The classification of micrometeorites

Abstract— Due to their small size, the mineralogical and chemical properties of micrometeorites (MMs) are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites. Identifying which groups of MM are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical, textural, and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies, albeit with minimal bias towards preconceived genetic models. Specifically, the interpretation of MMs requires a rigorous and meaningful classification scheme. At present the classification of MMs is, however, at best ambiguous. A unified petrological‐chemical classification scheme is proposed in the current study and is based on observations of several thousand MMs collected from Antarctic ice.     

Detailed analysis of cool stars

We present the detailed analysis of fourteen cool stars, nine of which have been associated by Eggen with four moving groups from the kinematics and the photometric properties; the five remaining stars are characterized by a large-spatial velocity. From the scatter of the chemical composition among the program stars belonging to a same group, we discuss that the moving groups Her, Wolf 630, and Kapteyn could really exist, and that Groombridge 1830 do not. No peculiar abundance relative to iron is found except a possible relative overabundance of Ni for the most metal-poor stars in this sample. The results of the detailed analyses are discussed in terms of the chemical abundances of our Galaxy.Based on observations collected at European Southern Observatory, La Silla, Chile and Observatoire de Haute-Provence, France.     

Structure and evolutionary history of the solar system,IV

In this fourth and last part of our analysis, the first section (14) contains a study of the chemical composition of the planets and satellites. A sharp distinction is made between the large quantity of speculations about the interiors of the bodies and the rather meagerfacts known with a reasonable degree of certainty. It is shown, however, that the latter are sufficient todisprove the old concept of a Laplacian disc of homogeneous chemical composition. There is asystematic variations in the chemical composition of planets (and probably also of satellites) so that heavy elements are more abundant in the outermost and in the innermost regions of the systems. Section 15 containsa study of meteorites. These have earlier been interpreted in terms of ‘exploded planets’ and condensation processes in thermodynamic equilibrium. It is shown that such models are irreconcilable with the laws of physics and also with the meteoritic observations. These instead are found toprovide abundant information on the processes in jet streams and on early fractionation and condensation. Further work along these lines supplemented with other solar system materials studies may lead to a detailed reconstruction of important events in the evolution of the solar system. Section 16 demonstrates that the location of the different groups of secondary bodies is a result of a plasma phenomenon occurring at the critical velocity limit. These have recently been studied in detail in the laboratory but have not yet been fully applied to astrophysics.Groups of bodies in the planetary and the satellite systems related by the critical velocity shouldhave the same gravitational potential. There are large chemical differences between groups of different gravitational potential. This is reconcilable with the chemical differentiation found in Section 14. Finally, Section 17 deals with thestructure of the different groups of bodies and shows that the mass distributionis a function of the spin of the central body. Summarizing the properties and distribution of bodies in the solar system against this background, it is shown that there isno need for ‘missing planets’ or to explode hypothetical large bodies. Nor is there any justification for involvingdrastic ad hoc changes in the orbits of existing bodies. The scheme is complete in the sense that in all places where groups of bodies are expected, such bodies are actually found. All of the existing bodies are accounted for (with the exception of the small Martian satellites!). The general conclusion is that already with the empirical material now availableit is possible to suggest a series of basic processes leading to the present structure of planet and satellite systems in an internally consistent way. With the expected flow of data from space research the evolution of the solar system may eventually be described with about the same confidence and accuracy as the geological evolution of the Earth.     

VANADIUM AND COPPER IN CHONDRITES

Emission spectrographic analyses for vanadium and copper in sixty-seven chondritic meteorites including members from ordinary, carbonaceous, and enstatite chondrite groups give a total range of 41–84 ppm vanadium and a total range of 77–180 ppm copper. The respective average values are 61 ± 7 ppm and 115 ± 12 ppm. Detailed patterns of vanadium and copper distributions between different chondrite groups are discussed in light of recent mechanisms for chemical fractionations in meteorites     

PAPUANITES: PSEUDO-TEKTITES FROM NEW GUINEA

Spherical translucent green glasses found in New Guinea for many years have the external appearance of stream abraded tektites, but are chemically distinct from known tektite groups. Their chemical composition is most similar to artificial glasses. If these objects are the source of reports of tektites occurring in New Guinea, the existence of New Guinea tektites must now be discounted.     

