Assessment of hafnium and iron isotope compositions of Chinese national igneous rock standard materials GSR-1 (granite), GSR-2 (andesite), and GSR-3 (basalt)

摘要

In this contribution, we investigate the suitability of three igneous rock standards distributed by the Chinese Academy of Geological Sciences as international standard reference materials for Hf-Fe isotope determinations. We report Lu-Hf concentrations and Hf-Fe isotope compositions of Chinese National Standards (CNS) GSR-1 (granite), GSR-2 (andesite), and GSR-3 (basalt) in order to assess their homogeneity and give the first benchmark values. Experiments using spiked (isotope dilution) and non-spiked samples reveal an overall homogeneous Hf isotope composition for CNS GSR-2 and GSR-3 with Hf-176/Hf-177=0.282641 +/- 6 (MSWD = 1.5, n = 10) and 0.282985 +/- 04 (MSWD = 0.48, n = 10), respectively. In contrast to GSR-2 and GSR-3, two out of ten analyses of GSR-1 show elevated Hf concentrations in conjunction with less radiogenic Hf isotope compositions, considerably lower to -10.6 and -16.8 epsilon(Hf) units, respectively, compared to the average of the remainder analyses. We ascribe this heterogeneity to inherited zircon grains so often observed in granitic rocks. Iron isotope determinations on a subset of samples yields values of delta Fe-57 = +0.35 +/- 0.02, +0.22 +/- 0.05, +0.43 +/- 0.02 (relative to IRMM-014, n = 3) for GSR-1, GSR-2, and GSR-3, respectively. Our results indicate that GSR-2 and GSR-3 appear suitable standards for Hf-Fe isotope analyses, but advise caution when using GSR-1 as a measure for quality control for Hf isotope compositions and concentrations. (C) 2015 Elsevier B.V. All rights reserved.