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1.
Titanomagnetites of basalts and diorites from an off-axis volcanic episode in the Pacific Ocean (Leg 61, DSDP) were studied for their ulvospinel composition and for changes in the iron-titanium ratio with progressive low-temperature oxidation. Thermomagnetic analysis and microscopy indicate that these Fe/Ti spinels show relatively little low-temperature oxidation, despite their Cretaceous age. Microprobe investigation shows that the Fe/Ti spinels have a wide range of ulvospinel content directly reflecting the range of parent rock chemistries. This is an important demonstration that ulvospinel contents are not constant but vary within a restricted range of compositions, namely at least x =0.5 to x=0.75 (Fe2?xTixO4). Recognition of this fact is important in attempting to elucidate changes in cation ratio as a result of progressive low-temperature oxidation.Examination of samples of different oxidation states for cation ratio changes as a result of low-temperature oxidation revealed lower titanium in the more oxidized samples and Ti enrichment in the less oxidized ones. This is probably a reflection of changing x values with progressive crystallization of individual sills, and thus unrelated to cation ratio changes with oxidation.In more than 100 grains analyzed, no evidence was found for a more oxidized shell on the exteriors of grains, suggesting that the oxidation process did not proceed uniformly with respect to grain geometry.  相似文献   

2.
Sized fractions of x = 0.6, 0.4, 0.2 and 0.0 titanomagnetites were studied with a vibration magnetometer. In the course particles (d > 150 μm), no compositional dependence of hysteresis parameters was found. HC was less than 50 Oe, HR/HC > 4 and JR/JS < 10?2, reflecting multi-domain behaviour. In contrast, fine particles (d ? 0.1 μm) revealed systematic grain-size dependence of parameters with coercive force as high as 2,000 Oe in x = 0.6 titanomagnetite. Grain-size dependence studies revealed broad transition sizes for the onset of true multi-domain behaviour depending upon which factor is chosen. In magnetite it varies from 10 to 20 μm. The experimental critical size for single-domain behaviour for magnetite is about 0.1 μm and for x = 0.6 titanomagnetite 1–2 μm.  相似文献   

3.
This paper focuses on the study of the correlation between magnetic parameters with the level of contamination by heavy metals in urban soils. We report a magnetic investigation of urban soil samples from Merida, state of Yucatan, Southern Mexico. It appears that most of our samples contain ferrimagnetic minerals as the magnetic carriers, probably coming from the titanomagnetites/titanomaghemites series. This is inferred by the acquisition of isothermal remanent magnetization, which shows that most of samples are almost completely saturated at about 200 mT. The S−200 value (factor characterizing stability of remanent magnetization) is between 0.8 and 1.0, characteristic of ferrimagnetic minerals. The susceptibility vs. temperature measurements also point to some titanomaghemites and titanomagnetites as probable responsible for magnetization. On the other hand, measurements of magnetic susceptibility at high and low frequencies helped us to determine the high content of superparamagnetic grains in the majority of the samples, although not all of these showed high values of magnetic susceptibility. We found that the most contaminated samples by Pb and Zn showed the higher saturation isothermal remanent magnetization values, whereas the higher values in magnetic susceptibility correspond to samples contaminated by Cr. Finally, we noted that a high level in Sr decreases the magnetic susceptibility.  相似文献   

4.
Methods for determining the Curie temperature (Tc) of titanomaghemites from experimental saturation magnetization-temperature (Js-T) data are reviewed.Js-T curves for many submarine basalts and synthetic titanomaghemites are irreversible and determining Curie temperatures from these curves is not a straightforward procedure. Subsequently, differences of sometimes over 100°C in the values ofTc may result just from the method of calculation. Two methods for determiningTc will be discussed: (1) the graphical method, and (2) the extrapolation method. The graphical method is the most common method employed for determining Curie temperatures of submarine basalts and synthetic titanomaghemites. The extrapolation method based on the quantum mechanical and thermodynamic aspects of the temperature variation of saturation magnetization nearTc, although not new to solid state physics, has not been used for estimating Curie temperatures of submarine basalts. The extrapolation method is more objective than the graphical method and uses the actual magnetization data in estimatingTc.  相似文献   

