Progressive aqueous alteration of CM carbonaceous chondrites

CM chondrites are aqueously altered rocks that contain ∼9 wt% H₂O⁺ (i.e., indigenous water) bound in phyllosilicates; also present are clumps of serpentine-tochilinite intergrowths (previously called “poorly characterized phases” or PCP), pentlandite and Ni-bearing pyrrhotite. We studied 11 CM chondrites that span the known range from least altered to most altered. We used various petrologic properties (many previously identified) that provide information regarding the degree of aqueous alteration. There are no known unaltered or slightly altered CM chondrites (e.g., rocks containing numerous chondrules with primary igneous glass). Some CM properties result from processes associated with early and intermediate stages of the alteration sequence (i.e., hydration of matrix, alteration of chondrule glass, and production of large PCP clumps). Other petrologic properties reflect processes active throughout the alteration sequence; these include oxidation of metallic Fe-Ni, alteration of chondrule phenocrysts, changes in PCP composition (reflecting an increase in the phyllosilicate/sulfide ratio), and changes in carbonate mineralogy (reflecting the development of dolomite and complex carbonates at the expense of Ca carbonate).On the basis of these parameters, we propose a numerical alteration sequence for CM chondrites. Because there are no known CM samples that display only incipient alteration, the least altered sample was arbitrarily assigned to subtype 2.6. The most altered CM chondrites, currently classified CM1, are assigned to subtype 2.0. These highly altered rocks have essentially no mafic silicates; they contain chondrule pseudomorphs composed mainly of phyllosilicate. However, their bulk compositions are CM-like, and they are closer in texture to other C2 chondrites than to CI1 chondrites (which lack chondrule pseudomorphs). Using several diagnostic criteria, we assigned petrologic subtypes (±0.1) to every CM chondrite in our study: QUE 97990, CM2.6; Murchison, CM2.5; Kivesvaara, CM2.5; Murray, CM2.4/2.5; Y 791198, CM2.4; QUE 99355, CM2.3; Nogoya, CM2.2; Cold Bokkeveld, CM2.2; QUE 93005, CM2.1; LAP 02277, CM2.0; MET 01070, CM2.0.The proposed CM numerical alteration sequence improves upon the existing scheme of Browning et al. (1996) in that it does not require a complicated algorithm applied to electron-microprobe data to determine the average matrix phyllosilicate composition. The new sequence is more comprehensive and employs petrologic subtypes that are easier to use and remember than mineralogic alteration index values.New neutron-activation analyses of QUE 97990, QUE 93005, MET 01070, Murchison and Crescent, together with literature data, confirm the compositional uniformity of the CM group; different degrees of alteration among CM chondrites do not lead to resolvable bulk compositional differences. This suggests that the textural differences among individual CM chondrites reflect progressive alteration of similar hypothetical CM3.0 starting materials in different regions of the same parent body, with minimal aqueous transport of materials over appreciable (e.g., meters) distances.     

The formation and alteration of the Renazzo-like carbonaceous chondrites I: Implications of bulk-oxygen isotopic composition

