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.     

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.     

U-Pb and Pb-Pb ages of zircons from basaltic eucrites: Implications for early basaltic volcanism on the eucrite parent body

We have undertaken petrologic and SHRIMP U-Th-Pb isotopic studies on zircons from basaltic eucrites (Yamato [Y]-75011, Y-792510, Asuka [A]-881388, A-881467 and Padvarninkai) with different thermal and shock histories. Eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites. Some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene, and typically have rounded to subrounded shapes due to recrystallization. Superchondritic Zr/Hf ratios of eucritic zircons indicate that they crystallized from incompatible element-rich melts after crystallization of ilmenite. Concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite Y-75011 are higher than those in metamorphosed eucrites.The U-Pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance. Radiogenic lead-loss up to 48% from zircons is observed in the shock-melted eucrite Padvarninkai. The ²⁰⁷Pb-²⁰⁶Pb ages of zircon in Y-75011 (4550 ± 9 Ma, n = 5) are nearly identical, within analytical uncertainty, to the ages of zircons from the metamorphosed eucrite Y-792510 (4545 ± 15 Ma, n = 13), the highly metamorphosed eucrites A-881388 (4555 ± 54 Ma, n = 5) and A-881467 (4558 ± 13 Ma, n = 8), and the shock-melted eucrite Padvarninkai (4555 ± 13 Ma, n = 18). The averaged ²⁰⁷Pb-²⁰⁶Pb age of zircon from five eucrites analyzed in this study is 4554 ± 7 Ma (95% confidence limits, n = 49), indistinguishable from the averaged U-Pb age (4552 ± 9 Ma) of the same samples. Because of the high closure temperature of lead in zircon (T_closure = ∼1050°C with a cooling rate of 0.2°C/yr), the ²⁰⁷Pb-²⁰⁶Pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas.This fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the Solar System history, 7-20 Ma after CAI formation (4567.2 ± 0.6 Ma), thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body. The U-Pb ages of eucritic zircons are older than the U-Pb, Sm-Nd and Rb-Sr ages of some basaltic eucrites, which is consistent with differences in closure temperatures of each isotopic system, and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene, plagioclase and phosphates.     

Cosmic-ray exposure ages of four acapulcoites and two differentiated achondrites and evidence for a two-layer structure of the acapulcoite/lodranite parent asteroid

We determined the He, Ne, and Ar isotopic abundances in the four acapulcoites Dhofar (DHO) 125, DHO 290, DHO 312, and Graves Nunataks 98028, the metal-rich diogenite Northwest Africa (NWA) 1982, and a unique achondrite, NWA 1058, that resembles the acapulcoites in its chemical composition. The noble gases in these meteorites consist of three components: trapped gases, cosmic-ray produced nuclides, and nuclides produced by K, Th, and U decay. The four acapulcoites yield cosmic-ray exposure (CRE) ages in the range of 5.0-5.7 Ma and confirm earlier conclusions concerning break-up of all acapulcoites from a common S-type parent asteroid, possibly in three events 4.9, 5.9, and 14.8 Ma ago. We also discuss the other characteristics (mineralogy, chemistry, formation ages, and oxygen and trapped noble gas isotopes) of all other acapulcoites and their relatives, the lodranites. We propose that the acapulcoite/lodranite parent asteroid had a shell structure similar to that of the H chondrites: The less metamorphosed acapulcoites correspond to the H3 and H4 chondrites and originate from the exterior layers, whereas the more severely metamorphosed lodranites, similar to the H5 and H6 chondrites, represent the inner regions of their parent body. Ungrouped achondrite NWA 1982, probably a diogenite, shows a CRE age of 18.9 ± 2.0 Ma that falls on the major exposure age cluster of the diogenites. The unique achondrite NWA 1058 differs in cosmic-ray exposure age (38.9 ± 4.0 Ma) and in oxygen-isotopic composition from the acapulcoites and lodranites and is probably a winonaite.     

