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1.
The bulk composition of metal (kamacite plus perryite) was determined in eleven E-chondrites and eight aubrites. The data are compatible with the subdivision of the E-chondrites into two groups (Yavnel, 1963; Anders, 1964), St Mark's and St Sauveur belonging to type I (Easton, 1985). The Ni content of kamacite plus perryite in Kota Kota (5.49%) is within the range covered by the remaining E-chondrites. Normative perryite, (Fe, Ni)x(Si,P)y constitutes 2.1% of Kota Kota and 2.7% of South Oman. The Ni content in the bulk metal of Aubres, Bishopville, Norton County and Peña Blanca Spring is about half the average Ni content in the metal of E-chondrites or the remaining aubrites (Bustee, Khor Temiki, Mayo Belwa and Shallowater). High Ga/Ni and Ge/Ni ratios distinguish the metal in E-chondrites and aubrites from that in ordinary chondrites. In a large metal grain from Aubres perryite formed on reheating, whereas in one from Khor Temiki there is evidence of shock and displacement of fragmented schreibersite (rhabdite). Thirty-eight metal grains (< 1.5 mm diameter) from Khor Temiki have a wide compositional range like that in Mayo Belwa (Graham, 1978). In Shallowater the distribution of Ni in the metal is bimodal (5.2 and 11.6%) and there is evidence of rapid cooling. The composition of both bulk metal and individual grains in aubrites makes it unlikely that they represent residual metal trapped during magmatic differentiation and/or fractional crystallization of E6 material. Compositional differences between metal grains strongly indicate that the aubrites are polymict breccias.  相似文献   

2.
The size distribution and morphology of metal grains have been examined in 11 sections of types I and II E-chondrites. The changes in the grain-size distribution and morphology of metal grains correspond with the petrologic types and define a series that reflects increase in thermal metamorphism in the following order: type I, Kota Kota-Indarch-South Oman-St. Mark's; and type II, Jajh deh Kot Lalu-Atlanta-Daniel's Kuil-Hvittis-Pillistfer-Khairpur-Blithfield. Concentrations of metal grains adjacent to the perimeters of chondrules are observable throughout the sequence and delineate relic chondritic structure in six of the seven type II E-chondrites; relic structures are absent from Blithfield.  相似文献   

3.
New bulk chemical analyses are given of Aubres, Bishopville, Bustee, Khor Temiki, Norton County, Peña Blanca Spring and Shallowater, Selective attack by dry chlorine (350°C) on magnetic and non-magnetic fractions was used to determine the distribution of some normally lithophile elements (Al, Ca, Cr, K, Mg, Mn, Na, P and Ti) between silicate and sulphide groups of minerals.  相似文献   

4.
I propose that the properties of the two outbursts observed in the X‐ray transient XTEJ1118+480 in 2000 are akin to superoutbursts of SU UMa stars. In these systems a ‘normal’ outburst immediately precedes a 5–10 times longer (‘super’) outburst. The optical light curve of the outbursts of XTEJ1118+480 is remarkably similar to that seen in some SU UMa stars, such as UVPer and TLeo, where the precursor outburst is distinct from the superoutburst, but the time scales are a factor of ∼15 different. The first outburst of XTEJ1118+480 was relatively short (∼1 month) while the second outburst was ∼5 times longer. During the second outburst superhumps were seen, a feature characteristic for superoutbursts. The gap of about a month between the two outbursts is longer in X‐rays with respect to the optical, a feature not previously recognized for X‐ray transients. Also in SU UMa stars the precursor outburst becomes more distinct at shorter wavelengths. Finally, I show that the time of appearance of the superhumps in XTEJ1118+480 is consistent with the expected superhump growth time, if the superhump mechanism was triggered during the first outburst. I conclude that the similarity in outburst behaviour in the two types of systems provides further support that a common mechanism is at work to start the long (‘super’) outbursts.  相似文献   

