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1.
Blithfield (EL6) is one of five known enstatite chondrite breccias. It consists of troilite-rich clasts (35 ± 5vol.%) embedded in an extremely metallic Fe,Ni-rich matrix (65 ± 5 vol.%) that contains metal nodules up to 17 mm in size. Clasts and matrix agglomerated independently in the solar nebula under conditions of high and lowpS2/pO2 ratios, respectively. The matrix accreted to an EL chondrite planetesimal and was metamorphosed to～ 1000°C, above the FeNiS eutectic; chondrule textures were obliterated. The S-rich eutectic melt was lost from the matrix. The matrix material was buried to a depth of at least 3 km; accreting troilite-rich material was incorporated into the matrix as clasts. The breccia cooled through～ 500°C at 1000–10,000°C/Myr. After cooling below～ 500°C, Blithfield was quenched, possibly by impact excavation from depth and deposition onto the surface.Clasts or inclusions that are enriched in sulfide and depleted in metallic Fe,Ni are common in brecciated enstatite chondrites. Variations in thepS2/pO2 ratio in the nebular regions where these materials formed may explain many of their petrologic properties. The silica-rich clasts in Adhi Kot (EH4) formed at very highpS2/pO2ratios(> 1027); niningerite, free silica and troilite were produced from the sulfurization of enstatite and metallic Fe. The troilite-rich clasts in Blithfield and Atlanta (EL6) as well as the troilite-rich regions of the Hvittis (EL6) matrix formed at somewhat lowerpS2/pO2 ratios where sulfurization of metalic Fe produced troilite. The Ni content of the residual metal increased, forming some metal of martensitic composition. The dark inclusions in Abee (EH 4), which contain up to 9 wt.% oldhamite, formed at highpS2/pO2 ratios in the presence of an additional Ca-rich component.  相似文献   

2.
The contents of the moderately volatile elements Ga, Ge, Cu and Sb in ordinary chondrites give us some clues with regard to the metal-silicate fractionation process. Their concentration in coexisting magnetic and non-magnetic portions of members of each ordinary chondrite group will be discussed. Germanium and Sb are mostly siderophilic, but Ga is strongly lithophilic in unequilibrated chondrites; its partition coefficient between magnetic and non-magnetic portions is positively correlated with petrologic type in L and LL chondrites, but not in H4–6 chondrites. From 25 to 50% of the total Cu is found in the non-magnetic fraction of chondrites, but there is no correlation between Cu content and petrologic type. The abundances of Ga, Cu and Sb (relative to Si) are constant in ordinary chondrites, independent of the amount of metal present, indicating that these elements were not in solid solution in the metal phase of chondrites when the metal-silicate fractionation process occurred. Germanium, which is the most volatile among the four elements analyzed, is more abundant in H than in L and LL chondrites, indicating that it was fractionated by this process. Nebular oxidation processes can be responsible for the behavior of Ga if this element was in oxidized form when loss of metal occurred, but cannot explain the results for Cu and Sb which are predicted to condense as metals and accrete mostly in metallic form. It is possible that Cu and Sb, upon condensation, did not form solid solutions with metallic Ni-Fe until after the separation of metal from silicates took place.  相似文献   

3.
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   

4.
Allan Hills (ALHA) 77081 is achondritic in texture while the mineral composition and the chemistry are chondritic with the exception of a few elements. An assignment to one specific group of ordinary chondrites is therefore difficult. In many respects this meteorite is similar to the unusual stone meteorite Acapulco. The REE pattern of ALHA 77081 is essentially flat and the distribution ratios of siderophile elements between metal and silicates are high compared to ordinary chondrites.Gas retention ages are 3.5±0.5 AE for U, Th-He and 4.50±0.15 AE for K-Ar. In spite of the high degree of recrystallisation the meteorite contains trapped noble gases in amounts comparable to type 4 chondrites.Cosmic ray tracks and spallogenic noble gases indicate a small preatmospheric radius of about 2–3 cm. Spallogenic nuclides produced by solar cosmic rays or stopped solar flare ions may be present.  相似文献   

