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1.
Three of the most highly metamorphosed meteorites of their respective classes, Shaw (LL7), Karoonda (C5), and Coolidge (C4), were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U, and Zn. Comparison with data by Lipschutz and coworkers on artificially heated primitive meteorites shows that the natural metamorphism of meteorites cannot have taken place in a system open to volatiles. Shaw, metamorphosed at 1300°C for >106 yr, is less depleted in In, Bi, Ag, Te, Zn, and Tl than Krymka heated at 1000°C for 1 week. Karoonda, metamorphosed at 600°C for many millennia, is less depleted in Bi and Tl than Allende heated at 600°C for 1 week.Data on primordial noble gases also show that the volatile-element patterns of ordinary and carbonaceous chondrites were established by nebular condensation, and changed little if at all during metamorphism. For enstatite chondrites, the evidence is still incomplete, but seems to favor a nebular origin of the volatile pattern.The general constancy of Tl/Rb, Tl/Cs and Tl/U ratios in terrestrial and lunar rocks suggests that loss of volatile metals such as Tl is rare during normal magmatism or metamorphism. Only impact melts show such loss with any frequency.  相似文献   

2.
Determination by neutron activation of 6 trace elements retained in Allende (C3) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~10?5 atm H2) atmosphere reveals loss of small proportions of Ga and Se and large proportions of Bi, In and Tl-Co being unaffected. The retentivity patterns for the 5 volatile elements differ and in no way duplicate a step-function. In contrast to these trace elements, sulfur is initially present in discrete mineral(s) and visually it appears to be released over a narrow temperature range. Elements are lost more easily from powder than from chips but the difference is ≤35 per cent. Above 600°C, the process of loss appears due to process(es) with apparent activation energies of 2 kcal/mole (Bi, Tl), 4 kcal/mole (Se) and 22 kcal/mole (In). Loss of Bi, Se and Tl below 600°C involves higher apparent activation energies. Two-element correlation diagrams involving Bi, In and Tl are consistent with the idea that trends among highly-volatile elements in enstatite chondrites arise from metamorphism.  相似文献   

3.
Six C2M chondrites (Boriskino, Cold Bokkeveld, Erakot, Essebi, Haripura and Santa Cruz) and the C2R chondrite Al Rais were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Sn, Te, Tl, U, and Zn. Abundances (relative to Cl chondrites) show a systematic dependence on volatility, apparently reflecting volatile loss during formation of chondrules and other high-T components. Elements of nebular condensation temperature (Tc) > 1200 K are undepleted, those of Tc < 700 K are depleted by a constant factor (0.482 ± 0.049 for C2M's) and elements of intermediate volatility are depleted by intermediate factors. The abundances do not “tend to fall monotonically as a function of [Tc],” as previously claimed by Wai and Wasson (1977) for a more restricted temperature range. For meteorites that have suffered little aqueous alteration (Mighei, Murchison, Murray), the mean abundance of volatiles agrees with the matrix content, but for the more altered meteorites, matrix contents are 20–30% higher. Only a few meteorites deviate appreciably from the mean abundance pattern. Al Rais, a C2R chondrite with a significant metal content, is systematically lower in 12 volatiles, but is enriched in Ni and Pd. Haripura and Erakot are enriched in Bi and Tl, possibly from the late condensate, mysterite.  相似文献   

4.
Extending our earlier work on 11 L-chondrites, we have measured 9 volatile elements (Ag, Bi, Cs, In, Rb, Tl, Se, Cd, Zn) by neutron activation analysis in 11 LL- and 10 E-chondrites; the first 6 elements also in 22 H-chondrites. The observed fractionation patterns are consistent with theoretical condensation curves and hence were apparently established during condensation from the solar nebula. Ordinary chondrites seem to have accreted between 420 and 500°K at P ≈ 10?5 atm; enstatite chondrites, at 460 to 520°K and P ≈ 5 x? 10?4 atm. The values for ordinary chondrites agree with O18-based temperatures by Onuma. et al. (1972) and with other characteristics such as Fe2+ content, presence of FeS and absence of Fe3O4.A few detailed trends were noted. Seven of the 54 meteorites seem to contain small amounts of a material enriched in Ag, Bi and especially T1; possibly a late condensate from a region depleted in metal. Silver shows considerable scatter, which suggests inhomogeneous distribution in the meteorites. Xenon correlates with In approximately as expected for equilibrium solubility, with some differences (petrologic type 3; E-chondrites) attributable to mineralogical factors. Meteorites of higher petrologic types are slightly deficient in Xe, probably due to gas losses during metamorphism. Cesium also appears to have been redistributed during metamorphism.Various features of the two-component model are critically examined in the light of the latest evidence. Apparently this model still is an adequate approximation of reality.  相似文献   