5.
Abstract— Fusion crusts develop on all meteorites during their passage through the atmosphere but have been little studied. We have characterized the textures and compositions of the fusion crusts of 73 stony meteorites to identify the nature of meteorite ablation spheres (MAS) and constrain the processes operating during the entry heating. Most chondrite fusion crusts are porphyritic and are dominated by olivine, glass, and accessory magnetite; whereas those of the achondrites are mainly glassy. Chondrite fusion crusts contain sulphide droplets with high-Ni contents (>55 wt%). The partially melted substrate of ordinary chondrites (underlying the outer melted crusts) are dominated by silicate glass and composite metal, sulphide, and Cr-bearing Fe-oxide droplets that form as coexisting immiscible liquids. Enstatite chondrite substrates contain Cr- and Mn- bearing sulphides. The substrates of the carbonaceous chondrites comprise a sulphide-enriched layer of matrix. The compositions of melted crusts are similar to those of the bulk meteorite. However, differences from whole rock suggest that three main processes control their chemical evolution: (1) the loss and reaction of immiscible Fe-rich liquids, (2) mixing between substrate partial melts and bulk melts of the melted crust, and (3) the loss of volatile components by evaporation and degassing. Data from fusion crusts suggest that MAS produced at low altitude have compositions within the range of those of silicate-dominated cosmic spherules that are formed by the melting dust particles. Meteorite ablation spheres produced at high altitude probably have compositions very different from bulk meteorite and will resemble cosmic spherules derived from coarse-grained precursors.  相似文献   

6.
We are carrying out a project to calculate power spectra of variability, assuming a model of a ‘spotted’ accretion disc near a black hole. We consider relativistic effects that change photon energy and produce light‐bending and time‐delays acting on the X‐ray signal received by an observer. We assume that the life‐time and the intrinsic emissivity of individual.aring events are described in terms of a simple stochastic process. This allows us to give approximate analytical formulae and compare them with numerical computations. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

7.
The Ashmore olivine-bronzite chondrite is a group H, type 5 stone which differs from other H5 chondrites mainly in its higher proportion of chromite (0.9 wt %) and in the relatively lower iron and higher magnesium content of the chromite. The modal proportions of opaque phases were obtained by point-counting in reflected light, and the modal proportions of nonopaque silicate phases in the matrix were estimated from traverses of a selected small area by electron microprobe. The consistency between the bulk chemical analysis and the chemical composition calculated from the modal mineral proportions implies that the bulk silicate composition of the chondrules is very similar to that of the silicate matrix and suggests a common source for both chondrules and matrix.  相似文献   

8.
Abstract— We report the discovery of four large (>50 μm) cosmic spherules (CSs) and a single scoriaceous micrometeorite (SMM) that contain evidence for the separation of immiscible Fe-Ni-S liquids during atmospheric entry heating. The particles contain segregated Fe-rich regions dominated by either Ni-S-bearing Fe-oxides or iron sulphides and have textural relations that suggest these separated from the silicate portions of the particles as metallic liquids. The oxides, which may be hydrous, are thought to result from alteration of metal and sulphide. The compositions of the silicate portions of the CSs are equivalent to spherules without Fe-rich regions, implying that metallic liquids are exsolved during the heating of most spherules, but completely separate. The single SMM has a very different composition from other scoriaceous particles, and the occurrence of an exsolved metallic liquid probably indicates extreme reduction during entry heating. The pyrolysis of carbonaceous materials is the most likely explanation for reduction and suggests that the precursor material of this particle was unusually C-rich. This SMM might be, therefore, an appropriate candidate for a large melted anhydrous or smectite interplanetary dust particle (IDP). The exsolution of immiscible Fe-Ni-S liquids during entry heating will result in systematic changes in the compositions of the remaining silicate melt.  相似文献   

9.
The Grier(b), New Mexico meteorite, a single mass of 929.4 grams, was found in 1969. This brecciated chondrite can be classified as an L-group from the bulk chemical analysis, ～ 8 wt % metal with an estimated total iron content of 25 wt %, and the constant olivine (Fa25.5) and orthopyroxene (Fs23) compositions. The main portion of the meteorite fits the criteria for an L5 (grey to intermediate hypersthene) chondrite. A conspicuous, large (several cm3) dense fragment, texturally an L6–7 chondrite, contains practically no metal or chondrules. However, there is little variation in the bulk silicate and individual phase compositions between the fragment and the matrix. In spite of this, it seems unlikely that the fragment was created in situ because metal and sulfide are not found in the fragment-matrix contact zone; thus the formation of olivines and pyroxenes in both parts, as well as the “draining” of metal from the fragment, occurred prior to accretion with little, if any, subsequent thermal metamorphism.  相似文献   

