A carbon and nitrogen isotope study of diamond from primitive chondrites

Abstract Diamonds isolated from primitive chondrites of the carbonaceous, ordinary and enstatite groups have been analysed by high-resolution stepped combustion, followed by measurement of their C and N isotopes using a newly adapted technique that allows quantitative measurements of C/N ratios. The δ¹³C of the diamond is shown to vary between meteorite groups from ?32 to ?38%0, and the measured C/N ratios suggest that the N concentration of diamond ranges over a factor of 7 from 1800 ppm (Tieschitz) to 13,000 ppm (Adrar 003). The δ¹⁵N of N released from diamond is constrained to ?348 ± 7%. The complexity of the C release pattern and C/N ratio during combustion implies the presence of more than one component, which suggests that either more than one type of diamond is present in the samples, or unidentified additional phases are located in the acid-resistant residue. The components are present in varying proportions between meteorite groups. The data are compatible with a model of a mix of different diamond populations (some probably presolar and some possibly solar) existing in the early solar nebula, where each population originally contributed a roughly equal amount to chondrites of every class. Subsequent metamorphism has resulted in overall variations in δ¹³C and C/N ratios in diamond isolated from meteorites of differing petrologic grade without significantly altering the N isotopic composition. Possible ways for this to be achieved are explored.     

Spectral properties of angrites

Abstract— Angrites are generally believed to be fragments of a basaltic asteroid that differentiated under relatively oxidizing conditions. Almost all angrites (e.g., D'Orbigny, Lewis Cliff [LEW] 86010, and Sahara 99555) are composed predominately of anorthite, Al‐Ti diopside‐hedenbergite, and Ca‐rich olivine, except for the type specimen, Angra dos Reis, which is composed almost entirely of Al‐Ti diopside‐hedenbergite. D'Orbigny, LEW 86010, and Sahara 99555 also have spectral properties very different from Angra dos Reis. These newly measured angrites all have broad absorption features centered near 1 μm with very weak to absent absorption bands at ?2 μm, which is characteristic of some clinopyroxenes. The spectrum of Angra dos Reis has the characteristic 1 and 2 μm features due to pyroxene. One asteroid, 3819 Robinson, has similar spectral properties to the newly measured angrites in the visible wavelength region, but does not appear to spectrally match these angrites in the near‐infrared.     

The origin of chondritic macromolecular organic matter: A carbon and nitrogen isotope study

Abstract— The N and C abundances and isotopic compositions of acid-insoluble carbonaceous material in thirteen primitive chondrites (five unequilibrated ordinary chondrites, three CM chondrites, three enstatite chondrites, a CI chondrite and a CR chondrite) have been measured by stepped combustion. While the range of C isotopic compositions observed is only ～δ¹³C = 30%, the N isotopes range from δ¹⁵N ' -40 to 260%. After correction for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3–4 wt% of the insoluble C in all the chondrites studied. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites, suggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in Renazzo and Semarkona. These two meteorites have almost M-shaped N isotope release profiles that can be explained most simply by the superposition of two components, one with a composition between δ¹⁵N = -20 and -40% and a narrow combustion interval, the other having a broader release profile and a composition of δ¹⁵N ～ 260%. Although isotopically more subdued, the CI and the three CM chondrites all appear to show vestiges of this M-shaped profile. How and where the components in the acid-insoluble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most ¹⁵N-rich material probably formed in the interstellar medium, and the fraction of organic N in Renazzo in this material ranges from 40 to 70%. The isotopically light component may have formed in the solar system, but the limited range in nanodiamond to total organic C ratios in the chondrite groups is consistent with most of the organic material being presolar.     

