Meteoroid streams: Evidence from meteorites recovered on Earth

The flux of small meteoroids, originating primarily from comets, consists of sporadic, random objects and others whose orbits are related. Here, we summarize data relevant to the question of whether the flux of large meteoroids of asteroidal origin (recoverable as meteorites) also consists of objects with random orbits, as well as coorbital objects. After reviewing some relevant properties of planetary materials, applications of two nuclear techniques - radiochemical neutron activation analysis (RNAA) and accelerator mass spectrometry (AMS) - to this question are discussed. Contents of ten thermally labile trace and ultratrace elements determined by RNAA (Ag, Bi, Cd, Cs, In, Rb, Se, Te, Tl, Zn) act as thermometers for thermal metamorphism in parent sources. These data, together with spectral reflectivity information, establish the nature of surfaces on abundant C-, G-, B- and F-class asteroids. Data for these ten cosmothermometers in H4-6 type ordinary chondrites, when treated by multivariate statistical techniques, demonstrate that a suite chosen by one set of criteria (the circumstances of their fall in May, between 1855 and 1895) is distinguishable by another set, i.e. compositionally, from all other such falls analyzed. Hence, this suite, H Cluster 1, has an average thermal history distinguishable from those of all other falls, demonstrating that near-Earth source regions for H chondrite falls changes rapidly. AMS measurements of cosmogenic³⁶Cl (301 kyr half-life), quantify nominal terrestrial ages for Antarctic H chondrites whose contents of thermometric trace elements were also established by RNAA. While multivariate statistical analysis of RNAA data from Antarctic H chondrites with nominal terrestrial ages 50 kyr are not distinguishable from those of falls, older Antarctic H chondrites are compositionally distinguishable from falls. Assertions that these highly significant compositional differences reflect terrestrial or methodologic causes are refutable. This result argues that near-Earth source regions of H chondrites have changed over a long time, as well. Thus, the Earth receives a highly biased sampling of planetary objects in the Solar System in any one time-period.     

Amino acid analyses of Antarctic CM2 meteorites using liquid chromatography‐time of flight‐mass spectrometry

Abstract— Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH) 83100 and Lewis Cliff (LEW) 90500 using liquid chromatography‐time of flight‐mass spectrometry (LC‐ToF‐MS) coupled with UV fluorescence detection revealed that these carbonaceous meteorites contain a suite of indigenous amino acids not present in Antarctic ice. Several amino acids were detected in ALH 83100, including glycine, alanine, β‐alanine, γ‐amino‐n‐butyric acid (γ‐ABA), and α‐aminoisobutyric acid (AIB) with concentrations ranging from 250 to 340 parts per billion (ppb). In contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a much higher total abundance of these amino acids (440–3200 ppb). In addition, ALH 83100 was found to have lower abundances of the α‐dialkyl amino acids AIB and isovaline than LEW 90500 and Murchison. There are three possible explanations for the depleted amino acid content in ALH 83100: 1) amino acid leaching from ALH 83100 during exposure to Antarctic ice meltwater, 2) a higher degree of aqueous alteration on the ALH 83100 parent body, or 3) ALH 83100 originated on a chemically distinct parent body from the other two CM2 meteorites. The high relative abundance of ?‐amino‐n‐caproic acid (EACA) in the ALH 83100 meteorite as well as the Antarctic ice indicates that Nylon‐6 contamination from the Antarctic sample storage bags may have occurred during collection.     

Acfer 217-A new member of the Rumuruti chondrite group (R)

