The abundance and isotopic composition of Cd in iron meteorites

Cadmium is a highly volatile element and its abundance in meteorites may help better understand volatility‐controlled processes in the solar nebula and on meteorite parent bodies. The large thermal neutron capture cross section of ¹¹³Cd suggests that Cd isotopes might be well suited to quantify neutron fluences in extraterrestrial materials. The aims of this study were (1) to evaluate the range and magnitude of Cd concentrations in magmatic iron meteorites, and (2) to assess the potential of Cd isotopes as a neutron dosimeter for iron meteorites. Our new Cd concentration data determined by isotope dilution demonstrate that Cd concentrations in iron meteorites are significantly lower than in some previous studies. In contrast to large systematic variations in the concentration of moderately volatile elements like Ga and Ge, there is neither systematic variation in Cd concentration amongst troilites, nor amongst metal phases of different iron meteorite groups. Instead, Cd is strongly depleted in all iron meteorite groups, implying that the parent bodies accreted well above the condensation temperature of Cd (i.e., ≈650 K) and thus incorporated only minimal amounts of highly volatile elements. No Cd isotope anomalies were found, whereas Pt and W isotope anomalies for the same iron meteorite samples indicate a significant fluence of epithermal and higher energetic neutrons. This observation demonstrates that owing to the high Fe concentrations in iron meteorites, neutron capture mainly occurs at epithermal and higher energies. The combined Cd‐Pt‐W isotope results from this study thus demonstrate that the relative magnitude of neutron capture‐induced isotope anomalies is strongly affected by the chemical composition of the irradiated material. The resulting low fluence of thermal neutrons in iron meteorites and their very low Cd concentrations make Cd isotopes unsuitable as a neutron dosimeter for iron meteorites.     

Rhenium and osmium systematics on iron and stony iron meteorites

Abstract— Re and Os abundances and ¹⁸⁷Os/¹⁸⁶Os isotopic ratios in 12 iron meteorites of various groups and five stony iron meteorites have been determined by an inductively coupled plasma mass spectrometry (ICP-MS). The series of iron meteorites studied have Re and Os concentrations ranging from 0.004 to 3.3 ppm and 0.03 to 41 ppm, respectively. The ¹⁸⁷Re/¹⁸⁶Os ratios in these meteorites fall between 3.0 and 6.1 and the ¹⁸⁷Os/¹⁸⁶Os between 1.0 and 1.2, giving an initial ¹⁸⁷Os/¹⁸⁶Os isotopic ratio of 0.790 and a Re-Os age of iron meteorites of 4.30 ± 0.28 Ga when employing the decay constant of 1.64 × 10^?11 yr^?1. The observed Re-Os age for iron meteorites appears somewhat younger than that for chondrites. The resultant younger age might be due either to a very slow cooling of the parental planetesimals or due to a secondary “shock” event. However, for definite conclusions about the Re-Os age, higher precisions of the Re and Os isotopic measurements and of the decay constant of ¹⁸⁷Re are required. Furthermore, the clear elucidation of the mechanisms for the fractionation of the Re/Os abundance ratios are related to the understanding of the meaning of the Re-Os age. The Re and Os abundances in pallasite stony iron meteorites are extremely low compared with those for most iron meteorites. On the other hand, the Re and Os abundances in mesosiderite stony iron meteorites show values comparable with those observed in most iron meteorites. The difference in Re and Os abundances in pallasite and mesosiderite stony iron meteorites strongly suggests that these stony iron meteorites are different in origin or history of chemical evolution. Re and Os abundances in the series of iron and stony iron meteorites were found to have a wide variation covering nearly four orders of magnitude, with a very high correlation coefficient (0.996), and a slope very slightly less than unity. The regression line observed here covers various groups of iron meteorites, stony iron meteorites and also chondrites. Masuda and Hirata (1991) suggested the possible direct mixing process of particles of most refractory metallic elements with gaseous clouds of less refractory matrix elements, since the Re and Os were predicted theoretically to be the first elements to condense as a solid phase from the high temperature solar nebula. The aims of this paper are to present a reliable technique for the Re-Os chronology and to study the cosmochemical sequences of the meteoritic metals.     

