Cosmic ray exposure ages of Rumuruti chondrites from North Africa

We analyzed noble gases and determined ³He, ²¹Ne, and ³⁸Ar cosmic ray exposure ages (CREAs) of Rumuruti chondrites from North West Africa (NWA) to rule on potential pairings and/or source pairings of North Africa R chondrite samples. The ²¹Ne exposure ages range between 10 and 74 Ma, with NWA 2897 and 1668 having the highest known exposure ages among R chondrites. We also include other R chondrites from North Africa (Schultz et al., 2005) and, based on their noble gas characteristics and their ²¹Ne CREAs, propose pairings of the following samples: NWA 2198, 5069, 755, 4615, 845, 851, 978, 1471, and possibly DaG 013 belonging to one fall with a CREA of ∼10 Ma, and NWA 753, 4360, 4419, 5606, 1472, 1476, 1477, 1478, and 1566 representing one fall with a CREA of ∼14 Ma. NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446 seem to belong to one single fall with a CREA of ∼20 Ma, and NWA 2897 and 1668 seem to be paired and show a common CREA of ∼66 Ma. Overall, all R chondrite samples from North Africa analyzed for noble gases so far represent ∼16 individual falls. Comparing falls from North Africa to literature CREAs of R chondrites worldwide, it seems possible that a significant number of all R chondrite falls studied for noble gases were ejected from the R chondrite parent body during one large collisional event between 15 and 25 Ma ago. However, the database is still too small to draw definitive conclusions. The large portion of brecciated R chondrites in collections suggests severe impact brecciation of the R chondrite parent body.     

Petrogenesis of acapulcoites and lodranites: A shock-melting model

Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.     

Ar-Ar ages of martian nakhlites

We report ³⁹Ar-⁴⁰Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of ³⁹Ar recoil and variable amounts of trapped ⁴⁰Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2σ errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial ⁴⁰Ar on weathered grain surfaces that was degassed along with the first several percent of ³⁹Ar. A total K-⁴⁰Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2σ). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of ∼12 Ma is also similar to CRE ages for other nakhlites.     

Noble gases in the unique Kaidun meteorite

Two examined fragments of the Kaidun meteorite principally differ in the concentrations of isotopes of noble gases and are very heterogeneous in terms of the isotopic composition of the gases. Because these fragments belong to two basically different types of meteoritic material (EL and CR chondrites), these characteristics of noble gases could be caused by differences in the cosmochemical histories of the fragments before their incorporation into the parent asteroid. As follows from the escape kinetics of all gases, atoms of trapped and cosmogenic noble gases are contained mostly in the structures of two carrier minerals in the samples. The concentrations and proportions of the concentrations of various primary noble gases in the examined fragments of Kaidun are obviously unusual compared to data on most currently known EL and CR meteorites. In contrast to EL and CR meteorites, which contain the primary component of mostly solar provenance, the elemental ratios and isotopic composition of Ne and He in the fragments of Kaidun correspond to those typical of the primary components of A and Q planetary gases. This testifies to the unique conditions under which the bulk of the noble gases were trapped from the early protoplanetary nebula. The apparent cosmic-ray age of both of the Kaidun fragments calculated based on cosmogenic isotopes from ³He to ¹²⁶Xe varies from 0.027 to 246 Ma as a result of the escape of much cosmogenic isotopes at relatively low temperatures. The extrapolated cosmic-ray age of the Kaidun meteorite, calculated from the concentrations of cosmogenic isotopes of noble gases, is as old as a few billion years, which suggests that the material of the Kaidun meteorite could be irradiated for billions of years when residing in an unusual parent body.     