Results of statistical analysis of primary components concentrations in bulk chemical composition of ordinary chondrite groups

We used two different methods of statistical analysis—cluster analysis and principal component analysis—to analyze the concentrations of principal chemical components (Si, Mg, Ca, Fe, Ni) and Co in ordinary chondrites. The analysis is based predominantly on published data (metadata). In total, chemical composition data from 646 ordinary chondrites were used in the statistical analysis. The aim of this analysis was to establish whether it would be possible or not to distinguish H, L, and LL chondrites based on the concentrations of major elements and Co in their bulk chemical compositions. It was also important to determine what conclusions such an analysis could enable to draw about matter differentiation in the formation environments of primordial parent bodies of particular ordinary chondrite groups (H, L, and LL). Another aim of the statistical analysis was to determine whether the distribution of Fe and Ni (with Co admixtures) is independent of petrographic types within particular groups of chondrites. This is of crucial importance for determining the distribution of FeNi(Co) ore occurrences in potential extraterrestrial deposits on modern asteroids—the sources of ordinary chondrites. The obtained results of statistical analyses confirmed that a clear-cut distinction between particular groups of ordinary chondrites is only possible for group H, while distinguishing L chondrites from LL chondrites is not always obvious. The results of the statistical analyses relating to the question of the possible existence of several primordial parent bodies (formation environments) of each group of ordinary chondrites are consistent with the results of contemporary astronomical spectroscopy research. What is particularly interesting is obtaining indications of the existence of common formation environments of the matter of L and LL chondrites, possibly on a few primordial parent bodies. The statistical analyses indicate that there is no correlation between the concentration of principal chemical components and the petrographic type of ordinary chondrites. This proves homogenous distributions of these elements within the parent bodies of each group of ordinary chondrites. Hence, the distribution of these elements in individual present-day asteroids is also homogenous.     

Temperature and abundance profiles of hot gas in galaxy groups – II. Implications for feedback and ICM enrichment

We investigate the history of galactic feedback and chemical enrichment within a sample of 15 X-ray bright groups of galaxies, on the basis of the inferred Fe and Si distributions in the hot gas and the associated metal masses produced by core-collapse and Type Ia supernovae (SNe). Most of these cool-core groups show a central Fe and Si excess, which can be explained by prolonged enrichment by SN Ia and stellar winds in the central early-type galaxy alone, but with tentative evidence for additional processes contributing to core enrichment in hotter groups. Inferred metal mass-to-light ratios inside r ₅₀₀ show a positive correlation with total group mass but are generally significantly lower than in clusters, due to a combination of lower global intracluster medium (ICM) abundances and gas-to-light ratios in groups. This metal deficiency is present for products from both SN Ia and SN II, and suggests that metals were either synthesized, released from galaxies or retained within the ICM less efficiently in lower mass systems. We explore possible causes, including variations in galaxy formation and metal release efficiency, cooling out of metals, and gas and metal loss via active galactic nuclei (AGN) – or starburst-driven galactic winds from groups or their precursor filaments. Loss of enriched material from filaments coupled with post-collapse AGN feedback emerges as viable explanations, but we also find evidence for metals to have been released less efficiently from galaxies in cooler groups and for the ICM in these to appear chemically less evolved, possibly reflecting more extended star formation histories in less massive systems. Some implications for the hierarchical growth of clusters from groups are briefly discussed.     

Contribution of the tully groups to the soft X-ray background

We examine a model for the expected contributions to the soft X-ray background (SXRB) due to bremsstrahlung and re-combination radiation of gas, enriched with heavy elements, located in groups of galaxies taken from Tully'sCatalog of Nearby Groups of Galaxies. It is shown that the contribution to the SXRB of groups of galaxies can explain up to 20% of the observed flux depending on the chemical composition and ratio of emitting mass to assumed virial mass.     

SEVEN NEW BULK CHEMICAL ANALYSES OF AUBRITES

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.     

E-CHONDRITES: SIGNIFICANCE OF THE PARTITION OF ELEMENTS BETWEEN ‘SILICATE’ AND ‘SULPHIDE’