Natural variations in the rhenium isotopic composition of meteorites

Rhenium is an important element with which to test hypotheses of isotope variation. Historically, it has been difficult to precisely correct the instrumental mass bias in thermal ionization mass spectrometry. We used W as an internal standard to correct mass bias on the MC‐ICP‐MS, and obtained the first precise δ¹⁸⁷Re values (~±0.02‰, 2SE) for iron meteorites and chondritic metal. Relative to metal from H chondrites, IVB irons are systematically higher in δ¹⁸⁷Re by ~0.14 ‰. δ¹⁸⁷Re for other irons are similar to H chondritic metal, although some individual samples show significant isotope fractionation. Since ¹⁸⁵Re has a high neutron capture cross section, the effect of galactic cosmic‐ray (GCR) irradiation on δ¹⁸⁷Re was examined using correlations with Pt isotopes. The pre‐GCR irradiation δ¹⁸⁷Re for IVB irons is lower, but the difference in δ¹⁸⁷Re between IVB irons and other meteoritic metal remains. Nuclear volume‐dependent fractionation for Re is about the right magnitude near the melting point of iron, but because of the refractory and compatible character of Re, a compelling explanation in terms of mass‐dependent fractionation is elusive. The magnitude of a nucleosynthetic s‐process deficit for Re estimated from Mo and Ru isotopes is essentially unresolvable. Since thermal processing reduced nucleosynthetic effects in Pd, it is conceivable that Re isotopic variations larger than those in Mo and Ru may be present in IVBs since Re is more refractory than Mo and Ru. Thus, the Re isotopic difference between IVBs and other irons or chondritic metal remains unexplained.     

Tungsten isotopes in bulk meteorites and their inclusions—Implications for processing of presolar components in the solar protoplanetary disk

We present high precision, low‐ and high‐resolution tungsten isotope measurements of iron meteorites Cape York (IIIAB), Rhine Villa (IIIE), Bendego (IC), and the IVB iron meteorites Tlacotepec, Skookum, and Weaver Mountains, as well as CI chondrite Ivuna, a CV3 chondrite refractory inclusion (CAI BE), and terrestrial standards. Our high precision tungsten isotope data show that the distribution of the rare p‐process nuclide ¹⁸⁰W is homogeneous among chondrites, iron meteorites, and the refractory inclusion. One exception to this pattern is the IVB iron meteorite group, which displays variable excesses relative to the terrestrial standard, possibly related to decay of rare ¹⁸⁴Os. Such anomalies are not the result of analytical artifacts and cannot be caused by sampling of a protoplanetary disk characterized by p‐process isotope heterogeneity. In contrast, we find that ¹⁸³W is variable due to a nucleosynthetic s‐process deficit/r‐process excess among chondrites and iron meteorites. This variability supports the widespread nucleosynthetic s/r‐process heterogeneity in the protoplanetary disk inferred from other isotope systems and we show that W and Ni isotope variability is correlated. Correlated isotope heterogeneity for elements of distinct nucleosynthetic origin (¹⁸³W and ⁵⁸Ni) is best explained by thermal processing in the protoplanetary disk during which thermally labile carrier phases are unmixed by vaporization thereby imparting isotope anomalies on the residual processed reservoir.     

