Cumulative exposure ages of Libyan desert glass determined from in situ production of /sup 10/Be and /sup 26/Al

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6577855

Giegengack, R; Klein, J; Middleton, R; Sharma, P; Underwood, Jr, J R; Weeks, R A

Libyan Desert Glass (LDG), a unique high-silica natural glass of unknown origin from the Western Desert of Egypt, has been distributed across at least 3500 km/sup 2/ of desert surface during a period of erosional lowering of that landscape that may have prevailed for much of the time since the glass was formed 28.5 m.y. ago; a minimum of 1.4 x 10/sup 9/ g of glass has survived that erosional process. All specimens examined in the course of three field investigations are interpreted to have been carried to their present positions by stream transportation or by the hand of early man. Two dozen samples were selected for measurement of the abundance of /sup 10/Be and /sup 26/Al produced in situ by cosmic-ray interaction with LDG. /sup 26/Al//sup 10/Be ratios >1 support the conclusion that these nuclides have been produced at the Earth's surface by spallation reactions within the glass itself, and that individual pieces have experienced disparate histories of exposure to cosmic rays. From models of cosmic-ray flux and from /sup 10/Be and /sup 26/Al concentrations and ratios, the authors have calculated that specimens of LDG have lain exposed to cosmic rays for cumulative periods of time ranging from 200,000 to the order of 10/sup 6/ years during the last 6 million years.

• Research Organization: Univ. of Pennsylvania, Philadelphia (USA)
• OSTI ID: 6577855
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

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ALUMINIUM ISOTOPES
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