Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematics

Abstract

Sm-Nd and Rb-Sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects. Tektites have large negative ε Nd(0) values and are uniform within each tektite group while the ε Sr(0) are large positive values and show considerable variation within each group. Chemical, trace element, and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust. Each tektite group is characterized by a uniform Nd model age, T CHUR Nd , interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites: (1) ∼1.15 AE for Australasian tektites; (2) ∼1.91 AE for Ivory Coast tektites; (3) ∼0.9 AE for moldavites; (4) ∼0.65 AE for North American tektites, and (5) ∼0.9 AE for high-Si irghizites. Sr model ages, T UR Sr , are variable within each group reflecting Rb-Sr fractionation and in the favorable limit of very high Rb/Sr ratios, approach the time of sedimentation of the parent material which melted to form the tektites. Australasian tektites are derived from ∼0.25 AE sediments, moldavites from ∼0.0 AE sediments, Ivory Coast tektites from ∼0.95 AE sediments. Possible parent sediments of other tektite groups have poorly constrained ages. Our data on moldavites and Ivory Coast tektites are consistent with derivation from the Ries and Bosumtwi craters, respectively. Irghizites are isotopically distinct from Australasian tektites and are probably not related. Sanidine spherules from a Cretaceous-Tertiary boundary clay have initial ε Nd ∼ +2; ε Sr ∼ +5 and are not derived from old continental crust or meteoritic feldspar. They may represent a mixture of basaltic oceanic crust and sediments, implying an oceanic impact. These isotopic results are also consistent with a volcanic origin for the spherules.