5.
A theoretical model of grain size variation of domain transitions in titanomagnetite (x = 0.6) as a function of oxidation (z) is presented. The superparamagnetic (SP) to single-domain (SD) transition ds, the SD to two-domain (TD) transition d0, the TD to three-domain (3D) transition and the pseudo-single domain (PSD) to multi-domain (MD) transition are calculated as a function of z. It is shown that all the transition grain sizes increase with z, except for the PSD-MD transition for z > 0.6. The calculations predict that ds increases from 0.044 to 0.197 μm, d0 increases from 0.54 to 13 μm, the TD-3D transition increases from 1.6 to 49 μm as z varies from 0 to 0.8. The PSD-MD transition increases from 42 μm at z = 0 to 150 μm at z = 0.6, whereas between z = 0.6 to z = 0.8, the PSD-MD transition decreases to 49 μm. Qualitatively, the model explains some of the trends in magnetic properties of submarine basalts with low-temperature oxidation. Quantitatively, the model does give reasonable estimates of the PSD-MD boundary and d0, which are close to the experimental values for x = 0.6 and z = 0. Furthermore, the model predicts that psarks or two-domain grains could be the major contributors to the remanence of oxidized submarine pillow basalts.  相似文献   

6.
A model mechanism has been derived for the evolution of the cation distribution in titanomaghemites, which have formed by the progressive removal of iron from the spinel lattice. According to the model, the cation distribution of a titanomaghemite is not a unique function of composition but depends on the path by which maghemitization has taken place. Cation distributions (and consequent magnetic properties) have been calculated and compared for titanomaghemite produced by the “removal of iron” and “addition of oxygen” mechanisms. For a given composition, the Curie point temperature is almost invariant, being, according to molecular field theory, more sensitive to composition than cation distribution. Therefore, the model cannot explain the variations observed in synthetic titanomaghemite of different provenances. According to the model, the spontaneous magnetization is significantly lower for the removal of iron mechanism, which may be helpful in explaining a decrease in the remanence of submarine basalts with age.Structurally metastable titanomaghemites invert to an intergrowth of phases on heating above approximately 350°C and it is expected that the inversion product would be independent of the mechanism by which maghemitization had previously taken place. Available X-ray, and other data, have been used to produce a simple model of the inversion process, and values of the Curie point of the inversion product and the ratio of the magnetization before and after the inversion derived. These model values compare favourably with experimental values for a synthetic titanomaghemite in which inversion was induced by progressive slow heating to approximately 600°C. The inversion process therefore has potential in determining the compositions of naturally occurring titanomaghemites.  相似文献   

7.
Studies of the effect of dissolved salts on the oxygen isotope activity ratio of water have been extended to 275°C. Dehydrated salts were added to water of known isotope composition and the solutions were equilibrated with CO2 which was sampled for analysis. For comparison similar studies were made using pure water. Results on water nearly coincide with earlier calculations. Salt effects diminish with increasing temperature only for solutions of MgCl2 and LiCl. Other salt solutions show complex behavior due to the temperature-dependent formation of ion pairs of changing character. Equilibrium fractionations (103 ln α) between 1 molal solutions and pure water at 25, 100, and 275°C are: NaCl 0.0, ?1.5, +1.0; KCl 0.0, ?1.0, +2.0; LiCl ?1.0, ?0.6, ?0.5; CaCl2 ?0.4, ?1.8, +0.8; MgCl2 ?1.1, ?0.7, ?0.3; MgSO4 ?1.1, +0.1, ?; NaF (0.8 m) 0.0, ?1.5, ?0.3; and NH4Cl (0.55 m) 0.0, ?1.2, ?1.3. These effects are significant in the isotope study of hot saline fluids responsible for ore deposition and of fluids found in certain geothermal systems. Minor modification of published isotope geothermometers may be required.  相似文献   

8.
In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/ titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S-200 values (S-200 = IRM-200/ SIRM, where IRM-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.  相似文献   