The Cu isotopic composition of iron meteorites

Abstract– High‐precision Cu isotopic compositions have been measured for the metal phase of 29 iron meteorites from various groups and for four terrestrial standards. The data are reported as the δ⁶⁵Cu permil deviation of the ⁶⁵Cu/⁶³Cu ratio relative to the NIST SRM 976 standard. Terrestrial mantle rocks have a very narrow range of variations and scatter around zero. In contrast, iron meteorites show δ⁶⁵Cu approximately 2.3‰ variations. Different groups of iron meteorites have distinct δ⁶⁵Cu values. Nonmagmatic IAB‐IIICD iron meteorites have similar δ⁶⁵Cu (0.03 ± 0.08 and 0.12 ± 0.10, respectively), close to terrestrial values (approximately 0). The other group of nonmagmatic irons, IIE, is isotopically distinct (?0.69 ± 0.15). IVB is the iron meteorite group with the strongest elemental depletion in Cu and samples in this group are enriched in the lighter isotope (δ⁶⁵Cu down to ?2.26‰). Evaporation should have produced an enrichment in ⁶⁵Cu over ⁶³Cu (δ⁶⁵Cu >0) and can therefore be ruled out as a mechanism for volatile loss in IVB meteorites. In silicate‐bearing iron meteorites, Δ¹⁷O correlates with δ⁶⁵Cu. This correlation between nonmass‐dependent and mass‐dependent parameters suggests that the Cu isotopic composition of iron meteorites has not been modified by planetary differentiation to a large extent. Therefore, Cu isotopic ratios can be used to confirm genetic links. Cu isotopes thus confirm genetic relationships between groups of iron meteorites (e.g., IAB and IIICD; IIIE and IIIAB); and between iron meteorites and chondrites (e.g., IIE and H chondrites). Several genetic connections between iron meteorites groups are confirmed by Cu isotopes, (e.g., IAB and IIICD; IIIE and IIIAB); and between iron meteorites and chondrites (e.g., IIE and H chondrites).     

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.     

Simulation of the interaction of galactic cosmic‐ray protons with meteoroids: On the production of radionuclides in thick gabbro and iron targets irradiated isotropically with 1.6 GeV protons

Abstract— Thick spherical targets made of gabbro (R = 25 cm) and of steel (R = 10 cm) were irradiated isotropically with 1.6 GeV protons at the Saturne synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay in order to simulate the interaction in space of galactic cosmic‐ray (GCR) protons with stony and iron meteoroids. Proton fluences of 1.32 × 10¹⁴ cm^?2 and 2.45 × 10¹⁴ cm^?2 were received by the gabbro and iron sphere, respectively, which corresponds to cosmic‐ray exposure ages of about 1.6 and 3.0 Ma. Both artificial meteoroids contained large numbers of high‐purity target foils of up to 28 elements at different depths. In these individual target foils, elementary production rates of radionuclides and rare gas isotopes were measured by x‐ and γ‐spectrometry, by low‐level counting, accelerator mass spectrometry (AMS), and by conventional rare gas mass spectrometry. Also samples of the gabbro itself were analyzed. Up to now, for each of the experiments, ～500 target‐product combinations were investigated of which the results for radionuclides are presented here. The experimental production rates show a wide range of depth profiles reflecting the differences between low‐, medium‐, and high‐energy products. The influence of the stony and iron matrices on the production of secondary particles and on particle transport, in general, and consequently on the production rates is clearly exhibited by the phenomenology of the production rates as well as by a detailed theoretical analysis. Theoretical production rates were calculated in an a priori way by folding depth‐dependent spectra of primary and secondary protons and secondary neutrons calculated by Monte Carlo techniques with the excitation functions of the underlying nuclear reactions. Discrepancies of up to a factor of 2 between the experimental and a priori calculated depth profiles are attributed to the poor quality of the mostly theoretical neutron excitation functions. Improved neutron excitation functions were obtained by least‐squares deconvolution techniques from experimental thick‐target production rates of up to five thick‐target experiments in which isotropic irradiations were performed. A posteriori calculations using the adjusted neutron cross sections describe the measured depth profiles of all these simulation experiments within 9%. The thus validated model calculations provide a basis for reliable physical model calculations of the production rates of cosmogenic nuclides in stony and iron meteorites as well as in lunar samples and terrestrial materials.     

Mass‐dependent fractionation of nickel isotopes in meteoritic metal

Abstract— We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non‐magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of ～0.4‰ amu^?1. The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (～0.0 to ～0.3‰ amu^?1) and chondrites (～0.0 to ～0.2‰ amu^?1) are similar, whereas the range in pallasite metal (～–0.1 to 0.0‰ amu^?1) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (～0.0 to ～0.3‰ amu^?1) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by ～0.4‰ amu^?1. In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe‐Ni alloy and the development of the Widmanstätten pattern.     

