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.     

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 gas record,collisional history,and pairing of CV,CO, CK,and other carbonaceous chondrites

Abstract— Concentration and isotopic composition of the light noble gases as well as of ⁸⁴Kr, ¹²⁹Xe, and ¹³²Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary‐type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high ¹²⁹Xe/¹³²Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects. Cosmic‐ray exposure ages are calculated from cosmogenic ²¹Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic‐ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ～9 and ～29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls. In general, we confirm the results of Mazor et al. (1970) regarding cosmic‐ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic‐ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar‐gas‐bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.     

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.     

Cosmogenic and trapped noble gases in individual chondrules: Clues to chondrule formation

Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped ²⁰Ne and ³⁶Ar, and the ratio (³⁶Ar/²⁰Ne)_t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (³⁶Ar/²⁰Ne)_t and trapped ³⁶Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (³⁶Ar/²⁰Ne)_t ratio, Ne‐isotopic composition, and excess of ⁴He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (²¹Ne/²²Ne)_c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.     

81Kr‐Kr age and multiple cosmic‐ray exposure history of the Vaca Muerta mesosiderite

Abstract– Noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate–metal mixture from the Vaca Muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods. Trapped noble gases were degassed completely by a high‐temperature thermal event, probably at the formation of the Vaca Muerta parent body (VMPB). The presence of fissiogenic Xe isotopes from extinct ²⁴⁴Pu in the bulk samples might be a result of rapid cooling from an early high‐temperature metamorphism. High concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic Kr and Xe. Spallogenic Ne from Na and unique Ar isotopic compositions were observed. The ⁸¹Kr‐Kr exposure age of 168 ± 8 Myr for the silicate pebble is distinctly longer than the age of 139 ± 8 Myr for the bulk samples. The precursor of the pebble had been irradiated on the surface of the VMPB for more than 60 Myr (first stage irradiation), with subsequent incorporation into bulk materials approximately 4 Gyr ago. The Vaca Muerta meteorite was excavated from the VMPB 140 Myr ago (second stage irradiation). Relative diffusion rates among the cosmogenic Ar, Kr, and Xe based on data obtained by stepwise heating indicate that Kr and Xe can be partially retained in pyroxene and plagioclase under the condition that resets the K‐Ar system. This result supports the presence of fission Xe and of excess concentration of cosmogenic Kr, which could have survived the thermal event approximately 3.8 Gyr ago.     

Noble gas study of the Saratov L4 chondrite

Abstract– We have determined the elemental abundances and the isotopic compositions of noble gases in a bulk sample and an HF/HCl residue of the Saratov (L4) chondrite using stepwise heating. The Ar, Kr, and Xe concentrations in the HF/HCl residue are two orders of magnitude higher than those in the bulk sample, while He and Ne concentrations from both are comparable. The residue contains only a portion of the trapped heavy noble gases in Saratov; 40 ± 9% for ³⁶Ar, 58 ± 12% for ⁸⁴Kr, and 48 ± 10% for ¹³²Xe, respectively. The heavy noble gas elemental pattern in the dissolved fraction is similar to that in the residue but has high release temperatures. Xenon isotopic ratios of the HF/HCl residue indicate that there is no Xe‐HL in Saratov, but Ne isotopic ratios in the HF/HCl residue lie on a straight line connecting the cosmogenic component and a composition between Ne‐Q and Ne‐HL. This implies that the Ne isotopic composition of Q has been changed by incorporating Ne‐HL (Huss et al. 1996) or by being mass fractionated during the thermal metamorphism. However, it is most likely that the Ne‐Q in Saratov is intrinsically different from this component in other meteorites. The evidence of this is a lack of correlation between the isotopic ratio of Ne‐Q and petrologic types of meteorites (Busemann et al. 2000). A neutron capture effect was observed in the Kr isotopes, and this process also affected the ¹²⁸Xe/¹³²Xe ratio. The ³He and ²¹Ne exposure ages for the bulk sample are 33 and 35 Ma, respectively.     