To better understand the role of aqueous alteration on the CR chondrite parent asteroid, a whole-rock oxygen isotopic study of 20 meteorites classified as Renazzo-like carbonaceous chondrites (CR) was conducted. The CR chondrites analyzed for their oxygen isotopes were Dhofar 1432, Elephant Moraine (EET) 87770, EET 92042, EET 96259, Gao-Guenie (b), Graves Nunataks (GRA) 95229, GRA 06100, Grosvenor Mountains (GRO) 95577, GRO 03116, LaPaz Ice Field (LAP) 02342, LAP 04720, Meteorite Hills (MET) 00426, North West Africa (NWA) 801, Pecora Escarpment (PCA) 91082, Queen Alexandra Range (QUE) 94603, QUE 99177, and Yamato-793495 (Y-793495). Three of the meteorites, Asuka-881595 (A-881595), GRA 98025, and MET 01017, were found not to be CR chondrites. The remaining samples concur petrographically and with the well-established oxygen-isotope mixing line for the CR chondrites. Their position along this mixing line is controlled both by the primary oxygen-isotopic composition of their individual components and their relative degree of aqueous alteration. Combined with literature data and that of this study, we recommend the slope for the CR-mixing line to be 0.70 ± 0.04 (2σ), with a δ¹⁷O-intercept of −2.23 ± 0.14 (2σ).Thin sections of Al Rais, Shi?r 033, Renazzo, and all but 3 samples analyzed for oxygen isotopes were studied petrographically. The abundance of individual components is heterogeneous among the CR chondrites, but FeO-poor chondrules and matrix are the most abundant constituents and therefore, dominate the whole-rock isotopic composition. The potential accreted ice abundance, physico-chemical conditions of aqueous alteration (e.g. temperature and composition of the fluid) and its duration control the degree of alteration of individual CR chondrites. Combined with literature data, we suggest that LAP 02342 was exposed to lower temperature fluid during alteration than GRA 95229. With only two falls, terrestrial alteration of the CR chondrites complicates the interpretation of their whole rock isotopic composition, particularly in the most aqueously altered samples, and those with relatively higher matrix abundances. We report that QUE 99177 is the isotopically lightest whole rock CR chondrite known (δ¹⁸O = −2.29‰, δ¹⁷O = −4.08‰), possibly due to isotopically light unaltered matrix; which shows that the anhydrous component of the CR chondrites is isotopically lighter than previously thought. Although it experienced aqueous alteration, QUE 99177 provides the best approximation of the pristine CR-chondrite parent body’s oxygen-isotopic composition, before aqueous alteration took place. Using this value as a new upper limit on the anhydrous component of the CR chondrites, water/rock ratios were recalculated and found to be higher than previously thought; ratios now range from 0.281 to 1.157. We also find that, according to their oxygen isotopes, a large number of CR chondrites appear to be minimally aqueously altered; although sample heterogeneity complicates this interpretation.     

Amino acids of the Nogoya and Mokoia carbonaceous chondrites

Amino acids were found in acid hydrolyzed, hot water extracts of the Nogoya (C2) and Mokoia (C3V) chondrites. About 40 n moles/g of amino acids were found in the Nogoya extract while Mokoia contained less than 1 n mole/g. The amino acid composition of Nogoya differs from that of other C2 chondrites studied earlier. The results from Mokoia are similar to previous data obtained from the C3V chondrite Allende.     

Planetesimal sulfate and aqueous alteration in CM and CI carbonaceous chondrites

Water-soluble sulfate salts extracted from six CM chondrites have oxygen isotope compositions that are consistent with an extraterrestrial origin. The Δ¹⁷O of sulfate are correlated with previously reported whole rock δ¹⁸O and with an index of meteorite alteration, and may display a correlation with the date of the fall. The enrichments and depletions for Δ¹⁷O of water-soluble sulfate from the CM chondrites relative to the terrestrial mass dependent fractionation line are consistent with sulfate formation in a rock dominated asteroidal environment, and from aqueous fluids that had undergone relatively low amounts of oxygen isotope exchange and little reaction with anhydrous components of the meteorites. It is unresolved how the oxidation of sulfide to sulfate can be reconciled with the inferred low oxidation state during the extraterrestrial alteration process. Oxygen isotope data for two CI chondrites, Orgueil and Ivuna, as well as the ungrouped C2 chondrite Essebi are indistinguishable from sulfate of terrestrial origin and may be terrestrial weathering products, consistent with previous assertions. Our oxygen isotope data, however, can not rule out a preterrestrial origin either.     