Grove Mountains (GRV) 020043: Insights into acapulcoite-lodranite genesis from the most primitive member

Although acapulcoites and lodranites played a key role in understanding partial differentiation of asteroids, the lack of samples of the chondritic precursor limits our understanding of the processes that formed these meteorites. Grove Mountains (GRV) 020043 is a type 4 chondrite, with abundant, well-delineated, pyroxene-rich chondrules with an average diameter of 690 μm, microcrystalline mesostasis, polysynthetically striated low-Ca pyroxene, and slightly heterogeneous plagioclase compositions. Similarities in mineralogy, mineral composition, and oxygen isotopic composition link GRV 020043 to the acapulcoite-lodranite clan. These features include a high low-Ca pyroxene to olivine ratio, high kamacite to taenite ratio, and relatively FeO-poor mafic silicates (Fa_10.3, Fs_10.4) relative to ordinary chondrites, as well as the presence of ubiquitous metal and sulfide inclusions in low-Ca pyroxene and ƒO₂ typical of acapulcoites. GRV 020043 shows that evidence of partial melting is not an essential feature for classification within the acapulcoite-lodranite clan. GRV 020043 experienced modest thermal metamorphism similar to type 4 ordinary chondrites. GRV 020043 suggests a range of peak temperatures on the acapulcoite-lodranite parent body similar to that of ordinary chondrites, but shifted to higher temperatures, perhaps consistent with earlier accretion. The mineralogy and mineral compositions of GRV 020043, despite modest thermal metamorphism, suggests that most features of acapulcoites previously attributed to reduction were, instead, inherited from the precursor chondrite. Although partial melting was widespread on the acapulcoite-lodranite parent body, ubiquitous Fe,Ni-FeS blebs in the cores of silicates were not implanted by shock or trapped during silicate melting, but were inherited from the precursor chondrite with subsequent overgrowths during metamorphism.     

Noble gases,nitrogen and cosmic ray exposure age of the Sulagiri chondrite

The Sulagiri meteorite fell in India on 12 September 2008,LL6 chondrite class is the largest among all the Indian meteorites.Isotopic compositions of noble gases(He,Ne,Ar,Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed.Low cosmogenic(~(22)Ne/~(21)Ne)_c ratio is consistent with irradiation in a large body.Cosmogenic noble gases indicate that Sulagiri has a 4πcosmic-ray exposure(CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of IX chondrites.Radiogenic ~4He and ~(40)Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and4.56 Ca,indicating the loss of He from the meteorite.Xenon and krypton are mixture of Q and spallogenic components.     

Hf-W chronometry of primitive achondrites

Metal segregation and silicate melting on asteroids are the most incisive differentiation events in the early evolution of planetary bodies. The timing of these events can be constrained using the short-lived ¹⁸²Hf-¹⁸²W radionuclide system. Here we present new ¹⁸²Hf-¹⁸²W data for major types of primitive achondrites including acapulcoites, winonaites and one lodranite. These meteorites are of particular interest because they show only limited evidence for partial melting of silicates and are therefore intermediate between chondrites and achondrites.For acapulcoites we derived a ¹⁸²Hf-¹⁸²W age of Δt_CAI = 4.1 ^+1.2/_−1.1 Ma. A model age for winonaite separates calculated from the intercept of the isochron defines an age of Δt_CAI = 4.8 ^+3.1/_−2.6 Ma (assuming a bulk Hf/W ratio of ∼1.2). Both ages most likely define primary magmatic events on the respective parent bodies, such as melting of metal, although metal stayed in place and did not segregate to form a core. A later thermal event is responsible for resetting of the winonaite isochron, yielding an age of Δt_CAI = 14.3 ^+2.7/_−2.2 Ma, significantly younger than the model age. Assuming a co-genetic relationship between winonaites and silicates present in IAB iron meteorites (based on oxygen isotope composition) and including data by Schulz et al. (2009), a common parent body chronology can be established. Magmatic activity occurred between ∼1.5 and 5 Ma after CAIs. More than 5 Ma later, intensive thermal metamorphism has redistributed Hf-W. Average cooling rates calculated for the winonaite/IAB parent asteroid range between ∼35 and ∼4 K/Ma, most likely reflecting different burial depths. Cooling rates obtained for acapulcoites were ∼40 K/Ma to ∼720 K and then ∼3 K/Ma to ∼550 K.Accretion and subsequent magmatism on the acapulcoite parent body occurred slightly later if compared to most achondrite parent bodies (e.g., angrites, ureilites and eucrites), in this case supporting the concept of an inverse correlation between accretion-age of asteroids and intensity of heating in their interiors as expected from heating by ²⁶Al and ⁶⁰Fe decay. However, the early accretion of the parent asteroid of primitive IAB silicates (∼1.0 Ma after CAIs; Schulz et al., 2009) and the possibly impact-induced melting-history of winonaites show that this concept is too simplistic. Parent body size, impact-driven melting as well as heat-insulating regolith cover also need to be considered in the early history of asteroid differentiation.     