Production rates and proton‐induced production cross sections of 129I from Te and Ba: An attempt to model the 129I production in stony meteoroids and 129I in a Knyahinya sample

Abstract— Proton‐induced production cross sections of ¹²⁹I from Te and Ba are presented. Earlier assumptions that Te is the most important target element in meteoroids are confirmed. Based on this data set and the experimental production rates of ¹²⁹I from thick‐target experiments, the production of ¹²⁹I in stony meteoroids is modeled using a GCR flux density of 4.06 cm^?2 s^?1. The results of this modeling must be considered preliminary because the contribution from neutron capture on ¹²⁸Te needs further investigation. We obtained modeled production rates that agree with experimental results for samples from two medium‐sized meteorites (Abee and Knyahinya). However, we find that this model does not describe ⁴¹Ca in lunar rocks well and seems to overestimate ¹²⁹I production in larger bodies, such as Allende. We present elemental production rates from Te and Ba based on our modeling as well as for a model that describes neutron capture products. For ¹²⁹I analysis of Knyahinya, a novel method to separate Te and analysis using ICP‐MS was used, enabling the use of experimental elemental concentrations obtained from the same meteorite to calculate ¹²⁹I production.     

Re-Os isotope systematics and fractionation of siderophile elements in metal phases from CBa chondrites

We report Os isotope compositions of metal grains in two CB_a chondrites (Bencubbin and Gujba) determined using a micromilling sampling coupled with thermal ionization mass spectrometry, together with the abundances of major and trace siderophile elements obtained by electron probe microanalysis and femtosecond laser ablation inductively coupled plasma–mass spectrometry. The CB_a metal grains presented ¹⁸⁷Os/¹⁸⁸Os ratios akin to carbonaceous chondrites with limited variations (0.1257–0.1270). Most of the CB_a metal grains were scattered along a ¹⁸⁷Re-¹⁸⁷Os reference isochron of IIIAB iron meteorites, indicating that the CB_a metals experienced limited Re-Os fractionation at the time of their formation. The Re/Os ratios of sampling spots for the CB_a metals, recast from the observed ¹⁸⁷Os/¹⁸⁸Os ratios, had a positive correlation with their Os/Ir ratios. In addition, the metal grains showed a positive correlation in a Pd/Fe versus Ni/Fe diagram. These correlations suggest that the CB_a metal grains have formed via equilibrium condensation or evaporation from a gaseous reservoir at ~10⁻⁴ bar with enhanced metal abundances. Compared to the Bencubbin metals, the Gujba metals are characterized by having systematically lower Pd/Fe and Ni/Fe ratios that span subchondritic values. Such a difference was most likely induced by the compositionally heterogeneous impact plume from which the metals were condensed.     

Differentiation of metal‐rich meteoritic parent bodies: I. Measurements of PGEs,Re, Mo,W, and Au in meteoritic Fe‐Ni metal

Abstract— We describe an analytical technique for measurements of Fe, Ni, Co, Mo, Ru, Rh, W, Re, Os, Ir, Pt, and Au in bulk samples of iron meteorites. The technique involves EPMA (Fe, Ni, Co) and LA‐ICP‐MS analyses of individual phases of iron meteorites, followed by calculation of bulk compositions based on the abundances of these phases. We report, for the first time, a consistent set of concentrations of Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in the iron meteorites Arispe, Bennett County, Grant, Cape of Good Hope, Cape York, Carbo, Chinga, Coahuila, Duchesne, Gibeon, Henbury, Mundrabilla, Negrillos, Odessa, Sikhote‐Alin, and Toluca and the Divnoe primitive achondrite. The comparison of our LA‐ICP‐MS data for a number of iron meteorites with high‐precision isotope dilution and INAA data demonstrates the good precision and accuracy of our technique. The narrow ranges of variations of Mo and Pd concentrations within individual groups of iron meteorites suggest that these elements can provide important insights into the evolution of parent bodies of iron meteorites. Under certain assumptions, the Mo concentrations can be used to estimate mass fractions of the metal‐sulfide cores in the parent bodies of iron meteorites. It appears that a range of Pd variations within a group of iron meteorites can serve as a useful indicator of S content in the core of its parent body.     

