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TERRESTRIAL COSMIC-RAY EXPOSURE HISTORY OF A 23-GRAM MOLDAVITE AND EVIDENCE FOR THE PRESENCE OF METEORIC BERYLLIUM-10. F. Serefiddin

Introduction: Impact-associated melting of nearsurface materials such as soils or loess probably produced the four main groups of tektites [1,2]. Measurements of cosmogenic nuclides in Australasian and Ivory Coast tektites support this view. In particular, both types have appreciable concentrations of Be (t1/2=1.5 My) [3,4] and, in a few cases, very low levels of the cosmogenic nuclides Al (t1/2=0.70 My) and Mn (t1/2=3.7 My) [5,6]. The relative and absolute concentrations indicate that cosmic rays produced the Be mainly in the Earth’s atmosphere rather than in situ. That these tektites incorporated relatively high levels of meteoric Be implies that they formed from small (<1 mm) precursor grains, a conclusion consistent with other petrographic and geochemical evidence [7-9] To extend these arguments, we decided to search for meteoric Be in a third major group of tektites. The Central European tektites formed ~ 15 My ago as a result of the Ries-Steinheim impact event into a region capped by a discontinuous layer of sand, clay, and limestone [10,11]. Assuming that the Be concentrations in this region resembled those inferred for Australasian tektites, we estimated that even after the passage of ten half-lives, enough Be might remain a fraction of ~0.001 for a measurement. To control for the possibility of recent, in situ production of Be, we decided to analyze for Cl (t1/2=0.3 My), certain that none of any original Cl (<10) would be detectable. Experimental methods: We obtained a moldavite with a mass of 23.1985 g from the Natural History Museum in Vienna. Wary of the possibility of meteoric contamination, we removed surface material in several steps. First we abraded ~ 3 g from the surface with a Dremel tool, recovering 1.2120 g of powder (J2388ZA) and several chips. The chips, which we reserved, consisted of what appeared to be colorless glass and of orange material resembling iron oxide. Next, we carried out 6 etches of J2388Z as follows: B, 20 mL 1M HCl, low heat for 15 min; C, 1 M HF for 15 minutes at 20°C; D-F, 1 M HF for 20 minutes at 20°C; G, 1 M HF for 30 minutes. The masses of the etched samples were determined by difference. At the end of the etching procedures a sample mass of 19.95 g remained of which we took 19.14 g for the analysis of Be. To each sample we added a known mass of Be carrier. In addition, to sample H we added ~10 mg of Cl ion as NaCl for a determination of Cl. Samples A and H were dissolved in HF and HNO3. We separated Cl from sample H and isolated Be from all samples as described by [12]. The concentrations of Be and Cl were measured by accelerator mass spectrometry (AMS) at PRIME Lab of Purdue University. In calculating the concentrations of Be and Cl, we made small corrections for the native concentrations of Be (2 ppm) and Cl (10 ppm) [9].