Possible age of crystallization of pyrope from the Stockdale kimberlite, Kansas, U. S. A.

A possible age of 745 ± 100m.y. (using 50b.y. = half life of 87Rb) has been determined for pyrope from the Stockdale kimberlite assuming an upper mantle initial 87Sr/86Sr ratio of 0.702 ± 0.001. Despite the large error associated with this age it clearly demonstrates that the pyrope crystallized long before its final emplacement which is set at ≤ 100 m.y. Allowance for crustal and mantle contamination and other sources of error have been carefully considered in this determination.