Abstract— Previously, three meteorites from Australia and Antarctica were described as a new chondritic “grouplet” (Carlisle Lakes, Allan Hills (ALH) 85151, Yamato (Y) ?75302; Rubin and Kallemeyn, 1989). This grouplet was classified as the “Carlisle Lakes-type” chondrites (Weisberg et al., 1991). Recently, one Saharan sample and four more Antarctic meteorites were identified to belong to this group (Acfer 217, Y-793575, Y-82002, PCA91002, PCA91241). The latter two are probably paired. With the meteorite Rumuruti, the first fall of this type of chondrite is known (Schulze et al., 1994). We report here on the Saharan meteorite Acfer 217 which has chemical and mineralogical properties very similar to Rumuruti and Carlisle Lakes. All eight members of this group, Rumuruti, Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, and the paired samples PCA91002 and PCA91241 justify the introduction of a new group of chondritic meteorites, the Rumuruti meteorites (R). Acfer 217 is a regolith breccia consisting of up to cm-sized clasts (～33 vol%) embedded in a fine-grained, well-lithified clastic matrix. The most abundant mineral is olivine (～72 vol%), which has a high Fa-content of 37–39 mol%. The major minerals (olivine, low-Ca pyroxene, Ca-pyroxene, and plagioclase) show some compositional variability indicating a slightly unequilibrated nature of the meteorite. Considering the mean olivine composition of Fa_{37.8 ± 5.7}, a classification of Acfer 217 as a R3.8 chondrite would result; however, Acfer 217 is a regolith breccia consisting of clasts of various petrologic types. Therefore, we suggest to classify Acfer 217 as a R3–5 chondrite regolith breccia. The bulk meteorite is very weakly shocked (S2). The bulk composition of Acfer 217 and other R-meteorites show that the R-meteorites are basically chondritic in composition. The pattern of moderately volatile elements is unique in R chondrites; Na and Mn are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The oxygen isotopic composition in Acfer 217 is similar to that of Rumuruti, Carlisle Lakes, ALH 85151, and Y-75302. In a δ¹⁷O vs. δ¹⁸O-diagram, the R-meteorites form a group well resolved from other chondrite groups. Acfer 217 was a meteoroid of common size with a radius between 15–65 cm and with a single stage exposure history. Based on ²¹Ne, an exposure age of about 35 Ma was calculated.     

Chemical studies of H chondrites-10: Contents of thermally labile trace elements are unaffected by late heating

Abstract— We have used radiochemical neutron activation analysis (RNAA) to determine 15 trace elements, including 10 moderately to highly volatile ones—Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, In (in increasing volatility order), in 6 H chondrite falls with low-³He contents. These (plus prior RNAA data) provide a compositional database of 92 H4-6 chondrite falls. Three suites of samples can be identified from their noble gas contents: 44 with “normal” contents and, therefore, “normal” orbits and cosmic-ray exposure histories; 8 that lost radiogenic gases, presumably by shock late in their histories; and 17 that lost cosmogenic gases by heating during close solar approach. We used the standard multivariate statistical techniques of linear discriminant analysis and logistic regression to compare contents of the 10 moderately and highly volatile trace elements, listed above, in these three suites. We found no significant differences. This contrasts sharply with similar comparisons involving random falls and H4-6 chondrites that landed on Earth at specific time intervals. Apparently, contents of volatile trace elements in H4-6 chondrites were established early in their histories, and they are so retentively sited that loss during later heating episodes did not occur.     

Thermal metamorphism of CI and CM carbonaceous chondrites: An internal heating model

Abstract— Infrared diffuse reflectance spectra were measured for several thermally metamorphosed carbonaceous chondrites with CI-CM affinities which were recently found from Antarctica. Compared with other CI or CM carbonaceous chondrites, these Antarctic carbonaceous chondrites show weaker absorption bands near 3 μm due to hydrous minerals, and weaker absorption bands near 6.9 μm due to carbonates, interpreted as thermal metamorphic features. These absorption bands also disappear in the spectra of samples of the Murchison (CM) carbonaceous chondrite heated above 500 °C, implying that the metamorphic temperatures of the Antarctic carbonaceous chondrites considered here were higher than about 500 °C. Model calculations were performed to study thermal metamorphism of carbonaceous chondrites in a parent body internally heated by the decay of the extinct nuclide ²⁶Al. The maximum temperature of the interior of a body more than 20 km in radius is 500–700 °C for the bulk Al contents of CI and CM carbonaceous chondrites, assuming a ratio of ²⁶Al/²⁷Al = 5 × 10^?6 which has been previously proposed for an ordinary-chondrite parent body. The metamorphic temperatures experienced by the Antarctic carbonaceous chondrites considered here may be attainable by an internally heated body with an ²⁶Al/²⁷Al ratio similar to that inferred for an ordinary-chondrite parent body.     