Molecular and compound-specific isotopic characterization of monocarboxylic acids in carbonaceous meteorites

Low molecular weight monocarboxylic acids are the most abundant water soluble organic compounds in the Murchison and many other CM type carbonaceous chondrites. In this study, we examined the monocarboxylic acids in Murchison and EET96029.20 carbonaceous meteorites using a new sample preparation and introduction technique for gas chromatograph recently developed for volatile, water-soluble organic compounds: solid phase micro-extraction (SPME). We identified more than 50 monocarboxylic acids from Murchison compared with the 18 compounds reported previously. Formic acid, a known interstellar molecule, has been fully analyzed in these carbonaceous meteorites, with its δD value suggesting an interstellar origin. We determined both carbon and hydrogen isotopic ratios of individual monocarboxylic acids in Murchison, to better define the origins and genetic relationships of these compounds. The compound-specific isotopic data reveal a large enrichment in ¹³C (δ¹³C up to + 32.5‰) and particularly D (δD up to + 2024‰). The branched acids are substantially enriched in both ¹³C and D relative to the straight chain acids, with those branched acids containing a quaternary carbon showing the greatest isotopic enrichment. The isotopic difference may be attributed to variations in the different synthetic regimes or terrestrial input of straight chain acids.     

Aqueous alteration in carbonaceous chondrites: Mass balance constraints on matrix mineralogy

Bulk chemical compositions of matrix material in Antarctic CM chondrites and other non-Antarctic CM and CI chondrites have been determined using microprobe defocused beam techniques. These are used, along with the results of previously published mineralogical studies, to provide mass balance constraints on the relative proportions of intergrown and intermixed phyllosilicate phases in carbonaceous chondrite matrices. Results of these calculations indicate differing amounts of PCP (a mixture of approximately 25% tochilinite and 75% cronstedtite) and serpentines (Mg-rich and Fe-rich varieties in varying proportions or intermediate compositional varieties). Additional sulfide phases are also probably necessary to account for excess Ni and S. Fe/Si ratios for matrices of individual meteorites range from 1.21 to 2.77, corresponding to PCP/(PCP + SERF) ratios of 0.16 to 0.58. Progressive aqueous alteration of matrix appears to have occurred by formation of tochilinite, then cronstedtite and Mg-rich serpentine, and finally Fe-rich serpentine and sulfides. CM matrix clearly did not behave as an isolated system during alteration. CI chondrite matrices appear to contain little if any PCP; this may be a natural consequence of the absence of chondrule-associated metal, from which PCP forms, in the unaltered precursor material. These data provide a more quantitative picture of low-temperature aqueous alteration processes in carbonaceous chondrite parent bodies than has heretofore been possible from TEM studies alone.     

Elemental composition of individual chondrules from carbonaceous chondrites,including Allende

Major and trace element analyses of over 180 individual chondrules from 12 carbonaceous chondrites are reported, including individual analyses of 60 chondrules from Pueblito de Allende. Siderophile elements in most chondrules are depleted, compared to the whole chondrite. Correlations of Al-Ir and Ir-Sc among chondrules high in Ca and Al were observed. A Cu-Mn correlation was also found for chondrules from some meteorites. No correlation was observed between Au and other siderophile elements (Fe, Ni, Co and Ir). It is suggested that these elemental associations were present in the material from which the chondrules formed. Compositionally, chondrules appear to be a multicomponent mixture of remelted dust. One component displaying an Al-Ir correlation is identified as Allende-type white aggregates. The other components are a material chemically similar to the present matrix and sulfides-plus-metal material. Abundances of the REE (rare earth elements) were measured in ‘ordinary’ Allende chondrules and were 50% higher than REE abundances in Mokoia chondrules; REE abundances in Ca-Al rich chondrules were similar to REE abundances in Ca-rich white aggregates.     

Siderophile-element anomalies in CK carbonaceous chondrites: Implications for parent-body aqueous alteration and terrestrial weathering of sulfides

