DETECTION OF A METEORITIC COMPONENT IN IVORY-COAST TEKTITES WITH RHENIUM-OSMIUM ISOTOPES

Measurement of rhenium (Re) and osmium (Os) concentrations and Os isot opic compositions in Ivory Coast tektites (natural glasses with upper crustal compositions that are ejected great distances during meteorite impact) and rocks from the inferred source crater, Lake Bosumtwi, Gha na, show that these tektites incorporate about 0.6 percent of a meteor itic component. Analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components bec ause the target rocks already have relatively large amounts of noble m etals. The Re-Os system is ideally suited for the study of meteorite i mpacts on old continental crust for three reasons. (i) The isotopic co mpositions of the target rocks and the meteoritic impactor are signifi cantly different. (ii) Closed-system mixing of target rocks and meteor ites is linear on Re-Os isochron diagrams, which thus permits identifi cation of the loss of Re or Os. (iii) Osmium isotopic compositions are not likely to be altered during meteorite impact even if Re and Os ar e lost.