The Rumuruti chondrite group

Since 1994, the Rumuruti (R) chondrites have been recognized as a new, well-established chondrite group differing from carbonaceous, ordinary, and enstatite chondrites. The first R chondrite, Carlisle Lakes, was found in Australia in 1977. Meanwhile, the number has increased to 107 (December, 2010). This group is named after the Rumuruti meteorite, the first and so far the only R chondrite fall. Most of the R chondrites are breccias containing a variety of different clasts embedded in a clastic matrix. Some textural and mineralogical characteristics can be summarized as follows: (a) the chondrule abundance in large fragments and in unbrecciated rocks is ∼35–50 vol%; (b) Ca,Al-rich inclusions are rare; (c) the olivine abundance is typically 65–78 vol%; (d) the mean chondrule diameter is ∼400 μm; (e) in unequilibrated R chondrites, low-Ca pyroxene is dominating, whereas in equilibrated R chondrites it is Ca-rich pyroxene; (f) the typical olivine in a metamorphosed lithology is ∼Fa_38–40; (g) matrix olivine in unequilibrated, type 3 fragments and rocks has much higher Fa (∼45–60 mol%) compared to matrix olivines in type 4–6 lithologies (∼Fa_38–41); (h) spinels have a high TiO₂ of ∼5 wt%; (i) abundant different noble metal-bearing phases (metals, sulfides, tellurides, arsenides) occur. The exception is the metamorphosed, type 5/6 R chondrite La Paz Icefield 04840 which contains hornblende, phlogopite, and Ca-poor pyroxene, the latter phase typically occurring in low-grade metamorphosed R chondrites only.In bulk composition, R chondrites have some affinity to ordinary chondrites: (a) the absence of significant depletions in Mn and Na in R chondrites and ordinary chondrites is an important feature to distinguish these groups from carbonaceous chondrites; (b) total Fe (∼24 wt%) of R chondrites is between those of H and L chondrites (27.1 and 21.6 wt%, respectively); (c) the average CI/Mg-normalized lithophile element abundances are ∼0.95 × CI, which is lower than those for carbonaceous chondrites (≥1.0 × CI) and slightly higher than those for ordinary chondrites (∼0.9 × CI); (d) trace element concentrations such as Zn (∼150 ppm) and Se (∼15 ppm) are much higher than in ordinary chondrites; (e) the whole rock Δ¹⁷O of ∼2.7 for R chondrites is the highest among all meteorite groups, and the mean oxygen isotope composition is δ¹⁷O = 5.36 ± 0.43, δ¹⁸O = 5.07 ± 0.86, Δ¹⁷O = +2.72 ± 0.31; (f) noble gas cosmic ray exposure ages of R chondrites range between ∼0.1 and ∼70 Ma. More than half of the R chondrites analyzed for noble gases contain implanted solar wind and, thus, are regolith breccias. The 43 R chondrites from Northern Africa analyzed so far for noble gases seem to represent at least 16 falls. Although the data base is still scarce, the data hint at a major collision event on the R chondrite parent body between 15 and 25 Ma ago.     

