ON THE SIGNIFICANCE OF CRATER AGES - NEW AGES FOR DELLEN (SWEDEN) AND ARAGUAINHA (BRAZIL)

Samples of a few grams in weight from the impact melt at the Dellen crater (Sweden) and the Araguainha dome (Brazil) were analyzed by Rb-Sr and Sm-Nd techniques. For Dellen, pure labradorite, orthopyroxene, and glass separates from a clast-free impact melt rock with holohyaline ground mass yield an internal Rb-Sr isochron with 89.0 +/- 2.7 Ma (I(Sr) = 0.83571 +/- 0.00007; 2sigma), compared to Ar-Ar plateau ages of 102 +/- 3.2 Ma and 109.6 +/- 2.0 Ma published for total melt rocks. Various whole-rock values not plotting on this Rb-Sr isochron indicate primary variations in Sr-87/Sr-86 over the melt sheet. The age of the Araguainha dome (Brazil) is bracketed by the Rb-Sr model age of 243 +/- 19 Ma (I(Sr) = 0.7226 +/- 0.0005), for altered cordierite in the impact melt, and by the internal Rb-Sr isochron age of 449 +/- 9 Ma (I(Sr) = 0.7140 +/- 0.0007), for an uplifted granite (shock stage Ib). In melt rocks from both impact structures Nd-143/Nd-144 was not homogenized during total rock melting: Sm-Nd data either scatter around the primary isochron (Araguainha) or point to a partial resetting (Dellen). This data array may be caused by tiny refractory REE-rich inclusions in the main mineral phases which dominate the Sm-Nd system of the impact melt rocks. The significant difference between the new isochron age for Dellen and Ar-Ar ''ages'' show that the data base for analysis Of periodicity in cratering record most probably lack precision and accuracy.