Characterization of insoluble organic matter in primitive meteorites by microRaman spectroscopy

Abstract— We have analyzed the chemically and isotopically well‐characterized insoluble organic matter (IOM) extracted from 51 unequilibrated chondrites (8 CR, 9 CM, 1 CI, 3 ungrouped C, 9 CO, 9 CV, 10 ordinary, 1 CB and 1 E chondrites) using confocal imaging Raman spectroscopy. The average Raman properties of the IOM, as parameterized by the peak characteristics of the so‐called D and G bands, which originate from aromatic C rings, show systematic trends that are correlated with meteorite (sub‐) classification and IOM chemical compositions. Processes that affect the Raman and chemical properties of the IOM, such as thermal metamorphism experienced on the parent bodies, terrestrial weathering and amorphization due to irradiation in space, have been identified. We established separate sequences of metamorphism for ordinary, CO, oxidized, and reduced CV chondrites. Several spectra from the most primitive chondrites reveal the presence of organic matter that has been amorphized. This amorphization, usually the result of sputtering processes or UV or particle irradiation, could have occurred during the formation of the organic material in interstellar or protoplanetary ices or, less likely, on the surface of the parent bodies or during the transport of the meteorites to Earth. D band widths and peak metamorphic temperatures are strongly correlated, allowing for a straightforward estimation of these temperatures.     

Nanodiamonds and silicate minerals in ordinary chondrites as determined by micro‐Raman spectroscopy

We present here the Raman spectroscopic study of silicate and carbonaceous minerals in three ordinary chondrites with the aim to improve our understanding the impact process including the peak metamorphic pressures present in carbon‐bearing ordinary chondites. The characteristic Raman vibrational peaks of olivines, pyroxenes, and plagioclase have been determined on three ordinary chondrites from India, Dergaon (H5), Mahadevpur (H4/5), and Kamargaon (L6). The Raman spectra of these meteorite samples show the presence of nanodiamonds at 1334–1345 cm^?1 and 1591–1619 cm^?1. The full‐width at half maximum (FWHM) of Raman peaks for Mahadevpur and Dergaon reflect the nature of shock metamorphism in these meteorites. The frequency shift in Raman spectra might be because of shock effects during the formation of the diamond/graphite grains.     

Formation of the parent bodies of the carbonaceous chondrites

We have carried out extensive particle track studies for several C2 chondrites. On the basis of these and the available data on spallogenic stable and radioactive nuclides in several C1 and C2 chondrites, we have constructed a scenario for the precompaction irradiation of these meteorites. We discuss the rather severe constraints which these data place on the events leading to the formation of the parent bodies of the carbonaceous chondrites. Our analyses suggest that the precompaction solar flare and solar wind irradiation of the individual components most probably occurred primarily while the matter had accreted to form swarms of centimeter- to meter-sized bodies. This irradiation occured very early, within a few hundred my of the birth of the solar system; the pressure in the solar system had then dropped below 10^?9 atm. Further, the model assumes that soon after the irradiation of carbonaceous matter as swarms, the small bodies coalesced to form kilometer-sized objects, in time scales of 10^5±1 years, a constraint defined by the low cosmogenic exposure ages of these meteorites. Collisions among these objects led to the formation of much-larger-sized parent bodies of the carbonaceous chondrites. Implicit in this model is the existence of “irradiated” components at all depths in the parent bodies, which formed out of the irradiated swarm material.     

Organic matter from comet 81P/Wild 2, IDPs,and carbonaceous meteorites; similarities and differences