5.
CO2 has been investigated up to 514 kbar at23 ± 2°C by both optical and in situ X-ray diffraction studies using a diamond-anvil pressure cell. CO2 solidifies in an unknown structure in the pressure range 5 to 23 kbar, and transforms to ordinary dry-ice structure above 23 kbar at room temperature. Isothermal compression data for dry ice have been obtained above about 24 kbar. These appear to be the first data at room temperature known in the literature. The data fitted to the Birch equation of state yieldK0 = 29.3 ± 1.0kbar andK0′ = 7.8 assuming the volume of the hypothetical dry ice at zero-pressure and room temperature is 31.4 ± 0.2 cm3/mole. The isothermal bulk modulus(K0) thus derived is consistent with the compression data and compressibilities for dry ice obtained at low temperatures using dilatometry and ultrasonic techniques, respectively, reported in the literature. By comparing shock-wave data for relevant materials, it is suggested that CO2 is not likely to transform to one of the crystalline forms of SiO2 which is otherwise expected from empirical grounds, but may instead decompose into C (diamond) + O2, at high pressures.  相似文献   

6.
Solar abundances based on recent laboratory oscillator strengths confirm the relationship between solar matter and carbonaceous chondrites. Within spectroscopic uncertainties (typically±40%) these meteorites contain refractory and volatile elements in solar proportions. Significant improvement of accuracy at present seems restricted to a few abundant elements having reliable quantum-mechanical oscillator strengths, and necessitates strictly differential spectrum analysis. Taking this into account, the solar abundance ratios Na/Ca and S/Ca have been determined to an accuracy of±15%. The results are:Na/Ca= 0.91and S/Ca= 6.8. These volatile/refractory ratios just match type 1 carbonaceous chondrites, but contrast with other types.These and related interstellar abundance features place constraints on the condensation process and a potential heterogeneity of the solar nebula. There is evidence that no drastic pre-solar separation of interstellar gas and grains has occurred, but minor imbalance may be a common mechanism co-determining stellar metal content.  相似文献   

7.
In a diamond-anvil press coupled with YAG laser heating, the spinels of Co2GeO4 and Ni2GeO4 have been found to disproportionate into their isochemical oxide mixtures at about 250 kbar and 1400–1800°C in the same manner as their silicate analogues. At about the same P-T conditions MnGeO3 transforms to the orthorhombic perovskite structure (space group Pbnm); the lattice parameters at room temperature and 1 bar are a0 = 5.084 ± 0.002, b0 = 5.214 ± 0.002, and c0 = 7.323 ± 0.003Å with Z = 4 for the perovskite phase. The zero-pressure volume change associated with the ilmenite-perovskite phase transition in MnGeO3 is ?6.6%. Mn2GeO4 disproportionates into a mixture of the perovskite phase of MnGeO3 plus the rocksalt phase of MnO at P = 250kbar and T = 1400–1800°C. The concept of utilizing germanates as high-pressure models for silicates is valid in general. The results of this study support the previous conclusion that the lower mantle comprises predominantly the orthorhombic perovskite phase of ferromagnesian silicate.  相似文献   

8.
A fragment found in soil from the Apollo 12 site (12037, from the rim of Bench Crater) appears to be a unique type of chondrite, petrologically and chemically distinct from other chondrites and lunar rocks. Inclusions consisting of shocked pyroxene rimmed by euhedral troilite crystals are set in a black aphanitic matrix. Abundant magnetite in the matrix exhibits microscopic morphologies (framboids and plaquets) characteristic of C1 chondrites. The bulk composition of this sample has high Mg/Si and low Fe/Si relative to other chondrites, and P and S are strongly enriched. Most compositional differences between this meteorite and other chondrites may be explained by fractionation of Fe phases, such as magnetite and troilite. Low refractory element contents preclude mixing with lunar materials. This sample may be a surviving fragment of the meteoritic component present in the lunar regolith. Its characteristics suggest that ancient meteoritic debris sampled by the moon may be significantly different from that captured by the present-day earth.  相似文献   

9.
SEM, optical and chemical observations have been performed on 12 H3-6 chondrites, 9 of them being also studied by other groups. Morphological features of chondrules and crystals (growth steps) are shown; the significance of the finely crystallised troilite in Menow and Ambapur Nagla is discussed in the light of the discovery that the NiFe blebs associated with it are Ni-rich (50–60% Ni). Sulphur should have been mobilized without shock evidence possibly as a result of solar heating. Pre-chondritic relict material is recognized by anomalous or variable mineral compositions, and in some cases, by the presence of overgrowths on relict cores. After short notes on individual chondrites, a tentative history of H chondrites is proposed. The chondrule-forming episode is considered as a remelting of pre-existing material. The accretion would immediately follow this event for type 6 (around 1000°C), and would occur at progressively lower temperature for types 5 and 4. Type 3 would represent material coming from an extended source region, an hypothesis consistent with the broader range composition of the particles and with their cooling before accretion to much lower temperatures (below 350°C).  相似文献   

