The Differentiationon History of the Earth by Rubidium-strontium Isotopic Relationships

The rubidium-strontium isotopic relationships in alkaline and subalkaline oceanic basalts have been determined by mass spectrometric techniques. The results are consistent with a complex model for the earth in which there is continuing convection. Sr87/Sr 8 6 ratios for all oceanic basalts fall in a narrow range from .702 to .705. This suggests that the oceanic upper mantle source regions are generally uniform with respect to their Rb/Sr ratio and have been so for Much 2f the earth's history. Inter-island variations in the Sr 7/Sr 6 ratio indicate that minor heterogeneities in the Rb/Sr ratio were developed at the time of major differentiation of the mantle early in the Earth's history. The subalkaline basalts dredged from the ocean floors and ridges have uniformly low strontium and rubidium concentrations and low Rb/Sr ratios These ratios are too low to account for the observed Srg7/Sr86 ratio and it is concluded that the subalkaline basalts are derivatives from material that had a significantly higher Rb/Sr ratio at an earlier stage of the Earth's development. To account for the present low Rb/Sr ratio, the suggestion is made that the source mater al, following the development of much of the radiogenic Sr 7 now observed in these basalts, passed through a residual stage prior to the generation of the subalkaline magmas.