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1.
Cooling rates of eight group IVA iron meteorites were determined using a modification of the Wood method where cooling rate curves are calculated as a function of central taenite Ni content and taenite half-width. The major modification of the Wood method was to include the effect of P on the Ni solubility limits and Ni diffusion coefficients in the kamacite phase for each meteorite studied. The Borg and Lai binary kamacite Ni diffusivities were judged to be the best quality data available for the calculations. The calculated cooling rates range between 3 and 65°C/Myr. A correlation between decreasing cooling rate and increasing Ni content within the group IVA irons is observed. This cooling rate variation agrees closely with the 6–70°C/Myr range calculated by the independent bulk Ni-kamacite bandwidth method. Such a large variation in cooling rate within the group IVA argues against formation within the core of a single parent body.Willis and Wasson in the preceding paper found only a factor of 2 variation in cooling rate for the six IVA irons studied. The differences between the results of Willis and Wasson and this study are due mainly to the choice of the Ni diffusion coefficients in α and to the choice of the expressions for the effect of P on both the diffusion coefficients and the Ni solubility in kamacite. The Hirano et al. diffusivities used by Willis and Wasson were judged to be incorrect particularly because they are significantly higher than diffusivities in kamacite and taenite measured by other investigators. The assumption of Willis and Wasson that the same P content (0.03 wt.%) can be used for the low-Ni IVA's and that the same P content (0.16 wt.%) can be used for the high-Ni IVA's was judged to be a serious oversimplification.  相似文献   

2.
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   

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.
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   

6.
Steinbach is a stony-iron meteorite with approximately equal amounts of silicate and metal that shows Widmanstätten structure. The silicate portion contains tridymite, orthobronzite, and clinobronzite that formed by inversion from high-temperature protobronzite. The assemblage orthobronzite-protobronzite-tridymite-metallic iron indicates an equilibrium temperature of 1200°C and an ?o2 of 10?12 under a total pressure of less than 2 kbar. Preservation of the high-temperature phase relations implies much more rapid cooling in the 1200-700°C range than the rates that have been deduced for the development of Widmanstätten structure in the 700-500°C range.  相似文献   

7.
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   

8.
We have analyzed the nickel isotopic composition of meteoritic materials by high-precision mass spectrometry. The samples analyzed include almost all meteorite types for which large isotopic anomalies have been reported for oxygen, silver, magnesium and titanium. These samples are C1, C3, L, LL, H and E chondrites, IVB irons, Eagle Station pallasite and inclusion, matrix and “whole rock” samples of the Allende meteorite. The result is that we have not found any anomaly for nickel isotopic compositions within our accuracy of 0.7‰ for61Ni/60Ni, 0.4-0.08‰ for62Ni/60Ni and 1–1.5‰ for64Ni/60Ni.  相似文献   

9.
The thermoluminescence (TL) levels of 45 ordinary chondrites were measured to obtain information about the meteorite orbits. The low-temperature TL reaches equilibrium while the meteorite is in space and reflects the temperature of the meteorite at perihelion. Samples of Pribram, Lost City, and Innisfree, whose orbits are accurately known, were used as control samples. The TL levels in 40 out ot 42 meteorites are similar to the three control samples, indicating that the vast majority of ordinary chondrites that survive atmospheric entry have perihelia similar to three known orbits, i.e., in the range 0.8–1 AU. The effects of albedo and rotation are also considered. A simple model indicates that temperature gradients of 1–2°K/cm are possible in slowly rotating meteoroids and such a temperature gradient is consistent with the unusually large TL gradient measured in the Farmville meteorite. Since slow rotation rates are improbable, other possibilities are examined but no satisfactory explanation has been found. The TL level measured in the Malakal meteorite is two orders of magnitude lower than control samples and is best explained by thermal draining due to solar heating in an orbit with a small perihelion distance. The perihelion is estimated to be～0.5–0.6AU.  相似文献   

