A noble record

Alexander B Verchovsky and Mark A Sephton review the origins and significance of noble gases in meteorites, focusing on what this unique record reveals about the early solar system.     

The fluid phase evolution during the formation of carbonatite of the Guli massif: Evidence from the isotope (C,N, Ar) data

The first data on variations of the isotope composition and element ratios of carbon, nitrogen, and argon in carbonatites of different generations and ultrabasic rocks of the Guli massif obtained by the method of step crushing are reported. It is shown that early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotope compositions of carbon (CO₂), argon, and hydrogen (H₂O). The data obtained allow us to conclude that the mantle component predominated in the fluid at the early stages of formation of rocks of the Guli massif, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon, and most likely a high portion of carbon dioxide with isotopically heavy carbon.     

Implanted radiogenic and other noble gases in crustal diamonds from Northern Kazakhstan

Noble gases were extracted in steps from grain size fractions of microdiamonds ( < 100 μm) from the Kokchetav Massif, Northern Kazakhstan, by pyrolysis and combustion. The concentration of ⁴He in the diamonds proper (liberated by combustion) shows a 1/r dependence on grain size. For grain diameters > 15 μm the concentration also decreases with the combustion step. Both results are clear evidence that ⁴He has been implanted into the diamonds from -decaying elements in the surrounding matrix. The saturation concentration of ⁴He(5.6 × 10⁻⁴ cm³ STP/g) is among the very highest observed in any terrestrial diamonds. Fission xenon from the spontaneous fission of ²³⁸U accompanies the radiogenic ⁴He; the ¹³⁶Xe_f/⁴He ratio of (2.5 ± 0.3) × 10⁻⁹ agrees well with the production ratio of 2.3 × 10⁻⁹ expected in a reservoir where Th/U 3.3. Radiogenic ⁴⁰Ar is predominantly ( > 90%) set free upon combustion; it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation.

³He, on the other hand is mainly released during pyrolysis and hence is apparently carried by ‘contaminants’. The concentration in the diamonds proper is of the order of 4 × 10⁻¹² cm³ STP/g, with a ³He/⁴He ratio of 1 × 10⁻⁸. Excess ²¹Ne, similarly, appears to be present in contaminants as well as in diamonds proper. These two nuclides in the contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations.     

Stable isotope analysis of carbon and nitrogen in angrites

Angrites are a small group of ancient basaltic achondrites, notable for their unusual chemistry and extreme volatile depletion. No comprehensive study of indigenous light elements currently exists for the group. Measurement of the abundances and isotopic composition of carbon and nitrogen could provide information pertaining to the evolution of the angrite parent body. Bulk‐sample stepped combustion analyses of five angrites and a glass separate from D'Orbigny were combined with earlier data and acid dissolution experiments of carbonates found in D'Orbigny to compile an inventory of indigenous carbon and nitrogen. Indigenous carbon combusted between 700 °C and 1200 °C, with abundances of 10–140 ppm and a mass‐weighted δ¹³C of ?25 to ?20‰ with the exception of D'Orbigny (δ¹³C approximately ?5‰). Nitrogen was released at 850–1200 ºC, 1–20 ppm with a δ¹⁵N ?3‰ to +4‰; again, D'Orbigny (δ¹⁵N approximately +20 to +25‰) was an exception. We interpret these components as largely indigenous and decoupled; the carbon in graphitic or amorphous form, while the nitrogen is present as a dissolved component in the silicates. No relationship with the textural sub‐classification of angrites is apparent. We suggest that the angrite parent body contains a reservoir of reduced carbon and thus may have undergone a change in redox conditions, although the timing and mechanism for this remain unclear.     

Effects of sonochemical treatment on meteoritic nanodiamonds

A nanodiamond‐rich fraction (NDF) separated from the Orgueil meteorite was subjected to a high‐intensity ultrasonic treatment in a weakly acidic aqueous solution. After sedimentation by centrifugation, two fractions of grains (suspension, designated as OD7C and sediment, designated as OD7D) with different properties have been obtained. The following effects of the sonication were revealed from comparison of the contents and isotope compositions of C, N, and Xe released during stepped pyrolysis and combustion of the fractions OD7C and OD7D, the initial NDF and two grain‐size fractions (OD10 and OD15) produced without sonication (a) surface layer of the sonicated diamond grains is modified to different extent in comparison with nontreated ones, (b) in some grains concentrations of the bulk N and Xe a reduced significantly, and (c) nondiamond nitrogen containing phases (e.g., Si₃N₄) have been destroyed. It is suggested that combined effects of the sonication and centrifugation observed for the fractions OD7C and OD7D are due to differences in surface chemistry of the nanodiamond grains, which statistically influences behavior of nanoparticles during the sonication resulting in their preferential modification in the different reaction zones of the cavitating fluid.     