CK chondrites constitute the most oxidized anhydrous carbonaceous chondrite group; most of the Fe occurs in magnetite and in FeO-rich mafic silicates. The two observed CK falls (Karoonda and Kobe), along with thirteen relatively unweathered CK finds, have unfractionated siderophile-element abundance patterns. In contrast, a sizable fraction of CK finds (9 of 24 investigated) shows fractionated siderophile abundance patterns including low abundances of Ni, Co, Se and Au; the most extreme depletions are in Ni (0.24 of normal CK) and Au (0.14 of normal CK). This depletion pattern has not been found in other chondrite groups. Out of the 74 CK chondrites listed in the Meteoritical Bulletin Database (2006; excluded considerably paired specimens; see http://tin.er.usgs.gov/meteor/metbull.php) we analyzed 24 and subclassified the CK chondrites in terms of their chemical composition and sulfide mineralogy: sL (siderophiles low; six samples) for large depletions in Ni, Co, Se and Au (>50% of sulfides lost); sM (siderophiles medium; two CKs) for moderately low Ni and Co abundances (sulfides are highly altered or partly lost); sH (siderophiles high; one specimen) for enrichments in Ni, Co, Se and Au; ‘normal’ for unfractionated samples (13 samples). The sole sH sample may have obtained additional sulfide from impact redistribution in the parent asteroid. We infer that these elements became incorporated into sulfides after asteroidal aqueous processes oxidized nebular metal; thermal metamorphism probably also played a role in their mineral siting. The siderophile losses in the sL and sM samples are mainly the result of oxidation of pentlandite, pyrite and violarite by terrestrial alteration followed by leaching of the resulting phases. Some Antarctic CK chondrites have lost most of their sulfides but retained Ni, Co, Se and Au, presumably as insoluble weathering products.     

The insoluble organic matter in carbonaceous chondrites: Chemical structure,isotopic composition and origin

Carbonaceous chondrites are characterized by their enrichment in organic matter, mainly represented by insoluble organic matter (IOM), which consists of small aromatic units linked by short-branched aliphatic chains. Furthermore, IOM contains organic radicals heterogeneously distributed along with diradicaloids. These chemical features discriminate IOM from terrestrial counterparts. Isotopic compositions, especially the D/H isotopic ratio, are also distinct. IOM is highly enriched in D (D/H > 350 × 10⁻⁶), and the D/H isotopic ratio is heterogeneous. The isotopic composition is the result of interstellar-like processes that could have taken place during the first ages of the protosolar nebula. Chemical structure and isotopic composition clearly show that IOM is synthesized by an abiotic process and is subsequently affected by aqueous alteration or high-temperature metamorphism on the parent body.     

Abundance of 17 trace elements in carbonaceous chondrites

Seventeen trace elements (Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Rb, Re, Sb, Se, Te, Tl, U and Zn) were measured by neutron activation analysis in 8 C1 samples (1 Alais, 3 Ivuna, 4 Orgueil) and in 3 C2 samples (one each of Mighei, Murchison, Murray). The results show far less scatter than earlier literature data. The standard deviation of a single measurement from the mean of 8 C1 samples lies between 2 and 14 per cent, except for the following 4 elements: Au ±18 per cent, Ag ±22 per cent, Rb ±19 per cent and Br ±33 per cent. The first two probably reflect contamination and sample heterogeneity, the last two, analytical error. Apparently C1 chondrites have a far more uniform composition than some authors have claimed.The new data suggest significant revisions in cosmic abundance for the following elements (old values in parentheses): Zn 1250 (1500), Cd 1.51 (2.12), Ir 0.72 (0.43) atoms/10⁶ Si atoms. The Br value is also lower, 6.8 vs 20.6, but may be affected by analytical error.Relative to C1 chondrites, the C2 chondrites Mighei, Murchison and Murray are depleted in volatile elements by a factor of 0.508 ± 0.038, much more constant than indicated by oldor data. Ordinary chondrites also show a more uniform depletion relative to the new C1 data. The mean depletion factor of Sb, F, Cu, Ga, Ge, Sn, S, Se, Te and Ag is 0.227 ± 0.027 in H-chondrites. This constancy further strengthens the case for the two-component model of chondrite formation.     