10.
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   

11.
Antimony concentrations determined by radiochemical neutron activation analysis in 60 iron meteorites range from 0.2 ng/g to 36 μg/g. The meteorites with the highest Sb concentrations are those of the non-magmatic groups IAB and IIICD, while the lowest Sb concentrations are found in groups IVA and IVB, the groups with the lowest concentrations of the other most volatile siderophiles Ge and Ga. In all groups Sb is positively correlated with Ni. In each of the magmatic groups slopes on log Sb vs. log Ni plots decrease with increasing Ni. This decrease may reflect an increasing tendency to avoid schreibersite during the analysis of high-Ni meteorites because Sb partitions strongly into schreibersite.Schreibersite from New Westville is enriched in Cr, Ni, Ge, As, Sb and Au and depleted in Fe, Co and Ir; the content of Sb in schreibersite is 540 × higher than the bulk metal value.The Sb abundances of the iron meteorite groups are as expected from volatility trends with the exception of IAB and IIAB in which abundances appear depleted. The most likely explanation for this and the decreasing slope in the magmatic groups is that one or more Sb-rich phases were not sampled during metal analyses.  相似文献   

12.
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   

13.
The paleomagnetic field intensity is estimated with the aid of the Koenigsberger-Thellier method for four ordinary chondrites and one carbonaceous chondrite by assuming that the stable NRM component of these meteorites is attributable to the TRM acquired in a low-temperature range (lower than about 400°C) during their extremely-slow cooling process. The results are summarized in Table IV, where the paleomagnetic field intensity ranges from 0.10 to 0.97 Oe.Possible effects of the extremely-slow cooling rate of meteorites and the secondary TRM acquisition of the surface fusion crust upon the original NRM of the meteorite interior are discussed.  相似文献   

14.
The Allende meteorite has been examined with a view to applying thermoluminescence (TL) to the study of a meteorite's passage through the atmosphere. At least three kinds of TL-bearing minerals are present. A strong peak at 140°C is due to forsterite, and one at 200°C is probably caused by cordierite. By far the most intense TL comes from an alteration product associated with gehlenite.In the 4-cm diameter meteorite examined the 200°C TL varied in intensity across the stone, showing it to be produced by fragmentation. Temperature gradients induced by atmospheric heating can also be derived, and indicate the orientation of the meteorite. Together with fusion crust measurements these results enable the final phase of the meteorite's passage through the atmosphere to be delineated.  相似文献   

15.
The warming trend observed during the last decades in the Bay of Biscay is put within the context of sea surface temperature (SST) changes observed in the area since 1854. Macroscopically, two consecutive warming–cooling cycles were detected during this period of time: cooling from 1867 to 1910; warming from 1910 to 1945; cooling from 1945 to 1974; and warming from 1974 to nowadays. Warming rates of 0.17 and 0.22 °C per decade were measured during the warming sub-periods and cooling rates of ?0.14 and ?0.10 °C per decade were measured during the cooling sub-periods. The present warming period is on the same order of magnitude although slightly more intense than the one observed from 1910 to 1945, which is consistent with previous analysis carried for the North Atlantic. Finally, the thermal amplitude defined as the difference between the maximum and minimum annual values has increased since 1974 at a rate of 0.06 °C per decade due to the different increasing rates of the maximum (0.26 °C per decade) and minimum (0.20 °C per decade) SSTs.  相似文献   

16.
Compositions of coexisting ilmenite and titano-magnetite commonly have been used to establish the conditions of formation for igneous and metamorphic rocks (Buddington-Lindsley geothermometer/oxybarometer). Temperatures so determined for some rocks are lower than might be expected for magmatic crystallization. These low temperatures are the result of re-equilibration of the Fe-Ti oxides during slow cooling. This process is one in which titano-magnetites are oxidized and ilmenites are reduced upon cooling along a buffer curve or in the presence of a fluid of constant composition. Textural studies indicate that the reduction of the ilmenite-hematite phase may be the slower reaction consequently, the rate-controlling step of the re-equilibration.The kinetics of the reduction reactions in the two-phase region between the joins for the ilmenite-hematite and magnetite-ulvöspinel solid solution series were studied by thermogravimetric analysis at controlledfO2. With experiments conducted near the Ni/NiO and QFM buffers, annealed ilmenite(ss) synthetic charges were reduced isothermally. The temperature range for the experiments was 900–1250°C, with total gas-flow rates of 0.35 and 1.0 cm/s. Kinetic data were collected by measuring the weight change during each experiment as a function of time. Additionally, a cooling-rate (0.5°C/hr) experiment was conducted near the QFM buffer from 1150 to 985°C starting with an ilmenite-saturated spinel phase.At 1200°C and 0.35 cm/s flow rate, the reaction mechanism is an interfacial process where mass transport of the reacting gases is the rate-controlling step. In isothermal experiments at 1100°C and lower, diffusion of reactants through a solid product layer was the controlling mechanism. The activation energy for this diffusion process is 59±6 kcal/mole. Significant enhancement of diffusion occurs along grain boundaries.Although the individual phases may re-equilibrate internally upon cooling, the Fe-Ti contents must remain unchanged for proper use of this mineral indicator-i.e. no external re-equilibration. It is imperative that the kinetics of the situation be appreciated. These preliminary kinetic experiments indicate that extreme caution must be exercised when applying this ilmenite/titano-magnetite geothermometer/oxybarometer. These phases may only truly “quench” in the parameters of formation in the case of certain hypabyssal and extrusive occurrences  相似文献   

