Separation of planetary noble gas carrier from bulk carbon in enstatite chondrites during stepped combustion

Abstract

The exact location of planetary noble gases in meteorites remains unknown but it is inferred to be closely associated with, if not precisely some portion of, the macromolecular organic material in carbonaceous chondrites [1]. Herein we show that for enstatite chondrites the major carbonaceous component is not the carrier of the gases. This may also be true for other chondritic groups. Rather, these gases are all contained in a minor combustible constituent. This situation has all the hallmarks of a Russian matryoshka doll problem, as was witnessed previously in the study of meteorites prior to the understanding of the presence of presolar grains. A possible conclusion, which is in line with previous suggestions [2], [3], is that planetary noble gases are also presolar and located in a new, and as yet unidentified, form of presolar material.

Introduction

The noble gas component known as ‘planetary’ (PNG, for planetary noble gases), which is also referred to as ‘Q’ [1] or P1 [4], is usually one of the most abundant primordial noble gas constituents found in primitive meteorites. The origin of PNG is still not completely understood [5], [6], [7], but it is generally accepted that they were acquired during the birth of the Solar System (e.g. [8], [9], [10]) or possibly earlier [3]. The PNG are present in variable amounts in almost all types of chondritic meteorites [11], [12], [13], [14] and some ureilites [15], [16], [17], and are closely associated with meteoritic carbon [1], [18], [19], [20]. Isotopically, the PNG are similar to, but nonetheless characteristically different from, those in the Sun [9], which we will refer to as SNG (for solar noble gases). Compared to the SNG, the PNG are strongly and systematically depleted in the light elements [7]. The most obvious isotopic differences between PNG and SNG are observed for Ne and Xe. The differences suggest that PNG could have formed from SNG as a result of isotope and element fractionation [9]. At present, the most generally accepted incorporation mechanism is the sorption of PNG onto the surfaces of carbon grains at low temperatures [21], [22]. It is thought that this material was then sampled by the accreting meteorite parent bodies. It is not possible to exclude, however, the possibility that trapping of PNG into their carrier took place outside of, and before formation of, the Solar System [2], [3]. In this scenario, presolar carbon-containing PNG was inherited by the early Solar System and then incorporated into the parent bodies of the primitive meteorites. The fact that noble gas components with isotopic compositions close to those in the Sun appear to be common galactic components (e.g. P3 [23] or N-noble gases [24] in presolar diamonds and SiC respectively), and the observation that the most abundant carbon phase in meteorites, the macromolecular material, is present in meteorites in rather constant ratios with respect to presolar grains [25], support this proposal.

The most important features of PNG are: (i) they are concentrated in acid (HF-HCl) resistant residues, which account for only a small fraction of the total mass of meteorites and consist mostly of carbon phases and oxide minerals [1]; (ii) the carrier of the noble gases is closely associated with meteoritic carbon and is combustible in an atmosphere of oxygen [1], [18], [19], [20], [26]; (iii) the gases are effectively removed from their carrier(s) by treatment with oxidizing acids and reagents (HNO₃, H₂O₂, HClO₄, etc. [1], [27]). The first two features point to a carbon phase as a carrier for PNG, while the interpretation of the third characteristic is not straightforward. Although oxidizing chemicals remove most of PNG, they destroy only a few percent of the total carbon.

Attempts to identify the carrier of the PNG (Q-phase, according to [1]) in meteorites have so far been unsuccessful. Perhaps the most satisfactory explanation [28], to date, is that there is no specific PNG carrier; rather the close association of PNG with carbon is the result of the surface adsorption of noble gases onto any carbonaceous material having a sufficiently high surface area. The interpretation is consistent with the chemical extraction procedures, as oxidizing acids would destroy the surfaces of carbon grains resulting in the release of surface-sited noble gases.

An important analytical observation that places some constraints on the nature of the PNG carrier, is that during stepped heating (i.e. vacuum pyrolysis) the noble gases are released at temperatures of 1200–1600°C, whilst under stepped gas-phase oxidation the gases are evolved at 400–550°C. To explain the high release temperature of PNG during pyrolysis, Zadnik et al. [28] developed the ‘labyrinth’ hypothesis where it was proposed that during initial trapping, noble gases found their way through a complex network of micropores on the surface of the carbon grains. Following adsorption, the labyrinth channels were plugged with organic molecules. This would have had the effect of preventing the release of PNG until the carrier was subjected to elevated temperatures (either on the parent body, or during laboratory heating experiments). While the PNG were trapped within the labyrinth, isotopic exchange with gases from other reservoirs, such as the terrestrial atmosphere, was also precluded. This explanation, based on a number of rather sophisticated experiments [22], [28], [29], and relying upon the observations mentioned above, also suggests that oxidation in an atmosphere of pure oxygen will release PNG in the same way as the oxidizing acids. However, when the PNG adsorption and labyrinth mechanisms were originally formulated, no direct measurements of the simultaneous release of CO₂ and PNG during stepped combustion were available.

The combustion experiments which had been done at that time included bulk sample (effectively one-step) combustion [20] and stepped combustion experiments in which noble gases and CO₂ were released from separate aliquots of the same sample in different experiments [19]. The first experiment where both yields of carbon and noble gases were determined simultaneously was performed later by Schelhaas et al. [13] in which the amount of CO₂ was measured with low precision using a Pirani gauge and the stepped combustion experiment was carried out with rather poor temperature resolution. Despite that, data from an HF-HCl residue of Dhajala (H3.8) showed that most of PNG were released before the main carbon release, which seemed to be in keeping with the labyrinth theory (though complete release of the noble gases required destruction of about 50% of the carbon which seems to be too high for the surface-sited noble gases).

In the present study, we undertook a detailed examination of the relationship between PNG and carbon in meteorites of different types through the use of high resolution stepped combustion experiments. A primary purpose of the investigation was to establish conclusively whether or not oxidation with pure O₂ gas would confirm the surface siting of PNG on carbonaceous components. To this end, seven meteorites (Murchison, Orgueil, Renazzo, Dhajala, LEW87223, Indarch and Yilmia) were analysed. By choosing chondrites of different chemical groups and petrological types it was considered that some of the subtleties of parent body processing (e.g. metamorphism and aqueous alteration) and the resulting structural transformations of the putative carbon carrier materials could also be studied.