Alkylation of polycyclic aromatic hydrocarbons in carbonaceous chondrites

Using microprobe laser-desorption, laser-ionization mass spectrometry (μL²MS), we measured the distributions of alkylated and unalkylated polycyclic aromatic hydrocarbons (PAHs) in the free organic material of 20 carbonaceous chondrites. These meteorites represent a variety of meteorite classes and alteration histories, including CI, CK, CM, CO, CR, CV, and Tagish Lake. This work provides information on free organic compounds that is complementary to studies of the structure and composition of meteoritic macromolecular content.For the nine CM2 meteorites analyzed, we observe that higher relative abundances of alkylated PAHs correlate with more intense aqueous activity. We attribute this correlation to the differences in solubility and volatility between unalkylated and alkylated PAHs. Naphthalene and its alkylation series are more susceptible to the effects of aqueous exposure than the less-soluble PAH phenanthrene and its alkylated derivatives. These observations are consistent with the possibility of chromatographic separations on the meteorite parent bodies. We identify six CM2 meteorites with similar PAH distributions that may represent the original, unaltered organic composition of the parent body.Increased metamorphic intensity reduces the abundance of all PAHs. The thermally metamorphosed CK chondrites had no detectable levels of typical meteoritic PAHs. This observation might be explained either by a loss of PAHs caused by volatilization or by a significantly different organic content of the CK parent body.     

The distribution of trace elements in carbonaceous chondrites

12 carbonaceous chondrites, amongst them representatives of nearly all known petrologic types were analyzed for twenty trace elements by spark source mass spectrography combined with the isotope dilution method. Data on different element groups (refractory, moderately volatile and volatile) show that the distribution of the trace elements in the carbonaceous chondrites, with the exception of Renazzo, can be well explained by Anders' two-component model. This is also valid for the highly metamorphosed CV5 chondrite Karoonda.Furthermore, it is observed that the $\text{Zr}\text{Hf}$-ratios in the carbonaceous chondrites increase with increasing petrologic type which is interpreted as the result of mixing two components with different $\text{Zr}\text{Hf}$-ratios     

Evolution of the iron-silicate and carbon material of carbonaceous chondrites

The observed consistence of the composition of chondrules and the matrix in chondrites is explained by their origin as a result of chondrule-matrix splitting of the material of primitive (not layered) planets. According to the composition of chondrites, two main stages in the evolution of chondritic planets (silicate-metallic and olivine) are distinguished. Chondritic planets of the silicate-metallic stage were analogs of chondritic planets, whose layering resulted in the formation of the terrestrial planets. The iron-silicate evolution of chondritic matter is correlated with the evolution of carbon material in the following sequence: diamond ± moissanite → hydrocarbons → primitive organic compounds.     

Investigating the variations in carbon and nitrogen isotopes in carbonaceous chondrites

The carbonaceous chondrites contain significant amounts of carbon- and nitrogen-bearing components, the most abundant of which is organic matter. Stepped combustion data of whole rock and HF/HCl residues of carbonaceous chondrites reveal that the organic material can be subdivided operationally into three components: (1) free organic matter (FOM), which is readily extractable from whole-rock meteorites and is enriched in ¹³C and ¹⁵N; (2) labile organic matter (LOM), which has a macromolecular structure but is liberated by hydrous pyrolysis; LOM is the parent structure for some FOM and is also enriched in ¹³C and ¹⁵N; and (3) refractory organic matter (ROM), which is also macromolecular but is virtually unaffected by hydrous pyrolysis and is relatively depleted in ¹³C and ¹⁵N. The macromolecular entities (LOM and ROM) are by far the most abundant organic components present, and as such, the relative abundances of the ¹³C- and ¹⁵N-enriched LOM and the ¹³C- and ¹⁵N-depleted ROM will have a major influence on the overall isotopic composition of the whole-rock meteorite. Laboratory experiments designed to simulate the effects of parent body aqueous alteration indicate that this form of processing removes LOM from the macromolecular material, allowing ROM to exert a stronger influence on the overall isotopic compositions. Hence, aqueous alteration of macromolecular materials on the meteorite parent body may have a significant control on the stable isotopic compositions of whole-rock carbonaceous chondrites. The enstatite chondrites are also carbon rich but have been subjected to high levels of thermal metamorphism on their parent body. Stepped combustion data of HF/HCl residues of enstatite chondrites indicate, that if they and carbonaceous chondrites inherited a common organic progenitor, metamorphism under reducing conditions appears to incorporate and preserve some of the ¹³C enrichments in LOM during graphitisation. However, when metamorphism is at its most extreme, the ¹⁵N enrichments in LOM are lost.     