5.
The diffusion of Xe in olivine, a major mineral in both meteorites and lunar samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500°C. The fraction of implanted Xe retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100% occurred for an implantion dose of 3 × 1015 Xe ions/cm2. Retention was less at lower doses, with ≥ 50% loss at 1 × 1014 Xe ions/cm2. Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 μm layer for 107 years was calculated as a function of metamorphic temperature. For example, an activation energy of 50 kcal/mole implies Xe retention may be possible for metamorphic temperatures below 500°C.  相似文献   

6.
Ten trace elements were determined by neutron activation analysis in Krymka (L3) chondrite samples heated for 1 week, at 100°C increments, from 400 to 1000°C in a low-pressure environment (initially 10?5 atm H2). As in other samples studied. Co seems unaffected by heating; ~50% of Cs and Ga are lost only at 1000°C and losses of other elements increase with temperature to extremes of ～ 25% for Se and 95–99% for Ag, Bi, In, Te, T1 and Zn. Where comparison is possible, ‘open-system’ losses are generally in the order Krymka (L3) > Abee (E4) > Allende (C3). Treating elemental mobilization as representing a kinetic process involving diffusion from spherical grains of uniform size Ag, Bi, In, Te, Tl and Zn are lost from a single host phase or by a single process. This differs from trends shown by many of these elements in Abee and Allende and terrestrial basalt BCR-1. Loss of Tl apparently involves a process with a low activation energy, perhaps desorption. Loss of other elements apparently reflects diffusion-controlled process(es).Trace element contents, patterns of statistically significant interelement relationships, factor analysis and two-element correlation diagrams for unheated and heated Krymka and ‘as-received’ L3-6 chondrites are very different. Thus, significant open-system metamorphism during the genesis of L-group chondrites are not supported by these data. This contrasts sharply with the picture for enstatite chondrites, indicating substantial differences in the origin of various chondritic groups.  相似文献   

7.
We have studied trapping of radioactive 127Xe in three types of carbon: carbon black (lamp black  LB), pyrolyzed polyvinylidene chloride (PVDC), and pyrolyzed acridine (C13H9N). A total of 86 samples were exposed to Xe at T between 100 and 1000°C, for times between 5 min and 240 hours, at pxe ~ 5 × 10?7 atm. Excess gas phase and loosely sorbed Xe were pumped away and the remaining, tightly bound Xe was measured by γ-spectrometry.At 100°C,× >90% of the Xe desorbs within a few minutes' pumping but a small amount remains even after 4000 min. Distribution coefficients for this tightly bound Xe are ~1 × 10?2, 1 and 10 ccSTP/g atm for LB, acridine and PVDC carbons. The tightly bound Xe consists of two components. One occurs over the entire range 100–1000°C, becoming less abundant at high T; it appears to be physisorbed. The other occurs only at T > 500°C and is probably due to volume diffusion. The adsorbed component in LB has an apparent ΔH between ?2.3 and ?5.7 kcal/mole. The diffused component, which occurs in LB and possibly in acridine carbon, has an activation energy Q = 27 ± 8 kcal/mole and a diffusion coefficient D = 1.3 × 10?17 cm2/sec at 1000°C. These values are comparable to those found for other types of amorphous carbon (Morrisonet al., 1963; Nakai et al., 1960).The low-T component displays two paradoxical features: low ΔHads, in the range for Xe physisorbed on carbon, but exceedingly long adsorption or desorption times (~103 min at 100–400 or 1000°C). Although these long times seem to suggest a high energy process such as chemisorption, our results are best explained by a model that invokes physisorption within a labyrinth of micropores—of atomic dimensions—known to exist in amorphous carbons. The long adsorption/desorption times reflect either the long distances (~5 cm) Xe atoms must migrate by random walk to enter or leave the labyrinth, or the long times needed for Xe atoms to traverse tight spots or constricted pores that connect interior and exterior surfaces of the carbon (activated entry). Both variants of this model predict long equilibration times for the observed ΔHads of ?2 to ?6 kcal/mole. Apparently, xenon can be tightly trapped in carbon without resorting to high-energy bonding or to exotic mechanisms.These results suggest that “planetary” type noble gases in meteorites, located at or near grain surfaces of amorphous carbon, may be trapped by adsorption in micropores, whereas components such as CCFXe, which are uniformly distributed in their carrier phases, may be trapped by mechanisms such as volume diffusion or ion implantation.  相似文献   