10.
We performed a LA-ICP-MS study of refractory lithophile trace elements in 32 individual objects selected from a single section of the reduced CV3 chondrite Leoville. Ingredients sampled include ferromagnesian type I and II chondrules, Al-rich chondrules (ARCs), calcium-aluminum-rich inclusions (CAIs), a single amoeboid olivine aggregate (AOA), and matrix. The majority of rare earth element (REE) signatures identified are either of the category “group II” or they are relatively flat, i.e., more or less unfractionated. Data derived for bulk Leoville exhibit characteristics of the group II pattern. The bulk REE inventory is essentially governed by those of CAIs (group II), ARCs (flat or group II), type I chondrules (about 90% flat, 10% group II), and matrix (group II). Leoville matrix also shows a superimposed positive Eu anomaly. The excess in Eu is possibly due to terrestrial weathering. The group II pattern, however, testifies to volatility-controlled fractional condensation from a residual gas of solar composition at still relatively high temperature. In principle, this signature (group II) is omnipresent in all types of constituents, suggesting that the original REE carrier of all components was CAI-like dust. In addition, single-element anomalies occasionally superimposing the group II signature reveal specific changes in redox conditions. We also determined the bulk chemical composition of all objects studied. For Mg/Si, Mg/Fe, and Al/Ca, Leoville's main ingredients—type I chondrules and matrix—display a complementary relationship. Both components probably formed successively in the same source region.  相似文献   

11.
The UVIS dark matter detector, proposed by Spooner and Smith (Phys. Lett. B 314 (1993) 430), consists of < 1000 ppm Tl doped NaI scintillator operated at 100-200 K, with measurement of UV and visible scintillation components used to discriminate between electrons (gammas) and Na or I recoils. Presented here are results of measurements of the gamma/nuclear recoil discrimination power of a Kyropolous NaI (0.5 ppm Tl) UVIS test detector operated at 160 K, with monoenergetic neutrons used to induce nuclear recoils via elastic scattering. Defining the statistical gain factor for N events as , the coefficient Cm was measured as a function of photoelectron pulse height. At 40 photoelectrons Cm was found to be 0.5 (corresponding to 90% rejection of gammas with 20% loss of neutron events). The scintillation efficiency for Na and I recoils relative to electrons was also measured at 160 K and found to be 35 ± 5% for Na recoils and 10 ± 2% for iodine.  相似文献   

12.
The diversity of silicate, glassy spherules analogous to chondrules, called microchondrules, and the implications for their presence in unequilibrated ordinary chondrites (UOCs) were investigated using different electron microscope techniques. Our observations show that the abundance of microchondrules in UOCs is much larger than the values proposed by previous studies. We identified two different types of microchondrules, porous and nonporous, embedded within fine‐grained matrices and type I chondrule rims. The porous microchondrules are characterized by distinctive textures and chemical compositions that have not been recognized previously. Additionally, we show detailed textures and chemical compositions of protuberances of silicate materials, connected to the chondrules and ending with microchondrules. We suggest that microchondrules and protuberances formed from materials splattered from the chondrules during stochastic collisions when they were still either completely or partially molten. The occurrence and distinct morphologies of microchondrules and protuberances suggest that rather than just a passive flash melting of chondrules, an additional event perturbed the molten chondrules before they underwent cooling. The bulk chemical compositions suggest that (1) nonporous microchondrules and protuberances were formed by splattering of materials that are compositionally similar to the bulk silicate composition of type I chondrules, and (2) the porous microchondrules could represent the splattered melt products of a less evolved, fine‐grained dust composition. The preservation of protuberances and microchondrules in the rims suggests that the cooling and accretion rates were exceptionally fast and that they represent the last objects that were formed before the accretion of the parent bodies of OCs.  相似文献   

13.
We comment briefly on a recent paper by Fuhrmann which claims that about half of the sample of halo stars in the solar neighbourhood presented by Fuchs and Jahreiß [A&A 329 (1998) 81] are actually thick disc stars. By referring to star counts in the CADIS survey we argue that this is rather unlikely.  相似文献   

14.
Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have 12C/13C ratios between 18 and 6800, 14N/15n ratios from 13 to 200, δ29Si/28Si between ?750 and +60%0, δ30Si/28Si from ?770 to ?10%0, and 54Fe/56Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic 26Mg (from the radioactive decay of 26Al, half‐life ? 7 times 105 years) and radiogenic 44Ca (from the radioactive decay of 44Ti, half‐life = 60 years). While all X grains but one have radiogenic 26Mg, only ～20% of them have detectable amounts of radiogenic 44Ca. Initial 26Al/27Al ratios of up to 0.36 and initial 44Ti/48Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct 44Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for 26Al/27Al ratios, except for the highest 26Al/27Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed 29Si/30Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured 14n/15n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 Modot; type II SN if rotational mixing, preferential trapping of N, or both from 15n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial 26Al/27Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.  相似文献   