10.
In order to understand the origin of iron-rich olivine in the matrices of type 3 ordinary chondrites, the reaction of metallic iron and enstatite, with and without forsterite and SiO2, has been experimentally reproduced at temperatures between 1150° and 800°C and PO2 between 10−11 and 10−16 atm (between the IQF and MW buffers). The olivine produced ranges from Fo58 to Fo34 and this composition does not change significantly with temperature and time of the runs. The magnesian olivine which forms does become more magnesian with increasing forsterite/enstatite ratio of the starting materials. Iron-rich olivine (Fo< 35) cannot be formed by the reaction of enstatite and metallic iron, with or without forsterite as starting materials but it can be formed in the presence of free silica. The composition of olivine becomes more iron-rich with increasing silica/enstatite ratio. The compositional range of olivine formed from each mixture is 25–30 mole% Fo regardless of the temperature, composition, mineral assemblage, and run duration.From these experimental results, two possibilities suggested for the origin of the iron-rich olivine in the matrices of type 3 ordinary chondrites: (1) free silica must have been present if the iron-rich olivine was formed by solid-state reactions under oxidizing condition in the solar nebula; (2) reaction of silicon-rich gas with metallic iron took place under oxidizing condition in the solar nebula. Though it is difficult to define which alternative was dominant, the formation of free silica or silicon-rich gas may be a result of fractional condensation. This is possible if there is a reaction relation between forsterite and gas to produce enstatite. The suggested fractional condensation is supported by the fact that the compositions of the fine-grained matrices of type 3 ordinary chondrites are more silica-rich than the bulk compositions of the chondrites. Though it is not known whether such conditions were established all over the nebula or locally in the nebula, both fractionation and more oxidizing conditions than the average solar nebula are required for the formation of matrix olivine.  相似文献   

11.
On the basis of18O/16O and17O/16O ratios, meteorites and planets can be grouped into at least six categories, as follows: (1) the terrestrial group, consisting of the earth, moon, differentiated meteorites and enstatite chondrites; (2) types L and LL ordinary chondrites; (3) type H ordinary chondrites; (4) anhydrous minerals of C2, C3, C4 carbonaceous chondrites; (5) hydrous matrix minerals of C2 carbonaceous chondrites; (6) the ureilites. Objects of one category cannot be derived by fractionation or differentiation from the source materials of any other category.  相似文献   

12.
Two comparative simulation experiments(a normal atmospheric-pressure opening system and a 20 MPa closed system)were conducted to study the geochemical evolution of n-alkane,sterane,and terpane biomarkers in the process of oil cracking into gas under different pressures.With an initial experimental temperature set at 300°C,the temperature was increased to 650°C at a heating rate of 30°C/h.The products were tested every 50°C starting at 300°C,and a pressure of 20 MPa was achieved using a water column.The low-maturity crude oil sample was from the Paleogene system in the Dongying sag in eastern China.The threshold temperature obtained for the primary oil cracking process in both pressure systems was 450°C.Before the oil was cracked into gas,some components,including macromolecular n-alkanes,were cracked into medium-or small-sized n-alkanes.The secondary oil cracking of heavy hydrocarbon gases of C2–5to methane mainly occurred between 550°C to 650°C,and the parameters Ln(C1/C2)and Ln(C1/C3),as well as the dry coefficients,increased.Overpressure inhibited the oil cracking process.In the 20 MPa system,the oil conversion rate decreased,the temperature threshold for gas generation rose,and oil cracking was inhibited.Compared with the normal pressure system,high-carbon n-alkanes and other compounds in the 20 MPa pressure system were reserved.Furthermore,the parameters∑C21-/∑22+,Ln(C1/C2),and Ln(C1/C3),as well as the dry coefficients,decreased within the main temperature range.During secondary oil cracking(550°C to 600°C),the Ph/nC18and Pr/nC17decreased.High pressure influenced the evolution of the biomarkers Ts and Tm,C31homohopane,C29sterane,and their related maturity parameters to different extents during oil cracking under different temperature ranges.  相似文献   

13.
The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α95 = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α95 = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α95 = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α95 = 4°, k = 131).  相似文献   

