Noble gases in enstatite chondrites I: Exposure ages,pairing, and weathering effects

Abstract— Cosmic‐ray exposure ages calculated from cosmogenic noble gas nuclides are reported for 57 enstatite (E) chondrites, 43 of them were measured for the first time. With a total of 62 individual E chondrites (literature and this data, corrected for pairing) the observed spectrum of ages ranges between 0.07 and 66 Ma. Three clusters seem to develop at about 3.5, 8, and 25 Ma, respectively. Since the uncertainty of ages is estimated to be ～20% (in contrast to 10 to 15% for ordinary chondrites) and the number of examined samples is still comparatively small, these peaks have to be confirmed by more measurements. Regarding the two subgroups, EH and EL chondrites, no systematic trend is apparent in the distribution of cosmic‐ray exposure ages. Several E chondrites yield significantly lower ³⁸Ar ages compared to those calculated from cosmogenic ³He and ²¹Ne. For these E chondrites, we suggest a reduction of cosmogenic ³⁸Ar as a result of weathering. In order to prove the possible influence of terrestrial alteration on the cosmogenic noble gas record of E‐chondritic material, we simulated terrestrial weathering in an experiment of 12 weeks duration. The treatment showed that a significant amount of cosmogenic ³⁸Ar is lost on Earth by the influence of water.     

Rumuruti chondrites: Noble gases,exposure ages,pairing, and parent body history

Abstract— In this paper, we present concentration and isotopic composition of the light noble gases He, Ne, and Ar as well as of ⁸⁴Kr, ¹³²Xe, and ¹²⁹Xe in bulk samples of 33 Rumuruti (R) chondrites. Together with previously published data of six R chondrites, exposure ages are calculated and compared with those of ordinary chondrites. A number of pairings, especially between those from Northwest Africa (NWA), are suggested, so that only 23 individual falls are represented by the 39 R chondrites discussed here. Eleven of these meteorites, or almost 50%, contain solar gases and are thus regolithic breccias. This percentage is higher than that of ordinary chondrites, howardites, or aubrites. This may imply that the parent body of R chondrites has a relatively thick regolith. Concentrations of heavy noble gases, especially of Kr, are affected by the terrestrial atmospheric component, which resides in weathering products. Compared to ordinary chondrites, ¹²⁹Xe/¹³²Xe ratios of R chondrites are high.     

Noble gases in twenty-one Saharan LL-chondrites: Exposure ages and possible pairings

Abstract— Nineteen LL-chondrites and two L/LL-chondrites (Adrar 003 and Tanezrouft 010) from the Saharan desert of Algeria and Libya have been analysed for their He, Ne and Ar composition as well as for their abundance of ⁸⁴Kr and ¹³²Xe. Calculated ²¹Ne cosmic-ray exposure ages vary between 2 and 35 Ma. The age distribution is consistent with that of modern LL-chondrite falls except that no dominant peaks can be observed, especially not the one related to a 15 Ma collisional event. However, the lack of young exposure ages of <8 Ma is obvious. This is a characteristic feature of LL-chondrites. Only one of the 21 LL-chondrites, namely Acfer 066, contains solar gases and is thus considered a regolith breccia. Three specimens reveal considerable loss of ³He, probably due to periods of elevated temperatures in orbits with small perihelion distances. Furthermore, severe loss of ⁴He and ⁴⁰Ar is found in two samples. Considering possible pairings, we suggest 14 individual falls basically on the basis of the noble gas data, the petrographic sub-classification and by taking the find location into consideration. However, there are constraints on confirming pairings solely on the basis of our studies. Thus, we can only exclude individual samples with a unique noble gas fingerprint from paired specimens.     

Weathering of ordinary chondrites from Oman: Correlation of weathering parameters with 14C terrestrial ages and a refined weathering scale

We have investigated 128 ¹⁴C‐dated ordinary chondrites from Oman for macroscopically visible weathering parameters, for thin section‐based weathering degrees, and for chemical weathering parameters as analyzed with handheld X‐ray fluorescence. These 128 ¹⁴C‐dated meteorites show an abundance maximum of terrestrial age at 19.9 ka, with a mean of 21.0 ka and a pronounced lack of samples between 0 and 10 ka. The weathering degree is evaluated in thin section using a refined weathering scale based on the current W0 to W6 classification of Wlotzka (1993), with five newly included intermediate steps resulting in a total of nine (formerly six) steps. We find significant correlations between terrestrial ages and several macroscopic weathering parameters. The correlation of various chemical parameters including Sr and Ba with terrestrial age is not very pronounced. The microscopic weathering degree of metal and sulfides with newly added intermediate steps shows the best correlation with ¹⁴C terrestrial ages, demonstrating the significance of the newly defined weathering steps. We demonstrate that the observed ¹⁴C terrestrial age distribution can be modeled from the abundance of meteorites with different weathering degrees, allowing the evaluation of an age‐frequency distribution for the whole meteorite population.     

