Geomagnetic field paleointensity in the cretaceous from Upper Cretaceous rocks of Georgia     

V. V. Shcherbakova  B. Z. Asanidze  V. P. Shcherbakov  G. V. Zhidkov 《Izvestiya Physics of the Solid Earth》2007,43(11):951-959

A representative collection of Upper Cretaceous rocks of Georgia (530 samples from 24 sites) is used for the study of magnetic properties of the rocks and the determination of the paleodirection and paleointensity (H _an) of the geomagnetic field. Titanomagnetites with Curie points of 200–350°C are shown to be carriers of natural remanent magnetization (NRM) preserving primary paleomagnetic information during heatings to 300–350°C. The characteristic NRM component of the samples is identified in the interval 120–350°C. The Thellier and Thellier-Coe methods are used for the determination of H _an meeting modern requirements on the reliability of such results. New paleointensity determinations are obtained and virtual dipole magnetic moment (VDM) values are calculated for four sites whose stratigraphic age is the Upper Cretaceous (Cenomanian-Campanian). It is shown that, in the interval 99.6–70.6 Ma, the VDM value was two or more times smaller than the present value, which agrees with the majority of H _an data available for this time period. According to our results, the H _an value did not change at the boundary of the Cretaceous normal superchron.  相似文献   

Extreme-Mg olivines from venancite lavas of Pian di Celle volcano (Italy)     

P. Yu. Plechov  N. A. Nekrylov  V. D. Shcherbakov  M. S. Tikhonova 《Doklady Earth Sciences》2017,474(1):507-510

Olivines with a forsterite minal of up to 99.5 mol % were discovered in ultra-potassium lavas of Pian di Celle volcano (Italy). Based on high-precision analyses of these olivines and the available literature data, a new type of extreme-Mg olivines with a forsterite content of >96 mol % was distinguished. These olivines could have formed as a result of oxidation of lower Mg olivines, crystallization from the carbonatite melt, or they may represent earlier crystallized phenocrysts in skarn. Oxidized olivines are easily diagnosed on the content of typomorphic admixtures, in particular, high MgO and NiO contents. It is difficult to recognize olivines from skarns and carbonatites due to the lack of high-precision analytical data. Extreme-Mg olivines from Pian di Celle (Italy) are close to the carbonatite subtype of olivines.  相似文献   

Composition and conditions of formation of the parental melts of Jurassic dolerites of southwestern Crimea: Evidence from melt inclusions in olivine phenocrysts     

D. V. Popov  N. Nekrylov  P. Yu. Plechov  V. D. Shcherbakov  M. V. Portnyagin  M. S. Serova 《Petrology》2017,25(3):272-303

This study focuses on Jurassic shallow intrusions and subvolcanic bodies from around Trudolyubovka village in the southwestern Crimea. All the rocks are similar in mineral composition and have similar geochemical features and occur in close spatial and geological association. This allows us to assign the intrusions to a single magmatic series and interpret them as differentiation products of a single parental melt. The investigation of melt inclusions in olivine from the most magnesian sample showed that the composition of igneous melts ranged from basalt to basaltic andesite of a moderately potassic subalkaline affinity. Compared with N-MORB, they are enriched in LILE, but have similar HFSE and REE contents. The early magmatic melts crystallized at temperatures ranging from 1240 to 1125°C, pressures of 6–8 kbar, and an oxygen fugacity of ΔQFM = +0.6; and later melts crystallized at 1090–940°C, ~1.5 kbar, and oxygen fugacity increasing from ΔQFM + 0.9 to ΔQFM + 2.3. The minimum pressure of groundmass crystallization was estimated as 40–60 bar. The primitive melts were formed in a mature island arc or an active continental margin setting by ~13% melting of a DMM-like source. The melting occurred at spinel-facies depths under the influence of a slab-derived fluid at a temperature 25°C below the dry peridotite solidus.  相似文献   

Petrology of mantle xenoliths in rocks of the Bezymyannyi Volcano (Kamchatka)     

V. D. Shcherbakov  P. Yu. Plechov 《Doklady Earth Sciences》2010,434(2):1317-1320

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Determination of the paleointensity in the early proterozoic from granitoids of the Shumikhinskii complex of the Siberian craton     

V. V. Shcherbakova  V. P. Shcherbakov  A. N. Didenko  Yu. K. Vinogradov 《Izvestiya Physics of the Solid Earth》2006,42(6):521-529