Stable carbon isotope ratios in tree rings of co-occurring species from semi-arid tropics in Africa: Patterns and climatic signals

Although several proxies have been proposed to trace the course of environmental and climatological fluctuations, precise paleoclimate records from the tropics, notably from Africa are still sorely lacking today. Stable carbon isotopes (δ¹³C) in tree rings are an attractive record of climate. In this study, the patterns and climatic signals of δ¹³C ratios were determined on tree rings of deciduous (Acacia senegal, Acacia tortilis, Acacia seyal) and an evergreen (Balanites aegyptiaca) species, from a semi-arid Acacia Woodland in Ethiopia. δ¹³C inter-annual patterns are synchronous among the co-occurring species. A declining trend with time was observed in δ¹³C, notably for B. aegyptiaca, which could be due to anthropogenic increases in atmospheric CO₂ concentration and decrease in atmospheric δ¹³C. Tree ring δ¹³C values of all the species revealed significant negative correlation with precipitation amount but not with temperature and relative humidity. The δ¹³C series of the deciduous species shows a higher correlation (r = − 0.70 to − 0.78) with precipitation than the evergreen species (r = − 0.55). A master δ¹³C series, composed of the average of the three Acacia species, displayed stronger significant correlation (r = − 0.82) than any of the individual species δ¹³C series. The weak relationship between temperature and δ¹³C in this study indicates that photosynthetic rate is not a significant factor. Moisture stress, however, may have a direct impact on the stomatal conductance and explain the strong negative relationship between δ¹³C and precipitation. The results demonstrate the potential of δ¹³C in tree rings to reflect physiological responses to environmental changes as a vehicle for paleoclimatic reconstruction, which is important to understand tree response to past and future climate change.     

A nitrogen isotope study of bencubbinites

Abstract— ‐Nitrogen‐isotopic compositions of the bencubbinites—Bencubbin, Hammadah al Hamra (HH) 237, and Queen Alexandra Range (QUE) 94411—and in a petrographically similar chondrite Grosvenor Mountains (GRO) 95551 were measured by stepped‐combustion static mass spectrometry. Hammadah al Hamra 237 and QUE 94411 contain isotopically heavy N, but not as heavy as that in Bencubbin or Weatherford. Grosvenor Mountains 95551 contains isotopically near‐normal N and light N and, hence, it is not related to the bencubbinites, which is also indicated by its O‐isotopic composition. The N carriers in these meteorites were investigated using secondary ion mass spectrometry. In the bencubbinites, N is mostly located around sulfide in metal clasts and in impact‐melt areas. The N carriers in the former are taenite, carbide, or both; whereas those in the latter are molten metal, tiny graphitic carbon in metal, oxi‐nitride glass, or both. In the various N carriers, N is isotopically equilibrated, and therefore the carriers are not pristine presolar grains. Isotopically near‐normal N in GRO 95551 is located in graphite. The carrier of isotopically light N in GRO 95551 has not been found.     

Stable isotope laser spectrometer for exploration of Mars

On Earth, measurements of the ratios of stable carbon isotopes have provided much information about geological and biological processes. For example, fractionation of carbon occurs in biotic processes and the retention of a distinctive 2-4% contrast in 13C/12C between organic carbon and carbonates in rocks as old as 3.8 billion years constitutes some of the firmest evidence for the antiquity of life on the Earth. We have developed a prototype tunable diode Laser spectrometer which demonstrates the feasibility of making accurate in situ isotopic ratio measurements on Mars. This miniaturized instrument, with an optical path length of 10 cm, should be capable of making accurate 13C/12C and 15N/14N measurements. Gas samples for measurement are to be produced by pyrolysis using soil samples as small as 50 mg. Measurements of 13C/12C, 18O/16O and 15N/14N have been made to a precision of better than 0.1% and various other isotopes are feasible. This laser technique, which relies on the extremely narrow emission linewidth of tunable diode lasers (<0.001 cm(-1)) has favorable features in comparison to mass spectrometry, the standard method of accurate isotopic ratio measurement. The miniature instrument could be ready to deploy on the 2003 or other Mars lander missions.     