Mineralogy and chemistry of Rumuruti: The first meteorite fall of the new R chondrite group

Abstract— The Rumuruti meteorite shower fell in Rumuruti, Kenya, on 1934 January 28 at 10:43 p.m. Rumuruti is an olivine-rich chondritic breccia with light-dark structure. Based on the coexistence of highly recrystallized fragments and unequilibrated components, Rumuruti is classified as a type 3–6 chondrite breccia. The most abundant phase of Rumuruti is olivine (mostly Fa_～39) with about 70 vol%. Feldspar (～14 vol%; mainly plagioclase), Ca-pyroxene (5 vol%), pyrrhotite (4.4 vol%), and pentlandite (3.6 vol%) are major constituents. All other phases have abundances below 1 vol%, including low-Ca pyroxene, chrome spinels, phosphates (chlorapatite and whitlockite), chalcopyrite, ilmenite, tridymite, Ni-rich and Ge-containing metals, kamacite, and various particles enriched in noble metals like Pt, Ir, arid Au. The chemical composition of Rumuruti is chondritic. The depletion in refractory elements (Sc, REE, etc.) and the comparatively high Mn, Na, and K contents are characteristic of ordinary chondrites and distinguish Rumuruti from carbonaceous chondrites. However, S, Se, and Zn contents in Rumuruti are significantly above the level expected for ordinary chondrites. The oxygen isotope composition of Rumuruti is high in δ¹⁷O (5.52 ‰) and δ¹⁸O (5.07 ‰). Previously, a small number of chondritic meteorites with strong similarities to Rumuruti were described. They were called Carlisle Lakes-type chondrites and they comprise: Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, PCA91002, and PCA91241, as well as clasts in the Weatherford chondrite. All these meteorites are finds from hot and cold deserts having experienced various degrees of weathering. With Rumuruti, the first meteorite fall has been recognized that preserves the primary mineralogical and chemical characteristics of a new group of meteorites. Comparing all chondrites, the characteristic features can be summarized as follows: (a) basically chondritic chemistry with ordinary chondrite element patterns of refractory and moderately volatile lithophiles but higher abundances of S, Se, and Zn; (b) high degree of oxidation (37–41 mol% Fa in olivine, only traces of Fe, Ni-metals, occurrence of chalcopyrite); (c) exceptionally high Δ¹⁷O values of about 2.7 for bulk samples; (d) high modal abundance of olivine (～70 vol%); (e) Ti-Fe³⁺?rich chromite (～5.5 wt% TiO₂); (f) occurrence of various noble metal-rich particles; (g) abundant chondritic breccias consisting of equilibrated clasts and unequilibrated lithologies. With Rumuruti, nine meteorite samples exist that are chemically and mineralogically very similar. These meteorites are attributed to at least eight different fall events. It is proposed in this paper to call this group R chondrites (rumurutiites) after the first and only fall among these meteorites. These meteorites have a close relationship to ordinary chondrites. However, they are more oxidized than any of the existing groups of ordinary chondrites. Small, but significant differences in chemical composition and in oxygen isotopes between R chondrites and ordinary chondrites exclude formation of R chondrites from ordinary chondrites by oxidation. This implies a separate, independent R chondrite parent body.     

The Meteoritical Bulletin,No. 80, 1996 July

Abstract— The Meteoritical Bulletin No. 80 lists data for 178 meteorites. Noteworthy are 3 HED meteorites (ALH 88102, Hammadah al Hamra (HaH) 059, and Monticello); 3 ureilites (HaH 064, HaH 126, and Dar al Gani (DaG) 084); 4 irons (Baygorria (IAB), Ste. Croix (IIIAB), Sargiin Gobi (IAB), and Tarahumara (IIE)); an unusual metal-rich meteorite (Vermillion); 8 carbonaceous chondrites (HaH 043 (C03), HaH 073 (C4), DaG 055 (C3) and 5 C03 chondrites (probably paired) from DaG); an R chondrite (DaG 013); and 6 unequilibrated ordinary chondrites (ALH 88105 (L3), Camel Donga 016 (L3), HaH 093 (LL3.9), HaH 096 (LL(L)3), Richfield (LL3.7), and Sarir Quattusah (LL(L)3)). Three recent falls of ordinary chondrites (Coleman (LL5), St. Robert (H5), and Tsukuba (H5-6)) are described.     