Differentiation processes in FeO‐rich asteroids revealed by the achondrite Lewis Cliff 88763

Olivine‐dominated (70–80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO‐rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Δ¹⁷O value of ?1.19 ± 0.10‰, and low bulk‐rock Mg/(Mg+Fe) (0.39), similar to the FeO‐rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk‐rock major‐, minor‐, and trace‐element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and ¹⁸⁷Os/¹⁸⁸Os (0.1262), implies a FeO‐ and volatile‐rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe‐Ni‐S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal‐rich partial melts from FeO‐rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to <<5%). As such, LEW 88763 represents the least‐modified FeO‐rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe,Ni‐FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO‐rich meteorites, such as brachinites, brachinite‐like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.     

Galactic cosmic ray‐produced 129Xe and 131Xe excesses in troilites of the Cape York iron meteorite

Abstract— The flux of galactic cosmic rays (GCR) in the solar system appears to change with time. Based on the abundances in iron meteorites of cosmogenic nuclides of different half lives, Lavielle et al. (1999) found that the GCR flux increased in recent times (<100 Ma) by about 38% compared to average flux in the past 150 Ma to 700 Ma ago. A promising technique for calibrating the GCR flux during the past ?50 Ma, based on the ¹²⁹I and ¹²⁹Xe pair of nuclides, was discussed earlier (Marti 1986; Murty and Marti 1987). The ¹²⁹I‐¹²⁹Xe_n chronometer provides a shielding‐independent system as long as the exposure geometry remained fixed. It is especially suitable for large iron meteorites (Te‐rich troilite) because of the effects by the GCR secondary neutron component. Although GCR‐produced Xe components were identified in troilites, several issues require clarifications and improvements; some are reported here. We developed a procedure for achieving small Xe extraction blanks which are required to measure indigenous Xe in troilites. The ¹²⁹Xe and ¹³¹Xe excesses (¹²⁹Xe_n, ¹³¹Xe_n) due to neutron reactions in Te are correlated in a stepwise release run during the troilite decomposition. Our data show that indigenous Xe in troilite of Cape York has isotopic abundances consistent with ordinary chondritic Xe (OC‐Xe), in contrast to a terrestrial signature which was reported earlier. Two methods are discussed which assess and correct for an interfering radiogenic ¹²⁹Xe_r component from extinct ¹²⁹I. The corrected ¹²⁹Xe_n concentration in troilite D4 of Cape York yields a cosmic ray exposure (CRE) age of 82 ± 7 Ma consistent, within uncertainties, with reported data (Murty and Marti 1987; Marti et al. 2004).     

Chlorine abundances in meteorites

Abstract— We used the nuclear reaction ³⁷Cl (n,γ) ³⁸Ar, achieved during neutron irradiation for dating meteorites by the ³⁹Ar‐⁴⁰Ar technique, to calculate the elemental Cl concentration of 132 samples of 94 different meteorites (mostly finds) representing several different classes. determined k and ca concentrations are also reported. Total [Cl] varies considerably, both among meteorites of the same class and among different meteorite classes. The range in [Cl] is approximately 15–177 ppm for ordinary chondrites; approximately 24–650 ppm for enstatite chondrites; approximately 4–177 ppm for eucrites; approximately 7–128 ppm for mesosiderites; approximately 35–268 ppm for acapulcoites and lodranites; and approximately 12–507 ppm for winonaites and iron silicates. As expected, most differentiated meteorites have lower [Cl] compared to chondrites and iron silicates. Analyses of 11 interior samples (～0.1 g each) of a large L6 chondrite varied over 68–129 ppm, which is a measure of the homogeneity of Cl distribution. By evaluating Ar release during stepwise sample degassing, we separated the Cl into low‐temperature and high‐temperature components, the former of which may consist of terrestrial contamination. Most samples show low‐temperature Cl concentrations of <40 ppm, but for several samples terrestrial Cl contamination constitutes significant fractions of the total Cl. Among most differentiated meteorites, finds show considerably greater low‐temperature [Cl] compared to falls.     

