Separation of Q-Noble gas carrier from bulk carbon in enstatite chondrites by stepped combustion

The nature of the planetary noble gas (PNG) carrier or Q-phase [1] in meteorites is still a subject of discussion. Failure to identify and isolate this carrier since the first successful concentration of it by a factor of ~200 in an Allende HF-HCl residue [1] and establishing it to be carbonaceous in nature [2], led eventually to the so-called “labyrinth” theory [3], according to which PNG initially found their way through a complex network of micropores on the surface of the carbon grains during adsorption. Following adsorption, the labyrinth channels could be plugged with organic molecules, preventing the release of PNG at elevated temperatures, or/and exchange with gases from other reservoirs such as the terrestrial atmosphere. In support of this theory a number of rather sophisticated experimental investigations on the sorption of noble gases on carbon grain surfaces have been undertaken [3-5], which have proved that the labyrinth mechanism can in principle be realised in natural conditions. However whether or not this is the case for PNG in meteorites remains uncertain. An important observation that fits the theory is the very effective removal of PNG from HF-HCl residues by treatment with oxidizing acids during which only a few percent of the total carbon is destroyed. Oxidation with pure oxygen should work in a similar way, although there have been no systematic investigations of the release of PNG and carbon from HF-HCl residues during stepped combustion. In the present study we decided to fill this gap by analysing a number of HF-HCl residues separated from meteorites of different types and metamorphic history for noble gases, nitrogen and carbon using detailed stepped combustion. We analysed three carbonaceous chondrites (Orgueil (CI), Murchison (CM) and Renazzo (CR)), three enstatite chondrites (LEW87223 (E3), Indurch (EH4) and Yilmia (EL6)) and one ordinary chondrite Dhajala (H3.8). Figure 1 shows a typical example of carbon and noble gases release from carbonaceous chondrites. As can be seen, carbon and noble gases are released in the same temperature range, so that the complete release of noble gases occurs only after most of the carbon has been oxidised. More precisely, when 80% of C is combusted only about 60% of planetary Ar and Xe are released. This release pattern is compatible with PNG being a volume, rather than surface-correlated component. A similar relationship between C and PNG is also observed in most ureilites [6], in which PNG are volume-correlated, as they located mostly in shock-produced diamonds and therefore cannot be removed by treatment with HNO3. Thus, oxidation with pure oxygen in a vacuum affects the release of PNG in carbonaceous chondrites in a way significantly different from what occurs during the treatment of the HF-HCl residues with nitric acid, and suggests that PNG are located not on the surface of any carbon grains but rather in a separate carrier (Q-phase). The specific chemical properties of this carrier remain, however, poorly understood.