THE CARBONACEOUS CHONDRITES—A SELECTIVE REVIEW

The presently known carbonaceous chondrites are listed, and their densities and contents of Fe, Si, and Mg recorded. A review of classification schemes indicate that Wiik's (1956) division into Types I, II, and III reflects basic differences in chemical and mineralogical composition and structure, and is preferable to the Van Schmus-Wood (1967) classification into C1, C2, C3, and C4 types. Renazzo is a Type II meteorite. The Van Schmus division of Type III meteorites into two subtypes is sustained, and is shown to involve not only textural differences but also differences in total iron content. The genesis of the different types of carbonaceous chondrites is ascribed to condensation in different temperature regions within the early solar nebula.     

THE CHEMICAL COMPOSITIONS OF THE STONE METEORITES YAMATO (a), (b), (c) and (d), AND NUMAKAÍ

The stone meteorites Yamato (a), (b), (c) and (d) were found in Antarctica in 1969, and the chondrite Numakai was seen to fall in Hokkaido, Japan, in 1925. The chemical compositions of these meteorites have been determined by classical and instrumental methods. With the help of the stepwise fractional dissolution method the chemical composition confirmed the author's previous classification of Yamato (a), (b), (c) and (d) as enstatite chondrite, Ca-poor achondrite, type III carbonaceous chondrite and H-group chondrite, respectively. Numakai is classified as an H-group olivine-bronzite chondrite. The distribution of the major elements in each phase of these chondrites is discussed.     

Chemical analyses of meteorites at the Smithsonian Institution: An update

Abstract— Thirteen new meteorites and three meteorite inclusions have been analyzed. Their results have been incorporated into earlier published data for a comprehensive reference to all analyzed meteorites at the Smithsonian Institution. The six tables facilitate a convenient overlook of meteorite data. Table 1 presents an alphabetical list of analyzed meteorites, Table 2 chemical analyses of stony meteorites, Table 3 chemical analyses of iron meteorites, Table 4 elemental composition of stony meteorites, Table 5 average composition of carbonaceous chondrites and achondrites (falls and finds), and Table 6 presents average composition of H, L, LL, and Antarctic chondrites (falls and finds). The tables are available online at the journal's Web site http:meteoritics.org .     

The Distribution and Significance of Evaporitic Weathering Products on Antarctic Meteorites

Abstract— The distribution of white evaporitic deposits differs among different meteorite compositional groups and weathering categories of Antarctic meteorites. Evaporites occur with unusual frequency on carbonaceous chondrites, and are especially common in carbonaceous chondrites of weathering categories A and B. Among achondrites, weathering categories A and A/B show the most examples of evaporite weathering. Unlike carbonaceous chondrites and achondrites, most evaporite-bearing ordinary (H and L) chondrites are from rustier meteorites of weathering categories B and, to a lesser degree, B/C and C. LL chondrites are conspicuous by their complete lack of any evaporitic weathering product. Almost two-thirds of all evaporite-bearing meteorites belong to weathering categories A, A/B, and B. Where chemical data are available, surficial evaporite deposits are associated with elemental anomalies in meteorite interiors. Meteorites of weathering classes B, A/B, and even A may have experienced significant element redistribution and/or contamination as a result of terrestrial exposure. Evaporite formation during terrestrial weathering cannot be neglected in geochemical, cosmochemical, and mineralogical studies of Antarctic meteorites. A lower-case “e” should be added to the weathering classification of evaporite-bearing Antarctic meteorites, to inform meteorite scientists of the presence of evaporite deposits and their associated compositional effects.     

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.     