New noble gas data of primitive and differentiated achondrites including Northwest Africa 011 and Tafassasset

Abstract— This work reports on the noble gas inventory of 3 new acapulcoites, 3 brachinites, 2 new eucrites from the Dar al Gani region in Libya, the unique achondrite Dar al Gani (DaG) 896 from the same locality, the new eucrite‐like achondrite Northwest Africa (NWA) 011, and the controversial sample Tafassasset. We determined cosmic ray exposure and gas retention ages, evaluated shielding conditions, and discuss the trapped noble gas component of the specimens. All exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters. Shielding conditions vary, suggesting substantial shielding for all 3 brachinites and Tafassasset. We cannot exclude, however, that the Mg‐rich composition of brachinites simply simulates heavy shielding. Regarding the trapped component, we found Q‐like compositions only for the acapulcoite Thiel Mountains (TIL) 99002. The brachinite Elephant Moraine (EET) 99402 yields a high, subsolar ³⁶Ar/¹³²Xe ratio of ?400 along with a slightly elevated ⁸⁴Kr/¹³²atio, indicating minor atmospheric contamination. All the other samples, particularly the eucrite DaG 983, are characterized by clearly elevated Ar/Kr/Xe ratios due to significant terrestrial alteration. Tafassasset exhibits noble gas parameters that are different from those of CR chondrites, including a relatively high cosmic ray exposure age, the absence of a solar component, low ¹³²Xe concentrations, a low trapped ³⁶Ar/¹³²Xe ratio of ?30, and a noticeable amount of radiogenic ¹²⁹Xe. Similar attributes have been observed for some primitive achondrites. These attributes are also consistent with the metamorphic character of the sample. We, therefore, consider Tafassasset's noble gas record to be inconclusive as to its classification (primitive achondrite versus metamorphosed CR chondrite).     

Noble gases in ten Nullarbor chondrites: Exposure ages,terrestrial ages,and weathering effects

Abstract— We present concentration and isotopic composition of He, Ne, and Ar in ten chondrites from the Nullarbor region in Western Australia as well as the concentrations of ⁸⁴Ke, ¹²⁹Xe, and ¹³²Xe. From the measured cosmogenic ¹⁴C concentrations (Jull et al. 1995), shielding‐corrected production rates of ¹⁴C are deduced using cosmogenic ²²Ne/²¹Ne ratios. For shielding conditions characterized by ²²Ne/²¹Ne >1.10, this correction becomes significant and results in shorter terrestrial ages. The exposure ages of the ten Nullarbor chondrites are in the range of values usually observed in ordinary chondrites. Some of the meteorites have lost radiogenic gases as well as cosmogenic ³He. Most of the analyzed specimens show additional trapped Ar, Kr, and Xe of terrestrial origin. The incorporation of these gases into weathering products is common in chondrites from hot deserts.     

Effects of experimental aqueous alteration on the abundances of argon‐rich noble gases in the Ningqiang carbonaceous chondrite

Abstract— Ar‐rich noble gases, the so‐called “subsolar” noble gases, are a major component of heavy primordial noble gases in unequilibrated ordinary chondrites and some classes of anhydrous carbonaceous chondrites, whereas they are almost absent in hydrous carbonaceous chondrites that suffered extensive aqueous alteration. To understand the effects of aqueous alteration on the abundance of Ar‐rich noble gases, we performed an aqueous alteration experiments on the Ningqiang type 3 carbonaceous chondrite that consists entirely of anhydrous minerals and contains Ar‐rich noble gases. Powdered samples and deionized neutral water were kept at 200 °C for 10 and 20 days, respectively. Mineralogical analyses show that, during the 10‐day alteration, serpentine and hematite formed at the expense of olivine, low‐Ca pyroxene, and sulfide. Noble gas analyses show that the 10‐day alteration of natural Ningqiang removed 79% of the primordial ³⁶Ar, 68% of the ⁸⁴Kr, and 60% of the ¹³²Xe, but only 45% of the ⁴He and 53% of the primordial ²⁰Ne. Calculated elemental ratios of the noble gases removed during the 10‐day alteration are in the range of those of Ar‐rich noble gases. These results indicate that Ar‐rich noble gases are located in materials that are very susceptible to aqueous alteration. In contrast, heavy primordial noble gases remaining in the altered samples are close to Q gas in elemental and isotope compositions. This indicates that phase Q is much more resistant to aqueous alteration than the host phases of Ar‐rich noble gases. In the 20‐day sample, the mineralogical and noble gas signatures are basically similar to those of the 10‐day sample, indicating that the loss of Ar‐rich noble gases was completed within the 10‐day alteration. Our results suggest that almost all of the Ar‐rich noble gases were lost from primitive asteroids during early, low‐temperature aqueous alteration.     