Bulk chemical analyses of six E-chondrites (Daniel's kuil, Khairpur, Kota Kota, Saint-Sauveur, South Oman and St Mark's) are given, together with partial analyses of a further five (Blithfield, Hvittis, Indarch, Jajh deh Kot Lalu and Pillistfer). The distribution of some normally lithophile elements (Al, Ca, Cr, K, Mg, Na, P and Ti) between silicate and sulphide groups of minerals was determined using the selective attack by dry chlorine (350°C) on magnetically separated fractions. Subdivision of the E-chondrites into types I and II (Yavnel;, 1963; Anders, 1964) is accepted and it is shown using chemical data that St Mark's and Saint-Sauveur should be included in type I. Sulphides contribute an unexpectedly high proportion of several elements to the bulk: e.g. Ca (av. 88.5% type I, 66.3% type II); Ti(av. 77.1% type I, 84.8% type II) and P as phosphide (av. 44.4% type I, > 83.2% type II). The proportion of Ti contributed to the bulk composition by the sulphides in types I and II increases with increae in ‘thermal metamorphic effect’ (Easton, 1983b) within each type. There is marked variability in the relative abundances of metal, phosphide, silicate and sulphide among the members of each type in keeping with their aggregate nature. The chemical composition of the ‘silicate’ and ‘sulphide’ in type IE-chondrites differs from that in type II (e.g. CaO in the silicates, Mg in the bulk sulphides) which therefore precludes the isochemical evolution of all E-chondrites from a common parent material. Partition of Ti between silicate and sulphide groups of minerals indicates that types I and II E-chondrites originated in separate, chemically distinct bodies.     

Influence of uncertainties in the rate constants of chemical reactions on astrochemical modeling results

We analyze the influence of errors in the rate constants of gas-phase chemical reactions on the model abundances of molecules in the interstellar medium using the UMIST 95 chemical database. By randomly varying the rate constants within the limits of the errors given in UMIST 95, we have estimated the scatters in theoretical abundances for dark and diffuse molecular clouds. All of the species were divided into six groups by the scatter in their model equilibrium abundances when varying the rate constants of chemical reactions. The distribution of the species in groups depends on the physical conditions. The scatters in the abundances of simple species lie within 0.5–1 order of magnitude, but increase significantly as the number of atoms in the molecule increases. We suggest a simple method for identifying the reactions whose rate constants have the strongest effect on the abundance of a selected species. This method is based on an analysis of the correlations between the abundance of species and the reaction rate constants and allows the extent to which an improvement in the rate constant of a specific reaction reduces the uncertainty in the abundance of the species concerned to be directly estimated.     

X-ray evidence for multiphase hot gas with nearly solar Fe abundances in the brightest groups of galaxies

We analyse the ASCA spectra accumulated within 100 kpc radii of 12 of the brightest groups of galaxies. Upon fitting isothermal models (1T) jointly to the ASCA SIS and GIS spectra we obtain fits for most groups that are of poor or at best marginal quality and give very subsolar metallicities similar to previous studies, Z =0.29±0.12 Z. Two-temperature models (2T) provide significantly better fits for 11 out of the 12 groups, and in every case have metallicities that are substantially larger than obtained for the 1T models, Z =0.75±0.24 Z. Though not very well constrained, for most of the groups absorption in excess of the Galactic value is indicated for the cooler temperature component of the 2T models. A simple multiphase cooling flow model gives results analogous to the 2T models including large metallicities, Z =0.65±0.17 Z. The nearly solar Fe abundances and also solar /Fe ratios indicated by the 2T and cooling flow models are consistent with models of the chemical enrichment of ellipticals, groups, and clusters which assume ratios of Type Ia to Type II supernovae and an initial mass function (IMF) similar to those of the Milky Way.
Thus we have shown that the very subsolar Fe abundances and Si/Fe enhancements obtained from most previous studies within r 100 kpc of galaxy groups are an artefact of fitting isothermal models to the X-ray spectra, which also has been recently demonstrated for the brightest elliptical galaxies. Owing to the importance of these results for interpreting X-ray spectra, in an appendix we use simulated ASCA observations to examine in detail the 'Fe bias' and 'Si bias' associated with the spectral fitting of ellipticals, groups and clusters of galaxies.     

Martian northern polar cap: Layering and possible implications for radar sounding

The propagation of electromagnetic waves in the northern polar ice sheet of Mars is considered. It is shown that the dispersion and attenuation of radio waves in the polar sheet are regulated by two groups of factors: the physical and chemical composition of the ice, and the geometrical parameters of the layered structure of polar sheet. Both analytical and numerical simulations of ultra wide band chirp radar pulse propagating through the cap are performed. Wide variety of combinations of the physical and geometrical parameters of the ice sheet, consistent with previously published observational data, has been considered. The frequency bands of transparency and opacity of the northern ice sheet for radar signals were found. The side clutter for this particular region of the planet is studied.     

