plate collision. Paleomagnetic data allow the reconstruction of the former positions of these zones relative to a geographic grid, their pre-deformation patterns being important to the solution of problems in fold belt tectonics (Van der Voo et al., 1980; Khramov et al., 1982).Fold structures within the south-western spurs of the Tien Shan Mountains and the intermontane basins separating them were the first to be studied (Rzhevsky, 1966). Paleomagnetic studies aimed at understanding the tectonic setting of the region have been undertaken during the ensuing years and also cover adjacent areas of Central Asia (Khramov et al., 1982).In this paper an attempt is made to summarize some paleomagnetic results, produced palinspastic reconstructions of the region and produce a framework to help understand the formation of one of the crucial parts of the Alpine-Himalayan belt. 相似文献