15.
Abstract— Minor element variations in MgAl2O4 spinel from the type B1 calcium‐aluminum‐rich inclusion (CAI) Allende TS‐34 confirm earlier studies in showing correlations between the minor element chemistry of spinels with their location within the inclusion and with the chemistry of host silicate phases. These correlations result from a combination of crystallization of a liquid produced by re‐melting event(s) and local re‐equilibration during subsolidus reheating. The correlation of the Ti and V in spinel inclusions with the Ti and V in the adjacent host clinopyroxene can be qualitatively explained by spinel and clinopyroxene crystallization prior to melilite, following a partial melting event. There are, however, difficulties in quantitative modeling of the observed trends, and it is easier to explain the Ti correlation in terms of complete re‐equilibration. The correlation of V in spinel inclusions with that in the adjacent host clinopyroxene also cannot be quantitatively modeled by fractional crystallization of the liquid produced by re‐melting, but it can be explained by partial re‐equilibration. The distinct V and Ti concentrations in spinel inclusions in melilite from the edge regions of the CAI are best explained as being affected by only a minor degree of re‐equilibration. The center melilites and included spinels formed during crystallization of the liquid produced by re‐melting, while the edge melilites and included spinels are primary. The oxygen isotope compositions of TS‐34 spinels are uniformly 16O‐rich, regardless of the host silicate phase or its location within the inclusion. Similar to other type B1 CAIs, clinopyroxene is 16O‐rich, but melilite is relatively 16O‐poor. These data require that the oxygen isotope exchange in TS‐34 melilite occurred subsequent to the last re‐melting event.  相似文献   

16.
We have performed statistical equilibrium calculations for Ca I–Ca II, Ti I–Ti II, and Fe I–Fe II by taking into account the nonequilibrium line formation conditions (the non-LTE approach) in model atmospheres of giant stars with effective temperatures 4000 K ≤ T eff ≤ 5000 K and metal abundances ?4 ≤ [Fe/H] ≤ 0. The dependence of departures from LTE on atmospheric parameters has been analyzed. We present the non-LTE abundance corrections for 28 Ca I lines, 42 Ti I lines, 54 Ti II lines, and 262 Fe I lines and a three-dimensional interpolation code to obtain the non-LTE correction online for an individual line and specified atmospheric parameters.  相似文献   

17.
Except for asteroid sample return missions, measurements of the spectral properties of both meteorites and asteroids offer the best possibility of linking meteorite groups with their parent asteroid(s). Visible plus near‐infrared spectra reveal distinguishing absorption features controlled mainly by the Fe2+ contents and modal abundances of olivine and pyroxene. Meteorite samples provide relationships between spectra and mineralogy. These relationships are useful for estimating the olivine and pyroxene mineralogy of stony (S‐type) asteroid surfaces. Using a suite of 10 samples of the acapulcoite–lodranite clan (ALC), we have developed new correlations between spectral parameters and mafic mineral compositions for partially melted asteroids. A well‐defined relationship exists between Band II center and ferrosilite (Fs) content of orthopyroxene. Furthermore, because Fs in orthopyroxene and fayalite (Fa) content in olivine are well correlated in these meteorites, the derived Fs content can be used to estimate Fa of the coexisting olivine. We derive new equations for determining the mafic silicate compositions of partially melted S‐type asteroid parent bodies. Stony meteorite spectra have previously been used to delineate meteorite analog spectral zones in Band I versus band area ratio (BAR) parameter space for the establishment of asteroid–meteorite connections with S‐type asteroids. However, the spectral parameters of the partially melted ALC overlap with those of ordinary (H) chondrites in this parameter space. We find that Band I versus Band II center parameter space reveals a clear distinction between the ALC and the H chondrites. This work allows the distinction of S‐type asteroids as nebular (ordinary chondrites) or geologically processed (primitive achondrites).  相似文献   