Cosmic‐ray exposure age and heliocentric distance of the parent bodies of enstatite chondrites ALH 85119 and MAC 88136

Abstract— We measured concentrations and isotopic ratios of noble gases in enstatite (E) chondrites Allan Hills (ALH) 85119 and MacAlpine Hills (MAC) 88136. These two meteorites contain solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated heliocentric distances, parent body exposure ages, and space exposure ages of the two meteorites. The parent body exposure ages are longer than 6.7 Ma for ALH 85119 and longer than 8.7 Ma for MAC 88136. The space exposure ages are shorter than 2.2 Ma for ALH 85119 and shorter than 3.9 Ma for MAC 88136. The estimated heliocentric distances are more than 1.1 AU for ALH 85119 and 1.3 AU for MAC 88136. Derived heliocentric distances indicate the locations of parent bodies in the past when constituents of the meteorites were exposed to the Sun. From the mineralogy and chemistry of E chondrites, it is believed that E chondrites formed in regions within 1.4 AU from the Sun. The heliocentric distances of the two E chondrite parent bodies are not different from the formation regions of E chondrites. This may imply that heliocentric distances of E chondrites have been relatively constant from their formation stage to the stage of exposure to the solar wind.     

Petrographic and mineralogical study of new EH melt rocks and a new enstatite chondrite grouplet

Abstract— Enstatite chondrites are classified into EH and EL groups, and some of them were melted once. In this paper, we report petrography and mineral chemistry of five new Antarctic enstatite chondrites. Yamato 793225 is characterized by intermediate properties between the EH and EL groups and probably represents a new grouplet of enstatite chondrites. The three paired meteorites (Y-8404, Y-8414 and Y-86004) may have cooled rapidly near the surface of the parent body as impact EH melt rocks. Yamato 82189 is also classified as EH melt rock, but it experienced slow cooling as did Y-793225. Yamato 82189 contains the first occurrence of phlogopite in enstatite meteorites.     

Carbonaceous chondrite clasts in the howardites Bholghati and EET87513

Abstract— Twenty-two carbonaceous chondrite clasts from the two howardites Bholghati and EET87513 were analyzed. Clast N from EET87513 is a fragment classified as CM2 material on the basis of texture, bulk composition, mineralogy, and bulk O isotopic composition. Carbonaceous chondrite clasts from Bholghati, for which less data are available because of their small size, can be divided into two petrologic types: C1 and C2. C1 clasts are composed of opaque matrix with rare coarse-grained silicates as individual mineral fragments; textures resemble CI meteorites and some dark inclusions from CR meteorites. Opaque matrix is predominantly composed of flaky saponite; unlike typical CI and CR meteorites, serpentine is absent in the samples we analyzed. C2 clasts contain chondrules, aggregates, and individual fragments of coarse-grained silicates in an opaque matrix principally composed of saponite and anhydrous ferromagnesian silicates with flaky textures similar to phyllosilicates. These anhydrous ferromagnesian silicates are interpreted as the product of heating of pre-existing serpentine. The carbonaceous chondrite clasts we have studied from these two howardites are, with one notable exception (clast N from EET87513), mineralogically distinct from typical carbonaceous chondrites. However, these clasts have very close affinities to carbonaceous chondrites and have also experienced thermal metamorphism and aqueous alteration, but to different degrees.     

Amino acids in Antarctic CM1 meteorites and their relationship to other carbonaceous chondrites

