Evaluation of the effects of alteration and leaching on Sm-Nd and Lu-Hf systematics in submarine mafic rocks

Nd and Hf isotope systematics of oceanic basaltic rocks are often assumed to be largely immune to the effects of hydrothermal alteration. We have tested this assumption by comparing Nd and Hf isotope data for acid-leached Cretaceous oceanic basalts front Gorgona and DSDP Leg 15 with unleached data on the same rocks. Hf isotope values and Lu/Hf ratios are relatively unaffected by leaching, but Nd-143/Nd-144 values of leached samples are significantly higher than those of unleached fractions of the same sample in most cases. Furthermore, the Sm/Nd ratios of the majority of leached samples are 10-40% greater than those of unleached samples. X-ray diffraction studies indicate that selective removal of secondary minerals, such as smectite, during the acid leaching process is responsible for the fractionation of Sm/Nd ratios. These results have implications for interpretation of the Hf-Nd isotope systematics of ancient submarine rocks (older than similar to 50 Ma), as (1) the age-corrected Nd-143/Nd-144 ratio may not be representative of the primary magmatic signature and (2) the uncertainty of the age-corrected epsilon(Nd) value may exceed the assumed analytical precision. (C) 2007 Elsevier B.V. All rights reserved.