Hafnium isotope variations in oceanic basalts

Abstract

Routine low-blank chemistry and 0.01-0.04% precision on the ratio 176 Hf/ 177 Hf allows study of Hf isotopic variations, generated by β − -decay of 176 Lu, in volcanic rocks derived from the suboceanic mantle. Normalized to 176 Hf/ 177 Hf = 0.7325, 176 Hf/ 177 Hf ranges from 0.2828 to 0.2835, based on 24 basalt samples. 176 Hf/ 177 Hf is positively correlated with 143 Nd/ 144 Nd, and negatively correlated with 87 Sr/ 86 Sr and 206 Pb/ 204 Pb. Along the Iceland-Reykjanes ridge traverse, 176 Hf/ 177 Hf increases southwards. The coherence of Hf, Nd and Sr isotopes in the oceanic mantle allows an approximate bulk Earth 176 Hf/ 177 Hf of 0.28295 to be inferred from the bulk Earth 143 Nd/ 144 . This requires the bulk Earth Lu/Hf to be 0.25, similar to that of the Juvinas eucrite. 60% of the Hf isotopic variation in oceanic basalts occurs among mid-ocean ridge samples. Lu-Hf fractionation probably decouples from Sm-Nd and Rb-Sr fractionation in very depleted source regions, with high Lu/Hf, and consequent high 176 Hf/ 177 Hf ratios developing in mantle residual from partial melting.