8.
Samples of a type 3.4 chondrite have been annealed at 400–1000°C for 1–200 hours, their thermoluminescence properties determined and analyzed for K, Na, Mn, Sc and Ca by instrumental neutron activation analysis. After annealing at ?900°C, the samples showed a 50% decrease in TL sensitivity, while after annealing at 1000°C it fell to 0.1-0.01 times its unannealed value and loss of Na and K occurred. The TL and compositional changes resemble those observed for the equilibrated Kernouve chondrite after similar annealing treatments, except that the sharp TL decrease, and element loss, occurred at ~ 1100°C; this difference is presumably due to petrographic differences in the feldspar of the two meteorites. The temperature and the width of the TL peak showed a discontinuous increase after annealing at 800°C; peak temperature jumped from 130 to 200°C and peak width increased from 90 to 150°C. The activation energies for these TL changes are 7–10 kcal/mole. Similar increases in the TL peak temperature have been reported in TL studies of Amelia, VA, albite, where they were associated with the low to high-temperature transformation. However, the activation energy for the transformation is ~80 kcal/mole. These changes in TL emission characteristics resemble trends observed in type 3 ordinary chondrites and it is suggested that type 3.3–3.5 chondrites have a low-feldspar as TL phosphor and > 3.5 have high-feldspar as the phosphor. Thermoluminescence therefore provides a means of palaeothermometry for type 3 ordinary chondrites.  相似文献   

9.
The abundance and isotopic composition of Hg was determined in bulk samples of both the Murchison (CM) and Allende (CV) carbonaceous chondrites using single- and multi-collector inductively coupled plasma mass spectrometry (ICP-MS). The bulk abundances of Hg are 294 ± 15 ng/g in Murchison and 30.0 ± 1.5 ng/g in Allende. These values are within the range of previous measurements of bulk Hg abundances by neutron activation analysis (NAA). Prior studies suggested that both meteorites contain isotopically anomalous Hg, with δ196/202Hg values for the anomalous, thermal-release components from bulk samples ranging from −260 ‰ to +440 ‰ in Murchison and from −620 ‰ to +540 ‰ in Allende Jovanovic and Reed 1976a, Jovanovic and Reed 1976b, Kumar and Goel 1992. Our multi-collector ICP-MS measurements suggest that the relative abundances of all seven stable Hg isotopes in both meteorites are identical to terrestrial values within 0.2 to 0.5 ‰.On-line thermal-release experiments were performed by coupling a programmable oven with the single-collector ICP-MS. Powdered aliquots of each meteorite were linearly heated from room temperature to 900°C over twenty-five minutes under an Ar atmosphere to measure the isotopic composition of Hg released from the meteorites as a function of temperature. In separate experiments, the release profiles of S and Se were determined simultaneously with Hg to constrain the Hg distribution within the meteorites and to evaluate the possibility of Se interferences in previous NAA studies. The Hg-release patterns differ between Allende and Murchison. The Hg-release profile for Allende contains two distinct peaks, at 225° and 343°C, whereas the profile for Murchison has only one peak, at 344°C. No isotopically anomalous Hg was detected in the thermal-release experiments at a precision level of 5 to 30 ‰, depending on the isotope ratio. In both meteorites the Hg peak at ∼340°C correlates with a peak in the S-release profile. This correlation suggests that Hg is associated with S-bearing phases and, thus, that HgS is a major Hg-bearing phase in both meteorites. The Hg peak at 225°C for Allende is similar to release patterns of physically adsorbed Hg on silicate and metal grains. Prior studies suggested that the isotopic anomalies reported from NAA resulted from interference between 203Hg and 75Se. However, the amount of Se released from both meteorites, relative to Hg, is insufficient to produce all of the observed anomalies.  相似文献   