Hydropyrolysis of insoluble carbonaceous matter in the Murchison meteorite: new insights into its macromolecular structure

The major organic component of carbonaceous chondrites is a solvent-insoluble, high molecular weight macromolecular material that constitutes at least 70% of the total organic content in these meteorites. Analytical pyrolysis is often used to thermally decompose macromolecular organic matter in an inert atmosphere into lower molecular weight fragments that are more amenable to conventional organic analytical techniques. Hydropyrolysis refers to pyrolysis assisted by high hydrogen gas pressures and a dispersed catalytically-active molybdenum sulfide phase. Hydropyrolysis of meteorites has not been attempted previously although it is ideally suited to such studies due to its relatively high yields. Hydropyrolysis of the Murchison macromolecular material successfully releases significant amounts of high molecular weight PAH including phenanthrene, carbazole, fluoranthene, pyrene, chrysene, perylene, benzoperylene and coronene units with varying degrees of alklyation. Analysis of both the products and residue from hydropyrolysis reveals that the meteoritic organic network contains both labile (pyrolysable) and refractory (nonpyrolysable) fractions. Comparisons of hydropyrolysis yields of Murchison macromolecular materials with those from terrestrial coals indicate that the refractory component probably consists of a network dominated by at least five- or six-ring PAH units cross-linked together.     

Xenon isotopes in size separated nanodiamonds from Efremovka: Xe*, Xe-P3, and Xe-P6

Xenon isotopic data were acquired by high resolution step pyrolysis and combined step pyrolysis/combustion of aliquots of size separated nanodiamonds. ¹²⁹Xe excess (¹²⁹Xe*) from in situ decay of ¹²⁹I is preferentially associated with the larger grain size separates. This observation rules out trapping by recoil from surrounding material. The releases of Xe-P3 and ¹²⁹Xe occur in the same low temperature pyrolysis steps and exhibit similar distributions among the size separates. These observations imply a common site for the components and, in consequence, suggest a common incorporation event.Whether one component or two, our observations require that ¹²⁹Xe* and Xe-P3 were incorporated into a subpopulation of nanodiamonds before nanodiamonds were mixed and incorporated into parent bodies. Their susceptibilities to loss during heating in the laboratory are similar, but the ratio of ¹²⁹Xe* to Xe-P3 varies among nanodiamond separates from different meteorites (literature data). We conclude that the ¹²⁹Xe* we observe today was present as ¹²⁹I during parent body processing. Furthermore, the range of ¹²⁹Xe*/¹³²Xe_P3 ratios across all the separates requires that even nanodiamonds from CI chondrites were at least 5-10× more rich in Xe-P3 during ¹²⁹I decay than they are today.We present a simple model involving one degassing event per parent body between incorporation of nanodiamonds and final decay of ¹²⁹I. The observed variations among parent bodies require degassing events separated by several ¹²⁹I half lives (∼50Ma), consistent with low-temperature processing on parent bodies but longer than expected for nebular processing. In this model, nanodiamonds from ALHA77307 degassed at an unusually early stage, suggesting they alone may retain the signature of processing in the nebula in their P3 and ¹²⁹Xe* abundances.The isotopic signature associated with Xe-P6 is also found only in the larger size separates. Concentration of Xe-HL increases with increasing grain size, but its relative abundance with respect to Xe-P3 and P6 is higher in smaller grain-size fractions. We argue that Xe-P6 is best seen as a variant of Xe-HL, and that they are both mixtures of a “normal” component akin to solar xenon and a slightly variable exotic component. We show that both current models of Xe-H formation can account for the observed variability, and propose a scenario according to which Xe-HL and P6 were implanted into separate diamond populations before incorporation of Xe-P3 and ¹²⁹I.     

Investigating the variations in carbon and nitrogen isotopes in carbonaceous chondrites

The carbonaceous chondrites contain significant amounts of carbon- and nitrogen-bearing components, the most abundant of which is organic matter. Stepped combustion data of whole rock and HF/HCl residues of carbonaceous chondrites reveal that the organic material can be subdivided operationally into three components: (1) free organic matter (FOM), which is readily extractable from whole-rock meteorites and is enriched in ¹³C and ¹⁵N; (2) labile organic matter (LOM), which has a macromolecular structure but is liberated by hydrous pyrolysis; LOM is the parent structure for some FOM and is also enriched in ¹³C and ¹⁵N; and (3) refractory organic matter (ROM), which is also macromolecular but is virtually unaffected by hydrous pyrolysis and is relatively depleted in ¹³C and ¹⁵N. The macromolecular entities (LOM and ROM) are by far the most abundant organic components present, and as such, the relative abundances of the ¹³C- and ¹⁵N-enriched LOM and the ¹³C- and ¹⁵N-depleted ROM will have a major influence on the overall isotopic composition of the whole-rock meteorite. Laboratory experiments designed to simulate the effects of parent body aqueous alteration indicate that this form of processing removes LOM from the macromolecular material, allowing ROM to exert a stronger influence on the overall isotopic compositions. Hence, aqueous alteration of macromolecular materials on the meteorite parent body may have a significant control on the stable isotopic compositions of whole-rock carbonaceous chondrites. The enstatite chondrites are also carbon rich but have been subjected to high levels of thermal metamorphism on their parent body. Stepped combustion data of HF/HCl residues of enstatite chondrites indicate, that if they and carbonaceous chondrites inherited a common organic progenitor, metamorphism under reducing conditions appears to incorporate and preserve some of the ¹³C enrichments in LOM during graphitisation. However, when metamorphism is at its most extreme, the ¹⁵N enrichments in LOM are lost.