Abstract— CM2 carbonaceous chondrites are the most primitive material present in the solar system, and some of their subtypes, the CM and CI chondrites, contain up to 2 wt% of organic carbon. The CM2 carbonaceous chondrites contain a wide variety of complex amino acids, while the CI1 meteorites Orgueil and Ivuna display a much simpler composition, with only glycine and β‐alanine present in significant abundances. CM1 carbonaceous chondrites show a higher degree of aqueous alteration than CM2 types and therefore provide an important link between the CM2 and CI1 carbonaceous chondrites. Relative amino acid concentrations have been shown to be indicative for parent body processes with respect to the formation of this class of compounds. In order to understand the relationship of the amino acid composition between these three types of meteorites, we have analyzed for the first time three Antarctic CM1 chondrites, Meteorite Hills (MET) 01070, Allan Hills (ALH) 88045, and LaPaz Icefield (LAP) 02277, using gas chromatography‐mass spectrometry (GC‐MS) and high performance liquid chromatography‐fluorescence detection (HPLC‐FD). The concentrations of the eight most abundant amino acids in these meteorites were compared to those of the CM2s Murchison, Murray, Mighei, Lewis Cliff (LEW) 90500, ALH 83100, as well as the CI1s Orgueil and Ivuna. The total amino acid concentration in CM1 carbonaceous chondrites was found to be much lower than the average of the CM2s. Relative amino acid abundances were compared in order to identify synthetic relationships between the amino acid compositions in these meteorite classes. Our data support the hypothesis that amino acids in CM‐ and CI‐type meteorites were synthesized under different physical and chemical conditions and may best be explained with differences in the abundances of precursor compounds in the source regions of their parent bodies in combination with the decomposition of amino acids during extended aqueous alteration.     

Distribution of aliphatic amines in CO,CV, and CK carbonaceous chondrites and relation to mineralogy and processing history

The analysis of water‐soluble organic compounds in meteorites provides valuable insights into the prebiotic synthesis of organic matter and the processes that occurred during the formation of the solar system. We investigated the concentration of aliphatic monoamines present in hot acid water extracts of the unaltered Antarctic carbonaceous chondrites, Dominion Range (DOM) 08006 (CO3) and Miller Range (MIL) 05013 (CO3), and the thermally altered meteorites, Allende (CV3), LAP 02206 (CV3), GRA 06101 (CV3), Allan Hills (ALH) 85002 (CK4), and EET 92002 (CK5). We have also reviewed and assessed the petrologic characteristics of the meteorites studied here to evaluate the effects of asteroidal processing on the abundance and molecular distributions of monoamines. The CO3, CV3, CK4, and CK5 meteorites studied here contain total concentrations of amines ranging from 1.2 to 4.0 nmol g^?1 of meteorite; these amounts are 1–3 orders of magnitude below those observed in carbonaceous chondrites from the CI, CM, and CR groups. The low‐amine abundances for CV and CK chondrites may be related to their extensive degree of thermal metamorphism and/or to their low original amine content. Although the CO3 meteorites, DOM 08006 and MIL 05013, do not show signs of thermal and aqueous alteration, their monoamine contents are comparable to those observed in moderately/extensively thermally altered CV3, CK4, and CK5 carbonaceous chondrites. The low content of monoamines in pristine CO carbonaceous chondrites suggests that the initial amounts, and not asteroidal processes, play a dominant role in the content of monoamines in carbonaceous chondrites. The primary monoamines, methylamine, ethylamine, and n‐propylamine constitute the most abundant amines in the CO3, CV3, CK4, and CK5 meteorites studied here. Contrary to the predominance of n‐ω‐amino acid isomers in CO3 and thermally altered meteorites, there appears to be no preference for the larger n‐amines.     

Weathering in Antarctic H and CR chondrites: Quantitative analysis through Mössbauer spectroscopy

Abstract— Mössbauer spectroscopy is a very useful tool for identifying ferric iron weathering products in meteorites because of the capability to quantify the relative amounts of ferric iron in them. Mössbauer measurements were made of 33 Antarctic H chondrites (predominately H5) and two paired Antarctic CR chondrites. The primary goals of this study are to determine if Mössbauer spectroscopy can be used to determine which phases are weathering in Antarctic meteorites and if the relative amounts of ferric iron correlate with terrestrial age. Determining which minerals are weathering in ordinary chondrites appears very difficult due to variations in composition for different ordinary chondrites of the same meteorite class and possible problems in preparing homogeneous samples. The analysis of the two paired CR chondrites appears to indicate that metallic iron is predominately weathering to produce ferric iron for this class of meteorite. No correlation is seen between the relative amounts of ferric iron and terrestrial age for ordinary chondrites. One Antarctic H5 chondrite (ALHA77294) with a short ¹⁴C age of 135 ± 200 years from the dating of interior carbonate weathering products does have a relatively low amount of ferric iron, which is consistent with this meteorite being exposed on the surface for a relatively short time.     