Abstract— During preliminary examination of 81P/Wild 2 particles collected by the NASA Stardust spacecraft, we analyzed seven, sulfur embedded and ultramicrotomed particles extracted from five different tracks. Sections were analyzed using a scanning transmission X‐ray microscope (SXTM) and carbon X‐ray absorption near edge structure (XANES) spectra were collected. We compared the carbon XANES spectra of these Wild 2 samples with a database of spectra on thirty‐four interplanetary dust particles (IDPs) and with several meteorites. Two of the particles analyzed are iron sulfides and there is evidence that an aliphatic compound associated with these particles can survive high temperatures. An iron sulfide from an IDP demonstrates the same phenomenon. Another, mostly carbon free containing particle radiation damaged, something we have not observed in any IDPs we have analyzed or any indigenous organic matter from the carbonaceous meteorites, Tagish Lake, Orgueil, Bells and Murchison. The carbonaceous material associated with this particle showed no mass loss during the initial analysis but chemically changed over a period of two months. The carbon XANES spectra of the other four particles varied more than spectra from IDPs and indigenous organic matter from meteorites. Comparison of the carbon XANES spectra from these particles with 1. the carbon XANES spectra from thirty‐four IDPs (<15 micron in size) and 2. the carbon XANES spectra from carbonaceous material from the Tagish Lake, Orgueil, Bells, and Murchison meteorites show that 81P/Wild 2 carbon XANES spectra are more similar to IDP carbon XANES spectra then to the carbon XANES spectra of meteorites.     

A propensity for n‐ω‐amino acids in thermally altered Antarctic meteorites

Abstract– Asteroids and their fragments have impacted the Earth for the last 4.5 Gyr. Carbonaceous meteorites are known to contain a wealth of indigenous organic molecules, including amino acids, which suggests that these meteorites could have been an important source of prebiotic organic material during the origins of life on Earth and possibly elsewhere. We report the detection of extraterrestrial amino acids in thermally altered type 3 CV and CO carbonaceous chondrites and ureilites recovered from Antarctica. The amino acid concentrations of the thirteen Antarctic meteorites ranged from 300 to 3200 parts‐per‐billion (ppb), generally much less abundant than in amino acid‐rich CI, CM, and CR carbonaceous chondrites that experienced much lower temperature aqueous alteration on their parent bodies. In contrast to low‐temperature aqueously altered meteorites that show complete structural diversity in amino acids formed predominantly by Strecker–cyanohydrin synthesis, the thermally altered meteorites studied here are dominated by small, straight‐chain, amine terminal (n‐ω‐amino) amino acids that are not consistent with Strecker formation. The carbon isotopic ratios of two extraterrestrial n‐ω‐amino acids measured in one of the CV chondrites (δ¹³C approximately ?25‰) are consistent with ¹³C‐depletions observed previously in hydrocarbons produced by Fischer‐Tropsch type reactions. The predominance of n‐ω‐amino acid isomers in thermally altered meteorites hints at cosmochemical mechanisms for the preferential formation and preservation of a small subset of the possible amino acids.     

Elastic collisions in ion irradiation experiments: A mechanism for space weathering of silicates

Ion irradiation experiments have been performed on silicates (bulk samples) rich of olivine, pyroxene, and serpentine to simulate the effects of space weathering induced on asteroids by solar wind ions. We have used different ions (H⁺, He⁺, Ar⁺, Ar²⁺) having different energies (from 60 to 400 keV) to weather the samples, probed by Raman spectroscopy and UV-vis-NIR reflectance spectroscopy. All the irradiated materials have shown reddening and darkening of reflectance spectra in the 0.25-2.7 μm spectral range. We have found that the increase of the spectral slope of the continuum across the 1-μm band is strongly related with the number of displacements caused by colliding ions because of elastic collisions with the target nuclei. The spectral slopes have been compared, at increasing ion fluence, with those from irradiated Epinal meteorite. We show that formation of nuclear displacements by solar wind ion irradiation is a physical mechanism that reddens the asteroidal surfaces on a time-scale lower than 10⁶ years.     

NanoSIMS analysis of organic carbon from the Tissint Martian meteorite: Evidence for the past existence of subsurface organic‐bearing fluids on Mars

Two petrographic settings of carbonaceous components, mainly filling open fractures and occasionally enclosed in shock‐melt veins, were found in the recently fallen Tissint Martian meteorite. The presence in shock‐melt veins and the deuterium enrichments (δD up to +1183‰) of these components clearly indicate a pristine Martian origin. The carbonaceous components are kerogen‐like, based on micro‐Raman spectra and multielemental ratios, and were probably deposited from fluids in shock‐induced fractures in the parent rock of Tissint. After precipitation of the organic matter, the rock experienced another severe shock event, producing the melt veins that encapsulated a part of the organic matter. The C isotopic compositions of the organic matter (δ¹³C = ?12.8 to ?33.1‰) are significantly lighter than Martian atmospheric CO₂ and carbonate, providing a tantalizing hint for a possible biotic process. Alternatively, the organic matter could be derived from carbonaceous chondrites, as insoluble organic matter from the latter has similar chemical and isotopic compositions. The presence of organic‐rich fluids that infiltrated rocks near the surface of Mars has significant implications for the study of Martian paleoenvironment and perhaps to search for possible ancient biological activities on Mars.     