18.
Abstract– Analysis of the mineralogy, isotopic, and bulk compositions of the eucrite meteorites is imperative for understanding their origin on the asteroid 4 Vesta, the proposed parent body of the HED meteorites. We present here the petrology, mineral compositions, and bulk chemistry of several lithic components of the new brecciated basaltic eucrite Northwest Africa (NWA) 3368 to determine if all the lithologies reflect formation from one rock type or many rock types. The meteorite has three main lithologies: coarse‐ and fine‐grained clasts surrounded by a fine‐grained recrystallized silicate matrix. Silicate compositions are homogeneous, and the average rare earth element pattern for NWA 3368 is approximately 10× CI chondrites with a slight negative Eu anomaly. Major and trace element data place NWA 3368 with the Main Group‐Nuevo Laredo trend. High‐Ti chromites with ilmenite exsolution lamellae provide evidence of NWA 3368’s history of intense metamorphism. We suggest that this meteorite underwent several episodes of brecciation and metamorphism, similar to that proposed by Metzler et al. (1995) . We conclude that NWA 3368 is a monomict basaltic eucrite breccia related to known eucrites in texture and in mineral, bulk, and oxygen isotopic composition.  相似文献   

19.
Abstract— Petrological and bulk geochemical studies were performed on a large silicate clast from the Mount Padbury mesosiderite. The silicate clast is composed mainly of pyroxene and plagioclase with minor amounts of ilmenite, spinel, and other accessory minerals, and it shows subophitic texture. Pyroxenes in the clast are similar to those in type 5 eucrites and could have experienced prolonged thermal metamorphism after rapid crystallization from a near‐surface melt. Ilmenite and spinel vary chemically, indicating growth under disequilibrium conditions. The clast seems to have experienced an episode of rapid reheating and cooling, possibly as a result of metal‐silicate mixing. Abundances of siderophile elements are obviously higher than in eucrites, although the clast is also extremely depleted in highly siderophile elements. The fractionated pattern can be explained by injection of Fe‐FeS melts generated by partial melting of metallic portions during metal‐silicate mixing. The silicate clast had a complex petrogenesis that could have included: 1) rapid crystallization from magma in a lava flow or a shallow intrusion; 2) prolonged thermal metamorphism to equilibrate the mineral compositions of pyroxene and plagioclase after primary crystallization; 3) metal‐silicate mixing probably caused by the impact of solid metal bodies on the surface of the mesosiderite parent body; and 4) partial melting of metal and sulfide portions (and silicate in some cases) caused by the collisional heating, which produced Fe‐FeS melts with highly fractionated siderophile elements that were injected into silicate portions along cracks and fractures.  相似文献   

20.
Abstract— Two dark lithic fragments and matrix of the Krymka LL3.1 chondrite were mineralogically and chemically studied in detail. These objects are characterised by the following chemical and mineralogical characteristics, which distinguish them from the host chondrite Krymka: (1) bulk chemical analyses revealed low totals (systematically lower than 94 wt%) due to high porosity; (2) enrichment in FeO and depletion in S, MgO and SiO2 due to a high abundance of Fe‐rich silicates and low sulfide abundance; (3) fine‐grained, almost chondrule‐free texture with predominance of a porous, cryptocrystalline groundmass and fine grains; (4) occurrence of a small amount of once‐molten material (microchondrules) enclosed in fine‐grained materials; (5) occurrence of accretionary features, especially unique accretionary spherules; (6) high abundance of small calcium‐ aluminium‐rich inclusions (CAIs) in one of the fine‐grained fragments. It is suggested that the abundance of CAIs in this fragment is one of the highest ever found in an ordinary chondrite. Accretionary, fine‐grained spherules within one of the fragments bear fundamental information about the initial stages of accretion as well as on the evolution of the clast, its incorporation, and history within the bulk rock of Krymka. The differences in porosity, bulk composition, and mineralogy of cores and rims of the fine‐grained spherulitic objects allow us to speculate on the following processes: (1) Low velocity accretion of tiny silicate grains onto the surface of coarse metal or silicate grains in a dusty region of the nebula is the beginning of the formation of accretionary, porous (fluffy) silicate spherules. (2) Within a dusty environment with decreasing silicate/(metal + sulfide) ratio the porous spherules collected abundant metal and sulfide particles together with silicate dust, which formed an accretionary rim. Variations of the silicate/(sulfide + metal) ratio in the dusty nebular environment result in the formation of multi‐layered rims on the surface of the silicate‐rich spherules. (3) Soft accretion and lithification of rimmed, fluffy spherules, fine‐grained, silicate‐rich dust, metal‐sulfide particles, CAIs, silicate‐rich microchondrules, and coarse silicate grains and fragments followed. (4) After low‐temperature processing of the primary, accretionary rock collisional fragmentation occurred, the fragments were subsequently coated by fine‐grained material, which was highly oxidized and depleted in sulfides. (5) In a final stage this accretionary “dusty” rock was incorporated as a fragment within the Krymka host.  相似文献   

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