Exposure history and terrestrial ages of ordinary chondrites from the Dar al Gani region,Libya

Abstract— We measured the concentrations of noble gases in 32 ordinary chondrites from the Dar al Gani (DaG) region, Libya, as well as concentrations of the cosmogenic radionuclides ¹⁴C, ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in 18 of these samples. Although the trapped noble gases in five DaG samples show ratios typical of solar or planetary gases, in all other DaG samples, they are dominated by atmospheric contamination, which increases with the degree of weathering. Cosmic ray exposure (CRE) ages of DaG chondrites range from ?1 Myr to 53 Myr. The CRE age distribution of 10 DaG L chondrites shows a cluster around 40 Myr due to four members of a large L6 chondrite shower. The CRE age distribution of 19 DaG H chondrites shows only three ages coinciding with the main H chondrite peak at ?7 Myr, while seven ages are <5 Myr. Two of these H chondrites with short CRE ages (DaG 904 and 908) show evidence of a complex exposure history. Five of the H chondrites show evidence of high shielding conditions, including low ²²Ne/²¹Ne ratios and large contributions of neutron‐capture ³⁶Cl and ⁴¹Ca. These samples represent fragments of two or more large pre‐atmospheric objects, which supports the hypothesis that the high H/L chondrite ratio at DaG is due to one or more large unrecognized showers. The ¹⁴C concentrations correspond to terrestrial ages <35 kyr, similar to terrestrial ages of chondrites from other regions in the Sahara but younger than two DaG achondrites. Despite the loss of cosmogenic ³⁶Cl and ⁴¹Ca during oxidation of metal and troilite, concentrations of ³⁶Cl and ⁴¹Ca in the silicates are also consistent with ¹⁴C ages <35 kyr. The only exception is DaG 343 (H4), which has a ⁴¹Ca terrestrial age of 150 ± 40 kyr. This old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr. A possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods, such as the current one, which started ?3000 years ago. Finally, based on the ²⁶Al/²¹Ne and ¹⁰Be/²¹Ne systematics in 16 DaG meteorites, we derived more reliable estimates of the ¹⁰Be/²¹Ne production rate ratio, which seems more sensitive to shielding than was predicted by the semi‐empirical model of Graf et al. (1990) but less sensitive than was predicted by the purely physical model of Leya et al. (2000).     

Noble gases in enstatite chondrites II: The trapped component

Abstract— The trapped noble gas record of 57 enstatite chondrites (E chondrites) has been investigated. Basically, two different gas patterns have been identified dependent on the petrologic type. All E chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases (Q) and a subsolar component (range of elemental ratios for E4–6 chondrites: ³⁶Ar/¹³²Xe = 582 ± 270 and ³⁶Ar/⁸⁴Kr = 242 ± 88). E3 chondrites usually contain Q gases, but also a composition with lower ³⁶Ar/¹³²Xe and ³⁶Ar/⁸⁴Kr ratios, which we call sub‐Q (³⁶Ar/¹³²Xe = 37.0 ± 18.0 and ³⁶Ar/⁸⁴Kr = 41.7 ± 18.1). The presence of either the subsolar or the sub‐Q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites. We therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in E chondrites. Trapped solar noble gases have been observed only in some E3 chondrites. About 30% of each group, EH3 and EL3 chondrites, amounting to 9% of all analyzed E chondrites show the solar signature. Notably, only one of those meteorites has been explicitly described as a regolith breccia.     