Strengths of meteorites—An overview and analysis of available data

We report all available measurements on strength of meteorites, primarily focusing on compressive and tensile strengths and supplementary data such as Young's modulus, Poisson's ratio, elastic sound wave velocities, density, porosity, and sample sizes. These data are solely taken from the original papers to avoid misprints and other issues. The data are provided as originally presented by the authors with the exception of standardization of units to the SI system. A brief overview of methods for each original work is also provided as a guide to “data quality” since individual papers go to varying levels of detail on their experimental setup and procedures. From this data set, we confirm that the compressive strength of ordinary chondrites (varying in the range of 10s to 100s of MPa) is about an order of magnitude larger compared to their tensile strength and the difference increases with iron content. For carbonaceous chondrites, the tensile strength seems to be about an order of magnitude below the tensile strength of ordinary chondrites and at least an order of magnitude below their compressive strength. We also provide a statistical relation between the strength of meteorites and their densities and porosities and discuss the role of strain rate and sample size on the resultant measured strength. Finally, the data do not provide sufficient statistics to support a size scale effect of strength of meteorites.     

Effect of hot desert weathering on the bulk‐rock iron isotope composition of L6 and H5 ordinary chondrites

Abstract– Although iron isotopes are increasingly used for meteorites studies, no attempt has been made to evaluate the effect of terrestrial weathering on this isotopic tracer. We have thus conducted a petrographic, chemical, and iron isotopic study of equilibrated ordinary chondrites (OC) recovered from hot Moroccan and Algerian Saharan deserts environment. As previously noticed, we observe that terrestrial desertic weathering is characterized by the oxidation of Fe‐Ni metal (Fe⁰), sulfide and Fe²⁺ occurring in olivine and pyroxene. It produces Fe‐oxides and oxyhydroxides that partially replace metal, sulfide grains and also fill fractures. The bulk chemical compositions of the ordinary chondrites studied show a strong Sr and Ba enrichment and a S depletion during weathering. Bulk meteoritic iron isotope compositions are well correlated with the degree of weathering and S, Sr, and Ba contents. Most weathered chondrites display the heaviest isotopic composition, by up to 0.1‰, which is of similar magnitude to the isotopic variations resulting from meteorite parent bodies’ formation and evolution. This is probably due to the release of isotopically light Fe²⁺ to waters on the Earth’s surface. Hence, when subtle Fe isotopic effects have to be studied in chondrites, meteorites with weathering grade above W2 should be avoided.     

SUMMARY OF SEVERAL RECENT CHONDRITE FINDS FROM THE TEXAS PANHANDLE

Eleven recent chondrite finds from the Texas Panhandle have been examined and classified according to mineralogical and petrological criteria: five H's, five L's, and one LL chondrite. Five are distinct from nearby finds, while three remain ambiguous and three are related to previously reported chondrites. In addition, data are provided to classify the Muleshoe, Silverton, and Vigo Park chondrites, all of which were previously undescribed in the literature.     

CHEMICAL COMPOSITION AND CLASSIFICATION OF 19 YAMATO METEORITES

The elements Na, Mg, Al, Si, S, K, Ca, (V), Cr, Mn, Fe, Co and Ni have been determined in 19 Yamato meteorites by spark source mass spectrometry. For comparison the chondrites Allan Hills 7603, Mt Baldr (b) and Holbrook and the achondrites Johnstown, Pasamonte and Stannern also have been analyzed by the same method. By virtue of their chemical composition the Yamato meteorites 74002 and 74144 prove to be ordinary chondrites of type L; 74001, 74103, 74155 and 74156 are ordinary chondrites of type H; 74662 is a carbonaceous chondrite; Yamato 74010, 74011, 74016, 74037, 74097, 74125 and 74136 are diogenites; Yamato 7308(1) is a howardite; and Yamato 74450 is a eucrite. This agrees with earlier classifications based on petrological and mineralogical arguments (Nagata, 1978; Motylewski, 1978). For the chondrites Yamato 74002, 74106, 74144 and the diogenite 74125, however, no previous classifications could be found in the literature. In a Mg-Al diagram the eucrites, the howardites, the diogenites and the ureilites fall into characteristic fields. This enabled not only the classification of the Yamato achondrites investigated in this paper but also confirmed the previous identification of Yamato 74123 as a ureilite (Hintenberger et al., 1978). A very high chromium content is characteristic of some Yamato diogenites, especially Yamato 74037 (3.4%). Chromium and vanadium are positively correlated in the achondrites investigated.     