Radiation-induced amorphization of crystalline ice

We study radiation-induced amorphization of crystalline ice, analyzing the results of three decades of experiments with a variety of projectiles, irradiation energy, and ice temperature, finding a similar trend of increasing resistance of amorphization with temperature and inconsistencies in results from different laboratories. We discuss the temperature dependence of amorphization in terms of the ‘thermal spike’ model. We then discuss the common use of the 1.65 μm infrared absorption band of water as a measure of degree of crystallinity, an increasingly common procedure to analyze remote sensing data of astronomical icy bodies. The discussion is based on new, high quality near-infrared reflectance absorption spectra measured between 1.4 and 2.2 μm for amorphous and crystalline ices irradiated with 225 keV protons at 80 K. We found that, after irradiation with 10¹⁵ protons cm⁻², crystalline ice films thinner than the ion range become fully amorphous, and that the infrared absorption spectra show no significant changes upon further irradiation. The complete amorphization suggests that crystalline ice observed in the outer Solar System, including trans-neptunian objects, may results from heat from internal sources or from the impact of icy meteorites or comets.     

A NanoSIMS and Auger Nanoprobe investigation of an isotopically primitive interplanetary dust particle from the 55P/Tempel‐Tuttle targeted stratospheric dust collector

Abstract– An IDP nicknamed Andric, from a stratospheric dust collector targeted to collect dust from comet 55P/Tempel‐Tuttle, contains five distinct presolar silicate and/or oxide grains in 14 ultramicrotome slices analyzed, for an estimated abundance of approximately 700 ppm in this IDP. Three of the grains are ¹⁷O‐enriched and probably formed in low‐mass red giant or asymptotic giant branch (AGB) stars; the other two grains exhibit ¹⁸O enrichments and may have a supernova origin. Carbon and N isotopic analyses show that Andric also exhibits significant variations in its N isotopic composition, with numerous discrete ¹⁵N‐rich hotspots and more diffuse regions that are also isotopically anomalous. Three ¹⁵N‐rich hotspots also have statistically significant ¹³C enrichments. Auger elemental analysis shows that these isotopically anomalous areas consist largely of carbonaceous matter and that the anomalies may be hosted by a variety of components. In addition, there is evidence for dilution of the isotopically heavy components with an isotopically normal endmember; this may have occurred either as a result of extraterrestrial alteration or during atmospheric entry. Isotopically primitive IDPs such as Andric share many characteristics with primitive meteorites such as the CR chondrites, which also contain isotopically anomalous carbonaceous matter and abundant presolar silicate and oxide grains. Although comets are one likely source for the origin of primitive IDPs, the presence of similar characteristics in meteorites thought to come from the asteroid belt suggests that other origins are also possible. Indeed the distinction between cometary and asteroidal sources is somewhat blurred by recent observations of icy comet‐like planetesimals in the outer asteroid belt.     

Polycyclic aromatic hydrocarbons and amino acids in meteorites and ice samples from LaPaz Icefield,Antarctica

