THE EXPOSURE HISTORY OF THE JaH 073 METEORITE

Introduction: Large stony meteorites are relatively rare because (in contrast to iron meteorites) they fragment during atmospheric entry, producing large meteorite showers. Interestingly, many of the large chondrites, such as Bur Gheluai, Gold Basin, Jilin and Tsarev, appear to have complex exposure histories with a first-stage exposure on the parent body. The question is, whether a complex exposure history is simply more easily detected in large objects (due to multi-nuclide studies on various aliquots) or whether large objects are really more likely experience a complex exposure [1,2]. In order to check whether the observation of complex exposure histories for large objects also holds for the L6 chondrite JaH 073 we analysed various samples from this extremely well documented strewnfield and also probed the main mass (~80 kg) of this meteorite. From JaH 073 about 3000 fragments scattered in an area of about 60 km were found. The total mass is approximately 550 kg, wheras the biggest fragment is about 80 kg. Experimental: The samples with masses between 100 – 150 mg were prepared by careful cutting and cleaning. The samples were then wrapped in ~20 mg of Ni foil and loaded into the storage positions of the noble gas extraction system. To release atmospheric surface contamination, the samples were preheated at ~80 °C for various days. Gas extraction was performed in a Mo crucible held at about 1700 °C for about 35 min. A second heating step at 1750 °C performed for some samples ensures complete degassing. Gases were cleaned on Ti getters working at temperatures between 700 °C and room temperature. Argon was separated from He and Ne by adsorption on activated charcoal held at the temperature of boiling nitrogen. The measurements of HeNeand Ar-fractions were performed using two self-made sector field mass spectrometers, one for HeNe and one for Ar. Sample gas amounts were determined by peak height comparison with signals from known amounts of He, Ne, and Ar, respectively. Results: System blanks were determined by analysing ~20 mg of Ni foil with the same heating schedules as used for the samples. The blanks usually were below 0.5% for Ne and 0.8% for Ar of sample gas amounts, adding only negligible uncertainties to the latter. Cosmogenic Ne and Ar concentrations were determined from measured gas amounts by subtracting the trapped components using a 2-component deconvolution technique. For the partitioning of the Ne and Ar components we assume Ne/Ne(tr) = 9.80, Ne/Ne(cos) = 0.85, Ar/Ar(tr) = 5.32, and Ar/Ar(cos) = 0.63. While the corrections for noncosmogenic Ne are always very minor, the corrections for trapped Ar significantly compromise exposure age studies of JaH 073. The trapped Ar component, which is of atmospheric composition, is most probable due to the large degree of terrestrial weathering. However, circumventing this problem was a major reason for probing the main mass of JaH 073 (see below). So far we analysed 13 samples of 8 JaH 073 fragments. Duplicate analyses always give reproducible results, i.e. within the experimental uncertainties. However, for some samples Ar show slightly more scatter than He and Ne, possibly due to the rather large trapped correction. In cases where two or more analysis have been performed for one sample, average values are used for further discussion.