Comparison of the Murchison CM2 and Allende CV3 chondrites

The size distribution, abundance, and physical and chemical characteristics of chondritic inclusions are key features that define the chondrite groups. We present statistics on the size and abundance of the macroscopic components (inclusions) in the Murchison (CM2) and Allende (CV3) chondrites and measure their general chemical trends using established X‐ray mapping techniques. This study provides a fine‐scale assessment of the two meteorites and a semiquantitative evaluation of the relative abundances of elements and their distribution among meteorite components. Murchison contains 72% matrix and 28% inclusions; Allende contains 57% and 43%, respectively. A broad range of inclusion sizes and relative abundances has been reported for these meteorites, which demonstrates the necessity for a more standardized approach to measuring these characteristics. Nonetheless, the characteristic mean sizes of inclusions in Allende are consistently larger than those in Murchison. We draw two significant conclusions (1) these two meteorites sampled distinct populations of chondrules and refractory inclusions, and (2) complementary Mg/Si ratios between chondrules and matrix are observed in both Murchison and Allende. Both support the idea that chondrules and matrix within each chondrite group originated in single reservoirs of precursors with approximately solar Mg/Si ratios, providing a constraint on astrophysical models of the origin of chondrite parent bodies.     

Amorphous silicates as a record of solar nebular and parent body processes—A transmission electron microscope study of fine‐grained rims and matrix in three Antarctic CR chondrites

Renazzo‐type (CR) carbonaceous chondrites belong to one of the most pristine meteorite groups containing various early solar system components such as matrix and fine‐grained rims (FGRs), whose formation mechanisms are still debated. Here, we have investigated FGRs of three Antarctic CR chondrites (GRA 95229, MIL 07525, and EET 92161) by electron microscopy techniques. We specifically focused on the abundances and chemical compositions of the amorphous silicates within the rims and matrix by analytical transmission electron microscopy. Comparison of the amorphous silicate composition to a matrix area of GRA 95229 clearly shows a compositional relationship between the matrix and the fine‐grained rim, such as similar Mg/Si and Fe/Si ratios. This relationship and the abundance of the amorphous silicates in the rims strengthen a solar nebular origin and rule out a primary formation mechanism by parent body processes such as chondrule erosion. Moreover, our chemical analyses of the amorphous silicates and their abundance indicate that the CR rims experienced progressive alteration stages. According to our analyses, the GRA 95229 sample is the least altered one based on its high modal abundance of amorphous silicates (31%) and close‐to‐chondritic Fe/Si ratios, followed by MIL 07525 and finally EET 92161 with lesser amounts of amorphous silicates (12% and 5%, respectively) and higher Fe/Si ratios. Abundances and chemical compositions of amorphous silicates within matrix and rims are therefore suitable recorders to track different alteration stages on a submicron scale within variably altered CR chondrites.     

Chemical and oxygen isotopic properties of ordinary chondrites (H5, L6) from Oman: Signs of isotopic equilibrium during thermal metamorphism

Mean bulk chemical data of recently found H5 and L6 ordinary chondrites from the deserts of Oman generally reflect isochemical features which are consistent with the progressive thermal metamorphism of a common, unequilibrated starting material. Relative differences in abundances range from 0.5–10% in REE (Eu = 14%), 6–13% in siderophile elements (Co = 48%), and >10% in lithophile elements (exceptions are Ba, Sr, Zr, Hf, U = >30%) between H5 and L6 groups. These differences may have accounted for variable temperature conditions during metamorphism on their parent bodies. The CI/Mg‐normalized mean abundances of refractory lithophile elements (Al, Ca, Sm, Yb, Lu, V) show no resolvable differences between H5 and L6 suggesting that both groups have experienced the same fractionation. The REE diagram shows subtle enrichment in LREE with a flat HREE pattern. Furthermore, overall mean REE abundances are ~0.6 × CI with enriched La abundance (~0.9 × CI) in both groups. Precise oxygen isotope compositions demonstrate the attainment of isotopic equilibrium by progressive thermal metamorphism following a mass‐dependent isotope fractionation trend. Both groups show a ~slope‐1/2 line on a three‐isotope plot with subtle negative deviation in ?¹⁷O associated with δ¹⁸O enrichment relative to δ¹⁷O. These deviations are interpreted as the result of liberation of water from phyllosilicates and evaporation of a fraction of the water during thermal metamorphism. The resultant isotope fractionations caused by the water loss are analogous to those occurring between silicate melt and gas phase during CAI and chondrule formation in chondrites and are controlled by cooling rates and exchange efficiency.     

Major element fractionation in chondrites by distillation in the accretion disk of a T Tauri Sun?