147sm–143Nd and 87Rb–87Sr ages of the eucrite Piplia Kalan

Prompted by the finding that the eucrite Piplia Kalan could have retained Pu fission Xe earlier than the eucrites so far studied and hence be very ancient, we have measured a precise internal ¹⁴⁷Sm–¹⁴³Nd isochron for this meteorite. The age and initial Nd ratio relative to CHUR are 4.570 ± 0.023 Ga and −1.3 ± 0.7 ϵu, respectively. A Rb–Sr whole rock (clast) isochron for this meteorite corresponds to an age of 3.963 ± 0.119 Ga and initial ⁸⁷Sr/⁸⁶Sr ratio of 0.69902 ± 3. But initial ⁸⁷Sr/⁸⁶Sr ratio calculated for an age of 4.57 Ga is 0.698956 ± 25, which is indistinguishable from 0.698970 ± 15 reported for the angrites LEW and ADOR dated at 4.5578 ± 0.0005 Ga. These results indicating that Piplia Kalan could have formed within only a few million years of the earliest condensates in the Solar System are strongly supported by the recent discovery of live ²⁶Al in it.     

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.     

REE patterns of eucrites and their genetic implications

Rare earth element (REE) abundances in eucrites were precisely determined on Juvinas, Pasamonte, Cachari, Stannem and Antarctic eucrites Y-74450,63,D+G; Y-75015,20,E+F; Y-75011,73; Y-790007,61,F; Y-75011,84,D-1 and Y-790007,61,E-4. The last two samples are eucrite clasts from polymict eucrites. The other Antarctic eucrite samples are matrix from polymict eucrites.Like the Stannern eucrite, all of the Antarctic eucrites have relatively high REE abundances. REE abundances in two clast samples are higher than those in the Stannern and matrix samples of the Antarctic eucrites. When REE abundances in the Antarctic eucrites and the Stannem eucrite are normalized by the REE abundance suite of the Juvinas or Pasamonte eucrites, highly linear REE patterns are obtained. For the Juvinas- or Pasamonte-normalized REE patterns of the Antarctic and Stannern eucrites, the inclination of the linear REE patterns increases with increasing REE abundances. These observations suggest that the eucrites with larger REE abundances such as the Stannern and Antarctic eucrites were produced as residual liquid in a fractional crystallization process from the main group of eucrites (e.g., the Juvinas eucrite). On the other hand, the Moore County cumulate eucrite is considered to be a solid phase formed in this process.     

The Braunschweig meteorite − a recent L6 chondrite fall in Germany

On April 23rd 2013 at 2:07 a.m., a 1.3 kg meteorite fell in the Braunschweig suburb Melverode (52° 13′ 32.19″ N. 10° 31′ 11.60″ E). Its estimated velocity was 250 km/h and it formed an impact pit in the concrete fall site with a diameter of 7 cm and a depth of 3 cm. Radial dust striae are present around the impact pit. As a result of the impact, the meteorite disintegrated into several hundred fragments with masses up to 214 g. The meteorite is a typical L6 chondrite, moderately shocked (S4) – but with a remarkably high porosity (up to 20 vol%). The meteorite was ejected from its parent body as an object with a radius of about 10–15 cm (15–50 kg). The U,Th-He gas retention age of ∼550 Ma overlaps with the main impact event on the L-chondrite parent body ∼470 Ma ago that is recorded by many shocked L chondrites. The preferred cosmic-ray exposure age derived from production of radionuclides and noble gas isotopes is (6.0 ± 1.3) Ma.     

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.     