Noble gases in presolar diamonds I: Three distinct components and their implications for diamond origins

Abstract— High-purity separates of presolar diamond were prepared from 14 primitive chondrites from 7 compositional groups. Their noble gases were measured using stepped pyrolysis. Three distinct noble gas components are present in diamonds, HL, P3, and P6, each of which is found to consist of five noble gases. P3 , released between 200 °C and 900 °C, has a “planetary” elemental abundance pattern and roughly “normal” isotopic ratios. HL , consisting of isotopically anomalous Xe-HL and Kr-H, Ar with high ³⁸Ar/³⁶Ar, and most of the gas making up Ne-A2 and He-A, is released between 1100 °C and 1600 °C. HL has “planetary” elemental ratios, except that it has much more He and Ne than other known “planetary” components. HL gases are carried in the bulk diamonds, not in some trace phase. P6 has a slightly higher median release temperature than HL and is not cleanly separated from HL by stepped pyrolysis. Our data suggest that P6 has roughly “normal” isotopic compositions and “planetary” elemental ratios. Both P3 and P6 seem to be isotopically distinct from P1, the dominant “planetary” noble-gas component in primitive chondrites. Release characteristics suggest that HL and P6 are sited in different carriers within the diamond fractions, while P3 may be sited near the surfaces of the diamonds. We find no evidence of separability of Xe-H and Xe-L or other isotopic variations in the HL component. However, because ～10¹⁰ diamonds are required to measure a Xe composition, a lack of isotopic variability does not constrain diamonds to come from a single source. In fact, the high abundance of diamonds in primitive chondrites and the presence of at least three distinct noble-gas components strongly suggest that diamonds originated in many sources. Relative abundances of noble-gas components in diamonds correlate with degree of thermal processing (see companion paper), indicating that all meteorites sampled essentially the same mixture of diamonds. That mixture was probably inherited from the Sun's parent molecular cloud.     

The Campos Sales meteorite from Brazil: A lightly shocked L5 chondrite fall

Abstract— The Campos Sales meteorite fell close to the town of Campos Sales in the northeastern Brazilian state of Ceará (7°2′ S, 40°10′ W) on 1991 January 31 at 10:00 P.M. (local time). Several fragments were recovered from an area estimated to be 1 × 3 km. The stone is an ordinary L5 chondrite (Fa_25.0 and FS_21.6) and is lightly shocked (S1). Metal phases present are kamacite, tetrataenite, and antitaenite. Noble gases He, Ne, Ar, Kr, and Xe have been analyzed in two bulk samples of Campos Sales. All exposure ages based on determination of cosmogenic ³He, ²¹Ne, ³⁸Ar, ⁸³Kr, and ¹²⁶Xe abundances and on the cosmogenic ⁸¹Kr/⁸³Kr ratio agree well, which suggests no gas loss during cosmic-ray exposure. The cosmic-ray exposure age is 23.3 ± 1.0 Ma, which falls in the range observed for L5 chondrites (20–30 Ma). The gas-retention ages indicate He loss that must have occurred prior to or during ejection from the L-chondrite parent body.     