14.
Sm-Nd and Lu-Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm-Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean 147Sm/144Nd of 0.1960±0.0005, and a mean 143Nd/144Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm-Nd reference parameters, 147Sm/144Nd=0.1966 and 143Nd/144Nd=0.512638 (when fractionation-corrected to 146Nd/144Nd=0.7219).Lu-Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of 176Lu/177Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10−11 year−1 is used for the decay constant of 176Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311-324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with 176Lu/177Hf less than 0.030 are employed, no combination of the main population of chondrite Lu-Hf data yields a regression with sufficiently low error to constrain the decay constant of 176Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu-Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu-Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average 176Lu/177Hf and 176Hf/177Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu-Hf characteristics results in a BSE Hf-Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial εHf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu-Hf.  相似文献   

15.
Plagioclase in cataclastic anorthosite 67075 occurs as angular matrix grains and as recrystallized clasts of micro-anorthosite. Olivines are Fe-rich and fall into two compositional groupings. Large grains of pyroxene show exceptionally well-developed exsolution lamellae analogous to those observed in pyroxenes from layered complexes. The low-Ca component in both pigeonites and augites shows varying degrees of inversion to orthopyroxene. The lattices of host and lamellae may deviate slightly (up to 2°) from the ideal orientation. Very slow cooling from magmatic temperatures is required to produce the coarse exsolution textures and inversion features. Augite macrocrystals are distinctly subcalcic indicating crystallization at temperatures around1100 ± 50°C while host-lamellae pairs and small grains in lithic clasts and matrix indicate reequilibration on a micron scale to temperatures less than 800°C. Pyroxene compositions tend to cluster into two groups both of which are among the most Fe-rich reported for highland pyroxenes. Ti and Al contents of pyroxenes are very low and Ti, Cr, and Mn follow well-established magmatic differentiation trends. The high Cr content may reflect low?O2 conditions and/or early crystallization of olivine and plagioclase.The87Sr/86Sr ratios in lunar anorthosites are the lowest reported for any lunar rock. It is likely that anorthosites formed as cumulates during the major differentiation episode which occurred prior to～4.3AE. Recrystallization features are common and39Ar/40Ar ages cluster around 4.0 AE. This may be the result of the intense bombardment prior to 4.0 AE which caused repeated cycles of in-situ fracturing and granulation followed by recrystallization. The low siderophile element content and the inferred slow cooling indicate a plutonic source region (10km) not frequently plumbed by impact events. The Fe-rich silicates indicate crystallization from a melt at an advanced stage of fractionation. However, the low REE abundances are not consistent with late-stage crystallization. Plagioclase apparently crystallized relatively early and was concentrated by flotation and/or convection currents while the mafic minerals crystallized from a fractionated trapped liquid. The chemical, isotopic, and mineralogical data place stringent constraints on the nature of genetically related rocks and the relationship of anorthosites to other members of the ANT suite does not appear to be one ofsimple fractionation. The data presented in this paper are consistent with the Taylor-Jake?model of lunar evolution.  相似文献   

16.
Cosmogenic14C has been measured in 12 chondrites and the stone phase of the mesosiderite Bondoc. For the chondrites analysed the activities vary between 44 and 72 dpm/kg; the low value of (4.5 ± 0.9) dpm/kg for Bondoc is essentially due to its large pre-atmospheric size and not to a terrestrial age of several half-lives of14C.In eight cases39Ar in the metal phase from the same meteorite specimens had been measured previously. The results are combined to derive the pre-atmospheric radiiR0 of the meteoroids and depth of burial of the samples investigated. Values ofR0 between 35 and 82 cm are obtained; of 14 samples ten came from a depth of 10 cm or less. The preponderance of samples from shallow depths is ascribed to asymmetrical ablation losses of the meteoroids during their passage through the atmosphere.A compilation of all published14C concentrations in chondrites shows that the variations between different specimens from thesame meteorites are almost as large as those for samples fromdifferent meteorites. Thus, there is no need to invoke different orbits of the meteoroids and a strong spatial gradient in the primary cosmic-ray intensity to explain variations of low-energy-produced cosmogenic nuclides in different meteorites.  相似文献   

17.
A new phase which is much denser than the component oxides of spinel (MgAl2O4) was synthesised at loading pressures greater than 250 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. The new phase (ε-MgAl2O4) was indexed on the basis of an orthorhombic cell with a = 8.507 ± 0.004, b = 2.740 ± 0.003, c = 9.407 ± 0.005Å, and Z = 4 at room temperature and 1 bar pressure. Thus the molar volume for ε-MgAl2O4 at the above conditions was calculated to be 33.01 ± 0.07 cm3, which is 10.3% less than that of the mixture corundum plus periclase. The dense phase of spinel found in shock-wave experiments can be reasonably interpreted as ε-MgAl2O4, and this may be a potentially important mineral component of the earth's lower mantle. The new structure may also provide a possible candidate for the dense phases of Fe3O4 and Mg2SiO4 which were found by shock experiments.  相似文献   