Abstract— –We have analyzed ice samples and meteorites from the LaPaz region of Antarctica to investigate the composition of polycyclic aromatic hydrocarbons (PAHs) and amino acids with the goal to understand whether or not there is a compositional relationship between the two reservoirs. Four LL5 ordinary chondrites (OCs) and one CK carbonaceous chondrite were collected as part of the 2003/2004 ANSMET season. Ice samples collected from directly underneath the meteorites were extracted. In addition, exhaust particles from the snowmobiles used during the expedition were collected to investigate possible contributions from this source. The meteorite samples, the particulate matter and solid‐state extracts of the ice samples and the exhaust filters were subjected to two‐step laser mass spectrometry (L2MS) to investigate the PAH composition. For amino acids analysis, the meteorites were extracted with water and acid hydrolyzed, and the extracts were analyzed with offline OPA/NAC derivatization combined with liquid chromatography with UV fluorescence detection and time of flight mass spectrometry (LC‐FD/ToF‐MS). PAHs in the particulate matter of the ice were found to be qualitatively similar to the meteorite samples, indicating that micron‐sized grains of the meteorite may be embedded in the ice samples. The concentration levels of dissolved PAHs in all the ice samples were found to be below the detection limit of the L2MS. The PAH composition of the snowmobile exhaust is significantly different to the one in particulate matter, making it an unlikely source of contamination for Antarctic meteorites. The amino acids glycine, β‐alanine and γ‐amino‐n‐butyric acid that were detected at concentrations of 3 to 19 parts per billion (ppb) are probably indigenous to the Antarctic meteorites. Some of the LaPaz ice samples were also found to contain amino acids at concentration levels of 1 to 33 parts per trillion (ppt), in particular α‐aminoisobutyric acid (AIB), an abundant non‐protein amino acid of extraterrestrial origin found in some carbonaceous chondrites. We hypothesize that this amino acid could have been extracted from Antarctic micrometeorites and the particulate matter of the meteorites during the concentration procedure of the ice samples.     

The labelling of meteoritic organic material using osmium tetroxide vapour impregnation

We present a method for labelling and locating organic material in extraterrestrial samples in order to determine its spatial arrangement in relation to inorganic components. We have taken a suite of meteorites including carbonaceous chondrites (CC) and Shergottite–Nakhlite–Chassignites (SNCs) and attempted to locate organic material within them by labelling with osmium tetroxide vapour impregnation. This technique is limited by the abundance of organic material within each sample, and the degree of terrestrial contamination, which may contribute additional organic components or surface contaminants that may mask indigenous organic components. Results confirm previous studies that phyllosilicates are key to organic matter accumulation in extraterrestrial environments.     

MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites: Structural refinement,cooling history,and trace element contents

MgAl₂O₄ spinels from Allende and NWA 763 carbonaceous chondrites were studied by X‐ray single crystal diffraction, SEM, electron microprobe, LA‐ICP‐MS, and Raman spectroscopy. Those from Allende are almost pure, but, in one case, we found a strong FeO_tot zonation. Spinels from NWA 763 show Mg‐Fe²⁺ substitutions. Almost pure MgAl₂O₄ spinels from both meteorites underwent slow cooling and reached their intracrystalline closure temperature (T_c) in the range 460–520 °C. The NWA 763 spinel with higher FeO content shows a T_c of about 720 °C. X‐ray single crystal diffraction and Raman spectroscopy suggest a slow cooling and an ordered structure with trivalent cations in M site and divalent in T site. Among the trace elements, Ti and Co are enriched with respect to the terrestrial analogs, while Mn, Ni, and Sn show intermediate values between different terrestrial occurrences. Vanadium cannot be used as a tracer of extraterrestrial origin as for Cr‐spinels, because its content is similar in extraterrestrial and terrestrial spinels. In the zoned crystal from Allende, Co show a strong zonation similar to that of FeO.     

A model for the internal structures of asteroids

Interpretation of reflectance spectra indicates that most belt asteroids are composed of materials similar to carbonaceous chondrites. Also, there is considerable evidence to support the origin of many, if not most, lunar and meteoritic chondrules by impact processes. The accretional histories of the carbonaceous asteroids must have influenced greatly their internal structures and textures. A model for this accretional history can be divided conveniently into three temporal stages that produce distinctly different lithologies: (1) low-velocity accretion of fine silicate and carbonaceous grains producing chondrule-free petrologic type 1 lithology; (2) continued accretion of low-velocity fine silicate and carbonaceous grains, but with a few larger, higher-velocity bodies also impacting the surface thereby producing both fluid drop and lithic chondrules (the resultant lithology would be that of petrologic type 2 and 3 carbonaceous chondrites); and (3) dominance of high-velocity low-mass meteoroid impacts, producing a sparse, thin, erosive lunar-like regolith. The lithologic product of stage 3 is not ideally represented among the presently described carbonaceous chondrites, but texturally analogous samples are known from the achondrites. The greater proportion of chondrules in the CV group meteorites, in contrast to the CM2 and CO3 groups, may be due to the origin of the CV chondrites on larger asteroid parent bodies that could withstand more numerous and higher-energy chondrule-producing impacts prior to fragmentation.     

