U-SERIES FRACTIONATION IN THE MANTLE

New insights into the origin of basalt could be gained if the principle of U-series activity balance were to be applied to the steady-state system: Mantle ® Basalt + Residuum.

In a closed system the decay chain daughters of the two long-lived uranium isotopes have the same alpha activity as their parents: A(²³⁸U)=A(²³⁴U)=A(²³⁰Th)=A(²²⁶Ra) and A(²³⁵U)=A(²³¹Pa).

The very large excesses of daughters, especially ²²⁶Ra and ²³¹Pa, usually observed in mid-ocean ridge basalts (and often at convergent plate margins and hot spots), require that, at the time of eruption, most of the parent uranium remains behind in the mantle. Prevailing incompatibility models of U-series fractionation during basalt generation violate this requirement. An alternative fractionation model is needed.

Although the mantle, as a whole rock system, must be in radioactive equilibrium, individual minerals are necessarily out of equilibrium if disparities in their U concentration exist. Two physical processes, daughter diffusion and daughter recoil, will tend to produce daughter deficiencies in the high-U (donor) minerals and daughter excesses in the low-U (receptor) minerals.

This condition of paired-disequilibria prevails in the mantle before melting begins; so that subsequent preferential melting of the low-U minerals will produce the disequilibria observed in the basalt. The extreme daughter excesses measured imply that U is preferentially sequestered in high-U accessory minerals in the mantle, rather than in the more abundant low-U minerals such as garnet and pyroxene.