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1.
Kamacite Ni profiles in low-Ni and high-Ni IVA irons are distinctly different, and cannot be fit with the same α/(α + γ) boundary in the low-temperature Fe-Ni phase diagram. This is attributed to an expansion of the α field to higher Ni contents resulting from the substantially higher P contents of the high-Ni irons. New α/(α + γ) boundaries are derived for P contents of 0.03 and 0.16%.Cooling rates of six group IVA iron meteorites were estimated by a taenite central Ni concentration-taenite half-width method similar to that of Wood [1]. Narrow (<20 μm) taenite lamellae were used to minimize uncertainties resulting from differences in nucleation temperatures. The calculated cooling rates range between 13 and 25°C/Myr, with an average of 20°C/Myr. No correlation between cooling rate and bulk Ni content is observed, and the data appear to be consistent with a uniform cooling rate as expected from an igneous core origin. This result differs from previous studies reporting a wide range in cooling rates that were strongly correlated with bulk Ni contents. The differences mainly result from differences in the phase diagram and the selected diffusion coefficients.Cooling rates inferred from taenite Ni concentrations at the interface with kamacite are consistent with those based on taenite central Ni contents. 相似文献
2.
Cooling rates have been determined for twelve group IVA iron meteorites using a ternary (Fe-Ni-P) model that simulates the growth of the Widmanstätten pattern. The new ternary model is governed by a set of differential diffusion equations that are coupled through the phase growth velocity and elemental concentration profiles. Measured ternary diffusivities and phase diagram solubilities were extrapolated below 500°C for use in the model. The model is more sophisticated than previous ones in that P as well as Ni gradients are calculated, ternary α-γ tie lines are allowed to vary, and ternary diffusivities are used.Output from the simulation is used to create a family of cooling rate curves on plots of central taenite Ni vs. log taenite half-width for each meteorite. A comparison of measured data to the cooling rate curves yields unique meteorite cooling rates. The measured cooling rates for the twelve IVA irons vary inversely with Ni content by over an order of magnitude (4–200°C/Myr). It is proposed that the group IVA irons were accommodated at various depths in an asteroidal-sized body. 相似文献
3.
Because uncertainties in experimental data are large, one has considerable latitude in choosing the input parameters needed to calculate iron meteorite cooling rates. The best way to test input parameters is by examining their ability to yield the observed properties of the meteorites. Our phase diagram yields fits to kamacite profiles that are superior to those based on the Moren-Goldstein phase diagram. Our method of allowing for the effect of P on the Ni diffusion coefficient takes into account the enhancement in this effect with decreasing temperature; Moren and Goldstein use a relationship derived for a temperature of 1100°C, well outside the 700–350°C range where kamacite growth occurs. Use of our input parameters yields cooling rates in IVA irons that are independent of composition, consistent with a core origin. Since the fractionation of siderophiles in group IVA also indicates a core origin, we conclude that this is the correct model for this group. 相似文献
4.
It has been an enigma that in the Tieschitz, H3, and other unequilibrated chondrites the silicates show quench textures yet their metallic minerals, according to the Wood [6] model, appear to have cooled extremely slowly.In Tieschitz, spherical metallic chondrules up to 500 μm in diameter, with textures indicating an origin as liquid droplets, consist of polycrystalline intergrowths of α(kamacite), γ(taenite) and troilite. Interface Ni compositions of contiguous α (～5 wt.%) and γ (～50 wt.%) grains define equilibrium tie-line relationships in the Fe-Ni system indicating equilibration to ～350°C (620 K). Polycrystalline γ(taenite) is multi-zoned with respect to Ni and is interpreted as the relict of a primary solidification structure. A mechanism whereby Ni compositional heterogeneities were produced in γ(taenite) by the rapid, non-equilibrium cooling of FeNiS melts during chondrule formation is discussed.Comparisons with lunar metal globules indicate solidification rates for Tieschitz metallic chondrules in the range 1–106 K/s. It is suggested that before or during aggregation, sub-solidus cooling in the temperature range ～700–1400°C with cooling times of days to weeks allowed the preservation of a relict solidification structure in metallic chondrules. At a temperature of ～700°C accretion and shallow burial (1–10 m) on the surface of the Tieschitz parent body provided insulation with slower cooling required to nucleate and grow α(kamacite) from the heterogeneous γ(taenite) under equilibrium conditions by the process of solid state diffusion proposed by Wood [6]. The cooling rate (1 K/106 yr) through 500°C derived using the Wood model is shown to be an underestimate of the real cooling rate of Tieschitz metal through that temperature, since it does not take into account Ni heterogeneities produced at higher temperatures. A rough estimate of the post-accretional cooling rate is obtained from the average size of α(kamacite) grains(<100 μm) andTeqα ～ 350°C indicating a cooling rate of the order of<1K/103yr through 500°C. 相似文献
5.