9.
The Zuccale fault is a regional, low-angle, normal fault, exposed on the Isle of Elba in central Italy that accommodated a total shear displacement of 6–8 km. The fault zone structure and fault rocks formed at <8 km crustal depth. The present-day fault structure is the final product of several deformation processes superposed during the fault history. In this study, we report results from a series of rotary shear experiments performed on 1-mm thick powdered gouges made from several fault rock types obtained from the Zuccale fault. The tests were done under conditions ranging from room temperature to in situ conditions (i.e., at temperatures up to 300 °C, applied normal stresses up to 150 MPa, and fluid-saturated.) The ratio of fluid pressure to normal stress was held constant at either λ = 0.4 or λ = 0.8 to simulate an overpressurized fault. The samples were sheared at a constant sliding velocity of 10 μm/s for at least 5 mm, after which a velocity-stepping sequence from 1 to 300 μm/s was started to determine the velocity dependence of friction. This can be represented by the rate-and-state parameter (a–b), which was determined by an inversion of the data to the rate-and-state equations. Friction of the various fault rocks varies between 0.3 and 0.8, similar to values obtained in previous studies, and decreases with increasing phyllosilicate content. Friction decreases mildly with temperature, whereas normal stress and fluid pressure do not affect friction values systematically. All samples exhibited velocity strengthening, conditionally stable behavior under room temperature conditions and (a–b) increased with increasing sliding velocity. In contrast, velocity weakening, accompanied by stick–slips, was observed for the strongest samples at 300 °C and sliding velocities below 10 μm/s. An increase in fluid pressure under these conditions led to a further reduction in (a–b) for all samples, so that they exhibited unstable, stick–slip behavior at low sliding velocity. The results suggest that phyllosilicate-bearing fault rocks can deform by stable, aseismic creep at low, resolved shear stress and low shear rate conditions. An increase in fluid pressure or loading of stronger portions could lead to a runaway instability. The runaway instability might be limited in size because of (1) the fault heterogeneity, (2) the observed strengthening at higher sliding velocities, and (3) a co-seismic drop in pore-fluid pressure.  相似文献   

10.
Volume measurements for magnesiowüstite (Mg0.6Fe0.4)O, were carried out up to pressures of 10.1 GPa in the temperature range 300–1273 K, using energy-dispersive synchrotron X-ray diffraction. These data allow reliable determination of the temperature dependence of the bulk modulus and good constraint on the thermal expansitivity at ambient pressure which was previously not known for magnesiowüstite. From these data, thermal and elastic parameters were derived from various approaches based on the Birch–Murnaghan equation of state (EOS) and on the relevant thermodynamic relations. The results from three different equations of state are remarkably consistent. With (∂KT/∂P)T fixed at 4, we obtained K0=158(2) GPa, (∂KT/∂T)P=−0.029(3) GPa K−1, (∂KT/∂T)V=−3.9(±2.3)×10−3 GPa K−1, and αT=3.45(18)×10−5+1.14(28)×10−8T. The K0, (∂KT/∂T)P, and (∂KT/∂T)V values are in agreement with those of Fei et al. (1992) and are similar to previously determined values for MgO. The zero pressure thermal expansitivity of (Mg0.6Fe0.4)O is found to be similar to that for MgO (Suzuki, 1975). These results indicate that, for the compositional range x=0–0.4 in (Mg1−xFex)O, the thermal and elastic properties of magnesiowüstite exhibit a dependence on the iron content that is negligibly small, within uncertainties of the experiments. They are consequently insensitive to the Fe–Mg partitioning between (Mg, Fe)SiO3 perovskite and magnesiowüstite when applied to compositional models of the lower mantle. With the assumption that (Mg0.6Fe0.4)O is a Debye-like solid, a modified equation of heat capacity at constant pressure is proposed and thermodynamic properties of geophysically importance are calculated and tabulated at high temperatures.  相似文献   

11.
Atmospheres of different oxidation potentials were used to equilibriate samples at 1300°C starting with a stoichiometric composition xFe2TiO4·(1 ? x) Fe3O4 where x = 0.3 and 0.6. Samples were selected which appeared homogeneous in polished section and gave only spinel-type X-ray reflections. All specimens were found to have small amounts of included material of higher Curie point than the bulk material. Specimens were made by dispersing grains of known size in a non-magnetic matrix and the hysteresis properties were measured for these speciméns.  相似文献   

12.
There are some puzzling features of the stress-dependence of magnetic susceptibility in rocks, although the behaviour in high fields appears to be well understood. As a test for the factors that cause low-field behaviour to differ from current theories we have made both high- and low-field torquemeter measurements of the stress-induced magnetic anisotropies of a number of rocks. Ratios of low- and high-field torques differing from expectation by factors up to 4 or so have been found in some rocks, although others conform well to expectation. A comprehensive study of magnetic and microscopic properties was undertaken to seek the cause(s) of the discrepancies. Stress sensitivity of susceptibility becomes particularly high in titanomagnetite with composition parameter x ≈ 0.6, at which the intrinsic anisotropy vanishes, and some other high values appear to be related to the presence of sulphide. It is concluded that current theory is satisfactory if but only if the magnetic minerals in a rock are simple titanomagnetites with x ? 0.4.  相似文献   