17.
Constraints are reported on the thermal history of the constituents of the Abee enstatite chondrite. From thermal experiments on laboratory-prepared alloys, and on actual samples of the meteorite, it is concluded that the metal phase of Abee cooled from above 700°C to room temperature in less than ten hours.  相似文献   

18.
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   

19.
Three physical quantities define the essentials of the cosmic ray exposure history of a sample of an iron meteorite: (1) the cosmic ray exposure age T, (2) the pre-atmospheric “size” S of the irradiated body, and (3) the location, i.e. the “depth” D, of the samples within the body. To establish these quantities for a given sample three independent quantities must be determined experimentally. In the present work T is ascertained by the 41K/40K method and the 4He and 21Ne concentrations (C4 and C21) are measured by the isotope dilution method. Signer and Nier's evaluation of the rare gas distribution in the meteorite Grant and the measured exposure age for this meteorite provide the relationships allowing to ascertain for any meteorite the quantities S and D from the 21Ne production rate (P21 = C21/T) and the 4He/21Ne ratio.Earlier measurements have provided data on the isotopic composition of potassium in 74 different iron meteorites. New rare gas measurements are reported for some 40 samples. Results on the age, size and depth are obtained for almost 60 samples. These data suggest that Signer and Nier's model is well suited for describing not only the rare gas distribution in a single selected meteorite (Grant) but also the exposure histories of the great majority of all irons. For a few samples, however, secondary breakups of the meteoroid and a two- or multiple-stage irradiation must be invoked. Further measurements are proposed for testing and, possibly, refining the still somewhat uncertain relationships between the abundances of cosmogenic nuclides and the quantities T, S, and D in very large meteorites.Histograms are presented showing the age distributions for irons of different chemical groups and of different size ranges.The feasibility and the relative merits of other methods for the determination of T, S, and D are discussed.  相似文献   

20.
A one-dimensional model is used to analyze, at the local scale, the response of the equatorial Atlantic Ocean under different meteorological conditions. The study was performed at the location of three moored buoys of the Pilot Research Moored Array in the Tropical Atlantic located at 10° W, 0° N; 10° W, 6° S; and 10° W, 10° S. During the EGEE-3 (Etude de la circulation océanique et de sa variabilité dans le Golfe de Guinee) campaign of May–June 2006, each buoy was visited for maintenance during 2 days. On board the ship, high-resolution atmospheric parameters were collected, as were profiles of temperature, salinity, and current. These data are used here to initialize, force, and validate a one-dimensional model in order to study the diurnal oceanic mixed-layer variability. It is shown that the diurnal variability of the sea surface temperatures is mainly driven by the solar heat flux. The diurnal response of the near-surface temperatures to daytime heating and nighttime cooling has an amplitude of a few tenths of degree. The computed diurnal heat budget experiences a net warming tendency of 31 and 27 W m−2 at 0° N and 10° S, respectively, and a cooling tendency of 122 W m−2 at 6° S. Both observed and simulated mixed-layer depths experience a jump between the nighttime convection phase and the well-stabilized diurnal water column. Its amplitude changes dramatically depending on the meteorological conditions occurring at the stations and reaches its maximum amplitude (~50 m) at 10° S. At 6° and 10° S, the presence of barrier layers is observed, a feature that is clearer at 10° S. Simulated turbulent kinetic energy (TKE) dissipation rates, compared to independent microstructure measurements, show that the model tracks their diurnal evolution reasonably well. It is also shown that the shear and buoyancy productions and the vertical diffusion of TKE all contribute to the supply of TKE, but the buoyancy production is the main source of TKE during the period of the simulation.  相似文献   

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