微量陨石激光熔样稀有气体测定方法

微量陨石激光熔样稀有气体测定方法是一种可以在微米尺度上对几毫克陨石样品进行准确稀有气体同位素分析的方法,克服了传统全岩熔融法在测量时存在样品用量大、前处理过程复杂和样品稀有气体分布不均导致不同组分的宇宙射线暴露历史无法进一步区分等问题。但是由于该方法所用样品体积小和样品用量低,要求实验室具有超低本底的稀有气体提取系统,目前国内在微量陨石稀有气体分析技术方面尚处于起步阶段。本文采用金刚石激光样品窗成功研制了超低本底的气体提取系统,通过系统体积标定和天平称量误差、热本底、干扰元素、质量歧视及质谱灵敏度等参数的校正,在中国科学院地质与地球物理研究所建立了微量陨石激光熔样稀有气体测定方法,并对毫克级微量钙长辉长无球粒陨石Millbillillie粉末标样进行了稀有气体同位素含量和比值测定,计算获得准确一致的宇宙暴露年龄。该方法的建立,将为我国迅速发展的比较行星学和深空探测提供重要技术支撑。     

Quench temperatures of Moore County and other eucrites: residence time on eucrite parent body

We report an electron probe investigation of the eucrite Moore County. Moore County consists predominantly of four pyroxene phases and plagioclase, with minor tridymite, ilmenite, chromite, troilite, iron metal and possibly apatite. Temperatures have been computed for Moore County and other eucrites using three independent techniques for coexisting pairs of pyroxenes. These computed temperatures are all subsolidus and approach those calculated for slowly cooled terrestrial igneous and metamorphic rocks. These temperatures conflict with the conclusions of earlier workers that Moore County was catastrophically removed from its cumulate environment at high temperatures. The eucrites cooled to temperatures substantially below the solidus on their parent body.Our results are consistent with the conclusion that Moore County resided for an extended period of time on the eucrite parent body, perhaps from the time of its crystallization from a basaltic melt (～-4.5AE) until a few tens of millions of years ago. This extended residence time is consistent with the conclusion that the eucrite parent body is still intact, but raises dynamical objections to the tentative identification of the eucrite parent body as asteroid 4 Vesta.An alternative scenario involves ejection from the eucrite parent body, at times significantly older than those indicated by cosmic ray exposure ages, of meteoroids of sufficient size that their interiors were shielded from cosmic rays. These meteoroids were removed by low-probability mechanisms into Earth-crossing orbits where they were disrupted and ultimately sampled by the Earth. This proposal appears to remove the dynamical objections to the tentative identification of the eucrite parent body as Vesta.     

Siderophile and other geochemical constraints on mixing relationships among HED-meteoritic breccias

We have used neutron activation and electron-probe fused-bead techniques to analyze the bulk major and trace-element compositions of 104 named HED meteorites (about 100–102 distinct meteorites, depending upon pairings), including 32 polymict eucrites, 30 howardites and six diogenites. Most were not previously analyzed for siderophile trace elements; many not even for major elements. Our typical sample was 350 mg, and in some cases two separate chips were analyzed as a test of meteorite heterogeneity. Meteorites with extraordinary compositions include Bluewing 001, an unequilibrated eucrite that is rich in Ti, Sm and other incompatible elements; Y-791192, a cumulate-dominated polymict eucrite; and LEW 87002, an oddly Sm-rich howardite dominated by a ferroan variety of diogenite. The eucrite:diogenite mixing ratio is the single most important factor determining the compositions of polymict HEDs, but wide ranges in eucrite incompatible element contents, in diogenite Cr and V contents, and in Sc contents of both eucrites and diogenites, make for diversity among the polymict HEDs.As our new siderophile data help to show, the common practice of describing the entire class of howardites as regolith breccias is erroneous. Most howardites are fragmental breccias showing no sign of origin from true (in the lunar sense, i.e., soil-like) near-surface regolith. Howardites are highly diverse in Ni content, often remarkably Ni-poor, compared to lunar regolith breccias. However, the few (8) howardites with between 300 and 1200 μg/g Ni consistently show some combination of other traits suggestive of regolith origin. Most importantly, all four cases (or five if we include Malvern, which appears to have been altered by annealing) of howardites known to have enrichments in solar-wind noble gases belong to the >300 μg/g Ni group. In many cases, an abundance of glasses, particularly in spheroidal or turbid-brown form, provides additional evidence for regolith origin. We propose that the important subset of howardites that are regolith breccias be formally distinguished by the designation regolithic howardite.Apart from high siderophile levels, the regolithic howardites are compositionally distinctive in having Al₂O₃ consistently near 8–9 wt%; corresponding to a eucrite:diogenite mixing ratio of precisely 2:1. Assuming the HEDs are reasonably representative of the ancient (i.e., pre-vestoid-launch) surface of Vesta, this clustering of regolith composition is difficult to explain unless most of the ancient diogenite component was brought to the surface in a single early episode (i.e., probably a single great impact), after which smaller-scale cratering (with no further major excavations of diogenite until the vestoid-forming event), efficiently homogenized the surface. Such a single-excavation model may also help to explain why diogenites, in marked contrast with eucrites, are seldom polymict; and why Al₂O₃-poor (diogenite-dominated) howardites consistently lack major siderophile enrichments. The low siderophile contents of polymict eucrites are most enigmatic. Possibly in the HED-asteroidal context (low collision velocities, etc.), only materials blended by multiple impacts consistently acquire major enrichments in siderophile elements.     