Primordial noble gases in a graphite‐metal inclusion from the Canyon Diablo IAB iron meteorite and their implications

Abstract— We have carried out noble gas measurements on graphite from a large graphite‐metal inclusion in Canyon Diablo. The Ne data of the low‐temperature fractions lie on the mixing line between air and the spallogenic component, but those of high temperatures seem to lie on the mixing line between Ne‐HL and the spallogenic component. The Ar isotope data indicate the presence of Q in addition to air, spallogenic component and Ar‐HL. As the elemental concentration of Ne in Q is low, we could not detect the Ne‐Q from the Ne data. On the other hand, we could not observe Xe‐HL in our Xe data. As the Xe concentration and the Xe/Ne ratio in Q is much higher than that in the HL component, it is likely that only the contribution of Q is observed in the Xe data. Xenon isotopic data can be explained as a mixture of Q, air, and “El Taco Xe.” The Canyon Diablo graphite contains both HL and Q, very much like carbonaceous chondrites, retaining the signatures of various primordial noble gas components. This indicates that the graphite was formed in a primitive nebular environment and was not heated to high, igneous temperatures. Furthermore, a large excess of ¹²⁹Xe was observed, which indicates that the graphite was formed at a very early stage of the solar system when ¹²⁹I was still present. The HL/Q ratios in the graphite in Canyon Diablo are lower than those in carbonaceous chondrites, indicating that some thermal metamorphism occurred on the former. We estimated the temperature of the thermal metamorphism to about 500–600 °C from the difference of thermal retentivities of HL and Q. It is also noted that “El Taco Xe” is commonly observed in many IAB iron meteorites, but its presence in carbonaceous chondrites has not yet been established.     

A New Martian Meteorite: the Dhofar 019 Shergottite with an Exposure Age of 20 Million Years

The isotopic composition of the noble gases of the new Martian meteorite, the Dhofar 019 shergottite, found in the desert in the territory of the Sultanate of Oman on January 24, 2001, was investigated. Stepwise thermal annealing with isotopic analysis of each of the noble-gas temperature fractions was employed to determine the component composition. The concentration of the trapped noble gases in the new Martian meteorite Dhofar 019 is relatively high, although it lies within the range of concentrations in known SNC meteorites. A characteristic feature of all the trapped noble gases is the presence of two main components: a low-temperature, probably terrestrial atmospheric, component, trapped during the weathering of the meteorite on Earth, and a high-temperature trapped Martian component. Owing to the different ratios of the quantities of the two components, the trapped neon, argon, krypton, and xenon differ markedly in the kinetics of their release. The isotopic composition of the noble gases varies accordingly. The trapped xenon was found to contain two Martian components. One of them, with typical ratios of ¹²⁹Xe/¹³²Xe and ¹³²Xe/⁸⁴Kr, is representative of xenon and krypton of the Martian atmosphere; the other, of gases of the Martian mantle. Variations of the isotopic compositions of helium, neon, and argon (and also, to a lesser extent, of krypton and xenon) during the thermal annealing of the Dhofar 019 meteorite clearly point to a large proportion of cosmogenic as well as trapped components. The concentration of cosmogenic neon and argon in the meteorite is unusually high. This corresponds to a maximum exposure age among other SNC meteorites: 20 million years. Estimates of the potassium–argon age (gas-retention age) yielded the figure of 560 million years, which is within the range of values obtained for SNC meteorites by other authors, who used the rubidium–strontium and the potassium–argon technique.     

