MECHANISMS OF MAGMA GENERATION BENEATH HAWAII AND MIDOCEAN RIDGES - URANIUM THORIUM AND SAMARIUM/NEODYMIUM ISOTOPIC EVIDENCE/

Measurements of uranium/thorium and samarium/neodymium isotopes and co ncentrations in a suite of Hawaiian basalts show that uranium/thorium fractionation varies systematically with samarium/neodymium fractionat ion and major-element composition; these correlations can be understoo d in terms of simple batch melting models with a garnet-bearing perido tite magma source and melt fractions of 0.25 to 6.5 percent, Midocean ridge basalts shows a systematic but much different relation between u ranium/thorium fractionation and samarium/neodymium fractionation, whi ch, although broadly consistent with melting of a garnet-bearing perid otite source, requires a more complex melting model.