Noble gases in enstatite chondrites released by stepped crushing and heating

Abstract– Enstatite chondrites (ECs) were subjected to noble gas analyses using stepped crushing and pyrolysis extraction methods. ECs can be classified into subsolar gas‐carrying and subsolar gas‐free ECs based on the ³⁶Ar/⁸⁴Kr/¹³²Xe ratios. For subsolar gas‐free ECs, elemental ratios, and Xe isotopic compositions indicate that Q gas is the dominant trapped component, the Q gas concentration can be correlated with the petrologic type, reasonably explained by gas release from a common EC parental material during subsequent heating. Atmospheric Xe with sub‐Q elemental ratios is found in Antarctic E3s at 600–800 °C and through crushing. The ¹³²Xe released in these fractions accounts for 30–60% of the bulk concentrations. Hence, the sub‐Q signature is generally due to contamination of elementally fractionated atmosphere. Subsolar gas is mainly released (up to 78% of the bulk ³⁶Ar) at 1300–1600 °C and through crushing, suggesting that enstatite and friable phases are the host phases. Subsolar gas is isotopically identical to solar gas, but elementally fractionated. These observations are consistent with a previous study, which suggested that subsolar gas could be fractionated solar wind having been implanted into chondrule precursors ( Okazaki et al. 2001 ). Unlike subsolar gas‐free ECs, the primordial gas concentrations of subsolar gas‐carrying ECs are not simply correlated with the petrologic type. It is inferred that subsolar gas‐rich chondrules were heterogeneously distributed in the solar nebula and accreted to form subsolar gas‐carrying ECs. Subsequent metamorphic and impact‐shock heating events have affected noble gas compositions to various degrees.     

Meteorites from the United Arab Emirates: Description,weathering, and terrestrial ages

We report on the first meteorite search campaign in the United Arab Emirates (UAE). The geology and proximity of our search region suggest that it is the north‐western extension of the Oman meteorite fields. We found 26 ordinary chondrites, bringing the total number of official meteorites from the UAE to 28. The campaign was organized and conducted in close cooperation with the UAE government and the main masses of the meteorites remained in the country where they will become part of an exhibition. The bulk composition of five meteorite and three soil samples indicates an uptake of U, Mo, Sr, Ba, Li, and Pb from the soil into the meteorites during terrestrial weathering. Terrestrial ages determined from ¹⁴C decay of 21 meteorites range from recent falls to 24.4 ka, with two meteorites having >37 ka and approximately 39 ka, respectively. Weak correlations between weathering degree, meteorite bulk chemical composition, and terrestrial age suggest highly localized weathering conditions, possibly related to abundant occurrences of sabkhas in the search region.     

Noble gas studies in CAIs from CV3 chondrites: No evidence for primordial noble gases

Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The ²⁰Ne/²²Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The ²¹Ne/²²Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure ²²Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The ³⁶Ar/³⁸Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic ³⁶Ar produced by neutron capture on ³⁵Cl with subsequent β^?‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.     

Carbon,nitrogen and hydrogen in Saharan chondrites: The importance of weathering

Abstract— The Sahara Desert is a region of high diurnal temperature variation and sporadic rainfall that has recently yielded over 450 meteorites. Eighteen of these Saharan samples are carbonaceous chondrites, of which we have analysed 17 for C content and isotopic composition. Ten of the 18 are paired CR chondrites, of which four have also had N and H contents and compositions determined. A primitive ordinary chondrite (L/LL3.2) found in the region has also been analysed for C, N and H contents and isotopic composition. Saharan samples contain between 21% and 45% of the light elements of their non-Saharan counterparts. Paired Saharan samples show a greater heterogeneity in both C content and isotopic composition than multiple analyses of non-Saharan samples. The cause of the observed isotopic and abundance effects is due to the hot desert weathering processes experienced by these samples. Peak temperatures of meteorites on the desert floor may be in excess of 100 °C, leading to low-temperature hydrous pyrolysis and oxidation reactions, liberating volatile organics and CO₂. This may also cause the remaining material to become partially solubilised and ultimately lost during rainfall. The low δD of the CR and ordinary chondrites can be attributed to the destruction and loss of organic material through dehydrogenation and exchange reactions on the desert surface. The increased ¹³C abundance suggests that the less tightly bound C from the macromolecular organic material is isotopically lighter than the remaining C. Carbon contents and isotopic compositions are also affected by the addition of terrestrial calcitic evaporite deposits, up to 10,000 ppm carbonate has been measured, with a δ¹³C of between 0 and ?10%₀.     