地幔脱气作用和大气圈惰性气体形成与演化

惰性气体同位素地球化学是研究地幔脱气作用和地球大气形成与演化的有效工具。根据惰性气体提供的信息,地球大气圈中惰性气体主要由地幔脱气形成。地球有三个截然不同的惰性气体储集库：ＭＯＲＢ（洋中脊玄武岩）型地幔（大量脱了气的地幔）,Ｌｏｉｈｉ（夏威夷洛尹黑海山）型地幔（少量脱了气的地幔）,和大气圈＋海洋＋大陆地壳。为了探索这三个气体储集库之间的联系,国外学者已建立了三种地幔脱气模式：①整体脱气（ＢＤ）模式,②溶解度控制脱气（ＳＣＤ）模式,和③稳定态脱气（ＳＳＤ）模式。其中ＳＣＤ模式能较好地解释各储集库中惰性气体同位素体系特征。ＳＣＤ模式认为地幔中各种气体的脱出程度和脱出历史不完全相同,主要受气体在硅酸盐熔体中的溶解度所控制,因而认为地球大气中的各种气体的演化史也不完全相同。从地球形成时算起,大气中惰性气体主要形成和演化的平均时间分别为：１３０Ｘｅ为（２１±７）Ｍａ,３６Ａｒ为（５６±１９）Ｍａ,３Ｈｅ为（３１０±１２０）Ｍａ,４０Ａｒ为１５００Ｍａ,４Ｈｅ为８００Ｍａ。     

Terrestrial Kr-Kr ages of Antarctic meteorites

The production rate of ³⁸Ar in meteorites—P(38)—has been determined, as a function of the sample's chemical composition, from ⁸¹Kr-Kr exposure ages of four eucrite falls. The cosmogenic ⁷⁸Kr/⁸³Kr ratio is used to estimate the shielding dependence of P(38).

From the “true” ³⁸Ar exposure ages and the apparent ⁸¹Kr-Kr exposure ages of nine Antarctic eucrite finds, terrestrial ages are calculated. They range from about 3 × 10⁵ a (Pecora Escarpment 82502) to very recent falls (Thiel Mountains 82502). Polymict eucrites from the Allan Hills (A78132, A79017 and A81009) have within the limits of error the same exposure age (15.2 × 10⁶ a) and the same terrestrial age (1.1 × 10⁵ a). This is taken as strong evidence that these meteorites are fragments of the same fall. A similar case are the Elephant Moraine polymict eucrites A79005, A79006 and 82600 with an exposure age of 26 × 10⁶ a and a terrestrial age of 1.8 × 10⁵ a. EETA79004 may be different from this group because its exposure age and terrestrial age are 21 × 10⁶ a and 2.5 × 10⁵ a, respectively.

