Rb-Sr ages of chondrules and carbonaceous chondrites

Determinations of Rb, Sr, and K contents and isotopic compositions of Sr are reported for the carbonaceous chondrites Orgueil, Murray, Mokoia, and Lancé, the hypersthene chondrite Peace River, and four chondrules from the Peace River meteorite. In accordance with previous work, the abundances of Rb and K in type I is greater than in type III; Sr is relatively constant. An isochron constructed from the carbonaceous chondrite data alone indicates an age of 4.46±0.35 by., taking λRb87 equal to 1.39×10−11/year, and an initial Sr87/Sr86 of 0.7007. The age becomes 4.74±0.15 b.y. if the initial Sr87/Sr86 is assumed to be 0.6985 (the mean initial value for achondrites), but the residual variance in the regression now becomes greater than experimental error. Most of the residual variance is contributed by the type III carbonaceous chondrites; one of several possible explanations is that type III formed some 0.3 b.y. later than type I by Rb depletion. Inclusion of the data for the Peace River chondrite changes the age only slightly (to 4.76 b.y.). The data from the four chondrules scatter widely about this isochron; ‘geological’ error and possibly increased experimental error may be the cause of this scatter.