An interdisciplinary study of weathering effects in ordinary chondrites from the Acfer region,Algeria

Abstract— Weathering effects on meteorite finds from the Acfer region were studied by various analytical techniques and in dependence on the depth of sampling. In thin sections of weathered meteorites, weathering effects usually decrease from the outside to the interior of the meteorite. The results of evolved gas analysis indicate that variation in weathering between surface and core is not significant in respect to the formation of Fe-oxyhydroxides. The secondary alteration effects in the noble gases are distributed unevenly throughout the specimens, as seen in the nonsystematic differences observed for the heavy noble gases. Chemical analyses show significant enrichment of Ba and Sr in the outer parts of the weathered samples due to element contamination through aqueous solution. Iron, Ni, and Co are partly flushed from the system as the metal oxidation proceeds. Oxygen isotopes show increases in δ¹⁸O and δ¹⁷O with increasing terrestrial age. For a set of H3 chondrites, the degree of weathering determined from the water content was correlated with terrestrial ages and is discussed with respect to possible weathering mechanisms.     

Comparison of cosmic‐ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary,enstatite, and carbonaceous chondrites

Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio ²⁰Ne/²²Ne of 0.80 ± 0.03 and a trapped solar ⁴He/³He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped ²⁰Ne and ³⁶Ar concentrations in the range of 10^?6–10^?9 cm³ STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced ³He and of radiogenic ⁴He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a ²⁰Ne/²²Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.     

Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages,trapped gases,and a re‐evaluation of the evidence for solar cosmic ray‐produced neon in shergottites and other achondrites

We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine‐phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (²¹Ne/²²Ne)_cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray‐produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray‐produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (²¹Ne/²²Ne)_cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina‐like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.     

Noble gases and nitrogen in Muong Nong tektites

Abstract— Three samples of Muong Nong tektites have been studied for N and noble gases. The isotopic composition of noble gases is airlike. The noble gas amounts are much higher in Muong Nong tektites than in splash-form tektites. As compared to air, He and Ne have been enriched, most likely due to inward diffuion from ambient air, subsequent to glass formation. Nitrogen contents range from 0.3 to 1.34 ppm, with a non-atmospheric δ¹⁵N ranging from 8 to 17%. The release pattern of δ¹⁵N clearly shows the presence of two N components. Higher N/³⁶Ar values than those of air, together with positive δ¹⁵N, show that a major portion of N in Muong Nong tektites is a remnant from the sedimentary source material.     

Light noble gases in 11 achondrites: Cosmic ray exposure ages,gas retention ages,and preatmospheric sizes

We report light noble gas (He, Ne, and Ar) concentrations and isotopic ratios in 11 achondrites, Tafassasset (unclassified primitive achondrite), Northwest Africa (NWA) 12934 (angrite), NWA 12573 (brachinite), Jiddat al Harasis (JaH) 809 (ureilite), NWA 11562 (ungrouped achondrite), four lodranites (NWA 11901, NWA 7474, NWA 6685, and NWA 6484), NWA 2871 (acapulcoite), and Sahara 02029 (winonaite), most of which have not been previously studied for noble gases. We discuss their noble gas isotopic composition, determine their cosmogenic nuclide content, and systematically calculate their cosmic ray exposure (CRE) and gas retention ages. In addition, we estimate their preatmospheric radii and preatmospheric masses based on the shielding parameter (²²Ne/²¹Ne)_cos. None of the studied meteorites shows evidence of contribution from solar cosmic rays (SCRs). JaH 809 and NWA 12934 show evidence of ³He diffusive losses of >90% and 40%, respectively. The winonaite Sahara 02029 has lost most of its noble gases, either during or before analysis. The average CRE age of Tafassasset of ~49 Ma is lower than that reported by Patzer et al. (2003), but is consistent with it within the uncertainties; this confirms that Tafassasset and CR chondrites are not source paired, CR chondrites having CRE ages from 1 to 25 Ma (Herzog & Caffee, 2014). The ureilite JaH 809 has a CRE age of ~5.4 Ma, which falls into the typical range of exposure ages for ureilites; the angrite NWA 12934 has a CRE age of ~49 Ma, which is within the main range of exposure ages reported for angrites (0.2–56 Ma). We calculate a CRE age of ~2.4 Ma for the brachinite NWA 12573, which falls into a possible “cluster” in the brachinite CRE age histogram around ~3 Ma. Three lodranites (NWA 11901, NWA 7474, and NWA 6685) have CRE ages higher than the average CRE ages of lodranites measured so far, NWA 11901 and NWA 6685 having CRE ages far higher than the CRE age already reported by Li et al. (2019) on NWA 8118. The measured ⁴⁰K-⁴⁰Ar gas retention ages fit well into established systematics. The gas retention age of Tafassasset is consistent, within respective uncertainties, with that previously calculated by Patzer et al. (2003). Our study indicates that Tafassasset originates from a meteoroid with a preatmospheric radius of ~20 cm, however discordant with the radius of ~85 cm inferred in a previous study (Patzer et al., 2003).     

