Deuterium Enrichments in Primitive Meteorites

Recent studies (1-4) of D/H ratios in primitive meteorites (unequilibrated ordinary chondrites, UOCs, and carbonaceous chondrites) have demonstrated the presence of large excesses of deuterium by comparision with terrestrial waters. Far greater D enrichments are found in certain carbon containing entities in the interstellar medium where they were thought to be due to ion-molecule reactions at low temperature (5). A possible conclusion is that the primitive meteorites could contain carbon or other species surviving from the earliest events taking place during the birth of the solar system in a dark molecular cloud. It is important to establish the frequency with which D enrichments occur, the nature of D-rich phases, their petrographic location and any relationship between the D-containing species in carbonaceous chondrites and UOCs. Such investigations are likely to have considerable bearing on the synthesis of meteorite organics and aid the understanding of future analyses of the most primitive solar system materials including cometary dust and gas. Our initial study concerned Bishunpur, Semarkona and Krymka, generally accepted as the least metamorphosed UOCs (2). Bulk specimens of the former two samples released water between 200 and 750'~ with variable 6DSMOW values up to +30009, whilst water from the third had a 6D of only +54%, . Chainpur, a sample considered by Robert et al, (1) afforded water of 6D = +200%, but when subjected to a separation study and investigated by stepwise heating, fractions whose 6D exceeded +4400%, were recognised. All the above samples ard LL group meteorites. We have now extended the survey to include ALHA 77299 and Tieschitz both only moderately unequilibrated but nevertheless relatively carbon rich, possibly an important criterion since D in carbonaceous chondrites is associated with acid insoluble residues (4). The former is an H group meteorite whilst the latter was recently reported as having closer affinities to L or LL rather than H although it has long been considered an example of the latter type (6). When subjected to our previously described extraction procedure neither sample provided water of exceptional deuterium content having 6D values of -25.6%, and -76.5%, respectively. We have now also performed a stepwise heating experiment using our published extraction method on a bulk sample of Bishunpur. The amounts of water released and the isotopic compositions are listed in Table 1. The release patterns closely resemble that reported by Robert .et al. for Chainpur (1) and the rapid increase in D/H ratio up to 750'~ parallels the carbon isotopic changes observed from the sample during step-wise combustion (7) although it must be appreciated that the extraction techniques used were appreciably different. Rather surpriscngly none of the deuterium isotopic compositions approaches the values we earlier obtained for Bishunpur total extracts over the temperature interval 200-7500C. The weighted average 6D for the step-wise heated sample is +419%,. The results of the step-wise heating experiment tdnd to confirm our suspicion that a minor very D enriched phase may be sporadically distributed in Bishunpur which is notably inhomogeneous, Such a conclusion is important to verify and it is desirable to demonstrate the reproducibility of our. technique. One possible flaw in the method used up to now is that only H20 is collected; any H2 or CHq formed during the pyrolysis is lost during the pump out of non-condensible gases. Accordingly the procedure has been modified along the lines employed by us for carbon isotopic analysis (8).