The distribution of terrestrial ages of Allan Hills meteorites is discussed. Meteorites from this blue ice field have two sources: Directly deposited falls and meteorites transported to the Allan Hills inside the moving Antarctic ice sheet. During the surface residence time meteorites decompose due to weathering processes. The weathering “half-life” is about 1.6 × 10⁵ a. From the different age distributions of Allan Hills and Yamato meteorites, it is concluded that meteorite concentrations of different Antarctic ice fields need different explanations.     

Chronology and shock history of the Bencubbin meteorite: A nitrogen, noble gas, and Ar-Ar investigation of silicates, metal and fluid inclusions

We have investigated the distribution and isotopic composition of nitrogen and noble gases, and the Ar-Ar chronology of the Bencubbin meteorite. Gases were extracted from different lithologies by both stepwise heating and vacuum crushing. Significant amounts of gases were found to be trapped within vesicles present in silicate clasts. Results indicate a global redistribution of volatile elements during a shock event caused by an impactor that collided with a planetary regolith. A transient atmosphere was created that interacted with partially or totally melted silicates and metal clasts. This atmosphere contained ¹⁵N-rich nitrogen with a pressure ?3 × 10⁵ hPa, noble gases, and probably, although not analyzed here, other volatile species. Nitrogen and noble gases were re-distributed among bubbles, metal, and partly or totally melted silicates, according to their partition coefficients among these different phases. The occurrence of N₂ trapped in vesicles and dissolved in silicates indicates that the oxygen fugacity (f_O2) was greater than the iron-wüstite buffer during the shock event. Ar-Ar dating of Bencubbin glass gives an age of 4.20 ± 0.05 Ga, which probably dates this impact event. The cosmic-ray exposure age is estimated at ∼40 Ma with two different methods. Noble gases present isotopic signatures similar to those of “phase Q” (the major host of noble gases trapped in chondrites) but elemental patterns enriched in light noble gases (He, Ne and Ar) relative to Kr and Xe, normalized to the phase Q composition. Nitrogen isotopic data together with ⁴⁰Ar/³⁶Ar ratios indicate mixing between a ¹⁵N-rich component (δ¹⁵N = +1000‰), terrestrial N, and an isotopically normal, chondritic N.Bencubbin and related ¹⁵N-rich meteorites of the CR clan do not show stable isotope (H and C) anomalies, precluding contribution of a nucleosynthetic component as the source of ¹⁵N enrichments. This leaves two possibilities, trapping of an ancient, highly fractionated atmosphere, or degassing of a primitive, isotopically unequilibrated, nitrogen component. Although the first possibility cannot be excluded, we favor the contribution of primitive material in the light of the recent finding of extremely ¹⁵N-rich anhydrous clasts in the CB/CH Isheyevo meteorite. This unequilibrated material, probably carried by the impactor, could have been insoluble organic matter extremely rich in ¹⁵N and hosting isotopically Q-like noble gases, possibly from the outer solar system.     

Petrology,phase equilibria modelling,noble gas chronology and thermal constraints of the El Pozo L5 meteorite

We present the results of physical properties, petrography, bulk chemistry, mineral compositions, phase relations modelling and Noble gases study of the meteorite El Pozo. The petrography and mineral compositions indicate that the meteorite is an L5 chondrite with a low shock stage of S2-S3. Heterogenous weathering was preferentially along shock structures. Thermobarometric calculations indicate thermal equilibrium conditions between 768?°C and 925?°C at ～4 to 6?kb, which are substantially consistent with the petrological metamorphism type 5. A pseudosection phase diagram is relatively consistent with the mineral assemblage observed and PT conditions calculated. Temperature vs. fO₂ diagram shows that plagioclase compositional stability is very sensitive to Tschermack substitution in orthopyroxene, clinopyroxene and X_An plagioclase during the high temperature metamorphic process. Based on noble gases He, Ne, Ar and K contents a cosmogenic exposure age CRE of 1.9?Myr was calculated. The ²¹Ne would be totally cosmogenic, with no primordial Ne. The ²¹Ne/²²Ne value (0.97) is higher than solar value. According to the cosmogenic Ne content, we argue that El Pozo chondrite originally had a pre-atmospheric mass of 9–10?kg, which would have been produced by a later collision after the recognized collision of the L-chondrite parent body ～470?Ma ago.     