Noble gases in mineral separates from three shergottites: Shergotty,Zagami, and EETA79001

Abstract— This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic‐ray exposure ages from ³He, ²¹Ne, and ³⁸Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic ⁴He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic‐ray effects. While a contribution of this low‐energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic‐ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near‐surface siting. We derive an estimate for the ⁴⁰Ar/³⁶Ar ratio of the Martian interior component by subtracting from measured Ar in the (K‐poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from ¹²⁹Xe, * excesses. Unless compromised by the presence of additional components, a high ratio of ～2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of ⁴⁰Ar/³⁶Ar in the Martian mantle.     

Carbon-14 terrestrial ages and weathering of 27 meteorites from the southern high plains and adjacent areas (USA)

Abstract We report on a series of 27 ¹⁴C terrestrial ages of meteorites from four states in the central and southwestern USA. These results were compared to the earlier work of Boeckl (1972). Our results showed that the weathering rate for destruction of meteorites is lower than suggested by Boeckl (1972). We estimated a “half-life” for removal of meteorites of about 10 to 15 ka, similar to that derived for Roosevelt County meteorites. We also studied the weathering of these meteorites compared to terrestrial age. Only a weak correlation was observed, and for these meteorites the degree of weathering can only be taken as a weak indicator of terrestrial residence time. We also measured the δ ¹³C and ¹⁴C and amount of weathering-product carbonates which show some interesting variations with the length of time the meteorites have been exposed to weathering.     

S-type asteroids,ordinary chondrites,and space weathering: The evidence from Galileo's fly-bys of Gaspra and Ida

Abstract— New observations of the S-type asteroids Gaspra and Ida, especially, by the Galileo spacecraft demonstrate that a “space weathering” process operates, which modifies the reflectance spectra of fresh material to be redder, straighter, and have shallower absorption bands. The weathering process appears similar to but less potent than that observed on the Moon. It operates in the sense that it would tend to convert spectra of ordinary chondrites (OC) to having the spectral traits of S-type asteroids. These results appear to resolve the major obstacles of the long standing “S-type conundrum” about the provenance of ordinary chondrite meteorites. A wide body of recent research, which is reviewed here, builds on previous meteoritical evidence to support a new, developing consensus that the larger S-type asteroids are a diverse assortment of silicaceous assemblages, which includes the ordinary chrondite parent bodies. Recent fairly realistic laboratory simulations of space weathering processes have changed OC spectra to resemble S-type spectra. J. F. Bell's highly regarded earlier paradigm that OC parent bodies would be found only among sub-10 km main-belt asteroids has been tested by an extensive observational survey by R. Binzel and his proteges; the result is that no small, main-belt analog for the OCs has been found, not even the somewhat OC-like object Boznemcová. This article reviews the history of the S-type conundrum, which set the stage for Galileo's historic flybys. I review the findings about Gaspra and Ida, including results about their geology, their geophysical structure and probable origins, and about Ida's small moonlet, Dactyl. Density constraints on Ida set by Dactyl's orbit rule out (for Ida, at least) the classic view of S-types as metallic, stripped cores of differentiated precursor asteroids. New analysis of the Galileo spectral images of Ida is presented that provides strong evidence that space weathering occurs on Ida, and that Ida, and the Koronis family asteroids in general, are plausibly OC-like in composition. After reviewing 1990s developments on the S-type conundrum, I advocate a new perspective that the ordinary chondrite parent bodies are among the S-type asteroids, a diverse grouping that also contains other silicaceous and silicate/metal assemblages, presumably including various stony-irons and primitive achondrites represented in meteorite collections; Gaspra may be such a metal-rich assemblage.     