Oxygen and carbon isotope ratios in the martian atmosphere

Oxygen and carbon isotope ratios in the martian CO₂ are key values to study evolution of volatiles on Mars. The major problems in spectroscopic determinations of these ratios on Mars are uncertainties associated with: (1) equivalent widths of the observed absorption lines, (2) line strengths in spectroscopic databases, and (3) thermal structure of the martian atmosphere during the observation. We have made special efforts to reduce all these uncertainties. We observed Mars using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope. While the oxygen and carbon isotope ratios on Mars were byproducts in the previous observations, our observation was specifically aimed at these isotope ratios. We covered a range of 6022 to 6308 cm⁻¹ with the highest resolving power of ν/δν=3.5×10⁵ and a signal-to-noise ratio of 180 in the middle of the spectrum. The chosen spectral range involves 475 lines of the main isotope, 184 lines of ¹³CO₂, 181 lines of CO¹⁸O, and 119 lines of CO¹⁷O. (Lines with strengths exceeding 10⁻²⁷ cm at 218 K are considered here.) Due to the high spectral resolution, most of the lines are not blended. Uncertainties of retrieved isotope abundances are in inverse proportion to resolving power, signal-to-noise ratio, and square root of the number of lines. Laboratory studies of the CO₂ isotope spectra in the range of our observation achieved an accuracy of 1% in the line strengths. Detailed observations of temperature profiles using MGS/TES and data on temperature variations with local time from two GCMs are used to simulate each absorption line at various heights in each part of the instrument field of view and then sum up the results. Thermal radiation of Mars' surface and atmosphere is negligible in the chosen spectral range, and this reduces errors associated with uncertainties in the thermal structure on Mars. Using a combination of all these factors, the highest accuracy has been achieved in measuring the CO₂ isotope ratios: ¹³C/¹²C = 0.978 ± 0.020 and ¹⁸O/¹⁶O = 1.018 ± 0.018 times the terrestrial standards. Heavy isotopes in the atmosphere are enriched by nonthermal escape and sputtering, and depleted by fractionation with solid-phase reservoirs. The retrieved ratios show that isotope fractionation between CO₂ and oxygen and carbon reservoirs in the solid phase is almost balanced by nonthermal escape and sputtering of O and C from Mars.     

Implantation of carbon and nitrogen ions in water ice

Solid surfaces of atmosphereless objects in the Solar System are continuously irradiated by energetic ions (from solar wind and flares, planetary magnetospheres, and cosmic rays). Reactive ions (e.g., H, C, N, O, S) induce all of the effects of any other ion including the synthesis of molecular species originally not present in the target. In addition, these ions have a chance, by implantation in the target, of forming new species containing the projectile. An ongoing research program performed at our laboratory aims at investigating the implantation of reactive ions in many relevant ices (and mixtures) by using IR spectroscopy. Here we present new results obtained by implanting carbon and nitrogen ions in water ice at 16 and 77 K. Carbon implantation produces carbon dioxide and the production yield has been measured. Nitrogen implantation does not produce any N-bearing species detectable by IR spectroscopy. Both ions are also capable of synthesizing hydrogen peroxide at the two investigated temperatures. We show that, although a relevant quantity of CO₂ can be formed by C implantation in the icy jovian moons, this is not the dominant formation mechanism of carbon dioxide.     

Impact histories of angrites,eucrites, and their parent bodies

Abstract– Eucrites, which are probably from 4 Vesta, and angrites are the two largest groups of basaltic meteorites from the asteroid belt. The parent body of the angrites is not known but it may have been comparable in size to Vesta as it retained basalts and had a core dynamo. Both bodies were melted early by ²⁶Al and formed basalts a few Myr after they accreted. Despite these similarities, the impact histories of the angrites and eucrites are very different: angrites are very largely unshocked and none are breccias, whereas most eucrites are breccias and many are shocked. We attribute the lack of shocked and unbrecciated angrites to an impact, possibly at 4558 Myr ago—the radiometric age of the younger angrites—that extracted the angrites from their original parent body into smaller bodies. These bodies, which may have had a diameter of approximately 10 km, suffered much less impact damage than Vesta during the late heavy bombardment because small bodies retain shocked rocks less efficiently than large ones and because large bodies suffer near‐catastrophic impacts that deposit vastly more impact energy per kg of target. Our proposed history for the angrites is comparable to that proposed by Bogard and Garrison (2003) for the unbrecciated eucrites with Ar‐Ar ages of 4.48 Gyr and that for unbrecciated eucrites with anomalous oxygen isotopic compositions that did not come from Vesta. We infer that the original parent bodies of the angrites and the anomalous eucrites were lost from the belt when the giant planets migrated and the total mass of asteroids was severely depleted. Alternatively, their parent bodies may have formed in the terrestrial planet region and fragments of these bodies were scattered out to the primordial Main Belt as a consequence of terrestrial planet formation.     