The Meteoritical Bulletin,No. 82, 1998 July

Abstract— Meteoritical Bulletin No. 82 lists information for 974 new meteorites, including 521 finds from Antarctica, 401 finds from the Sahara, 21 finds from the Nullarbor region of Australia, and 7 falls (Ban Rong Du, Burnwell, Fermo, Jalanash, Juancheng, Monahans (1998), and Silao). Many rare types of meteorites are reported: counting pairing groups as one, these include one CR chondrite, two CK chondrites, two CO chondrites, four CV chondrites, one CH chondrite or Bencubbin-like, six C2 (unclassified) chondrites, two EH chondrites, two EL chondrites, three R chondrites, thirty unequilibrated ordinary chondrites, one un-grouped chondrite, three eucrites, six howardites, one diogenite, eleven ureilites, nine iron meteorites, one mesosiderite, two brachinites, one lodranite, one winonaite, and two lunar meteorites (Dar al Gani 400 and EET 96008). All italicized abbreviations refer to addresses tabulated at the end of this document.     

Chemical and physical studies of type 3 chondrites XII: The metamorphic history of CV chondrites and their components

Abstract— The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CK chondrites and Renazzo (CR2) have been measured in order to investigate their metamorphic history. The petrographic, mineralogical and bulk compositional differences among the CV chondrites indicate that the TL sensitivity of the ～130 °C TL peak is reflecting the abundance of ordered feldspar, especially in chondrule mesostasis, which in turn reflects parent-body metamorphism. The TL properties of 18 samples of homogenized Allende powder heated at a variety of times and temperatures, and cathodoluminescence mosaics of Axtell and Coolidge, showed results consistent with this conclusion. Five refractory inclusions from Allende, and separates from those inclusions, were also examined and yielded trends reflecting variations in mineralogy indicative of high peak temperatures (either metamorphic or igneous) and fairly rapid cooling. The CK chondrites are unique among metamorphosed chondrites in showing no detectable induced TL, which is consistent with literature data that suggests very unusual feldspar in these meteorites. Using TL sensitivity and several mineral systems and allowing for the differences in the oxidized and reduced subgroups, the CV and CV-related meteorites can be divided into petrologic types analogous to those of the ordinary and CO type 3 chondrites. Axtell, Kaba, Leoville, Bali, Arch and ALHA81003 are type 3.0–3.1, while ALH84018, Efremovka, Grosnaja, Allende and Vigarano are type 3.2–3.3 and Coolidge and Loongana 001 are type 3.8. Mokoia is probably a breccia with regions ranging in petrologic type from 3.0 to 3.2. Renazzo often plots at the end of the reduced and oxidized CV chondrite trends, even when those trends diverge, suggesting that in many respects it resembles the unmetamorphosed precursors of the CV chondrites. The low-petrographic types and low-TL peak temperatures of all samples, including the CV3.8 chondrites, indicates metamorphism in the stability field of low feldspar (i.e., <800 °C) and a metamorphic history similar to that of the CO chondrites but unlike that of the ordinary chondrites.     

Iron‐rich aureoles in the CM carbonaceous chondrites Murray,Murchison, and Allan Hills 81002: Evidence for in situ aqueous alteration