Acapulcoite-lodranite meteorites: Ultramafic asteroidal partial melt residues

Acapulcoites (most ancient Hf-W ages are 4,563.1?±?0.8 Ma), lodranites (most ancient Hf-W ages are 4,562.6?±?0.9 Ma) and rocks transitional between them are ancient residues of different degrees of partial melting of a chondritic source lithology (e.g., as indicated by the occurrence of relict chondrules in 9 acapulcoites), although the precise chondrite type is unknown. Acapulcoites are relatively fine- grained (～150–230?μm) rocks with equigranular, achondritic textures and consist of olivine, orthopyroxene, Ca-rich clinopyroxene, plagioclase, metallic Fe,Ni, troilite, chromite and phosphates. Lodranites are coarser grained (540–700?μm), with similar equigranular, recrystallized textures, mineral compositions and contents, although some are significantly depleted in eutectic Fe,Ni-FeS and plagioclase- clinopyroxene partial melts. The acapulcoite-lodranite clan is most readily distinguished from other groups of primitive achondrites (e.g., winoanites/IAB irons) by oxygen isotopic compositions, although more than 50% of meteorites classified as acapulcoites currently lack supporting oxygen isotopic data. The heat source for melting of acapulcoites-lodranites was internal to the parent body, most likely ²⁶Al, although some authors suggest it was shock melting. Acapulcoites experienced lower temperatures of ～980–1170?°C and lower degrees of partial melting (～1–4?vol.%) and lodranites higher temperatures of ～1150–1200?°C and higher degrees (～5?≥?10?vol.%) of partial melting. Hand-specimen and thin section observations indicate movement of Fe,Ni-FeS, basaltic, and phosphate melts in veins over micrometer to centimeter distances. Mineralogical, chemical and isotopic properties, Cosmic Ray Exposure (CRE) ages which cluster around 4–6 Ma and the occurrence of some meteorites consisting of both acapulcoite and lodranite material, indicate that these meteorites come from one parent body and were most likely ejected in one impact event. Whereas the precise parent asteroid of these meteorites is unknown, there is general agreement that it was an S-type object. There is nearly total agreement that the acapulcoite-lodranite parent body was <～100?km in radius and, based on the precise Pb–Pb age for Acapulco of 4555.9?±?0.6 Ma, combined with the Hf/W and U/Pb records and cooling rates deduced from mineralogical and other investigations, that the parent body was fragmented during its cooling which the U/Pb system dates at precisely 4556?±?1 Ma. Hf-W chronometry suggests that the parent body of the acapulcoites-lodranites and, in fact, the parent bodies of all “primitive achondrites” accreted slightly later than those of the differentiated achondrites and, thus, had lower contents of ²⁶Al, the heat producing radionuclide largely responsible for heating of both primitive and differentiated achondrites. Thus, the acapulcoite-lodranite parent body never experienced the high degrees of melting responsible for the formation of the differentiated meteorites, but arrested its melting history at relatively low degrees of ～15?vol.%.     

Pb-Pb dating constraints on the accretion and cooling history of chondrites

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouvé (H6), Bjurböle (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Séverin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. Science297, 1679-1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene-olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouvé show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from ²⁶Al and ⁶⁰Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10-50 km radius accreting within 1-3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid (r > 100 km) or possibly the product of collisions of smaller planetary bodies.     

Neutron capture records of mesosiderites and an iron meteorite

The Sm and Gd isotopic compositions of silicates from six mesosiderites (Dalgaranga, Estherville, Morristown, Northwest Africa (NWA) 1242, NWA 2932, and Vaca Muerta) and one iron meteorite (Udei Station) were determined to elucidate the cosmic-ray exposure records. All seven samples showed significant ¹⁵⁰Sm/¹⁴⁹Sm and ¹⁵⁸Gd/¹⁵⁷Gd isotopic shifts from neutron capture reactions corresponding to neutron fluences of (1.3-21.8) × 10¹⁵ n cm⁻². In particular, Vaca Muerta showed a significantly higher neutron fluences than the other six samples. The parameter for the degree of neutron thermalization (ε_Sm/ε_Gd) also showed a significant difference between Vaca Muerta (0.76) and the other samples (0.93-1.20). These results suggest a two-stage irradiation of the Vaca Muerta silicates in the parent body (>50 Ma) before formation of the mesosiderite and during its transit to Earth (138 Ma). This is consistent with the ⁸¹Kr-Kr cosmic-ray exposure age data of a Vaca Muerta pebble from a previous noble gas isotopic study.     

