Isheyevo Meteorite: Genetic link between CH and CB chondrites?

Based on the mineralogy, petrography, bulk chemical, oxygen, and nintrogen isotopic compositions and 40Ar-39Ar age, Isheyevo is genetically related to CH and CB carbonaceous chondrites and provides a link between these group of pristine meteorites

Distribution of mantle and atmospheric argon in mantle xenoliths from the Western Arabian peninsula: Constraints on timing and composition of metasomatizing agents in the lithospheric mantle

To investigate the geochemical behaviour of argon isotopes during mantle metasomatism and to obtain chronological information on the age of metasomatic events under the Arabian Shield, we analyzed mantle xenoliths and hornblende megacrysts from Saudi Arabian volcanic fields (Uwayrid, Al Birk) applying the 40Ar-39Ar dating technique. Two hornblende megacrysts yield plateau ages indicating formation or total resetting of the K/Ar system 1-2 Myr ago. The ultramafic xenoliths trapped mantle-derived and atmospheric argon in different proportions, resulting in variable isotopic compositions: 40Ar/36Ar ratios range from 296 (i.e. atmospheric) to 10 500, reflecting interactions with isotopically and genetically different fluids and/or melts during recent mantle metasomatism. One such episode of metasomatism led to the formation of Ba-rich phlogopite, which yielded a saddle-shaped age spectrum with a maximum age estimate of 18 Ma. Another episode, inducing formation of secondary pargasite in the lithospheric mantle, was dated to 4 Ma. In the mantle xenoliths the concentration of mantle argon is clearly related to the intensity of metasomatism. Argon extraction by high-resolution stepwise heating allowed us to deconvolve various argon components distributed heterogeneously within single xenoliths and ascribe them to specific carrier phases. Pyroxenes generally preserve much higher 40Ar/36Ar ratios than olivine, as they contain up to 100 times higher concentrations of mantle argon, which also correlates with a higher fluid inclusion content in pyroxenes. Hydrous phases (phlogopite/amphibole) have more variable 40Ar/36Ar ratios. K and Cl concentrations and the argon isotope compositions of the Uwayrid xenoliths indicate distinct metasomatic agents, causing elemental and isotopic disequilibrium on a local scale. On the basis of correlations between Ar isotope composition and K and Cl concentration in the samples most strongly affected by the late metasomatic fluids, we suggest that metasomatic processes in the local mantle occurring simultaneously with the opening of the Red Sea were accompanied by the introduction of saline-water saturated fluids into deep lithospheric zones

The Lunar Breccia Dhofar 1442: Noble Gases, Nitrogen, and Carbon Released by Stepwise Combustion and Crushing

Stepwise crushing and combustion methods were applied to study the KREEP-rich lunar breccia Dhofar 1442, the clastic material of which is cemented by porous matrix. The stepwise crushing released significant amount of gases of extraterrestrial origin from gas voids. Argon, nitrogen, and carbon are simultaneously released by stepwise combustion at 1100°С. The simultaneous high-temperature degassing of these gases, as well as the coincidence of С/N ratio and nitrogen and carbon contents in high-temperature combustion steps with those of crushing indicate that the gas carriers are voids in high-temperature phases (in particular, minerals, glasses), which are decomposed/melted at these temperatures. Helium and neon are released from the same positions at lower temperatures. The isotopic composition of neon obtained by stepwise combustion and crushing corresponds to the composition of fractionated solar wind. The fraction of argon in the first crushing steps is higher than that of any other of studied gases. The 40Ar/36Ar in the trapped lunar argon is ~18, which is not consistent with empirical model implying that 40Аr is implanted from lunar atmosphere (McKay et al., 1986; Eugster et al., 2001; Joy et al., 2011). We believe that the entrapment of volatile elements in gas voids of the meteorite Dhofar 1442 was caused by the redistribution of gases from one structural sites into others during impact events that accompanied the cratering, in particular, leading to the formation of the impact melt breccia Dhofar 1442. The trapped gases of the meteorite Dhofar 1442 contain not only typical volatile components (solar, radiogenic, cosmogenic, re-implanted 40Ar of lunar breccias, but also nitrogen and carbon formed through the oxidation of organic matter of metamorphosed chondrites, which are present in the breccia. With increasing number of strokes and, correspondingly, a degree of crushing, the elemental ratios change. A slight decrease of 4He/20Ne ratio during crushing is likely related to the different diffusion ability and permeability of helium relative to neon under temperature influence and/or to the heterogeneous distribution of these gases in voids of different size. The 4He//36Ar, 20Ne/36Ar, 14N/36Ar, and 12С/36Ar ratios increase by factors of 10–100 during crushing. This can be explained by the combination of dynamically different processes leading to the argon fractionation relative to other gases and uneven redistribution of gases from different positions in voids of different sizes during impact metamorphism

Evidence for silicate dissolution on Mars from the Nakhla meteorite

Veins containing carbonates, hydrous silicates and sulphates that occur within and between grains of augite and olivine in the Nakhla meteorite are good evidence for the former presence of liquid water in the Martian crust. Aqueous solutions gained access to grain interiors via narrow fractures, and those fractures within olivine whose walls were oriented close to (001) were preferentially widened by etching along [001]. This orientation selective dissolution may have been due to the presence within olivine of shock-formed [001](100) and [001]{110} screw dislocations. The duration of etching is likely to have been brief, possibly less than a year, and the solutions responsible were sufficiently cool and reducing that laihunite is absent and Fe liberated from the olivine was not immediately oxidised. The pores within olivine were mineralised in sequence by siderite, nanocrystalline smectite, a Fe-Mg phyllosilicate, and then gypsum, whereas only the smectite occurs within augite. The nanocrystalline smectite was deposited as sub-micrometre thick layers on etched vein walls, and solution compositions varied substantially between and sometimes during precipitation of each layer. Together with microcrystalline gypsum the Fe-Mg phyllosilicate crystallised as water briefly returned to some of the veins following desiccation fracturing of the smectite. These results show that etching of olivine enhanced the porosity and permeability of the nakhlite parent rock and that dissolution and secondary mineralization took place within the same near-static aqueous system

