The Cu isotopic signature of granites from the Lachlan Fold Belt, SE Australia

This contribution reports our preliminary work to determine Cu isotope ratios for various granite rocks and examine the Cu isotope systematics within granite suites. A chemical procedure, modified from Marechal [Marechal, C.N., Telouk, R and Albarede, F., 1999. Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry. Chemical Geology, 156(1-4): 251-273.], Was used to separate Cu from rock matrix. Quantitative recovery (100.6 +/- 1.6%), with a low total procedural blank (2.65 +/- 0.66 ng) for Cu, has been achieved, allowing Cu isotopic measurements on samples with as little as 10 ppm Cu. The Cu isotope ratios (delta Cu-65 relative to NIST SRM 976) of 32 rock samples, ranging from mafic to felsic compositions, from 3 batholiths (2 I-type, 1 S-type) from the Lachlan Fold Belt in southeastern Australia. vary from -0.46 parts per thousand to 1.51 parts per thousand. Most of them cluster around zero, With mean values for the I-type and S-type granites of 0.03 +/- 0.15 parts per thousand and - 0.03 +/- 0.42 parts per thousand (2 sigma) respectively. These data, together with Cu isotope ratios of two loess samples, provide preliminary evidence that the baseline Cu isotopic composition of the crystalline part of upper continental crust is close to zero. The tight clustering of Cu isotope ratios of rocks from the I-type suites Suggests that high-temperature magmatic processes do not produce significant Cu isotope fractionation. However, two granites with abnormally heavy Cu isotope signatures (up to 1.51 parts per thousand) appears to be the result of localized hydrothermal alteration. Measurable variation in Cu isotopic composition of the S-type granite may reflect isotopic heterogeneity in the sedimentary Source region as a result of redox processes or may be due to hydrothermal overprinting. Thus, Cu isotope geochemistry may be a useful tracer for studying hydrothermal alteration and Source heterogeneity of granitic rocks. (C) 2008 Elsevier B.V. All rights reserved.