Nd isotopic variations and petrogenetic models

Abstract

The decay of 147 Sm to 143 Nd allows 143 Nd/ 144 Nd to be used to trace Sm/Nd fractionation in long time‐scale geologic processes. 143 Nd/ 144 Nd has been measured in terrestrial rock samples of different ages to establish the characteristics of Nd isotopic evolution in the crust and mantle. The evolution of 143 Nd/ 144 Nd in the mantle indicates Sm/Nd essentially equal to that of chondrites, and implies a chondritic REE distribution for the earth. Variations in 143 Nd/ 144 Nd do exist in the mantle, however, indicating Sm/Nd heterogeneity and the existence of distinct mantle reservoirs with characteristic 143 Nd/ 144 Nd. 143 Nd/ 144 Nd in average crustal rocks today is much lower than found in recent mantle samples and reflects their age and low Sm/Nd. Oceanic tholeiites and alkali basalt are derived from sources with Sm/Nd which has been 5‐10% greater than chondritic over the age of the earth. Alkali basalt can not be derived from mantle reservoirs which have been light REE‐enriched for long times.