Ion probe measurements of carbon and nitrogen in iron meteorites

Abstract— Carbon and nitrogen distributions in iron meteorites, their concentrations in various phases, and their isotopic compositions in certain phases were measured by secondary ion mass spectrometry (SIMS). Taenite (and its decomposition products) is the main carrier of C, except for IAB iron meteorites, where graphite and/or carbide (cohenite) may be the main carrier. Taenite is also the main carrier of N in most iron meteorites unless nitrides (carlsbergite CrN or roaldite (Fe, Ni)₄N) are present. Carbon and N distributions in taenite are well correlated unless carbides and/or nitrides are exsolved. There seem to be three types of C and N distributions within taenite. (1) These elements are enriched at the center of taenite (convex type). (2) They are enriched at the edge of taenite (concave type). (3) They are enriched near but some distance away from the edge of taenite (complex type). The first case (1) is explained as equilibrium distribution of C and N in Fe-Ni alloy with M-shape Ni concentration profile. The second case (2) seems to be best explained as diffusion controlled C and N distributions. In the third case (3), the interior of taenite has been transformed to the α phase (kamacite or martensite). Carbon and N were expelled from the α phase and enriched near the inner border of the remaining γ phase. Such differences in the C and N distributions in taenite may reflect different cooling rates of iron meteorites. Nitrogen concentrations in taenite are quite high approaching 1 wt% in some iron meteorites. Nitride (carlsbergite and roaldite) is present in meteorites with high N concentrations in taenite, which suggests that the nitride was formed due to supersaturation of the metallic phases with N. The same tendency is generally observed for C (i.e., high C concentrations in taenite correlate with the presence of carbide and/or graphite). Concentrations of C and N in kamacite are generally below detection limits. Isotopic compositions of C and N in taenite can be measured with a precision of several permil. Isotopic analysis in kamacite in most iron meteorites is not possible because of the low concentrations. The C isotopic compositions seem to be somewhat fractionated among various phases, reflecting closure of C transport at low temperatures. A remarkable isotopic anomaly was observed for the Mundrabilla (IIICD anomalous) meteorite. Nitrogen isotopic compositions of taenite measured by SIMS agree very well with those of the bulk samples measured by conventional mass spectrometry.     

Production of nitrogen and carbon species by thunderstorms on Venus

The effects of the newly discovered thunderstorms on Venus upon the nitrogen and carbon species in its atmosphere were calculated. An Earth-like lightning frequency of 100 sec^?1 was used for Venus, in accord with recent optical measurements by Pioneer-Venus (W. J. Borucki, J. W. Dyer, G. Z. Thomas, J. C. Jordon, and D. A. Comstock, submitted for publication). The rate of NO production by thunder shock waves, 2.5 × 10¹¹ g year^?1, is about an order of magnitude smaller than on the Earth. But on Venus, in the absence of precipitation, which is the major removal mechanism of odd nitrogen from the Earth's atmosphere, the mixing ratios of odd nitrogen species might be considerably higher. The global CO production is governed by CO₂ photolysis rather than by CO₂ pyrolysis by lightning. However, thunderstorms produce about 2.5 × 10¹¹ g year^?1 of CO in the cloud layer, far from the high altitude CO₂ photolysis region.     

Compound‐specific carbon isotope compositions of aldehydes and ketones in the Murchison meteorite

Compound‐specific carbon isotope analysis (δ¹³C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Strecker‐cyanohydrin synthesis in the early solar system. Previous δ¹³C investigations have targeted α‐amino acid and α‐hydroxy acid Strecker products and reactant HCN; however, δ¹³C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O‐(2,3,4,5,6‐pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification, and δ¹³C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde, and acetone, were detected in the sample. δ¹³C values, ranging from ?10.0‰ to +66.4‰, were more ¹³C‐depleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar ¹²C/¹³C ratios. These relatively ¹³C‐depleted values suggest that chemical processes taking place in asteroid parent bodies (e.g., oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between δ¹³C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alteration‐driven chemical pathways.     