Abstract— Redistribution or loss of batches of condensate from a cooling protosolar nebula is generally thought to have led to the formation of the chemical groups of chondrites. This demands a nebula hot enough for silicate vaporization over 1–3 AU, the region where chondrites formed. Alternatively, heating of a protosolar accretion disk may have been confined to an annular zone at its inner edge, ?0.06 AU from the protosun. Most infalling matter was accreted by the protosun, but a proportion was heated and carried outwards by an x‐wind. Shu et al. (1996, 1997) proposed that larger objects such as chondrules and calcium‐aluminum‐rich inclusions (CAIs) were returned to the disk at asteroidal distances by sedimentation from the x‐wind. Fine dust and gas were lost to space. The model implies that solids were not lost from the cold disk. The chemical compositions of the chondrite groups were produced by mixing different proportions of CAIs and chondrules with disk solids of CI composition. Heating at the inner edge of the disk was accompanied by particle irradiation, which synthesized nuclides including ²⁶Al. The x‐wind model can produce CAIs, not chondrules, and allows survival of presolar grains >0.06 AU from the protosun. Normalization to Al and CI indicates that non‐carbonaceous chondrites may be disk material that gained a Si‐ and Mg‐enriched fraction. Carbonaceous chondrites are different; they appear to be CI that lost lithophile elements more volatile than Ca. Five carbonaceous chondrite groups also lost Ni and Fe but the CH group gained siderophiles. Elemental loss from CI is incompatible with the x‐wind model. Silicon and CI normalization confirms that the CM, CO, CK and CV groups may be CI that gained refractories as CAIs. The Si‐, Mg‐rich fraction may have formed by selective vaporization followed by precipitation on grains in the x‐wind. This fractional distillation mechanism can account for lithophile element abundances in non‐carbonaceous chondrite groups, but an additional process is required for the loss of Ca and Mn in the EL group and for fractionated siderophile abundances in the H, L and LL groups. Heated and recycled fractions were not homogenized across the disk so the chondrite groups were established in a single cycle of enhanced protosolar activity in lt;10⁴ years, the time for a millimeter‐sized particle to drift into the Sun from 2 to 3 AU, due to gas‐drag.     

The histories of ordinary chondrite parent bodies: U,Th-He age distributions

Abstract— Age patterns observed in meteorite groups reflect the different thermal or impact histories experienced by their parent bodies. To assess the number of ordinary chondrite (OC) parent bodies rare-gas data in the Schultz and Kruse (1989) data base were used to calculate U,Th-He gas-retention ages. Most H- and LL-chondrite ages are high; ～81% are >2.2 Ga. In contrast, most L-chondrite ages are low; ～69% are ≤2.2 Ga, and ～35% are ≤0.9 Ga. The latter fraction is substantially lower than the value of 44% given by Heymann (1967). The difference is attributed to the preferential inclusion of shocked L chondrites in early studies. Broad age peaks in the H and LL groups near 3.4 Ga probably reflect thermal loss during metamorphism, but in the H distribution there is a hint of minor outgassing “events” near 1 Ga. The L/LL chondrites have chemical properties intermediate between and unresolvable from L and LL chondrites. The high ages of most L/LL chondrites are evidence against these originating on the L parent body; the L/LL age distribution is consistent with an origin on the LL parent body or on an independent body.     

Subtype 3.0 chondrites: Petrologic classification criteria

Type 3 chondrites are subdivided into 3.0–3.9. Subtype 3.0 chondrites nearly preserve all of their primitive features. Many criteria have been proposed to distinguish such primitive chondrites. Here, we compiled mineral data and reconsider the petrologic classification criteria for subtype 3.0. Chondrites are classified into subtypes by the minor element distribution of olivine and textural and chemical features of Fe-Ni metal. The []Si₄O₈ and MgO components of feldspar also distinguish subtype 3.0 from subtypes ≥3.1. Other features, such as the occurrence of near pure chromite, are also indicators of subtype 3.0. It is difficult to distinguish between subtypes 3.0 and ≤2.9 based on mineral chemistry. Therefore, we propose the following criteria to distinguish between subtypes 3.0 and ≤2.9. In type 3.0 chondrites, major silicate (olivine, pyroxene, and plagioclase), oxide, metal, and sulfide minerals do not show aqueous alteration features. Melilite, anorthite, and glass show no or mild aqueous alteration features. Subtype 3.0 has not been identified in all chondrite groups. The absence of subtype 3.0 from some groups mainly reflects differences in the degrees of secondary parent body processes among the chondrite groups.