The extreme scarcity of data on the behavior of the paleointensity H _an in the geological past from rocks older than 400 Ma significantly hinders the development of our ideas of the geomagnetic field evolution and the geological history of the Earth as a planet. This work presents H _an determinations for the Early Proterozoic using the Thellier method and meeting modern requirements for their reliability. The data are obtained from 1850-Ma rocks of granite intrusions sampled in the south of the Siberian platform. The rocks are virtually unaltered granites and granitoids. The paleointensity was determined on 15 samples; results from 11 samples were found to be suitable for the calculation of H _an, which is good for experiments of this type. The common feature in the behavior of the natural remanent magnetization (NRM) is a very narrow interval of blocking temperatures: destruction of (60–90)% NRM often took place between 500 and 550°C. Because of the large thickness of the sampled magmatic body, the paleointensity estimates were corrected for its slow cooling rate. With regard for this correction, the probable value of the virtual dipole moment (VDM) from the given collection amounts to 5 × 10²² A m². Analysis of all published data obtained by the Thellier method for the Precambrian and satisfying the well-known minimal criteria of reliability showed that the average VDM value is about 2 × 10²² A m², which is four times smaller than the VDM value of the last million years. This phenomenon can be interpreted in terms of the hypothesis that the solid inner core formed only in Proterozoic and, in its absence, the generation of the geomagnetic field was relatively weak, which yielded a small intensity value of the geomagnetic field at early stages of the Earth’s evolution.  相似文献   

On the theory of low-temperature oxidation of titanomagnetite grains     

V.P. Shcherbakov 《Physics of the Earth and Planetary Interiors》1982,30(4):324-329

It is shown that the space-time distribution of vacancies in the process of formation of cation-deficient spinels is determined mainly by the boundary kinetics. At low rates of vacancy appearance in the surface layer (e.g., owing to oxygen deficit), the degree of grain oxidation remains invariant over the entire volume. The rate of oxidation in this case does not depend on the diffusion coefficient, but is determined by the boundary processes. With an excess of oxygen, the process of oxidation occurs inhomogeneously over the grain volume. In this case the rate of oxidation is determined by the diffusion coefficient. Approximate estimates have been made of the physical parameters characterising the two types of oxidation.  相似文献   

Polarimetry of stellar active regions and flares     

I. Tuominen  J. Huovelin  Yu. S. Efimov  N. M. Shakhovskoy  A. G. Shcherbakov 《Solar physics》1989,121(1-2):419-429

Observations of regular and irregular polarimetric variability in late-type stars are reviewed, and the related physical and geometrical effects are discussed. There are indications that the irregular part of the variability could be caused by transient events, possibly associated with flares. Polarimetric observations during flares are reviewed, and preliminary results of new observations of a well-known flare star, YY Geminorum, are presented. The results show that the small flare in YY Gem did not cause any significant variations in linear polarization, while the binary eclipse evidently causes an enhancement in the polarization. The reasons for the difficulties in stellar flare polarimetry are discussed. Finally, future prospects for the observations of flaring stars and for the utilization of linear polarimetry as a complementary method to other techniques of surface imaging of stellar activity and flares are presented.  相似文献   

Domain state stabilization by iterated thermal magnetization processes     

Karl Fabian  Valera P. Shcherbakov 《Geophysical Journal International》2004,159(2):486-494

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On the theory of depositional remanent magnetization in sedimentary rocks     

V. P. Shcherbakov  V. V. Shcherbakova 《Surveys in Geophysics》1983,5(4):369-380

The effects of various factors such as thermal agitation, coagulation, anisotropy of susceptibility, and shape irregularity on the alignment of magnetic carrier grains during the process of acquisition of depositional remanent magnetization in sedimentary rocks is discussed.  相似文献   

A Damage Mechanics Model for Aftershocks     

Robert Shcherbakov  Donald L. Turcotte 《Pure and Applied Geophysics》2004,161(11-12):2379-2391

— All earthquakes are followed by an aftershock sequence. A universal feature of aftershock sequences is that they decay in time according to the modified Omori’s law, a power-law decay. In this paper we consider the applicability of damage mechanics to earthquake aftershocks. The damage variable introduced in damage mechanics quantifies the deviation of a brittle solid from linear elasticity. We draw an analogy between the metastable behavior of a stressed brittle solid and the metastable behavior of a superheated liquid. The nucleation of microcracks is analogous to the nucleation of bubbles in the superheated liquid. In this paper we obtain a solution for the evolution of damage after the instantaneous application of a constant strain to a rod. We show that the subsequent stress relaxation can reproduce the modified Omori’s law. It is argued that the aftershocks themselves cause random fluctuations similar to the thermal fluctuations associated with phase transitions.  相似文献