Solar-proton-produced neon in shergottite meteorites and implications for their origin

Abstract— Measured Ne isotopes in samples of shergottite ALHA77005 show variations in ²¹Ne/²²Ne ratios and ²¹Ne abundances that are consistent with the presence of two cosmogenic components: a component produced by nuclear interactions of galactic cosmic rays (GCR) and a component produced at shallow shielding depths (～0–3 cm) by energetic solar flare protons (SCR). We suggest that the ²¹Ne/²²Ne ratio generally can be used to distinguish between SCR and GCR components in many meteorite types. Analysis of cosmogenic Ne produced in chondrite mineral separates, eucrites, and anorthositic lunar rocks, all having diverse major element compositions, indicate that the GCR ²¹Ne/²²Ne ratio increases modestly with relative Mg content. Data for hundreds of chondrite analyses suggest that SCR Ne is present in no more than a very small fraction of chondrites. Examination of literature data for other shergottites, however, indicate that all of these meteorites contain SCR Ne but that it is apparently absent in other SNC meteorites. The ubiquitous presence of SCR Ne in shergottites, in contrast to most other types of meteorites, suggests that the martian origin of shergottites gave them different orbital parameters compared to other meteorites. This in turn may have contributed to slower entry velocities and lesser surface ablation in the atmosphere or even to higher SCR production rates.     

Ejection ages from krypton‐81‐krypton‐83 dating and pre‐atmospheric sizes of martian meteorites

Abstract— Cosmic‐ray exposure (CRE) ages and Mars ejection times were calculated from the radionuclide ⁸¹Kr and stable Kr isotopes for seven martian meteorites. The following ⁸¹Kr‐Kr CRE ages were obtained: Los Angeles = 3.35 ± 0.70 Ma; Queen Alexandra Range 94201 = 2.22 ± 0.35 Ma; Shergotty = 3.05 ± 0.50 Ma; Zagami = 2.98 ± 0.30 Ma; Nakhla = 10.8 ± 0.8 Ma; Chassigny = 10.6 ± 2.0 Ma; and Allan Hills 84001 = 15.4 ± 5.0 Ma. Comparison of these ages with previously obtained CRE ages from the stable noble gas nuclei ³He, ²¹Ne, and ³⁸Ar shows excellent agreement. This indicates that the method for the production rate calculation for the stable nuclei is reliable. In all martian meteorites we observe effects induced by secondary cosmic‐ray produced epithermal neutrons. Epithermal neutron fluxes, φ_n (30–300 eV), are calculated based on the reaction ⁷⁹Br(n, γβ)⁸⁰Kr. We show that the neutron capture effects were induced in free space during Mars‐Earth transfer of the meteoroids and that they are not due to a pre‐exposure on Mars before ejection of the meteoritic material. Neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre‐atmospheric sizes are estimated. We obtain minimum radii in the range of 22–25 cm and minimum masses of 150–220 kg. These results are in good agreement with the mean sizes reported for model calculations using current semiempirical data.     

Accretion of differentiated achondritic and aqueously altered chondritic materials in the early solar system—Significance of an igneous fragment in the CM chondrite NWA 12651