The Meteoritical Bulletin,No. 100, 2014 June

Meteoritical Bulletin 100 contains 1943 meteorites including 8 falls (Boumdeid [2011], Huaxi, Ko?ice, Silistra, So?tmany, Sutter's Mill, Thika, Tissint), with 1575 ordinary chondrites, 139 carbonaceous chondrites, 96 HED achondrites, 25 ureilites, 18 primitive achondrites, 17 iron meteorites, 15 enstatite chondrites, 11 lunar meteorites, 10 mesosiderites, 10 ungrouped achondrites, 8 pallasites, 8 Martian meteorites, 6 Rumuruti chondrites, 3 enstatite achondrites, and 2 angrites, and with 937 from Antarctica, 592 from Africa, 230 from Asia, 95 from South America, 44 from North America, 36 from Oceania, 6 from Europe, and 1 from an unknown location. This will be the last Bulletin published in the current format. Information about approved meteorites can be obtained from the Meteoritical Bulletin Database (MBD) available online at http://www.lpi.usra.edu/meteor/     

A coordinated spectral, mineralogical, and compositional study of ordinary chondrites

Mineral compositions and abundances derived from visible/near-infrared (VIS/NIR or VNIR) spectra are used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid belt. Using a suite of 48 equilibrated (types 4-6) ordinary (H, L, and LL) chondrites containing orthopyroxene, clinopyroxene, and olivine, new relationships between spectra and mineralogy have been established. Contrary to previous suggestions, no meaningful correlation is observed between band parameters and cpx/(opx + cpx) ratios. We derive new calibrations for determining mineral abundances (ol/(ol + px)) and mafic silicate compositions (Fa in olivine, Fs in pyroxene) from VIS/NIR spectra. These calibrations confirm that band area ratio (BAR) is controlled by mineral abundances, while Band I center is controlled by mafic silicate compositions. Spectrally-derived mineralogical parameters correctly classify H, L and LL chondrites in ∼80% of cases, suggesting that these are robust relationships that can be applied to S(IV) asteroids with ordinary chondrites mineralogies. Comparison of asteroids and meteorites using these new mineralogical parameters has the advantage that H, L and LL chemical groups were originally defined on the basis of mafic silicate compositions.     

Application of Mössbauer spectroscopy,multidimensional discriminant analysis,and Mahalanobis distance for classification of equilibrated ordinary chondrites

Mössbauer spectra of equilibrated ordinary chondrites consist of two doublets due to paramagnetic iron present in olivines and pyroxenes and two sextets due to magnetically ordered iron present in metallic phases and troilite. The spectral areas of the different mineralogical phases found by Mössbauer spectroscopy in meteorites are proportional to the number of iron atoms in this mineralogical phase. This property of Mössbauer spectra can be the basis for constructing a method for the classification of ordinary chondrites. This idea was first explored at the Mössbauer Laboratory in Kanpur. This group suggested a qualitative method based on 2‐dimensional plots of Mössbauer spectral areas and thus classified properly some meteorites. We constructed a quantitative method using Mössbauer spectral areas, multidimensional discriminant analysis, and Mahalanobis distance (4M method) to determine the probability of a meteorite to be of type H, L, or LL. Based on 59 Mössbauer spectra, we calculated by the 4M method, S _cluster , the level of similarity of the Goronyo meteorite to the clusters. On the plot of ferrosilite versus fayalite, the point representing Goronyo is located on the border between H and L areas. Calculated by the 4M method, the meteorite Goronyo is 32% similar to type H, 75% to type L, and 11% to type LL. Additional mineralogical analyses suggested that the Goronyo meteorite would be classified as type L, although it was originally reported as type H in the Meteoritical Bulletin Database.     