Noble gas record and cosmic‐ray exposure history of the new CO3 chondrite Moss––Comparison with Lancé and other CO chondrite falls

Abstract– The Moss meteorite is the first CO chondrite fall after a time period of 70 yr and the least terrestrially contaminated member of its group. Its cosmic‐ray exposure (CRE) age (T₃ ～ 13.5 Ma; T₂₁ ～ 14.6 Ma) is distinct among CO chondrites and, within witnessed falls is the shortest after Lancé, which we have reanalyzed. Gas retention ages are approximately 3.95 × 10⁹ yr (U/Th‐He) and approximately 4.43 × 10⁹ yr (K/Ar), respectively. Trapped Ar, Kr, and Xe are present in Moss in abundances typical for CO chondrites, with “planetary” elemental and isotopic compositions. Presence of HL‐xenon from presolar diamonds is observed in the stepwise release analysis of Lancé. It may also be present in Moss, but it is difficult to ascertain in single‐step bulk analyses. It follows from our new data combined with a survey of the literature that the abundance of trapped gases in CO chondrites is not a good indicator of their petrological subtype.     

Primitive xenon in diogenites and plutonium-244-fission xenon ages of a diogenite,a howardite,and eucrites

Abstract The ²⁴⁴Pu-fission-¹³⁶Xe retention ages of howardites, eucrites, and diogenites (HEDs) show that these meteorites have retained Xe since they were formed about 4500 Ma ago. For the Garland diogenite and the Millbillillie eucrite, we obtain fission Xe ages of 4525 ± 40 Ma and 4486 ± 40 Ma, respectively. If Xe isotope data reported by other workers are also considered, we conclude that the monomict equilibrated eucrites Camel Donga, Juvinas, and Millbillillie formed about 40 Ma later than Pasamonte, a polymict unequilibrated eucrite. Stannern, a monomict equilibrated brecciated eucrite, yields a ²⁴⁴Pu-¹³⁶Xe age of 4442 Ma. The ⁴⁰K-⁴⁰Ar retention ages fall, for most HEDs, into the 1000–4000 Ma age range, indicating that ⁴⁰Ar is generally not well retained. The good retentivity for Xe of HEDs allows us to study primordial trapped Xe in these meteorites. Except for Shalka, in which other authors found Kr and Xe from terrestrial atmospheric contamination only, we present for the first time Kr and Xe isotopic data for diogenites. We studied Ellemeet, Garland, Ibbenbühren, Shalka, and Tatahouine. We show that Tatahouine contains two types of trapped Xe: a terrestrial contamination acquired by an irreversible adsorption process and released at pyrolysis temperatures up to 800 °C, and indigenous primordial Xe released primarily between 800 °C and 1200 °C. The isotopic composition of this primordial Xe is identical to that proposed earlier to be present in primitive achondrites and termed U-Xe or “primitive” Xe, but it has not been directly observed in achondrites until now. This type of primitive Xe is important for understanding the evolution of other Xe reservoirs in the Solar System. Terrestrial atmospheric Xe (corrected for fission Xe and radiogenic Xe from outgassing of the Earth) is related to it by a mass dependent fractionation favoring the heavier Xe isotopes. This primitive Xe is isotopically very similar to solar Xe except for ¹³⁴Xe and ¹³⁶Xe. Solar Xe appears to contain an enrichment of unknown origin for these isotopes relative to the primitive Xe.     

Fractionated martian atmosphere in the nakhlites?

Abstract— Considerable evidence points to a martian origin of the SNC meteorites. Noble gas isotopic compositions have been measured in most SNC meteorites. The ¹²⁹Xe/¹³²Xe vs. ⁸⁴Kr/¹³²Xe ratios in Chassigny, most shergottites, and lithology C of EETA 79001 define a linear array. This array is thought to be a mixing line between martian mantle and martian atmosphere. One of the SNC meteorites, Nakhla, contains a leachable component that has an elevated ¹²⁹Xe/¹³²Xe ratio relative to its ⁸⁴Kr/¹³²Xe ratio when compared to this approximately linear array. The leachable component probably consists in part of iddingsite, an alteration product produced by interaction of olivine with aqueous fluid at temperatures lower than 150 °C. The elevated Xe isotopic ratio may represent a distinct reservoir in the martian crust or mantle. More plausibly, it is elementally fractionated martian atmosphere. Formation of sediments fractionates the noble gases in the correct direction. The range of sediment/atmosphere fractionation factors is consistent with the elevated ¹²⁹Xe/¹³²Xe component in Nakhla being contained in iddingsite, a low temperature weathering product. The crystallization age of Nakhla is 1.3 Ga. Its low-shock state suggests that it was ejected from near the surface of Mars. As liquid water is required for the formation of iddingsite, these observations provide further evidence for the near surface existence of aqueous fluids on Mars more recently than 1.3 Ga.     

