Attempts to analyse D/H ratios of sub-micromole quantities of hydrogen : applications in the study of ordinary chondrites

Abstract

Semarkona is an unequilibrated ordinary chondrite which has experienced the least amount of thermal metamorphism and has the highest whole-rock D/H ratio of the ordinary chondrites. It is also unusual in that it has undergone some mild aqueous alteration, previously considered a secondary process restricted to some deuterium-rich carbonaceous chondrites. Semarkona was chosen to be the object of a more detailed study of secondary alteration and to determine whether the deuterium-rich phase was related in any way to the aqueous alteration process. A procedure was developed involving three analytical techniques (chemical/petrographic, thermoluminescence and hydrogen isotope) to study individual fragments of this meteorite. The deuterium-rich phase was found to be concentrated in the chondrules, average δD = +3890‰, with a maximum measured δD of +7500‰ in one of the chondrules. The matrix had a moderate deuterium enrichment, of average 3D = +2670‰. The isotope results in conjunction with the other techniques employed indicated that the chondrules that had experienced aqueous alteration are deuterium-rich. The water involved in the aqueous alteration was either deuterium-rich or transported deuterium-rich organics to the sites of aqueous alteration. In order to reduce sample size requirements, it was necessary to develop an inlet and mass spectrometer system capable of the isotopic analysis of less than 0.1 µmol hydrogen. Two inlet systems were developed for use with a dynamic mass spectrometer and compared with a conventional inlet system. The inlet system with greatest sensitivity, capable of analysing 0.1 µmol hydrogen, was used for the isotopic study of Semarkona; however, the accuracy of this system was poor due to the presence of a memory effect. Experiments, using the conventional inlet, showed that this memory effect was due to sample water remaining on the surfaces of the inlet system. The presence of the memory effect could not be eliminated, but a correction procedure was obtained. These experiments have shown various methods in which the memory effect can be overcome by careful design of an inlet system. Finally a static mass spectrometer system was developed, capable of analysing less than lnmol hydrogen, in the form of methane. Initial results show that this system is capable of the isotopic analysis 0.01 μmol of water with δD in the range of -500‰ to +5000‰ to an accuracy of ±20‰.