Noble gases and strontium isotopes in the unique meteorite Divnoe

Abstract— We present an isotope study of noble gases in Divnoe, an anomalous meteorite, and also Rb-Sr and K-Ar dating of this meteorite. The relatively young Rb-Sr age obtained (3.39 Ga) seems doubtful and, most probably, results from weathering or contamination. The ancient K-Ar age (4.67^+0.20_–0.40), together with clear excess of ¹²⁹Xe, allows the suggestion of very early formation of the Divnoe meteorite. Concentrations and isotope ratios of noble gases in Divnoe are: 17.9 ≤ ³He ≤ 29.0 × 10^?8; ²⁰Ne = 6.22 × 10^?8; 2.44 ≤ ³⁶Ar ≤ 5.10 × 10^?8; ¹³⁰Xe = 41.3 × 10^?12 cm³/g; 0.079 ≤ ³He/⁴He ≤ 0.193; ²⁰Ne/²²Ne = 0.860; ²¹Ne/²²Ne = 0.927; 3.47 ≤ ⁴⁰Ar/³⁶Ar ≤ 9.47; 2.22 ≤ 36Ar/³⁸Ar ≤ 3.27; ¹²⁹Xe/¹³²Xe = 1.09. The exposure age calculated from cosmogenic ³He, ²¹Ne, and ³⁸Ar is 17.9 ± 0.9 Ma. On the basis of the isotope data for the noble gases and O, and abundances of K, Rb, and Sr, an attempt was made to estimate the relationship of Divnoe to other meteorite types. The O-isotope characteristics of Divnoe are clearly distinct from those of ordinary chondrites, acapulcoites/lodranites, and SNC meteorites (Petaev et al., 1994, Clayton, 1993). In plots of ¹³⁶Xe vs. ¹²⁹Xe/¹³⁰Xe, the Divnoe data fall outside of the data fields for carbonaceous and enstatite chondrites. The light noble gas data, especially the ⁴⁰Ar/³⁸Ar ratio, and the ⁴⁰Ar, ³⁸Ar, ³He, and ⁴He contents of Divnoe differ significantly from those of all meteorite types except diogenites. The K, Rb, and Sr abundances in Divnoe are substantially lower than in most other meteorites. In the concentrations of these elements, as well as in the REE pattern, the Divnoe meteorite is similar only to diogenites. Divnoe probably should be treated as a restite remaining after partial melting of the chondritic mantle of a parent asteroid body.     

Weathering of meteorites from Oman: Correlation of chemical and mineralogical weathering proxies with 14C terrestrial ages and the influence of soil chemistry

Abstract— Fifty‐four fragments of ordinary chondrites from 50 finds representing all searched areas in central Oman and all weathering stages were selected to compare the physical, chemical, and mineralogical effect of terrestrial weathering with ¹⁴C terrestrial ages. ¹⁴C ages range from 2.0 to >49 kyr with a median value of 17.9 kyr. The peak of the age range, which is between 10–20 kyr, falls in an arid climate period. A comparison of the chemical composition of Omani chondrites with literature data for unweathered H and L chondrites demonstrates a strong enrichment in Sr and Ba, and depletion in S during weathering. Water contents in H chondrites increase with terrestrial age, whereas L chondrites show a rapid initial increase followed by nearly constant water content. Correlating Sr, Ba, and H₂O with age indicates two absorption trends: i) an initial alteration within the first 20 kyr dominated by H₂O uptake, mainly reflecting Fe‐Ni metal alteration, and ii) a second Ba‐and Sr‐dominated stage correlated with slower and less systematic weathering of troilite that starts after H₂O reaches ?2 wt%. Sulfur released from troilite partly combines with Ba and Sr to form sulfate minerals. Other parameters correlated with ¹⁴C age are degree of weathering, color of powdered meteorites, and the Ni/Fe ratio. Chemical analyses of 145 soils show a high degree of homogeneity over the entire interior Oman Desert, indicating large‐scale mixing by wind. Soil samples collected from beneath meteorite finds typically are enriched in Ni and Co, confirming mobilization from the meteorites. High Cr and Ni concentrations in reference soil samples, which decrease from NE to SW, are due to detrital material from ultramafic rocks of the Oman Mountains.