Are Large Time Differences in Meteorite Formation Real?

Abstract

IN recent years two types of cosmochronological dating have been interpreted to indicate that there are large time intervals, a few times 10⁷ yr, between the formation of various meteorite samples. For example, extrapolation backwards along isochrons in seven basaltic achondrites led Papanastassiou and Wasserburg¹ to establish a ‘standard’ initial ⁸⁷Sr/⁸⁶Sr ratio of 0.69898±0.00003 (BABI≡‘basaltic achondrite best initial’). Similar isochrons in another basaltic achondrite, Angra dos Reis, led, however, to an initial ratio of 0.69884±0.0004 (ADOR≡‘Angra dos Reis’)². The difference between these two values is attributed to the accumulation of different amounts of the ⁸⁷Sr decay product of ⁸⁷Rb. If this reflects a difference in the times of fractionation between Rb and Sr in the meteoritic material, then the time interval is 14×10⁶ yr. More recently, Tatsumoto, Knight, and Allegre³ found significant differences in the radiogenic ²⁰⁷Pb/²⁰⁶Pb ratios in Angra dos Reis and in two other basaltic achondrites, the value for Angra dos Reis being 0.6197 and the average of the other two being 0.6081. These nuclides result from the decay of ²³⁵U and ²³⁸U. If the differences are attributed to different times for the events of lead–uranium fractionation in the meteoritic materials, then the indicated time interval is 27×10⁶ yr.