26Al/10Be in terrestrial impact glasses

Abstract

There are three potential sources of cosmogenic nuclides in terrestrial impact glasses: (i) that in extraterrestrial “contamination” from the surface of the impacting body; (ii) that produced in the atmosphere and deposited in the terrestrial parent soil or sediment; (iii) “in situ” production at the earth's surface. With observation of only a single isotope, it is difficult, if not impossible, to uniquely identify its origin. However, since each of these sources gives rise to a particular distribution of isotopes, measurement of two or more of them improves the situation considerably. Using the technique of accelerator mass spectrometry we huave measured 26Al (half-life 0.72 My) and 10Be (1.5 My) in Libyan desert glass, Darwin glass and an Australasian tektite. We have in this way been able to demonstrate two of the sources mentioned above. In the case of the Australite the results prove the atmospheric origin of 10Be and thus give additional - and we believe conclusive - evidence for the terrestrial origin of tektites. The “in situ” production of 26Al and 10Be in Libyan desert glass, and 26Al in Darwin glass can be used to determine the surface exposure ages of these specimens.