An estimate of the isotopic composition of strontium in rocks of the Precambrian Shield of North America

Abstract

An examination of the Sr extracted from the calcareous shells of fresh-water mollusks from localities on the Precambrian shield of North America has revealed significant enrichment of Sr87 relative to primary basalt Sr. This enrichment had been predicted on the basis of the average Rb/Sr ratio and estimated age of the crystalline basement rocks. The average Sr87/Sr86 ratio for the calcareous shells, grouped according to the geologic age province in which they were found, are as follows: Kenoran age province (2.5 b.y), 0.721±0.005; Hudsonian age province (1.7 b.y.), 0.720±0.002; and Grenville age province (1.0 b.y.), 0.717±0.004. The isotopic composition of Sr in calcareous shells from the Great Lakes and the general drainage system of the St. Lawrence River seems to be in reasonable correlation with the bedrock geology of the drainage basin. No evidence of fractionation of Sr isotopes during incorporation in the calcareous shells was found.