根据40Ar/39Ar年龄谱探讨陨石冲击事件的分期

⁴⁰Ar/³⁹Ar年龄谱是研究陨石冲击事件的重要资料。根据对55块陨石⁴⁰Ar/³⁹Ar冲击年龄和陨石暴露年龄的分析,发现陨石的冲击年龄与陨石类型之间存在对应关系。据此,将陨石冲击事件划分为九期。其中3900-4000Ma、470-540Ma和小于65Ma是陨石母体的三个重要演化阶段。阶段Ⅰ、Ⅱ和Ⅲ(冲击年龄大于30亿年)主要涉及高钙型无球粒陨石。所有球粒陨石的冲击年龄均小于30亿年。陨石暴露年龄因类型而异,铁陨石最大,石铁陨石次之,石陨石最小。     

Noble gas and oxygen isotope studies of aubrites: A clue to origin and histories

Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mount Egerton, Norton County, Peña Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the ⁸¹Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the ⁸¹Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show “exposure at the surface” on their parent body(ies): (i) neutron capture ³⁶Ar, ⁸⁰Kr, ⁸²Kr and/or ¹²⁸Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Peña Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Peña Blanca Spring and ALHA 78113). The concentrations of ¹²⁸Xe from neutron capture on ¹²⁷I vary among four measured specimens of Cumberland Falls (0.5-76 × 10⁻¹⁴ cm³STP/g), but are correlated with those of radiogenic ¹²⁹Xe, implying that the concentrations of (¹²⁸Xe)_n and (¹²⁹Xe)_rad reflect variable abundances of iodine among specimens. The ratios of (¹²⁸Xe)_n/(¹²⁹Xe)_rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015) and Shallowater (0.001), meaning that neutron fluences, radiogenic ¹²⁹Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 × 10⁻⁷, 9.4 × 10⁻¹⁰ and 2.8 × 10⁻¹⁰ cm³STP/g, respectively) as reported previously (Busemann and Eugster, 2002). Isotopic compositions of Kr and Xe in Shallowater are consistent with those of Q (a primordial noble gas component trapped in chondrites). The Ar/Kr/Xe compositions are somewhat fractionated from Q, favoring lighter elements. Because of the unbrecciated nature of Shallowater, Q-like noble gases are considered to be primordial in origin. Fission Xe is found in Cumberland Falls, Mayo Belwa, Peña Blanca Spring, ALHA 78113 and LAP 02233. The majority of fission Xe is most likely ²⁴⁴Pu-derived, and about 10-20% seems to be ²³⁸U-derived at ¹³⁶Xe. The observed (¹³⁶Xe)_Pu corresponds to 0.019-0.16 ppb of ²⁴⁴Pu, from which the ²⁴⁴Pu/U ratios are calculated as 0.002-0.009. These ratios resemble those of chondrites and other achondrites like eucrites, suggesting that no thermal resetting of the Pu-Xe system occurred after ∼4.5 Ga ago. We also determined oxygen isotopic compositions for four aubrites with chondritic noble gases and a new aubrite LAP 02233. In spite of their chondritic noble gas signatures, oxygen with chondritic isotopic compositions was found only in a specimen of Cumberland Falls (Δ¹⁷O of ∼0.3‰). The other four aubrites and the other two measured specimens of Cumberland Falls are concurrent with the typical range for aubrites.     