Fission-track ages of four meteorites

A method for selective annealing of cosmic-ray tracks has been developed, permitting determination of fission-track ages in the presence of a large background of cosmic-ray tracks. The mesosiderite Bondoc contains 41 fission tracks/cm², of which about 75% are due to neutron-induced fission of U²³⁵ during cosmic-ray exposure. Its net fission-track age is 140 ± 40 Myr, nearly identical to its cosmic-ray exposure age of 150 Myr. The mesosiderite Mincy has a fission-track age of 1500 ± 400 Myr.Nakhla (nakhlite) contains an excess of apparent fission tracks, which may be either genuine fission tracks from Pu²⁴⁴ or etch pits mimicking fission tracks in length, thermal stability, random orientation, and other characteristics. On the assumption that they are fission tracks, the Pu²⁴⁴/U²³⁸ ratio at the onset of track retention in Nakhla was (3.1 ± 1.3) × 10^?3, nearly an order of magnitude lower than the initial solar system ratio. This may reflect a chemical fractionation of Pu and U, or a late impact or magmatic event. Different minerals of the Washougal howardite have different Pu²⁴⁴/U²³⁸ ratios, from (24 ± 7) × 10^?3 to (2.3 ± 0.7) × 10^?3. This may imply a succession of impacts over a period of time. Additionally, Pu and U may have been chemically fractionated from each other in this meteorite.Shocked meteorites show no consistent pattern in the retentivity of fission tracks and of fissiogenic or radiogenic noble gases. Some meteorites, e.g. Bondoc, Serra de Magé, and Mincy, retain gases more completely than tracks; others, e.g. Nakhla and Allende, retain them less completely.Uranium was determined in feldspar and/or pyroxene from 19 Ca-rich achondrites and mesosiderites. For most, only upper limits of 0.01–0.03 ppb were obtained. Apparently the uranium in these meteorites resides almost exclusively in minor phases, as in terrestrial and lunar rocks.     

Impact melting of the Cachari eucrite 3.0 Gy ago

Both the host phase and glass veins of the Cachari eucrite have been analyzed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their ³⁹Ar-⁴⁰Ar gas retention ages. Cachari is chemically similar to other non-cumulate eucrites. The vesicular glass veins vary from pure glass, to devitrified glass, to areas that are substantially crystalline. The glassy areas have nearly the same concentrations of major and trace elements as the unmelted portions of Cachari, but some differences, probably due to preferential dissolution, occur along melt contacts. The glass formed by shock melting of Cachari host or of rock identical to it. ³⁹Ar-⁴⁰Ar data for the host and glass suggest distinctly different ages of 3.04 ±.07 Gy and 3.47 ±.04 Gy, respectively. The time of glass formation, which may also be the time of brecciation, is most likely given by the 3.0 Gy age of the host. The higher age for the glass is interpreted to represent incomplete Ar degassing during the 3.0 Gy event due to the greater resistance to Ar diffusion shown by the glass compared to the host. Event ages significantly younger than 4.5 Gy have now been determined for several eucrites and howardites and suggest a long dynamic regolith history for the parent body.     