Extraterrestrial chromite in latest Maastrichtian and Paleocene pelagic limestone at Gubbio,Italy: The flux of unmelted ordinary chondrites

Abstract— –The distribution of sediment‐dispersed extraterrestrial (ordinary chondritic) chromite (EC) grains (>63 μm) has been studied across the latest Maastrichtian and Paleocene in the Bottaccione Gorge section at Gubbio, Italy. This section is ideal for determining the accumulation rate of EC because of its condensed nature and well‐constrained sedimentation rates. In a total of 210 kg of limestone representing eight samples of 14–28 kg distributed across 24 m of the Bottaccione section, only 6 EC grains were found (an average of 0.03 EC grains kg^?1). In addition, one probable pallasitic chromite grain was found. No EC grains could be found in two samples at the Cretaceous‐Tertiary (K‐T) boundary, which is consistent with the K‐T boundary impactor being a carbonaceous chondrite or comet low in chromite. The average influx of EC to Earth is calculated to ～～0.26 grain m^?2 kyr^?1. This corresponds to a total flux of ～～200 tons of extraterrestrial matter per year, compared to ～～30,000 tons per year, as estimated from Os isotopes in deep‐sea sediments. The difference is explained by the EC grains representing only unmelted ordinary chondritic matter, predominantly in the size range from ～～0.1 mm to a few centimeters in diameter. Sedimentary EC grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere. The average of 0.03 EC grain kg^?1 in the Gubbio limestone contrasts with the up to ～～3 EC grains kg^?1 in mid‐Ordovician limestone that formed after the disruption of the L chondrite parent body in the asteroid belt at ～～470 Ma. The two types of limestone were deposited at about the same rate, and the difference in EC abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup.     

The distribution of aluminum-26 in the early Solar System—A reappraisal

Abstract— A compilation of over 1500 Mg-isotopic analyses of Al-rich material from primitive solar system matter (meteorites) shows clearly that ²⁶Al existed live in the early Solar System. Excesses of ²⁶Mg observed in refractory inclusions are not the result of mixing of “fossil” interstellar ²⁶Mg with normal solar system Mg. Some material was present that contained little or no ²⁶Al, but it was a minor component of solar system matter in the region where CV3 and CO3 carbonaceous chondrites accreted and probably was a minor component in the accretion regions of CM chondrites as well. Data for other chondrite groups are too scanty to make similar statements. The implied long individual nebular histories of CAIs and the apparent gap of one or more million years between the start of CAI formation and the start of chondrule formation require the action of some nebular mechanism that prevented the CAIs from drifting into the Sun. Deciding whether ²⁶Al was or was not the agent of heating that caused melting in the achondrite parent bodies hinges less on its widespread abundance in the nebula than it does on the timing of planetesimal accretion relative to the formation of the CAIs.     

Correlated accretion ages and ε54Cr of meteorite parent bodies and the evolution of the solar nebula

We look at the relationship between the value of ε⁵⁴Cr in bulk meteorites and the time (after calcium‐aluminum‐rich inclusion, CAI) when their parent bodies accreted. To obtain accretion ages of chondrite parent bodies, we estimated the maximum temperature reached in the insulated interior of each parent body, and estimated the initial ²⁶Al/²⁷Al for this temperature to be achieved. This initial ²⁶Al/²⁷Al corresponds to the time (after CAI formation) when cold accretion of the parent body would have occurred, assuming ²⁶Al/²⁷Al throughout the solar system began with the canonical value of 5.2 × 10^?5. In cases of iron meteorite parent bodies, achondrite parent bodies, and carbonaceous chondrite parent bodies, we use published isotopic ages of events (such as core formation, magma crystallization, and growth of secondary minerals) in each body's history to obtain the probable time of accretion. We find that ε⁵⁴Cr correlates with accretion age: the oldest accretion ages (1 ± 0.5 Ma) are for iron and certain other differentiated meteorites with ε⁵⁴Cr of ?0.75 ± 0.5, and the youngest ages (3.5 ± 0.5 Ma) are for hydrated carbonaceous chondrites with ε⁵⁴Cr values of 1.5 ± 0.5. Despite some outliers (notably Northwest Africa [NWA] 011 and Tafassasset), we feel that the correlation is significant and we suggest that it resulted from late, localized injection of dust with extremely high ε⁵⁴Cr.     

