Cu isotope signature of granites

Weiqiang Li, Simon E. Jackson, Bruce W. Chappell, Olivier Alard, Norman J. Pearson

Research output: Contribution to journalMeeting Abstractpeer-review

Abstract

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 Maréchal
[Maréchal, C.N., Télouk, P. and Albarède, 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 (δ65Cu 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‰ to
1.51‰. Most of them cluster around zero, with mean values for the I-type and S-type granites of 0.03±0.15‰
and −0.03±0.42‰ (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‰) 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.
Original languageEnglish
Pages (from-to)38-49
Number of pages12
JournalGeochimica et Cosmochimica Acta
Volume71
Issue number15
Publication statusPublished - 24 Jun 2008
Event17th Annual V M Goldschmidt Conference - Cologne, Germany
Duration: 1 Aug 2007 → …

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