H.J. Axon 《Earth and Planetary Science Letters》1979,42(2):237-238
A number of group IVA iron meteorites show metallographic features suggesting that a shock annealing event intervened during the formation of the Widmanstatten structure. This could complicate the estimation of cooling rates based on diffusion profile methods but would have less influence on methods based upon macroscopic measurements of kamacite band widths. 相似文献
6.
The metal in seven Type III carbonaceous chondrites has been measured for concentrations of Ni, Co and Cr. Cobalt in kamacite is 3.2 to 5.5 times greater than in taenite on composite grains containing both phases. No correlation was found between the metal compositions and sub-type classification. Ni and Co contents of kamacite from several of the Type III's studied fall outside of the range for these elements in bulk meteoritic metal and are relevant to the assignment of a meteoritic vs a non-meteoritic origin for lunar metal particles in the fines and breccias. 相似文献
7.
铁镍合金是陨石中重要的磁性物质,其中铁纹石、镍纹石和四方镍纹石是球粒陨石中的主要铁镍合金.然而,迄今针对陨石中铁镍合金的磁学性质研究仍非常缺乏.本文研究了吉林陨石中的铁纹石、四方镍纹石、以及陨硫铁的磁学特征.实验表明,镍含量为6%~7%的铁纹石是该陨石中最主要的铁镍合金物质,它具有低矫顽力和高的热稳定性,居里温度~750 ℃.镍含量为~48%的四方镍纹石具有高矫顽力和高的热稳定性,居里温度~565 ℃,它是剩磁的主要载体.陨硫铁在室温为反铁磁性,不具有载剩磁能力,在60 K左右存在一个低温转换,在氩气中加热较稳定而在空气中加热被氧化转化为磁铁矿.这些研究结果为鉴定球粒陨石中的磁性物质提供了依据. 相似文献
8.
D.W. Sears 《Earth and Planetary Science Letters》1978,41(2):128-138
The pressure-temperature conditions in the primordial nebula which could produce the observed Ni, Ga and Ge abundances in the major iron meteorite groups have been calculated assuming equilibrium condensation. Included in these calculations are the effect on the metal composition of Fe oxidation and sulphide formation during accretion, GeS and GaCl in the nebula gases and pressure variations in the nebula. It was found that the IIAB irons had their abundances of these elements fixed at the low-pressure extreme of the range which gives the IAB irons, but at 50 ± 10K higher temperatures. IIIAB and IVA formed over the same temperature range as IAB (600–670?40+60 K) in regions where the pressure was lower by a factor of 20 and 104 respectively. Group IVB accreted soon after condensation of the metal and at pressures of less than 10?3 atmosphere. The distribution of sulphur and carbon are consistent with this. The abundance of carbon in group IAB suggests that this and group IIAB accreted at about 10?4 atmosphere, so that IIIAB and IVA accreted where the pressure was 5 × 10?6 and 10?8 atmosphere, respectively. 相似文献
9.
E. Rambaldi 《Earth and Planetary Science Letters》1976,31(2):224-238
Eight L- and one LL-group chondrites were selected for a major and trace element content study of their metals by instrumental neutron activation techniques. The elements Ni, Co, Fe, Cu, As, Ga, W, Au and Ir were determined. For each meteorite three metallic fractions were analyzed: (1) coarse, >100 mesh; (2) intermediate, 100–200 mesh; (3) fine, <200 mesh. The composition of the metals varies considerably with grain size, as a result of a preferential concentration of kamacite in the coarse and of taenite in the intermediate and fine fractions.A third metallic component, consisting of very fine plessitic inclusions, was observed in chondrules of equilibrated chondrite types 5 and 6. This component is probably responsible for the decrease of Ni, Cu, Ga and Au observed in the fine metallic fractions of the equilibrated chondrite types.W, as well as Ga, increases in the bulk metals with the petrologic type, suggesting that a substantial amount of this element, as already observed for Ga by previous authors, is not in the metal, but in some silicate phases in the lower metamorphic petrologic types 3 and 4.Ir is always concentrated in the fine metallic fractions of all meteorites, independent of petrologic type, suggesting the presence of a fine-grained metallic component enriched in this element. 相似文献
10.