One approach to decipher the dynamics of material transport and planetary accretion in the early solar system is to investigate xenolithic fragments in meteorites. In this work, we examined an igneous fragment from the NWA 12651 meteorite—the first igneous fragment found in any CM chondrite—by analyzing its mineralogy, rare earth elements (REEs), and O‐isotopes. The study shows that the exsolution lamellae of the igneous fragment consist of Fe‐rich and Ca‐rich pyroxene. Thus, the fragment was part of a progressive crystallization in a closed system, such as in a depleted magma reservoir or mantle. In this environment, the pyroxene co‐crystallized with plagioclase, resulting in a negative Eu anomaly and enrichment of the heavy REEs compared to the light REEs. The O‐isotopes of the fragment are more ¹⁶O‐enriched than the mafic minerals in the matrix or in other bulk CM chondrites; therefore, the fragment was formed in a different region than the NWA 12651 parent body. The iron meteorites Tucson and Deep Springs, the pallasite Milton, and the CB chondrites have similar O‐isotopes as the igneous fragment. However, no direct connection can be drawn and it is questionable if the fragment shares a same parent body with one of these meteorites. The close formation region to the CB chondrites may suggest a formation of the fragment in the carbonaceous chondrite region. Thus, a wide transport through the nebula of the early solar system may not have been necessary to move the fragment to the CM chondrite formation region.     

Ordinary Chondrites: 1. Short Life of Small Meteorites

The data on the concentration of cosmogenic neon isotopes and the density of cosmic-ray tracks in about 600 ordinary chondrites were analyzed. For ordinary chondrites of all chemical classes, the dispersion of the ratios between cosmogenic isotopes of neon, (²²Ne/²¹Ne) _c , and a fraction of meteorites with high rates of cosmic-ray-track formation were demonstrated to decrease with increasing cosmic-ray-exposure age. Most likely, these effects are related to the fact that chondrites of small exposure ages are more frequent among meteorites with low degrees of shielding (small sizes), probably because smaller meteorites are placed into Earth-crossing orbits faster than larger ones. This, in turn, is attributed to more effective insertion of small asteroid-belt bodies into resonances, most likely due to the diurnal Yarkovsky effect.     

Production of krypton and xenon isotopes in thick stony and iron targets isotropically irradiated with 1600 MeV protons

Abstract— Two spherical targets made of gabbro with a radius of 25 cm and of steel with a radius of 10 cm were irradiated isotropically with 1600 MeV protons at the SATURNE synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay, in order to simulate the production of nuclides in meteorites induced by galactic cosmic‐ray protons in space. These experiments supply depth‐dependent production rate data for a wide range of radioactive and stable isotopes in up to 28 target elements. In this paper, we report results for ⁷⁸Kr, ^80–86Kr isotopes in Rb, Sr, Y and Zr and for ¹²⁴Xe, ¹²⁶Xe, ^128–132Xe, ¹³⁴Xe, ¹³⁶Xe isotopes in Ba and La. Krypton and xenon concentrations have been measured at different depths in the spheres by using conventional mass spectrometry. Based on Monte‐Carlo techniques, theoretical production rates are calculated by folding depth‐dependent spectra of primary and secondary protons and secondary neutrons with the excitation functions of the relevant nuclear reactions. The comparison of the model calculation results with experimental data in the thick target experiments performed at LNS and previously at CERN have allowed adjustments of the poorly known excitation functions of neutron‐induced reactions. Thus, for the two experiments at SATURNE, excellent agreement is obtained between experimental and calculated production rates for most Kr and Xe isotopes in all investigated target elements. Only Xe production in Ba in the gabbro is underestimated by the calculations by ?25%. This work validates the approach of the thin‐target model calculations of cosmogenic nuclide production rates in the attempt of modeling the interaction of galactic cosmic‐ray protons with stony and iron meteorites in space as well as with lunar samples.     

Stony meteorite porosities and densities: A review of the data through 2001

Abstract— In this review, we summarize the data published up to December 2001 on the porosity and density of stony meteorites. These data were taken from 925 samples of 454 different meteorites by a variety of techniques. Most meteorites have densities on the order of 3 to 4 g/cm³, with lower densities only for some volatile‐rich carbonaceous meteorites and higher densities for stony irons. For the vast majority of stones, porosity data alone cannot distinguish between different meteorite compositions. Average porosities for most meteorite classes are around 10%, though individual samples can range as high as 30% porosity. Unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present. The measured density of ordinary chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment. A theoretical grain density based on composition can model “pre‐weathered” porosities. The average model porosity for H and LL chondrites is 10%, while L chondrite model porosities average only 6%, a statistically significant difference.