Petrography,classification, oxygen isotopes,noble gases,and cosmogenic records of Kamargaon (L6) meteorite: The latest fall in India

A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L‐chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low‐Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two‐pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991 ); however, local presence of quenched metal‐sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996 ). Based on noble gas isotopes, the cosmic‐ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ¹⁵N = 18‰) also correspond well with the L‐chondrite group. The He‐U, Th, and K‐Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He‐U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of ²⁶Al (39 dpm/kg) and the absence of ⁶⁰Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (⁸²Kr/⁸⁴Kr) further argue that the meteorite was derived from a shallow depth.     

Noble gases and nitrogen in Raghunathpura (IIAB) and Nyaung (IIIAB) iron meteorites

Noble gases and nitrogen were measured in two adjacent samples each from the Raghunathpura (IIAB) and the Nyaung (IIIAB) iron meteorite falls. Light noble gases in both the meteorites were of pure cosmogenic origin. Using (³He/⁴He)_c ratios and the production systematic of Ammon et al. ( 2009 ), we estimated the sample depth and meteoroid size for Nyaung (~8 cm depth in a ~15 cm radius object) and Raghunathpura (~12–14 cm depth in a ~25 cm object). We derived cosmic ray exposure ages of 1710 ± 256 Ma (for Nyaung, the highest reported so far for the IIIAB group) and 224 ± 34 Ma (for Raghunathpura). Variable amounts of trapped Kr and Xe were found in both meteorites. The phase Q‐like elemental ratio (⁸⁴Kr/¹³²Xe) suggests that the trapped component is of indigenous origin, and most likely hosted in the heterogeneously distributed micro‐inclusions of troilite/schreibersite. Trapped phase Q component is being reported for the first time, for a IIAB iron meteorite. Both meteorites showed light isotopic composition for nitrogen, and need at least two N components to explain the observed N isotopic systematic. Variable amounts of trapped noble gases and the presence of more than one N component suggest that the magmatic process that formed the parent body of these meteorites either could not completely homogenize or completely degas all the phases.     

Solar and galactic cosmic‐ray records of the Fermo (H) chondrite regolith breccia

Abstract— ‐We demonstrate the presence of solar flare as well as neutron capture effects in the isotopic composition of rare gases in the Fermo regolith breccia acquired on its parent body based on the measurements of tracks, rare gases and radionuclides. The track density along a 3.2 cm long core decreases by a factor of about 6 and by more than a factor of 13 within the meteorite, indicating small (2–9 cm) and asymmetrical ablation. Rare gases show a large trapped component; the isotopic ratios, particularly ²⁰Ne/²²Ne ? 11 and ²⁰Ne/³⁶Ar = 10 are indicative of a solar component. The galactic cosmic‐ray exposure age is determined to be 8.8 Ma. Activities of a dozen radionuclides ranging in half‐life from 16 day ⁴⁸V to 0.73 Ma ²⁶Al are consistent with their expected production rates. Track, rare gas and radionuclide data show that the meteoroid was a small body (≤ 120 kg) and had a simple, one‐stage exposure history to cosmic rays in the interplanetary space. However, ⁸²Kr and ¹²⁸Xe show an excess due to neutron irradiation on the parent body of the meteorite. The presence of solar gases and the neutron capture effects indicate several stages of irradiation on the parent asteroid. The chemical composition of Fermo confirms that it belongs to the H group of ordinary chondrites with lithic clasts having varying compositions. δ¹⁵N is found to be 8.3 ± 1.2%0, close to the typical values observed in H chondrites.