The lunar Dhofar 1436 meteorite: ⁴⁰Ar‐ ³⁹Ar chronology and volatiles, revealed by stepwise combustion and crushing methods

The lunar meteorite Dhofar 1436 is dominated by solar wind type noble gases. Solar argon is equilibrated with “parentless” 40Ar commonly known as lunar orphan argon. Ar‐Ar isochron analyses determined the lunar trapped 40Ar/36Ar ratio to 2.51 ± 0.04, yielding a corrected plateau age of 4.1 ± 0.1 Ga, consistent with the lunar Late Heavy Bombardment period. Lunar trapped and radiogenic argon components are all released at high temperatures (1200–1400 °C). Surprisingly, solar noble gases and lunar trapped argon can largely be released by crushing. Initial crushing steps mainly release elementally fractionated solar wind gases, while in advanced crushing steps, cosmogenic components dominate. Cosmogenic noble gases indicate irradiation at the lunar surface; they are less fractionated than solar wind species. We favor a scenario in which both solar and a large fraction of cosmogenic gases were acquired before the 4.1 Ga event, which caused shock metamorphism and formation of the regolith breccia. Sintering and agglutination along grain boundaries resulted in mobilization of solar wind, reimplanted, radiogenic, and cosmogenic noble gases, and resulted in their partial homogenization, fractionation, and retrapping in voids and/or defects accessible by crushing. An alternative scenario would be complete reset of the K‐Ar system 4.1 Ga ago accompanied by loss of all previously accumulated solar and cosmogenic noble gases. Later, the precursor of Dhofar 1436 became lunar regolith and accumulated solar and cosmogenic noble gases and reimplanted 40Ar before its final formation of the polymict impact breccia. The C abundance of the step‐combusted Dhofar 1436 is 555.3 ppm, with δ13C of −28‰ to +11‰. Nitrogen contents released by crushing and combustion are 3.2 ppm and 20.8 ppm, respectively. The lightest nitrogen composition (δ15N = −79‰) is likely due to release from voids of shock metamorphic phases and is rather a result of the mobilization of nitrogen components that accumulated prior to the 4.1 Ga event

Sierra Gorda 009: A New Member of the Metal-Rich G Chondrites Grouplet

We investigated the metal-rich chondrite Sierra Gorda (SG) 009, a member of the new G chondrite grouplet (also including NWA 5492, GRO 95551). G chondrites contain 23% metal, very reduced silicates, and rare oxidized mineral phases (Mg-chromite, FeO-rich pyroxene). G chondrites are not related to CH-CB chondrites, based on bulk O, C, and N isotopic compositions, mineralogy, and geochemistry. G chondrites have no fine-grained matrix or matrix lumps enclosing hydrated material typical for CH-CB chondrites. G chondrites’ average metal compositions are similar to H chondrites. Siderophile and lithophile geochemistry indicates sulfidization and fractionation of the SG 009 metal and silicates, unlike NWA 5492 and GRO 95551. The G chondrites have average O isotopic compositions Δ17O'0‰ ranging between bulk enstatite (E) and ordinary (O) chondrites. An Al-rich chondrule from SG 009 has Δ17O'0‰ indicating some heterogeneity in oxygen isotopic composition of G chondrite components. SG 009’s bulk carbon and nitrogen isotopic compositions correspond to E and O chondrites. Neon isotopic composition reflects a mixture of cosmogenic and solar components, and cosmic ray exposure age of SG 009 is typical for O, E, and R chondrites. G chondrites are closely related to O, E, and R chondrites and may represent a unique metal-rich parent asteroid containing primitive and fractionated material from the inner solar system. Oxidizing and reducing conditions during SG 009 formation may be connected with a chemical microenvironment and possibly could indicate that G chondrites may have formed by a planetesimal collision resulting in the lack of matrix. © The Meteoritical Society, 2020.We thank M. Weisberg, H. Downes, an anonymous reviewer, and Associate Editor C. Goodrich, for their thoughtful reviews which helped to improve this paper. The authors thank Sasha Krot for very fruitful discussions. This work was supported by the Russian Fond of Basic Research no. 20-05-00117A, by Klaus Tschira Stiftung gGmbH, by the NASA Emerging Worlds program (80NSSC18K0595, MH), and we thank the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1644779* and the State of Florida. This work was also supported?by the Project No. FEUZ-2020-0059 of the Ministry of Science and Higher Education of the Russian Federation. This study was a partial contribution to research theme no. 0137-2019-0002

Cosmic ray exposure ages of nakhlites - Nakhla, Lafayette, Governador Valadares - and Chassigny: one ejection event?

An important question currently vi-gorously discussed is if all nakhlites and possibly Chassigny were ejected by the same impact event from a common Martian location. However, the uncertainties of cosmic-ray exposure (CRE) ages are usually high and thus the crucial point in this discussion. Here we present CRE ages for Nakhla (Nak), Lafayette (Laf) and Chassigny (Chas) studied by 40Ar-39Ar dating of irradiated mineral separates and whole rocks. We also report CRE ages for these meteorites and Governador Valadares (GV) using the cosmogenic nuclides 3He, 21Ne, and 38Ar in unirradiated whole rock samples