Lithium,sodium and potassium abundances in carbonaceous chondrites

Concentrations of lithium, sodium, and potassium in 18 carbonaceous chondrites were determined in the same sample solution by atomic absorption. Mean abundances in carbonaceous Type I chondrites are, in atoms 10⁶ Si: Li = 60.1, Na = 5800, K = 3700. Relative to Type I carbonaceous chondrites, abundances in Type II's are: Li = 0.87, Na = 0.61, K = 0.58; and in Type III's Li = 0.82, Na = 0.49, K = 0.36. Evidently there is a differential depletion of potassium relative to sodium in Type III's, suggesting a fractionation after accretion.     

Petrographic variations among carbonaceous chondrites of the Vigarano type

The Vigarano subtype is a petrographically complex class of meteorites. Oxidized and reduced groups can be distinguished on the basis of metal vs magnetite abundances and Ni contents of sulfide minerals. These meteorites also differ in the proportions of matrix and chondrules and in polymict character. Slight bulk chemical differences correlate with the recognized petrologic groupings. It is likely that the Vigarano subtype includes several previously unrecognized subgroups. Metamorphism has affected Coolidge, Mulga (West) and, to a lesser extent, Allende, as evidenced by ferromagnesian mineral equilibration, Fe-enrichment of fine-grained inclusions, and loss of some volatile gases. Because of the metamorphic effects in the Allende chondrite (the only meteorite of the group that has been intensively studied) and the petrographie differences among all meteorites of the Vigarano subtype, it is suggested that Allende alone may not adequately reflect the wide spectrum of properties in this important class of meteorites.     

Chemical and petrographic constraints on the origin of chondrules and inclusions in carbonaceous chondrites

Bulk chemical compositions of the various petrographie types of chondrules and inclusions in Type 3 carbonaceous chondrites (excluding those affected by metamorphism) have been determined by microprobe defocused beam analysis. Inclusion compositions follow approximately the theoretical compositional trajectory for equilibrium condensation. Analyses of chondrules occurring in the same meteorites have higher silica contents and show only slight overlap with inclusion compositions. Dust fusion is apparently an inadequate mechanism for producing the wide chemical variations observed among chondrules. Impact melting models require sampling of complex target rocks which are unknown as components of meteorites; this mechanism also demands efficient mechanical processing of chondrules before accretion. A genetic relationship between chondrules and inclusions in carbonaceous chondrites is suggested by the compositional continuum between these objects. A condensation sequence which dips into the liquid stability field at lower temperatures is advocated for the production of both inclusions and chondrules. Textural relationships between intergrown chondrules and inclusions support such a sequence. This model suggests that the assembled components (inclusions and chondrules) of carbonaceous chondrites are related by a common process.     

Presolar spinel grains from the Murray and Murchison carbonaceous chondrites

With a new type of ion microprobe, the NanoSIMS, we determined the oxygen isotopic compositions of small (<1μm) oxide grains in chemical separates from two CM2 carbonaceous meteorites, Murray and Murchison. Among 628 grains from Murray separate CF (mean diameter 0.15 μm) we discovered 15 presolar spinel and 3 presolar corundum grains, among 753 grains from Murray separate CG (mean diameter 0.45 μm) 9 presolar spinel grains, and among 473 grains from Murchison separate KIE (mean diameter 0.5 μm) 2 presolar spinel and 4 presolar corundum grains. The abundance of presolar spinel is highest (2.4%) in the smallest size fraction. The total abundance in the whole meteorite is at least 1 ppm, which makes spinel the third-most abundant presolar grain species after nanodiamonds (if indeed a significant fraction of them are presolar) and silicon carbide. The O-isotopic distribution of the spinel grains is very similar to that of presolar corundum, the only statistically significant difference being that there is a larger fraction of corundum grains with large ¹⁷O excesses (¹⁷O/¹⁶O > 1.5 × 10⁻³), which indicates parent stars with masses between 1.8 and 4.5 M_⊙.