Laboratory experiments were performed on chalk samples from Denmark and Israel to determine diffusion and distribution coefficients. Batch tests were used to define sorption isotherms for naphthalene and o-xylene. Linear sorption isotherms were observed and described with Henry-isotherms. Because of the high purity and low contents of clay minerals and organic carbon, Danish and white Israeli chalk generally have low retardation capacities. Contrarily, gray Israeli chalk, with organic carbon fractions as high as 1.092%, remarkably retards organic contaminants. The K(OC) concept is not applicable to predicting distribution coefficients based on the organic carbon content in the chalk samples. Effective diffusivities of o-xylene, naphthalene, and several artificial tracers were determined using through-diffusion experiments. Based on measured diffusion coefficients and available literature values, a chalk specific exponent of 2.36 for Archie's law was derived, allowing a satisfactory estimate of relative diffusivities in chalk. A field-scale tracer test with uranine and lithium was performed in the Negev desert (Israel) to examine the transferability of diffusivities determined on small rock samples in the laboratory. Due to low recovery rates of the tracer, a modified single fissure dispersion model was used for inverse modeling of the breakthrough curves. Resulting diffusivities deviate insignificantly from the laboratory values, which are considered to be representative for the investigated part of the aquifer and applicable in transport models. 相似文献
11.
H. Voshage 《Earth and Planetary Science Letters》1978,40(1):83-90
The cosmic ray exposure ages of 16 iron meteorites were determined by the41K/40K-4He/21Ne method. The ages measured in the present and in previous experiments are summarized and presented in form of various histograms characterizing the age distributions of the different chemical groups separately. Age clustering at 650 Ma (mega years) is typical for the group IIIAB. Age clustering at 400 Ma is observed for the IVA irons. Quasi-continuous age distributions are found for the groups IA, IIA, IIB, IVB and for the anomalous irons. The following interpretation is offered. The IIIA and IIIB irons have initially been core material of the same parent asteroid and were ejected in consequence of a single impact event about 650 Ma ago. The IVA irons represent core material of another asteroid which was hit and partially disrupted in consequence of an impact event about 400 Ma ago. The group IA exhibits meteorites with ages between 200 and 1200 Ma. The quasi-continuous character of this age distribution and cosmochemical evidence indicate for these irons a raisin-bread-like character of their initial distribution within the silicate mantle of their parent asteroid. In consequence of several or, perhaps, of many crater-forming impact events the mantle material was gradually destructed and ejected. In the age distribution of the IIA hexahedrites, ages <300 Ma predominate and ages >600 Ma seem to be missing. In attempting to understand this, the possibility must be taken into consideration that the mean life-time of hexahedrites in the interplanetary space might be shorter than that of other irons. The cause might be that the hexahedrite single crystals are perhaps easier cleavable in the space environment. A similar kind of selective mass wastage appears also to be the cause for the absence of stone meteorites with high exposure ages. 相似文献
12.
The ion microprobe has been used to analyse olivine in seven pallasites for Ni. The results are in the range 22–41 μg/g and are significantly lower than published electron probe analyses. The olivine-metal equilibrium temperatures deduced from thermodynamic calculations are correspondingly lower, and it is likely that equilibrium between olivine and metal was maintained to somewhat below the temperature of the γ-α transformation in metal (～700°C). The results appear to be consistent with cooling rates of ～1K/Ma derived from metal phase compositions, and certainly more so than the previous electron probe data. A Ni profile across an olivine grain in Eagle Station shows a decrease from 40 μg/g at the centre to 25 μg/g at the edge, illustrating the effect of limited diffusion of Ni in olivine during cooling. A correlation between grain size and central Ni content was also observed in Eagle Station, the smallest grains having the lowest Ni content. 相似文献
13.