18.
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   

19.
A systematic calibration of the production rate of one specific cosmic-ray-produced nuclide in chondrites, that of21Ne, was achieved by using four independent methods:P21(1.11) = 0.507 ± 0.039, 0.302 ± 0.013, 0.312 ± 0.017and0.292 ± 0.019 (in units of 10?8 cm3 STP/g My) based on26Al-age,53Mn-age,81Kr-83Kr and22Na-22Ne methods, respectively. These production rates are all normalized to a shielding parameter ratio22Ne/21Ne= 1.11 and to the chemical composition of L chondrites. The results obtained by the latter three methods are in good agreement, but they disagree in a systematic way with the26Al-age calibration. Based on these results, we recommend a valueP21(1.11) = 0.31 and a production rate equation:P21 = 4.845 P21 (1.11) F[21.77(22Ne/21Ne) ? 19.32]?, whereF = 1.00 for L and LL, andF = 0.93 for H chondrites, for the calculation of cosmic ray exposure ages on the basis of Ne concentrations. In an attempt to assess possible causes for this discrepancy, we discuss the26Al half-life measurements, we evaluate effects resulting from pre-irradiation of meteorites, and we discuss the evidence regarding the constancy of the cosmic ray flux in the past, in the light of some recent astronomical observations.  相似文献   

20.
The Adhi Kot EH4 enstatite chondrite breccia consists of silica-rich clasts (12+mn; 5 vol.%), chondrule-rich clasts (55+mn; 10 vol.%) and matrix (35+mn; 10 vol.%). The silica-rich clasts are a new kind of enstatite chondritic material, which contains more cristobalite (18–28 wt.%) than enstatite (12–14 wt.%), as well as abundant niningerite and troilite. The bulk atomic Mg/Si ratios of the clasts (0.22–0.40) are much lower than the average for enstatite chondrites (0.79). Kamacite and martensite (with 8–11 wt.% Ni and a martensitic structure) occur in all three breccia components. The clasts have kamacite-rich rims, and kamacite-rich aggregates occur in the matrix.A unidirectional change in the ambient pS2/pO2 ratio in the region of the solar nebula where Adhi Kot agglomerated can explain many of the breccia's petrologic features. If this region initially had a very high pS2/pO2 ratio in a gas of non-cosmic composition, sulfurization of enstatite and metallic Fe (e.g., MgSiO3 + 2Fe + C + 3H2S = MgS + SiO2 + 2FeS + H2O + CH4) may have occurred, producing abundant niningerite, free silica and troilite at the expense of enstatite and metallic Fe. The Ni content of the residual metal would have increased, perhaps to ～ 8–10 wt.%. The silica-rich clasts agglomerated under these conditions; a significant fraction of the originally produced niningerite was lost (perhaps by aerodynamic size-sorting processes), lowering the clasts' bulk Mg/Si ratios.The pS2/pO2 ratio then decreased (perhaps because of infusion of additional H2O) and sulfurization of metallic Fe and enstatite ceased. The chondrule-rich clasts agglomerated under these conditions, acquiring little free silica and niningerite. An episode of chondrule formation occurred at this time (by melting millimeter-sized agglomerates of this relatively silica-poor enstatite chondrite material and concomitant fractionation of an immiscible liquid of metallic Fe,Ni and sulfide). The chondrule-rich clasts agglomerated many such chondrules. Subsequently, the matrix agglomerated, acquiring the few remaining chondrules. Kamacite-rich aggregates formed, after the cessation of metal sulfurization, and agglomerated with the matrix. The kamacite-rich clast rims were acquired at this time.The components of Adhi Kot accreted to the EH chondrite parent body, where the breccia was assembled, buried beneath additional accreting material, and metamorphosed at temperatures of ? 700°C. Impact-excavation of the breccia and deposition onto the surface caused the formation of martensite from taenite inside the clasts and the matrix. At the surface, impact-melting produced an albite glass spherule, which was incorporated into the matrix. However, the absence of solar-wind-implanted rare gases in bulk Adhi Kot indicates that the breccia spent little time in a regolith.  相似文献   

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