Potassium isotopic compositions of enstatite meteorites

Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass‐independent and mass‐dependent isotopic fractionations. Due to the analytical challenges to obtain high‐precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high‐precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from ?2.34‰ to ?0.18‰). However, K isotopic compositions of nearly all enstatite meteorites scatter around the bulk silicate earth (BSE) value. The average K isotopic composition of the eight enstatite chondrites (?0.47 ± 0.57‰) is indistinguishable from the BSE value (?0.48 ± 0.03‰), thus further corroborating the isotopic similarity between Earth's building blocks and enstatite meteorite precursors. We found no correlation of K isotopic compositions with the chemical groups, petrological types, shock degrees, and terrestrial weathering conditions; however, the variation of K isotopes among enstatite meteorite can be attributed to the parent‐body processing. Our sample of the main‐group aubrite MIL 13004 is exceptional and has an extremely light K isotopic composition (δ⁴¹K = ?2.34 ± 0.12‰). We attribute this unique K isotopic feature to the presence of abundant djerfisherite inclusions in our sample because this K‐bearing sulfide mineral is predicted to be enriched in ³⁹K during equilibrium exchange with silicates.     

Chemical characterization of a unique chondrite: Allan Hills 85085

Abstract— Allan Hills 85085 is a new and very important addition to the growing list of unique carbonaceous chondrites because of its unique chemical and mineralogical properties. Previously published data on ALH85085 have established its bulk composition, gross levels of volatile depletion, siderophile enrichment, and the unique character of this unique meteorite. This chemical study provides more precise data on the major, minor, and trace element characteristics of ALH85085. ALH85085 has compositional, petrological, and isotopic affinities to Al Rais and Renazzo, and to Bencubbin-Weatherford. The similarities to Al Rais and Renazzo, in particular, suggest similar formation locations and thermal processing, possibly in the vicinity of CI chondrites. Petrologic, compositional and isotopic studies indicate that the components that control the abundance of the various refractory and volatile elements were not allowed to equilibrate with the nebula as conditions changed. This lack of equilibration could explain some of the compositional and petrologic variations within and between these meteorites and the inconsistencies in the classification of these meteorites using known taxonomic parameters. It is also apparent that the compositional trends implied by the application of taxonomic parameters probably did not vary simply with heliocentric distance. In addition, variations in the agglomeration of diverse nebular phases along with local and regional temperature events may strongly influence the bulk composition of meteorites.     

Yamato 792947, 793408 and 82038: The most primitive H chondrites,with abundant refractory inclusions

Abstract— In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)‐792947, Y‐93408 and Y‐82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole‐rock oxygen isotopes and the chemical compositions are indicative of H chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2–3.4 (S1), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.     

The Morávka meteorite fall: 3. Meteoroid initial size,history, structure,and composition

Abstract— The properties and history of the parent meteoroid of the Morávka H5–6 ordinary chondrites have been studied by a combination of various methods. The pre‐atmospheric mass of the meteoroid was computed from fireball radiation, infrasound, seismic signal, and the content of noble gases in the meteorites. All methods gave consistent results. The best estimate of the pre‐atmospheric mass is 1500 ± 500 kg. The fireball integral bolometric luminous efficiency was 9%, and the acoustic efficiency was 0.14%. The meteoroid cosmic ray exposure age was determined to be (6.7 ± 1.0) × 10⁶ yr. The meteorite shows a clear deficit of helium, both ³He and ⁴He. This deficit can be explained by solar heating. Numerical backward integration of the meteoroid orbit (determined in a previous paper from video records of the fireball) shows that the perihelion distance was probably lower than 0.5 AU and possibly as low as 0.1 AU 5 Ma ago. The collision which excavated Morávka probably occurred while the parent body was on a near‐Earth orbit, as opposed to being confined entirely to the main asteroid belt. An overview of meteorite macroscopic properties, petrology, mineralogy, and chemical composition is given. The meteorites show all mineralogical features of H chondrites. The shock level is S2. Minor deviations from other H chondrites in abundances of trace elements La, Ce, Cs, and Rb were found. The ablation crust is enriched with siderophile elements.     