K–Ar ages of meteorites: Clues to parent-body thermal histories

Whereas most radiometric chronometers give formation ages of individual meteorites >4.5 Ga ago, the K–Ar chronometer rarely gives times of meteorite formation. Instead, K–Ar ages obtained by the ³⁹Ar–⁴⁰Ar technique span the entire age of the solar system and typically measure the diverse thermal histories of meteorites or their parent objects, as produced by internal parent body metamorphism or impact heating. This paper briefly explains the Ar–Ar dating technique. It then reviews Ar–Ar ages of several different types of meteorites, representing at least 16 different parent bodies, and discusses the likely thermal histories these ages represent. Ar–Ar ages of ordinary (H, L, and LL) chondrites, R chondrites, and enstatite meteorites yield cooling times following internal parent body metamorphism extending over ∼200 Ma after parent body formation, consistent with parent bodies of ∼100 km diameter. For a suite of H-chondrites, Ar–Ar and U–Pb ages anti-correlate with the degree of metamorphism, consistent with increasing metamorphic temperatures and longer cooling times at greater depths within the parent body. In contrast, acapulcoites–lodranites, although metamorphosed to higher temperatures than chondrites, give Ar–Ar ages which cluster tightly at ∼4.51 Ga. Ar–Ar ages of silicate from IAB iron meteorites give a continual distribution across ∼4.53–4.32 Ga, whereas silicate from IIE iron meteorites give Ar–Ar ages of either ∼4.5 Ga or ∼3.7 Ga. Both of these parent bodies suffered early, intense collisional heating and mixing. Comparison of Ar–Ar and I–Xe ages for silicate from three other iron meteorites also suggests very early collisional heating and mixing. Most mesosiderites show Ar–Ar ages of ∼3.9 Ga, and their significantly sloped age spectra and Ar diffusion properties, as well as Ni diffusion profiles in metal, indicate very deep burial after collisional mixing and cooling at a very slow rate of ∼0.2 °C/Ma. Ar–Ar ages of a large number of brecciated eucrites range over ∼3.4–4.1 Ga, similar to ages of many lunar highland rocks. These ages on both bodies were reset by large impact heating events, possibly initiated by movements of the giant planets. Many impact-heated chondrites show impact-reset Ar–Ar ages of either >3.5 Ga or <1.0 Ga, and generally only chondrites show these younger ages. The younger ages may represent orbital evolution times in the asteroid belt prior to ejection into Earth-crossing orbits. Among martian meteorites, Ar–Ar ages of nakhlites are similar to ages obtained from other radiometric chronometers, but apparent Ar–Ar ages of younger shergottites are almost always older than igneous crystallization ages, because of the presence of excess (parentless) ⁴⁰Ar. This excess ⁴⁰Ar derives from shock-implanted martian atmosphere or from radiogenic ⁴⁰Ar inherited from the melt. Differences between meteorite ages obtained from other chronometers (e.g., I–Xe and U–Pb) and the oldest measured Ar–Ar ages are consistent with previous suggestions that the ⁴⁰K decay parameters in common use are incorrect and that the K–Ar age of a 4500 Ma meteorite should be possibly increased, but by no more than ∼20 Ma.     

Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy

²¹Ne cosmic-ray exposure ages have been calculated from literature data for 201 H, 203 L and 38 LL chondrites, corrected for shielding differences when possible. The distributions of exposure ages again show the familiar peaks at 4.5 and 20 Myr for the H's, but no outstanding events for the L's and LL's. If the L-chondrite distribution is interpreted as a series of discrete events, then at least 6 peaks between 1 and 35 Myr are needed to model it. The observations, that every petrologic type occurs in every large peak and that even the higher petrologic types contain solar wind gases, suggest that the parent bodies have been fragmented and reassembled into a megabreccia. For the H chondrites, both large and small peaks contain about 15% solar-gas bearing meteorites, which could mean that surface material has been mixed to depths represented by the largest event, on the order of a kilometer. In contrast, only 3% of the L's contain solar wind, which may be related to breakup of their parent planet. Those L's with especially low radiogenic He (U,Th-He ages < 1 AE) tend to have low exposure ages: their distribution may be biased by a subgroup that had orbits coupling short capture lifetimes with significant solar heating.     

The Yamato-type (CY) carbonaceous chondrite group: Analogues for the surface of asteroid Ryugu?

We report new mineralogical, petrographic and noble gas analyses of the carbonaceous chondrite meteorites Y-82162 (C1/2_ung), Y-980115 (CI1), Y-86029 (CI1), Y-86720 (C2_ung), Y-86789 (C2_ung), and B-7904 (C2_ung). Combining our results with literature data we show that these meteorites experienced varying degrees of aqueous alteration followed by short-lived thermal metamorphism at temperatures of >500 °C. These meteorites have similar mineralogy, textures and chemical characteristics suggesting that they are genetically related, and we strongly support the conclusion of Ikeda (1992) that they form a distinct group, the CYs (“Yamato-type”). The CY chondrites have the heaviest oxygen isotopic compositions (δ¹⁷O ˜12‰, δ¹⁸O ˜22‰) of any meteorite group, high abundances of Fe-sulphides (˜10 ‒ 30 vol%) and phosphates, and contain large grains of periclase and unusual objects of secondary minerals not reported in other carbonaceous chondrites. These features cannot be attributed to parent body processes alone, and indicate that the CYs had a different starting mineralogy and/or alteration history to other chondrite groups, perhaps because they formed in a different region of the protoplanetary disk. The short cosmic-ray exposure ages (≤1.3 Ma) of the CY chondrites suggest that they are derived from a near-Earth source, with recent observations by the Hayabusa2 spacecraft highlighting a possible link to the rubble-pile asteroid Ryugu.     