Composition and evolution of the eucrite parent body: evidence from rare earth elements

Eucrites are extraterrestrial plagioclase-pigeonite basalts. Experimental studies suggest that they were produced by partial melting of an olivine (Fo₆₅)-pigeonite (Wo₅En₆₅)-plagioclase (An₉₄)-spinel-metal source region. Quantitative modeling of the evolution of REE abundances in the eucrites indicates that the main group of eucrites (e.g. Juvinas) may be produced by approximately 10% equilibrium partial melting of a source region with initial REE abundances which were chondritic relative and absolute. Other eucrites appear to represent greater (e.g. Sioux County—15%) or smaller (e.g. Stannern—4%) degrees of melting. Moore County and Serra de Magé appear to be cumulates of pyroxene and plagioclase produced by fractional crystallization of a Juvinas-like melt. Nuevo Laredo may represent a residual liquid after such fractional crystallization. Our calculations are consistent with the conclusion that the eucrites were derived from a single type of source region. The close correspondence of the age of the eucrites (? 4.6 AE) to the age of the solar system appears to preclude the possibility of extensive chemical differentiation of the eucrite parent body prior to the event which produced the eucritic melts. Thus our calculations have yielded not only the mode of the source region but, assuming homogeneous accretion, the mode and hence the bulk composition of the eucrite parent body as well. We are unable to estimate quantitatively the ratio of metal to olivine in the parent body. If no metal is present, the bulk composition (in oxide wt%) is Na₂O—0.04, MgO—29.7, Al₂O₃—1.8, SiO₂—39.0, CaO—1.2, FeO—28.3. If, in contrast, the parent body contained 30% metal, the bulk composition of the silicate portion of the eucrite parent body is Na₂O—0.06, MgO—28.0, Al₂O₃—2.6, SiO₂—41.3, CaO—1.9, FeO—26.3. Relative abundances of the meteorites suggest that the eucrite parent body is still intact. The solar system object most closely resembling the eucrites is asteroid 4 Vesta. Because Vesta is unique among the asteroids, we have license to conclude that it is the source of the eucrites and its bulk composition is close to the analyses given above.     

Study of melt and a clast of 546 Ma magmatic arc rocks in the 65 Ma Chicxulub bolide breccia,northern Maya block,Mexico: western limit of Ediacaran arc peripheral to northern Gondwana

The basement of the Maya block of eastern Mexico is generally covered by Mesozoic and Cenozoic platformal carbonate rocks. However, the 65.5 Ma Chicxulub meteorite impact in the northern Yucatan Peninsula excavated deep into the crust and brought crystalline basement fragments into the impact breccias. Common Pb isotopic data from impact melt and a granitic clast from drill core (Y6) are highly radiogenic, consistent with the Archaean derivation. A granodiorite clast in this breccia from drill core (Yaxcopoil-1) yielded a continuous range of concordant ²⁰⁶Pb/²³⁸U laser ablation inductively coupled plasma mass spectrometry zircon ages between 546 ± 5 Ma and 465 Ma, with three discordant zircons having ²⁰⁶Pb/²³⁸U ages between 130 Ma and 345 Ma. The ca. 546 Ma age is interpreted as the age of granodiorite intrusion, with younger ages representing variable Pb loss during melting associated with the meteorite impact. This is consistent with previous U–Pb zircon data that gave an upper intercept age of 550 ± 15 Ma at Chicxulub, which becomes 545 ± 5 Ma when combined with the zircon data from distal ejecta. Such arc rocks are absent in the southern Maya block, and in the neighbouring Oaxaquia terrane (s.s.) they are replaced by a 546 ± 5 Ma plume-related dike swarm. On the other hand, Ediacaran arc rocks continue through the peri-Gondwanan terranes of the Appalachians and Europe (Florida, Carolinia, Avalonia, Iberia, Armorica, Massif Central, Bohemia, and NW Africa). Arc magmatism in these areas ended between 570 Ma (Newfoundland) and 540 Ma (Carolinia/UK) as the subduction zone was replaced by a transform fault along the northern Gondwanan margin. This age range is synchronous with the two-stage birth of Iapetus, suggesting that both are related to major plate reorganization. The source of plume-related dikes may have been located at the rift–rift–transform triple junction between Laurentia, Baltica, and Gondwana.