A nondestructive analytical method for stone meteorites—and a controversial discrepancy

Abstract— A method is described for whole rock analyses of major elements in stone meteorites using the electron microprobe and requiring only powdering of the sample, most of which can be retrieved after analysis for additional analytical studies, such as instrument neutron activation analysis (INAA), radiochemical neutron activation analysis (RNAA) and O-isotopic analysis. Whole individual chondrules of > 1 mg can be analyzed. The method is especially attractive for meteorites in short supply or of great rarity. A total of 398 meteorites were analyzed by this method. The results compare favorably with wet chemical analyses. A study was made of seventeen ordinary chondrites to compare their whole rock (metal free) compositions with the averaged compositions of eleven to thirty-eight of their respective individual chondrules (a total of 374 chondrules). The oxide ratio Al₂O₃/CaO is generally lower in chondrules than in their respective chondrites, the disparity being larger for petrographic grade 5 than for grade 3. Ordinary chondrites are not simply the sum of their respective chondrules. Furthermore, correlations between CaO, Al₂O₃ and TiO₂ are strong for chondrules in unequilibrated chondrites and nonexistent in equilibrated chondrites. Also H, L and LL chondrite groups have similar bulk compositions within their respective groups, in spite of the different proportions of chondrules, kinds of chondrules, chondrule debris, and matrix. All this brings into question the metamorphic classification in which high petrographic grades are the metamorphosed equivalents of low petrographic grades.     

Black ordinary chondrites: An analysis of abundance and fall frequency

Abstract— Black ordinary chondrite meteorites sample the spectral effects of shock on ordinary chondrite material in the space environment. Since shock is an important regolith process, these meteorites may provide insight into the spectral properties of the regoliths on ordinary chondrite parent bodies. To determine how common black chondrites are in the meteorite collection and, by analogy, the frequency of shock-alteration in ordinary chondrites, several of the world's major meteorite collections were examined to identify black chondrites. Over 80% of all catalogued ordinary chondrites were examined and, using an optical definition, 61 black chondrites were identified. Black chondrites account for approximately 13.7% of ordinary chondrite falls. If the optically altered gas-rich ordinary chondrites are included, the proportion of falls that exhibit some form of altered spectral properties increases to 16.7%. This suggests that optical alteration of asteroidal material in the space environment is a relatively common process.     

The role of presolar dust in the formation of the solar system

Isotopic studies have revealed several types of presolar material in chondritic meteorites (e.g., Ne-E, various components of O, Ti, Ca, Mg). In fact, examples of presolar material are found in all meteorites whose components have not been completely altered by secondary processing. This paper suggests that presolar dust was the primary building material for the meteorites and terrestrial planets. To make this case, the characteristics of presolar dust are discussed and the material in the sun's parent molecular cloud is divided into eight reservoirs. Then the meteorites most likely to preserve their original constituents are identified, and it is shown that dust from several presolar material reservoirs is present in the primitive chondrites. Components that may have formed directly from presolar dust are also identified. Presolar dust and objects made from processed dust make up the vast majority of the material in primitive chondrites. Since there is no obvious reason to believe that other meteorites formed from fundamentally different material than did the primitive chondrites, it is reasonable to conclude that presolar dust, thermally processed but not evaporated and recondensed, was the parent material for the meteorites.In the second part of the paper, various processes that could have affected the presolar dust are identified. It is then shown that: (1) the chemical and oxygen isotopic variations between meteorite classes; (2) the formation of chondrules; and (3) accretion of chondrites and parent body metamorphism are consistent with relatively simple models that use presolar dust as the starting material. These models are presented, not as detailed solutions to the problems, but to exemplify a way of looking at the solar system that may lead to significant advances in our understanding.     

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.