Angular distribution of photoelectrons from atomic oxygen,nitrogen and carbon

The angular distributions of photoelectrons from atomic oxygen, nitrogen and carbon are calculated. Both Hartree-Fock and Hartree-Slater (Herman-Skillman) wave functions are used for oxygen and the agreement is excellent; thus only Hartree-Slater functions are used for carbon and nitrogen. The pitch angle distribution of photoelectrons is discussed and it is shown that previous approximations of energy independent isotropic or sin² θ distributions are at odds with our results, which vary with energy. This variation with energy is discussed as is the reliability of these calculations.     

Petrology and geochemistry of D'Orbigny,geochemistry of Sahara 99555, and the origin of angrites

Abstract— We have done a detailed petrologic study of the angrite, D'Orbigny, and geochemical study of it and Sahara 99555. D'Orbigny is an igneous‐textured rock composed of Ca‐rich olivine, Al‐Ti‐diopside‐hedenbergite, subcalcic kirschsteinite, two generations of hercynitic spinel and anorthite, with the mesostasis phases ulvöspinel, Ca‐phosphate, a silico‐phosphate phase and Fe‐sulfide. We report an unknown Fe‐Ca‐Al‐Ti‐silicate phase in the mesostasis not previously found in angrites. One hercynitic spinel is a large, rounded homogeneous grain of a different composition than the euhedral and zoned grains. We believe the former is a xenocryst, the first such described from angrites. The mafic phases are highly zoned; mg# of cores for olivine are ?64, and for clinopyroxene ?58, and both are zoned to Mg‐free rims. The Ca content of olivine increases with decreasing mg#, until olivine with ?20 mol% Ca is overgrown by subcalcic kirschsteinite with about 30–35 mol% Ca. Detailed zoning sequences in olivine‐subcalcic kirschsteinite and clinopyroxene show slight compositional reversals. There is no mineralogic control that can explain these reversals, and we believe they were likely caused by local additions of more primitive melt during crystallization of D'Orbigny. D'Orbigny is the most ferroan angrite with a bulk rock mg# of 32. Compositionally, it is virtually identical to Sahara 99555; they are the first set of compositionally identical angrites. Comparison with the other angrites shows that there is no simple petrogenetic sequence, partial melting with or without fractional crystallization, that can explain the angrite suite. Angra dos Reis remains an anomalous angrite. Angrites show no evidence for the brecciation, shock, impact metamorphism, or thermal metamorphism that affected the howardite, eucrite, diogenite (HED) suite and ordinary chondrites. This suggests that the angrite parent body may have followed a fundamentally different evolutionary path than did these other parent bodies.     

Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon‐rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ¹³C from ?7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ¹³C = ca ?25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ¹⁵N values of ?53 to ?94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen‐rich and isotopically heavier. Thus, although the parent asteroid 2008TC₃ was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ¹³C and δ¹⁵N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.     

Micro-XCT chondrule classification for subsequent isotope analysis

Chondrules are microscopic, recrystallized melt droplets found in chondritic meteorites. High-resolution isotope analyses of minor elements require large enough element quantities which are obtained by dissolving entire chondrules. This work emphasizes the importance of X-ray computed tomography (XCT) to detect features that can significantly affect the bulk chondrule isotope composition. It thereby expands on other works by looking into chondrules from a wide range of chondrites including CR, CV, CB, CM, L, and EL samples before turning toward complex and time-consuming chemical processing. The features considered are metal and igneous rims, compound chondrules, matrix remnants, and metal contents. In addition to the identification of these features, computed tomography prevents the inclusion of non-chondrule samples (pure matrix or metal) as well as samples where two different chondrule fragments with potentially different isotope compositions are held together by matrix. Matrix surrounding chondrules is also easily detected and the affected chondrules can be omitted or reprocessed. The results strongly encourage to perform XCT before dissolution of chondrules for isotope analysis as a non-invasive method.     