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.     

Clay mineral‐organic matter relationships in the early solar system

Abstract— As the solar system formed, it inherited and perpetuated a rich organic chemistry, the molecular products of which are preserved in ancient extraterrestrial objects such as carbonaceous chondrites. These organic‐rich meteorites provide a valuable and tangible record of the chemical steps taken towards the origin of life in the early solar system. Chondritic organic matter is present in the inorganic meteorite matrix which, in the CM and CI chondrites, contains evidence of alteration by liquid water on the parent asteroid. An unanswered and fundamental question is to what extent did the organic matter and inorganic products of aqueous alteration interact or display interdependence? We have used an organic labelling technique to reveal that the meteoritic organic matter is strongly associated with clay minerals. This association suggests that clay minerals may have had an important trapping and possibly catalytic role in chemical evolution in the early solar system prior to the origin of life on the early Earth.     

Thermal neutron capture effects in radioactive and stable nuclide systems

Neutron capture effects in meteorites and lunar surface samples have been successfully used in the past to study exposure histories and shielding conditions. In recent years, however, it turned out that neutron capture effects produce a nuisance for some of the short‐lived radionuclide systems. The most prominent example is the ¹⁸²Hf‐¹⁸²W system in iron meteorites, for which neutron capture effects lower the ¹⁸²W/¹⁸⁴W ratio, thereby producing too old apparent ages. Here, we present a thorough study of neutron capture effects in iron meteorites, ordinary chondrites, and carbonaceous chondrites, whereas the focus is on iron meteorites. We study in detail the effects responsible for neutron production, neutron transport, and neutron slowing down and find that neutron capture in all studied meteorite types is not, as usually expected, exclusively via thermal neutrons. In contrast, most of the neutron capture in iron meteorites is in the epithermal energy range and there is a significant contribution from epithermal neutron capture even in stony meteorites. Using sophisticated particle spectra and evaluated cross section data files for neutron capture reactions we calculate the neutron capture effects for Sm, Gd, Cd, Pd, Pt, and Os isotopes, which all can serve as neutron‐dose proxies, either in stony or in iron meteorites. In addition, we model neutron capture effects in W and Ag isotopes. For W isotopes, the GCR‐induced shifts perfectly correlate with Os and Pt isotope shifts, which therefore can be used as neutron‐dose proxies and permit a reliable correction. We also found that GCR‐induced effects for the ¹⁰⁷Pd‐¹⁰⁷Ag system can be significant and need to be corrected, a result that is in contrast to earlier studies.     

A preliminary investigation into the nature of carbonaceous material in ordinary chondrites

Abstract— The nature and isotopic composition of carbonaceous components in a variety of ordinary chondrites have been studied using stepped combustion. The samples were chosen to include falls, finds and Antarctic meteorites; specimens from all three chemical groups (H, L and LL) have been analysed. Effort was concentrated mostly on the low petrologic type meteorites (i.e., type 3); however, types 4–6 were also included in the study. Apart from terrestrial contaminants and weathering products, some of the unequilibrated ordinary chondrites appear to contain an indigenous organic component. In addition, most of the samples studied show evidence for an amorphous/graphitic component. This exists as C-rich aggregates or as carbon associated with “Huss” matrix. There does not appear to be any difference in δ¹³C for this carbon between Antarctic and non-Antarctic meteorites. In contrast, low temperature carbon in Antarctic samples is characterized by a ¹³C-enrichment. This is thought to be due to the influence of terrestrial weathering products introduced in the Antarctic. Curiously, the low temperature carbon in non-Antarctic finds appears to be intermediate in δ¹³C between Antarctic finds and non-Antarctic falls. This suggests that the weathering processes which are so obviously apparent from Antarctic samples may also extend, albeit in a more limited way, to non-Antarctic meteorites.