Heavily fractionated noble gases in an acid residue from the Klein Glacier 98300 EH3 chondrite

Noble gases were measured both in bulk samples (stepped pyrolysis and total extraction) and in a HF/HCl residue (stepped pyrolysis and combustion) from the Klein Glacier (KLE) 98300 EH3 chondrite. Like the bulk meteorite and as seen in previous studies of bulk type 3 E chondrites (“sub-Q”), the acid residue contains elementally fractionated primordial noble gases. As we show here, isotopically these are like those in phase-Q of primitive meteorites, but elementally they are heavily fractionated relative to these. The observed noble gases are different from “normal” Q noble gases also with respect to release patterns, which are similar to those of Ar-rich noble gases in anhydrous carbonaceous chondrites and unequilibrated ordinary chondrites (with also similar isotopic compositions). While we cannot completely rule out a role for parent body processes such as thermal and shock metamorphism (including a later thermal event) in creating the fractionated elemental compositions, parent body processes in general seem not be able to account for the distinct release patterns from those of normal Q noble gases. The fractionated gases may have originated from ion implantation from a nebular plasma as has been suggested for other types of primordial noble gases, including Q, Ar-rich, and ureilite noble gases. With solar starting composition, the corresponding effective electron temperature is about 5000 K. This is lower than inferred for other primordial noble gases (10,000-6000 K). Thus, if ion implantation from a solar composition reservoir was a common process for the acquisition of primordial gas, electron temperatures in the early solar system must have varied spatially or temporally between 10,000 and 5000 K.Neon and xenon isotopic ratios of the residue suggest the presence of presolar silicon carbide and diamond in abundances lower than in the Qingzhen EH3 and Indarch EH4 chondrites. Parent body processes including thermal and shock metamorphism and a late thermal event also cannot be responsible for the low abundances of presolar grains. KLE 98300 may have started out with smaller amounts of presolar grains than Qingzhen and Indarch.     

Origin of fossil meteorites from Ordovician limestone in Sweden

Based on the analysis of data in [1, 2] on the concentrations of noble gases and the cosmic ray exposure age (CREA) of chromite grains in fossil meteorites, it was demonstrated in [3] that the distributions of gas concentrations and cosmic ray exposure ages can be explained under the assumption of the fall of a single meteorite in the form of a meteorite shower in southern Sweden less than 0.2 Ma after the catastrophic destruction of the parental body (asteroid) of L chondrites in space at approximately 470 Ma. This assumption differs from the conclusion in [1, 2, 4] about the long-lasting (for 1–2 Ma) delivery of L chondrites to the Earth, with the intensity of the flux of this material one to two orders of magnitude greater than now. The analysis of newly obtained data on samples from the Brunflo fossil meteorite [5] corroborates the hypothesis of a meteorite shower produced by the fall of a single meteorite. The possible reason for the detected correlations between the cosmic ray exposure ages of meteorites and the masses of the samples with the ²⁰Ne concentrations can be the occurrence of Ne of anomalous isotopic composition in the meteorites.     

Cosmic ray exposure age and time of formation of a new meteorite,Dhofar 007 achondrite from Oman: A rock fragment from the asteroid Vesta

The isotopic composition of noble gases was investigated in the Dhofar 007 meteorite. Petrographic and mineralogical observations suggested that it is a brecciated cumulate eucrite with high contents of siderophile elements. The concentrations of noble gases in Dhofar 007 are identical to those of other eucrites. Its cosmic ray exposure age was estimated as 11.8 ± 0.8 Ma, which coincides with a maximum on the histogram of comic ray exposure ages of eucrite meteorites. It can be supposed that, similar to other eucrites, Dhofar 007 was ejected from the surface of their parent body (presumably, asteroid Vesta) about 12.0 Ma ago. The crystallization age of the Dhofar 007 eucrite was estimated from the ratio of plutonogenic Xe to Nd as 4476 ± 22 Ma. The potassium-argon age is much younger, 3.7–4.1 Ga, which indicates partial loss of radiogenic argon during the history of the meteorite, most likely related to impact metamorphic events.