10.
The rate of CaAl-NaSi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400° to 1000°C in calcic plagioclase (An80?81) by homogenizing coherent exsolution lamellae. The dependence of the average interdiffusion coefficient on temperature is given by the expression: D? = 10.99 (cm2/sec) exp(?123.4(kcal/mol)/RT), (T in °K). This value is for diffusion perpendicular to the (03 1?) interface of the lamellae. CaAl-NaSi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400° to 1200°C and possibly 10 orders of magnitude slower at subsolidus temperatures.The large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions (e.g., the Skaergaard), which retain delicate Ca, Na compositional zoning profiles on the micron scale, but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks. CaAl-NaSi diffusion is slow, the closure temperature is high (within the solidus-liquidus interval), and Ca-Na zoning is preserved. Oxygen diffusion is faster, the closure temperature is lower (350°-400°C) and the feldspars exchange oxygen with the low-temperature hydrothermal fluids.The complex micron-scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase. Cooling histories typical of large mafic intrusions (e.g. the Stillwater) are slow, begin at high initial temperatures (1200°C) and result in homogenization of oscillatory zones on the scale of 10 microns. The oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature (1000°C) limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion.  相似文献   

11.
Oxygen diffusion in albite has been determined by the integrating (bulk 18O) method between 750° and 450° C, for a P H2O of 2 kb. The original material has a low dislocation density (<106 cm?2), and its lattice diffusion coefficient (D 1), given below, agrees well with previous determinations. A sample was deformed at high temperature and pressure to produce a uniform dislocation density of 5 × 109 cm?2. The diffusion coefficient (D a) for this deformed material, given below, is about 0.5 and 0.7 orders of magnitude larger than D 1 at 700° and 450° C, respectively. This enhancement is believed due to faster diffusion along the cores of dislocations. Assuming a dislocation core radius of 4 Å, the calculated pipe diffusion coefficient (D p), given below, is about 5 orders of magnitude larger than D 1. These results suggest that volume diffusion at metamorphic conditions may be only slightly enhanced by the presence of dislocations. $$\begin{gathered} D_1 = 9.8 \pm 6.9 \times 10^{ - 6} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 33.4 \pm 0.6(kcal/mole)/RT] \hfill \\ \end{gathered} $$ $$\begin{gathered} D_a = 7.6 \pm 4.0 \times 10^{ - 6} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 30.9 \pm 1.1(kcal/mole)/RT] \hfill \\ \end{gathered} $$ $$\begin{gathered} D_p \approx 1.2 \times 10^{ - 1} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 29.8(kcal/mole)/RT]. \hfill \\ \end{gathered} $$   相似文献   

12.
We determined ten trace elements by neutron activation analysis in Tieschitz (H3) chondrite powder heated in a low-pressure environment (initially ~ 10?5 atm H2) for 1 week at 100°C increments from 400–1000°C. Of these, Co seems unaffected by heating, 20% of Ga is lost only at 1000°C and losses of other elements progress with temperature to extremes of 25% for Se, 75% for Cs and 90–97% for Ag, Bi, In, Te, Tl and Zn. Treating elemental mobilization as kinetically-controlled by diffusion from spherical grains of uniform size, Ag, Cs, In and Se are lost from a single site by a single process while Bi, Te, Tl and Zn are lost from two sites or from one site by different processes at high and low temperatures. Magnitudes of apparent activation energies for loss of the first four elements at all temperatures and the last four at low temperatures are consistent with volume diffusion; at high temperatures Bi, Te, Tl and Zn are lost by a low-energy process, like desorption.We compared trace element abundances, patterns of statistically-significant correlations, factor analysis and two-element correlations between Tieschitz and heated Krymka (L3) and, except for factor analysis, “as-received” H3–6 chondrites. Trends for heated ordinary chondrites are similar though small differences occur; those for Tieschitz and H3–6 chondrites differ markedly indicating that H3–6 chondrites—unlike E3–6 chondrites—probably escaped substantial open-system metamorphism. Sharp contrasts in pictures for E-, L- and H-group chondrites indicate substantial differences in genetic histories.  相似文献   

13.
The pressure and temperature dependence of the composition of sphalerite in equilibrium with troilite + metallic iron has been determined experimentally at 2.5 and 5.0 kbar between 400° and 800°C using both the aqueous and anhydrous alkali halide flux recrystallization techniques. The measured pressure effect is larger than that calculated by us and by Schwarczet al. (1975a), and is described by the equation (T in Kelvins), P (kbar) = ?3.576 + 0.0551T ?0.0296Tlogmole % FeS.Assuming temperatures of final equilibration between sphalerite and troilite of 350°C for iron meteorites and 600°C for enstatite chondrites, published analyses of sphalerites provide estimates of pressures of formation and possible radii of parent objects of meteorites as follows: IA irons (Landes, Sardis, Gladstone, Bogou, Odessa, Toluca) 0.0 to 3.5 kbar, 0 to 442 km; E6 enstatite chondrites (Yilmia, Pillistfer) ?0.2 to 0.7 kbar, 0 to 198 km.  相似文献   

