Evaluating the effect of leaching on trace element and Nd-Pb isotopic systematics in continental basalts

Abstract

Leaching technique was mostly carried out on submarine rock powder to minimize or eliminate secondary effect of seawater interaction on Sr and Pb isotope magmatic signatures. However, the effect of leaching on trace element contents is comparatively poorly established. To investigate such effects, we have compared trace element and Nd-Pb isotopic data for acid leached ∼290 Ma Keping basalts in the Tarim Large Igneous Province (NW China) with unleached data on the same rocks. Rare earth elements (REE), Y, Th and U contents of leached sample grains (0.6–1.0 mm size) are 10–65% lower than those of unleached grains of the same sample, whereas other trace elements are not affected. Sm/Nd, Lu/Hf, U/Pb and Th/Pb ratios are fractionated by acid leaching between leached and unleached sample grains. Nd isotope values are unaffected by leaching, but 208Pb/204Pb and 206Pb/204Pb values of leached samples are significantly lower than those of unleached samples. X-ray diffraction results suggest that removal of secondary epidote during the acid leaching process could result in the fractionation of REE, Y, Th and U contents, and Sm/Nd, Lu/Hf, U/Pb and Th/Pb ratios, and 208Pb/204Pb and 206Pb/204Pb values. These results suggest that when the samples have been leached with strong acid (>1N HCl), their trace element cannot be used to trace the sources and petrogenesis. If leaching is applied to old (e.g., >100 Ma) continental basalts in order to eliminate contamination during sample collection, storage, sawing and crushing, Th/Pb and U/Pb ratios used to calculate the initial Pb isotope values, should be obtained on the same leached samples. Keywords: Tarim Large Igneous Province, Trace element, Nd-Pb isotopes, Alteration, Acid leaching