Hafnium isotope variations in oceanic basalts

Routine low-blank chemistry and 0.01-0.04% precision on the ratio ¹⁷⁶Hf/¹⁷⁷Hf allows study of Hf isotopic variations, generated by β⁻-decay of ¹⁷⁶Lu, in volcanic rocks derived from the suboceanic mantle. Normalized to ¹⁷⁶Hf/¹⁷⁷Hf = 0.7325, ¹⁷⁶Hf/¹⁷⁷Hf ranges from 0.2828 to 0.2835, based on 24 basalt samples. ¹⁷⁶Hf/¹⁷⁷Hf is positively correlated with ¹⁴³Nd/¹⁴⁴Nd, and negatively correlated with ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb. Along the Iceland-Reykjanes ridge traverse, ¹⁷⁶Hf/¹⁷⁷Hf increases southwards. The coherence of Hf, Nd and Sr isotopes in the oceanic mantle allows an approximate bulk Earth ¹⁷⁶Hf/¹⁷⁷Hf of 0.28295 to be inferred from the bulk Earth ¹⁴³Nd/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 ¹⁷⁶Hf/¹⁷⁷Hf ratios developing in mantle residual from partial melting.