13.
—The seismicity in the territory of China, a seismotectonically complicated region, has been examined by using three complete samples of earthquakes which occurred during the last two centuries (1800–1995). The b value of the Gutenberg-Richter relation was estimated by using this data sample. Taking into account the fact that the b value is spatially more stable than the a value, the b values were calculated at the nodes of a normal grid superposing on the entire area studied, and their distribution was examined. The results showed that the b value increases smoothly from 0.4 in inner-Mongolia to 0.8 in the east, south and southwest of China with higher values (b>0.8) in the Taiwan region. Furthermore, keeping fixed the obtained b values, the a value distribution was also examined. In order to display more detailed information about the seismicity, smaller cell surface (10,000 km2) for the calculation of the a values was chosen. The mean return periods for different cutoff magnitudes were also calculated for each of these small cells. It was observed that the mean return periods are the shortest ones in China, which are 10 and 50 years for the magnitude larger than or equal to 6.0 and 7.0, respectively.  相似文献   

14.
We investigated the infrared stimulated luminescence (IRSL) and post-infrared IRSL (pIRIR) signals emitted by K-feldspars from sedimentary samples from Asia, Europe and Africa using a single-aliquot multiple elevated temperature (MET) stimulation procedure. For separate aliquots of the same sample, we show that variation among the dose response curves (DRCs), or growth curves, constructed from the regenerative dose signal (Lx), the test dose signal (Tx, an indicator of luminescence sensitivity) and the sensitivity-corrected signal (Lx/Tx) can be largely eliminated by normalising the DRCs using one of the regenerative dose signals; we call this procedure ‘regenerative-dose normalisation’ or re-normalisation. Furthermore, for the MET-pIRIR signals measured at 250 °C, we find that different samples have re-normalised DRCs that follow the same growth function, despite the samples differing significantly in terms of their geological provenance, sedimentary context, equivalent dose (De) and luminescence sensitivity. This common feature offers the potential to establish a ‘global standardised growth curve’ (gSGC) for different samples of K-feldspar, and thereby enable De values to be estimated for a large number of single aliquots by projecting the re-normalised natural signals on to the gSGC. For the 18 samples investigated in this study, we find that De estimates obtained from the gSGC are consistent with those obtained using full single-aliquot regenerative dose (SAR) procedures for doses of up to ∼1600 Gy. The establishment of a gSGC would greatly reduce the time required to date older samples using K-feldspar, as regenerative doses of several hundreds to a few thousands of Gy are typically delivered to each aliquot in each SAR cycle.  相似文献   

15.
Dredged rocks from an area of about 15 km2 within the inner floor and on the adjacent walls of the Rift Valley were collected. Based on petrographic and chemical data, four types of basaltic rocks were recognized: (1) picritic basalts with olivine xenocrysts, TiO2 < 0.6%, K2O < 0.1%, (2) olivine basalts with olivine megacrysts, TiO2 = 0.8–1.5%,K2O = 0.1–0.2%, (3) highly phyric and moderately phyric plagioclase basalts with megacrystic plagioclase, TiO2 < 1.3%, K2O < 0.3%, and (4) pyroxene basalts with pyroxene > plagioclase, TiO2 = 0.8–1%,K2O = 0.2–0.4%. The Cr and Ni having high partition coefficients show different variation trends for each type of rock and their values decrease continuously as crystallization proceeds within each type of basalt. It is speculated that two different magmas have given rise to the above-mentioned rocks. One has yielded the picritic basalts and subsequently the olivine basalts after a separation of the olivine cumulates; the other gave rise to the plagioclase basalts.  相似文献   

16.
Stoichiometric titanomagnetites Fe3?xTixO4 with compositions between x = 0 (magnetite) and x = 0.72 (a titanomagnetite having a Curie temperature of 60°C) have been synthesised using the double-sintering technique in controlled atmospheres. The quality of these materials was tested by various mineralogical and magnetic measurements. Isolated small multidomain (MD) and pseudo-single-domain (PSD) particles within pores of the bulk material were investigated with respect to their domain structures, and threshold sizes for the transition from the PSD to the SD stage determined for titanomagnetites of various compositions by extrapolation from the domain state of small MD grains. The threshold size was found to be 0.7 and 0.5 μm, respectively, for TM72 (x = 0.72) and TM62 (x = 0.62). The threshold size decreases slightly for smaller x values; however, the experimental data obtained to date are not sufficiently reliable to yield precise results.Preliminary experiments concerning hysteresis loops and TRM generation are also reported.  相似文献   