ABSTRACTTurbulence in the Earth's outer core not only increases all diffusive coefficients, but it can lead to their anisotropic properties. Therefore, the model of rotating magnetoconvection in horizontal plane layer rotating about vertical axis and permeated by homogeneous horizontal magnetic field, influenced by anisotropic diffusivities, viscosity and thermal diffusivity, is advanced by considering the magnetic diffusivity as anisotropic too. The case of full anisotropy, i.e. all coefficients anisotropic, is compared with both the case possessing isotropic diffusion coefficients and the case of partial anisotropy, i.e. mixed case with isotropic and anisotropic diffusive coefficients (viscosity and thermal diffusivity anisotropic and magnetic diffusivity isotropic). The existence and preference of instabilities is sensitive to all non-dimensional parameters, as well as on anisotropic parameter, the ratio of horizontal and vertical diffusivities. Two types of anisotropy, BM (introduced by Braginsky and Meytlis) and SA (stratification anisotropy) are studied. BM as well as SA were applied by ?oltis and Brestenský to the study of the partial anisotropy; this study is extended, in this paper, to full anisotropy cases (full SA and full BM) and it is shown that the style of convection given by the onset of stationary modes is more affected by anisotropic diffusivities in BM than in SA anisotropy. The important influence of strong anisotropies in the Earth's core dynamics is stressed. 相似文献
14.
Soret diffusion rise times were observed and resolved for Fe–Ni–S–P melts at 10 kbar in thermal gradients between 1300 and 1450°C. A steady-state separation of S-rich components to the hot end of the 2 mm long liquid charges is achieved in about 20 minutes. Chemical diffusivities of about 10−5 cm2/s are consistent with these observations and those of the compaction velocity of aggregates of crystalline Fe metal against the cold end of our experiments. These rapid diffusivities are not rapid enough for diffusion to be an important agent of bulk mass transfer in the addition or extraction of light elements in the Earth's core. 相似文献
15.
P. Pellas C. Perron G. Crozaz V.P. Perelygin S.G. Stetsenko 《Earth and Planetary Science Letters》1983,64(3):319-326
Nuclear tracks were studied in olivine and merrillite (phosphate previously called whitlockite) from the Marjalahti pallasite. The merrillite contains an important fission contribution due mainly to the spontaneous decay of now extinct244Pu. The U contents of 29 merrillite grains range from 60 to 140 ppb (median value: 85 ppb). Assuming a reasonable fractionation temperature of ～ 1750 K for the pre-pallasitic material, a lower limit of ～ 5 K/Myr is obtained for the cooling rate, in strong contrast with the previous metallographic result (～ 0.5 K/Myr). This disagreement, together with those observed in the case of mesosiderites, strengthens the need for a revision of the metallographic method of retracing the cooling histories of meteorites, as suggested by Narayan and Goldstein [31]. 相似文献
16.
D. M. Pechersky G. P. Markov V. A. Tsel’movich Z. V. Sharonova 《Izvestiya Physics of the Solid Earth》2012,48(7-8):653-669
Thermomagnetic and microprobe analyses are carried out and a set of magnetic characteristics are measured for 25 meteorites and 3 tektites from the collections of the Vernadsky Geological Museum of the Russian Academy of Sciences and Museum of Natural History of the North-East Interdisciplinary Science Research Institute, Far Eastern Branch of the Russian Academy of Sciences. It is found that, notwithstanding their type, all the meteorites contain the same magnetic minerals and only differ by concentrations of these minerals. Kamacite with less than 10% nickel is the main magnetic mineral in the studied samples. Pure iron, taenite, and schreibersite are less frequent; nickel, various iron spinels, Fe-Al alloys, etc., are very rare. These minerals are normally absent in the crusts of the Earth and other planets. The studied meteorites are more likely parts of the cores and lower mantles of the meteoritic parent bodies (the planets). Uniformity in the magnetic properties of the meteorites and the types of their thermomagnetic (MT) curves is violated by secondary alterations of the meteorites in the terrestrial environment. The sediments demonstrate the same monotony as the meteorites: kamacite is likely the only extraterrestrial magnetic mineral, which is abundant in sediments and associated with cosmic dust. The compositional similarity of kamacite in iron meteorites and in cosmic dust is due to their common source; the degree of fragmentation of the material of the parent body is the only difference. 相似文献
17.
