URANIUM THORIUM DISEQUILIBRIA AND PARTITIONING ON MELTING OF GARNET PERIDOTITE

THE abundances of isotopes in the U-238 decay series can be used as bo th tracers and chronometers of magmatic processes. In the subsolidus a sthenosphere, the activity of each daughter isotope (defined as the pr oduct of its concentration and decay constant, and denoted by parenthe ses) is assumed to be equal to that of its parent. By contrast, (Th-23 0/U-238) is greater than unity in most recent mid-ocean-ridge and ocea n-island basalts1, implying that thorium is more incompatible (that is , it is partitioned into the melt phase more strongly) than uranium. M elting of spinel peridotite cannot produce the (Th-230) excesses, beca use measured partition coefficients for pyroxenes and olivine demonstr ate that uranium is more incompatible than thorium for this rock2. Her e I report garnet-melt partitioning data which show that for this mine ral-melt pair thorium does behave more incompatibly than uranium, thus supporting the suggestion that mid-ocean-ridge basalts (MORB) are pro duced by melting initiated at depths where garnet is stable3-6. Using these data, I show that the observed (Th-230/U-238) ratios of MORB and most ocean-island basalts can be explained by slow, near-fractional m elting initiated in the garnet stability field.