Abstract— Iron‐rich aureoles in CM carbonaceous chondrites are previously unidentified domains of aqueously altered matrix material, whose FeO content may exceed that of the surrounding matrix by up to more than 15 wt%. We describe the petrography and mineralogy of these objects in the CM chondrites Murray, Murchison, and Allan Hills (ALH) 81002. The size of Fe‐rich aureoles ranges from a few hundred microns to several millimeters in diameter and appears to be a function of the degree of alteration of the host chondrite. The origin of Fe‐rich aureoles is related to the alteration of large metal grains that has resulted in the formation of characteristic PCP‐rich reaction products that are frequently observed at the centers of the aureoles. This suggests that Fe‐rich aureoles in CM chondrites are the result of the mobilization of Fe from altering metal grains into the matrix. The fact that Fe‐rich aureoles enclose numerous chondritic components such as chondrules, calcium‐aluminum‐rich inclusions (CAIs), and mineral fragments, as well as their radial symmetric appearance, are strong evidence that they formed in situ and that significant directional fluid flow was not involved in the alteration process. This and additional constraints, such as the distribution of S and other elements, as well as the inferred alteration conditions, are consistent with in situ parent‐body alteration. The observations are, however, entirely incompatible with preaccretionary alteration models in which the individual CM chondrite components have experienced diverse alteration histories. The presence of numerous intact aureoles in the brecciated CM chondrites Murray and Murchison further suggests that the alteration occurred largely after brecciation affected these meteorites. Therefore, the progressive aqueous alteration of CM chondrites may not be necessarily coupled to brecciation as has been previously proposed.     

CHEMICAL ANALYSES OF THE MURCHISON AND LOST CITY METEORITES

The abundances of 22 elements have been determined in the recently fallen Murchison and Lost City meteorites. Analyses were performed by 14-MeV neutron activation, thermal neutron activation, and in a few cases by wet chemical techniques. On the basis of these data the composition of the Murchison chondrite is intermediate between previously reported analyses of Type II and Type III carbonaceous chondrites. The data for the Lost City chondrite in general agree well with mean values reported for H-group ordinary chondrites. The low oxygen content of the Lost City chondrite suggests that previously reported oxygen abundances in H-group falls may be too high due to oxidation in storage or weathering prior to collection     

The onset of metamorphism in ordinary and carbonaceous chondrites

Abstract— Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by X‐ray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The distribution of Cr in FeO‐rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Igneous zoning patterns are replaced by complex ones and Cr‐rich coatings develop on all grains. Cr distributions in olivine are controlled by the exsolution of a Cr‐rich phase, probably chromite. Cr in olivine may have been partly present as tetrahedrally coordinated Cr³⁺. Separation of chromite is nearly complete by petrologic type 3.2. The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type. TEM shows this to be the result of crystallization of albite. Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K. Glass in type I chondrules also gains alkalis during metamorphism. Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re‐enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised.     

Dhofar 225 and Dhofar 735: Relationship to CM2 chondrites and metamorphosed carbonaceous chondrites,Belgica‐7904 and Yamato‐86720

Abstract– Dhofar (Dho) 225 and Dho 735 are carbonaceous chondrites found in a hot desert and having affinities to Belgica‐like Antarctic chondrites (Belgica [B‐] 7904 and Yamato [Y‐] 86720). Texturally they resemble CM2 chondrites, but differ in mineralogy, bulk chemistry and oxygen isotopic compositions. The texture and main mineralogy of Dho 225 and Dho 735 are similar to the CM2 chondrites, but unlike CM2 chondrites they do not contain any (P, Cr)‐sulfides, nor tochilinite 6Fe_0.9S*5(Fe,Mg)(OH)₂. H₂O‐contents of Dho 225 and Dho 735 (1.76 and 1.06 wt%) are lower than those of CM2 chondrites (2–18 wt%), but similar to those in the metamorphosed carbonaceous chondrites of the Belgica‐like group. Bulk compositions of Dho 225 and Dho 735, as well as their matrices, have low Fe and S and low Fe/Si ratios relative to CM2 chondrites. X‐ray powder diffraction patterns of the Dho 225 and Dho 735 matrices showed similarities to laboratory‐heated Murchison CM2 chondrite and the transformation of serpentine to olivine. Dho 225 and 735’s oxygen isotopic compositions are in the high ¹⁸O range on the oxygen diagram, close to the Belgica‐like meteorites. This differs from the oxygen isotopic compositions of typical CM2 chondrites. Experimental results showed that the oxygen isotopic compositions of Dho 225 and Dhofar 725, could not be derived from those of typical CM2 chondrites via dehydration caused by thermal metamorphism. Dho 225 and Dho 735 may represent a group of chondrites whose primary material was different from typical CM2 chondrites and the Belgica‐like meteorites, but they formed in an oxygen reservoir similar to that of the Belgica‐like meteorites.