14.
Samples studied were residual, carbonaceous /Alates—a coined word to designate colloids prepared sometimes before and sometimes after acid demineralization—from Murray, Murchison, Cold Bokkeveld (type C2s) and Allende (type CV3) meteorites. Characterization: C2 /Alates, comprising 0.5% of the bulk meteorite are fine-grained (< 100 Å), amorphous, sulfide-free, oxidizable, 95% carbonaceous materials which pyrolyze bimodally at 200–700 and 800–1200°C. Allende /Alates are similar but with traces of inferred spinel and chromite and of sulfur, Rare gas results: Elemental: Release from stepwise heated Murray is bimodal with maximum release and upper temperature peak at 1000°C, probably accompanying chemical reaction. All /Alates studied had very nearly the same elemental concentrations, distinctly planetary in pattern. Isotopic: Trapped neon compositions are unprecedentedly close to Pepin's neon-A corner but nevertheless show signs of complexity, as if accompanied by neon-E. The trapped 3He/4He ratio is essentially constant at (1.42 ± 0.2 × 10?4. The isotopically anomalous heavy noble gases, easily detected in the residues of oxidized /Alates, were not conspicuous in this particular study. Comparison and Chicago results: Concentrations of heavy rare gases in our /Alates agree with concentrations measured directly (as opposed to inferred by difference) in acid resistant residues at Chicago. Alone, our results support the idea of a carbonaceous gas-carrier uniformly present in meteorites of various types, but Chicago characterizations of the samples can apply to both their samples and ours provided that the right amount of gas was lost in the Berkeley procedures to make the uniform gas contents in various samples a coincidence.  相似文献   

15.
The temperature dependence of diffusion is usually found to follow the Arrhenius law: D = D0e?E/RT Winchell (1969) showed that there is commonly an inter-dependence between D0 and E (for diffusion in silicate glasses), such that diffusion of different species show a positive correlation on a log D0 vs E plot. A similar effect was noted by Hofmann (1980) for cation diffusion in basalt. This implies that diffusion rates of different species tend to converge at a particular temperature; this effect is known as the ‘compensation effect’. I will show that this effect is also present for diffusion in feldspars and olivines. The equations for the compensation lines (with E given in kcal/mol) are: basalt—E = 50 + 7.5 log D0 feldspar—E = 50.7 + 3.4 log D0 olivine—E = 78.0 + 7.5 log D0 The convergence, or crossover, temperatures for diffusion in various materials are: obsidian—3400°C basalt—1370°C olivine—1360°C feldspar—460°C Compensation plots are useful for evaluating and comparing experimental diffusion data (though of limited usefulness in a predictive sense) and for understanding ‘closure temperatures’ for diffusion in petrogenetic processes (since closure temperature, the temperature at which natural diffusion processes are frozen in, is dependent on E, log d0, and cooling rate). I show that most diffusing species in feldspar have a closure-temperature close to the crossover or convergence temperature, implying that all species in feldspars can be expected to ‘freeze-in’ simultaneously at temperatures in the range 400–600°C (for cooling rates in the range 101–105°C/myr). Closure temperatures of various species in olivine, on the other hand, span a much larger range (800°C) for a similar range in cooling rates, implying that different elements in olivine will record different time-temperature stages in petrogenetic processes.  相似文献   

16.
4He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4?C6 times on each sample, and indicate a bulk 4He diffusion coefficient of 3.5?±?1.3?×?10?C8 cm2?s?C1 at 21°C, compared to previously published diffusion coefficients of 1.2?×?10?C18 cm2?s?C1 (21°C) to 3.0?×?10?C15 cm2?s?C1 (150°C) in the sands and clays. Correcting the diffusion coefficient of 4Hewater for matrix porosity (??3%) and tortuosity (??6?C13) produces effective diffusion coefficients of 1?×?10?C8 cm2?s?C1 (21°C) and 1?×?10?C7 (120°C), effectively isolating pore fluid 4He from the 4He contained in the rock matrix. Model calculations indicate that <6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8?±?0.4% (SD, n?=?4) and mudstones 3.1?±?0.8% (SD, n?=?4).  相似文献   