17.
—Regional seismograms were collected to image the lateral variations of Lg coda Q at 1 Hz (Q 0?) and its frequency dependence <(eta)> in the Middle East using a back-projection method. The data include 124 vertical-component traces recorded at 10 stations during the period 1986–1996. The resulting images reveal lateral variations in both Q 0 and <eta>. In the Turkish and Iranian Plateaus, a highly deformed and tectonically active region, Q 0 ranges between about 150 and 300, with the lowest values occurring in western Anatolia where extremely high heat flow has been measured. The low Q 0 values found in this region agree with those found in other tectonically active regions of the world. Throughout most of the Arabian Peninsula, a relatively stable region, Q 0 varies between 350 and 450, being highest in the shield area and lowest in the eastern basins. All values are considerably lower than those found in most other stable regions. Low Q values throughout the Middle East may be caused by interstitial fluids that have migrated to the crust from the upper mantle, where they were probably generated by hydrothermal reactions at elevated temperatures known to occur there. Low Q 0 values (about 250) are also found in the Oman folded zone, a region with thick sedimentary deposits. <eta> varies inversely with Q 0 throughout most of the Middle East, with lower values (0.4–0.5) in the Arabian Peninsula and higher values (0.6–0.8) in Iran and Turkey. Q 0 and <eta> are both low in the Oman folded zone and western Anatolia.  相似文献   

18.
—We determined the response to P- and S-wave incidence of the permanent stations of the seismic network of Baja California (RESNOM) using two independent methods. We selected 65 events with magnitudes between 2.2 and 4.8 and hypocentral distances ranging between 5 and 330 km. The site response of the ten stations analyzed was first estimated using average spectral ratios between the horizontal and the vertical components of motion (H/V ratios). As a second approach we performed a simultaneous inversion for source and site. In order to invert the spectral records to determine the site response, we made an independent estimate of the attenuation for two different source-station path regions. Then we corrected the spectral records for the attenuation effect before we made the inversion. Although the average H/V ratio of many sites is inside the error bars of the site response estimated with the spectral inversion, the spectral inversion tends to give higher values. For the S wave some sites show similar frequency of predominant peak when comparing the responses obtained with both methods. In contrast, for the P waves the H/V ratios disagree with the results of the inversion. In general, the site response of the stations is strongly frequency dependent for both P and S waves. We also found that the natural frequency of resonance of the sites is near 0.5 Hz for P and near 0.8 Hz for the S waves.  相似文献   

19.
We have carried out in situ X-ray diffraction experiments on the FeS–H system up to 16.5 GPa and 1723 K using a Kawai-type multianvil high-pressure apparatus employing synchrotron X-ray radiation. Hydrogen was supplied to FeS from the thermal decomposition of LiAlH4, and FeSHx was formed at high pressures and temperatures. The melting temperature and phase relationships of FeSHx were determined based on in situ powder X-ray diffraction data. The melting temperature of FeSHx was reduced by 150–250 K comparing with that of pure FeS. The hydrogen concentration in FeSHx was determined to be x = 0.2–0.4 just before melting occurred between 3.0 and 16.5 GPa. It is considered that sulfur is the major light element in the core of Ganymede, one of the Galilean satellites of Jupiter. Although the interior of Ganymede is differentiated today, the silicate rock and the iron alloy mixed with H2O, and the iron alloy could react with H2O (as ice or water) or the hydrous silicate before the differentiation occurred in an early period, resulting in a formation of iron hydride. Therefore, Ganymede's core may be composed of an Fe–S–H system. According to our results, hydrogen dissolved in Ganymede's core lowers the melting temperature of the core composition, and so today, the core could have solid FeSHx inner core and liquid FeHx–FeSHx outer core and the present core temperature is considered to be relatively low.  相似文献   

20.
A review of experiments in which arrays of recording magnetometers deployed over an area of land to study induction problems is presented. Intensive array activities take place on a continual basis in North America, Australia, Scotland and Africa and array studies have been conducted in India, Scandinavia, and Russia. The main results are summarised. Analysis, presentation and interpretation of array data have also enjoyed significant developments which are discussed with illustrations. The use of multi-techniques in the analysis of array data has improved the reliability of the interpretational inferences. One-dimensional conductivity profiles can now be deduced by the simple inversion of the inductive response functionC(w, t) estimated from the vertical field Z and spatial gradient of the horizontal field components (ΔX/Δx+ΔY/Δy) of magnetometer array data with large gradient. Bounds can be placed on accepted profiles using the Monte Carlo process just as it is done in magnetotelluric data inversion. The results from array studies continue to improve our understanding of the synthesis of realistic tectonic models of the continents. The structure under some geothermal zones are now known through a number of recent studies.  相似文献   

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