Isotopic-geochemical study of nitrogen and carbon in peat from the Tunguska Cosmic Body explosion site

Isotopic-geochemical investigations were carried out on peat samples from the 1908 Tunguska Cosmic Body (TCB) explosion area. We analyzed two peat columns from the Northern peat bog, sampled in 1998, and from the Raketka peat bog, sampled during the 1999 Italian expedition, both located near the epicenter of the TCB explosion area. At the depth of the “catastrophic” layer, formed in 1908, and deeper, one can observe shifts in the isotopic composition of nitrogen (up to Δ¹⁵N = +7.2‰) and carbon (up to Δ¹³C = +2‰) and also an increase in the nitrogen concentration compared to those in the normal, upper layers, unaffected by the Tunguska event. One possible explanation for these effects could be the presence of nitrogen and carbon from TCB material and from acid rains, following the TCB explosion, in the “catastrophic” and “precatastrophic” layers of peat. We found that the highest quantity of isotopically heavy nitrogen fell near the explosion epicenter and along the TCB trajectory. It is calculated that 200,000 tons of nitrogen fell over the area of devastated forest, i.e., only about 30% of the value calculated by Rasmussen et al. (1984). This discrepancy is probably caused by part of the nitrogen having dispersed in the Earth’s atmosphere. The isotopic effects observed in the peat agree with the results of previous investigations [Kolesnikov et al 1998a], [Kolesnikov et al 1998b], [Kolesnikov et al 1999] and [Rasmussen et al 1999] and also with the increased content of iridium and other platinoids found in the corresponding peat layers of other columns [Hou et al 1998] and [Hou et al 2000]. These data favor the hypothesis of a cosmochemical origin of the isotopic effects.     

The non‐igneous genesis of angrites: Support from trace element distribution between phases in D'Orbigny

Abstract— D'Orbigny is an exceptional angrite. Chemically, it resembles other angrites such as Asuka‐881371, Sahara 99555, Lewis Cliff (LEW) 87051, and LEW 86010, but its structure and texture are peculiar. It has a compact and porous lithology, abundant glasses, augite‐bearing druses, and chemical and mineralogical properties that are highly unusual for igneous rocks. Our previous studies led us to a new view on angrites: they can possibly be considered as CAIs that grew to larger sizes than the ones we know from carbonaceous chondrites. Thus, angrites may bear a record of rare and special conditions in some part of the early solar nebula. Here we report trace element contents of D'Orbigny phases. Trace element data were obtained from both the porous and the compact part of this meteorite. We have confronted our results with the popular igneous genetic model. According to this model, if all phases of D'Orbigny crystallized from the same system, as an igneous origin implies, a record of this genesis should be expressed in the distribution of trace elements among early and late phases. Our results show that the trace element distribution of the two contemporaneous phases olivine and plagioclase, which form the backbone of the rock, seem to require liquids of different composition. Abundances of highly incompatible elements in all olivines, including the megacrysts, indicate disequilibrium with the bulk rock and suggest liquids very rich in these elements (>10,000 x CI), which is much richer than any fractional crystallization could possibly produce. In addition, trace element contents of late phases are incompatible with formation from the bulk system's residual melt. These results add additional severe constraints to the many conflicts that existed previously between an igneous model for the origin of angrites and the mineralogical and chemical observations. This new trace element content data, reported here, corroborate our previous results based on the shape, structure, mineralogy, chemical, and isotopic data of the whole meteorite, as well as on a petrographic and chemical composition study of all types of glasses and give strength to a new genetic model that postulates that D'Orbigny (and possibly all angrites) could have formed in the solar nebula under changing redox conditions, more akin to chondritic constituents (e.g., CAIs) than to planetary differentiated rock.     

A nitrogen and argon stable isotope study of Allan Hills 84001: Implications for the evolution of the Martian atmosphere

Abstract— The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] ～ 0.34 ppm; δ¹⁵N ～ +23%); although during stepped combustion, ¹⁵N-enriched N (δ¹⁵N ～ +143%) is released in a narrow temperature interval between 700 °C and 800 °C (along with ¹³C-enriched C (δ¹³C ～ +19%) and ⁴⁰Ar). Cosmogenic species are found to be negligible at this temperature; thus, the iso-topically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH 84001. The N and Ar data show that ALH 84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess ⁴⁰Ar from K decay. Deconvolution of radiogenic ⁴⁰Ar from other Ar components, on the basis of end-member ³⁶Ar/¹⁴N and ⁴⁰Ar/³⁶Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5–4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmos-phere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: δ¹⁵N ≥ +200%, ⁴⁰Ar/³⁶Ar ≤ 300 and ³⁶Ar/¹⁴N ≥ 17 × 10^?5. In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.