On a simplified radiative-conductive heat transfer equation 总被引：2，自引：0，他引：2
H. L. Kuo 《Pure and Applied Geophysics》1973,109(1):1870-1876
Summary A simplified equation purported to represent the joint influence of radiative and turbulent transfers of heat in the atmosphere is derived by dividing the absorption spectrum of terrestrial radiation into strongly and weakly absorbed regions, classified according to the local scale of variation or to the local heating rate, and introducing two mean absorption coefficients for these two groups of regions. Assurning the validity of theK-theory of turbulent diffusion of heat, it is found that the temperature of the atmosphere is governed by a sixth-order partial differential equation in the heightz. This equation can be simplified to the second order if the mean absorption coefficient of the strongly absorbed regions is much larger while that of the weakly absorbed regions is much smaller than the local scale variation, and the influence of the former is equivalent to an added diffusion while that of the latter is a newtonian cooling, and these two influences are present simultaneously. The values of the two coefficients and their dependencies on the concentration of the absorbing material have been obtained. The equation has been applied to the problem of the thermal interaction between the atmosphere and the underlying earth, as related to the diurnal heat wave, and it was found that the temperature changes within the first few hundred meters from the earth can be predicted accurately by the model when the partition of the two groups is adjusted to the heating rate and the eddy transfer coefficient is allowed to increase very rapidly within the lowest 10 m. 相似文献
18.
Takamoto Okudaira Yasutaka Hayasaka Osamu Himeno Koichiro Watanabe Yasuhiro Sakurai Yukiko Ohtomo 《Island Arc》2001,10(2):98-115
Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole-rock–mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis. 相似文献
19.
Solute exchange between the interstitial waters and overlying waters on the Washington continental shelf was investigated based on measurements of the pore-water sulfate distribution and sulfate reduction rates as well as through models describing the distribution of sulfate in anaerobic pore waters. The depth-integrated sulfate reduction rate greatly exceeded the influx of sulfate attributable to molecular diffusion and sediment accumulation acting on the measured vertical sulfate gradients, and indicated that additional transport mechanisms must have been operating. Sediment mixing was probably not the primary mechanism since high eddy diffusivities would be required to depths of 30 cm to maintain the observed sulfate distribution, whereas previously measured210Pb distributions indicated sediment mixing is primarily restricted to depths <7 cm. Irrigation of bottom water through animal burrows was the most likely mechanism. To describe this process, a general diffusive irrigation coefficient, B, was formulated. Vertical profiles of B showed the main irrigation zone occurred between 2 and 10 cm with reduced irrigation rates occurring below this. These coefficients calculated from the sulfate distribution were similar to ones calculated from previously published radon measurements at the same stations, indicating that this formulation of irrigation exchange may be useful in modelling the exchange of dissolved solutes between the pore water and the bottom water. 相似文献
20.
James E. Gardner Kenneth S. Befus James Watkins Marc Hesse Nathan Miller 《Bulletin of Volcanology》2012,74(8):1865-1879
Spherical masses of crystal fibers (spherulites) crystalize from rhyolitic melt/glass mainly in response to significant undercooling while lava cools. Spherulite growth should induce compositional gradients in the surrounding glass from expulsion of incompatible constituents and diffusion of those constituents away from the spherulite. Finite-difference numerical modeling of one-dimensional diffusion, in which diffusivities are allowed to vary with temperature, is used to investigate how compositional gradients reflect spherulite growth and lava cooling. Overall, three forms of gradients are identified. Elements that diffuse quickly are expelled from the spherulite but then migrate away too quickly to become enriched at the boundary of the spherulite. Elements that diffuse slowly are trapped within the growing spherulite. Between those endmembers are elements that are not trapped, yet diffuse slow enough that they become enriched at the contact. Their slow diffusion away then elevates their concentrations in the surrounding glass. How enriched those elements are at the spherulite-matrix interface and how far their enrichments extend outwards into the glass reflect how spherulites grow and thermal conditions during growth. Concentrations of H2O, Rb, F, Li, Cl, Na, K, Sr, Cs, Ba, and Be were measured in and around spherulites in obsidian from a 4.7?±?1?km3 rhyolite lava dome erupted from Tequila volcano, Mexico. Measurable concentration gradients are found for H2O, Rb, and F. Attributes of those gradients and the behaviors of the other elements are in accord with their experimentally constrained diffusivities. Spherulites appear to have grown following radial, rather than volumetric, growth. The observed gradients (and lack of others) are more consistent with growth mainly below the glass transition, which would necessitate the dome cooling at ca. 10?5 to 10?7?°C?s?1. Such slow cooling is consistent with the relatively large volume of the dome. 相似文献