17.
We used neutron activation analysis to determine ten trace elements retained in Abee (E4) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~ 10?5atm H2) environment. Eight elements generally are lost progressively with increasing temperature although gas(es) evolved from the samples apparently affect retention of some elements. In the extreme, ‘open-system’ losses are: Se—23%, Cs—40%; Te—87%; Ag, Bi, In, Tl, Zn— ≥93%. Under these conditions Co is not lost; Ga is lost only at 1000°C. At 900°C elements are lost from Abee chips in the same relative order as from Abee powder but the loss is somewhat less facile. Three of the most mobile elements—Bi, In, Tl—are lost more readily from Abee than from Allende (C3), the only other primitive chondrite studied to date. Assuming that elemental loss is a kinetic process involving mobilization from spherical grains, Bi, In, Se, Tl and Zn have different activation energies at high and low temperatures either because each element was originally present in two different sites or each has more than one loss mechanism (diffusion or desorption) in different temperature ranges.Comparison of elemental abundance patterns, patterns of statistically-significant correlations, factor analysis results and two-element correlation diagrams indicate strong similarities between heated Abee and ‘as-received’ enstatite chondrites for mobile elements. These results are consistent with a two-stage evolutionary model for enstatite chondrites involving condensation of cosmochemically fractionated primitive nebular material and subsequent loss of mobile elements from parent material by metamorphism.  相似文献   

18.
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19.
In this study kamacite was experimentally grown in taenite grains of Fe-Ni-P alloys containing between 5 and 10 wt% Ni and 0 and 1.0 wt% P. Both isothermal heat treatments and non-isothermal heat treatments at cooling rates of 2 to 5°C/day were carried out. Analytical electron microscopy was used to examine the orientation and chemical composition of the kamacite and the surrounding taenite matrix. The kamacite so produced is spindle or rod shaped and has a Widmanstätten pattern orientation. The presence of heterogeneous sites such as phosphides is necessary for the nucleation of the intergranular kamacite. During kamacite growth both Ni and P partition between kamacite and taenite with chemical equilibrium at the two phase interface. The growth kinetics are limited by the diffusion of Ni in taenite. Additional diffusion experiments showed that the volume diffusion coefficient of Ni in taenite is raised by a factor of 10 at 750°C in the presence of only 0.15 wt% P.A numerical model to simulate the growth of kamacite in Fe-Ni-P alloys, based on our experimental results, was developed and applied to estimate the cooling rates of the iron meteorites. The cooling rates predicted by the new model are two orders of magnitude greater than those of previous studies. For example the cooling rates of chemical groups I, IIIAB and IVA are 400–4000°C/106years, 150–1400°C/ 106 years and 750–6000°C/106years respectively. Previous models gave 1–4°C/106 years, 1–10°C/106 years and 3–200°C/106 years. Such fast cooling rates can be interpreted to indicate that meteorite parent bodies need only be a few kilometers in diameter or that iron meteorites can be formed near the surface of larger asteroidal bodies.  相似文献   

20.
The release kinetics of Xe of the isotopically normal component of noble gases (P3 component) from the coarse-grained fraction of nanodiamonds from the Orgueil (CI) meteorite and the kinetics of 4He release from lunar soil were studied by means of a numerical simulation. It is demonstrated that the release of these gases as a peak with a single pronounced maximum may not correspond to the diffusion model with a single activation energy and can in fact be controlled by a spectrum of activation energies with a number of peaks a number of peaks remaining unresolved at stepped pyrolysis. In particular, the amount of Xe-P3 preserved in nanodiamonds during thermal metamorphism of the Orgueil meteorite calculated using parameters of the diffusion process (activation energy and frequency factor) that were determined in the model with a single activation energy indicates that practically all Xe should be lost during a very short time. These losses are inconsistent with both the duration of thermal metamorphism of the meteorite parent bodies and the Xe-P3 concentrations measured in these meteorites. A much higher preservation of Xe-P3 during thermal metamorphism lasting for hundreds of years follows from calculations based on diffusion with a spectrum of activation energiesa for Xe release. The results of isothermal pyrolysis of a nanodiamonds fraction from Orgueil confirms a presence of several activation energies for Xe-P3 release from the nanodiamonds. The application of the diffusion model with a spectrum of activation energies to He release from lunar soil samples also shows that He can be retained in these samples at 20°C during a much longer time than it follows from the model with a single activation energy (Anufriev, 2010).  相似文献   

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