Analytical techniques

Besides petrographic studies of the thin sections, all rock samples were investigated by XRF for major and trace elements at the Institute of Mineralogy and Geochemistry, University of Lausanne (Table 1) using a Philips PW 2400 spectrometer. The BHVO-1, NIMN and GA basaltic and rhyolitic standards were used for quality control. The 2 uncertainties based on repeated measurements of these standards are <1% for all major elements and <5% for trace elements. Concentrations of rare earth elements (REE) and other trace elements such as U, Th, Ta, Hf, Cs, Sc (Table 1) were determined by LA-ICP-MS on glass beads (used for major element XRF analyses) at the Institute of Mineralogy and Geochemistry, University of Lausanne, using an ArF 193 nm Excimer Laser System (Geolas®) associated with an ICP-MS Perkin-Elmer ELAN 6100 DRC. The laser system is characterized by a laterally homogeneous energy distribution, permitting depth-controlled ablation at a rate of 0.1-0.2mm/shot, depending on laser energy and matrix chemistry. Operating conditions of the laser were: 10 Hz frequency, 120-160 mJ energy, 120 µm spot size. The sample was loaded together with an SRM612 glass from NIST in an ablation cell and placed under a petrographic microscope. The Laser ablation aerosol was transported to the ICP-MS instrument by a mixed Ar-He carrier gas. Two analyses on the external standard at the beginning and the end of each set bracketing up to 16 analyses of unknowns are required for the off-line data reduction. The measurement time for blank correction was about 90 seconds after which the laser was switched on and the signal was measured for about 40 seconds. For each sample 3 to 4 points were measured and results were then averaged. Uncertainties between results of repeat points of the same sample were <10%. CaO contents previously determined by XRF on the glass beads were used for internal standardization by reference to SRM612 from NIST. Raw data were reduced off-line using the LAMTRACE program. Limits of detection were calculated for each element as three times the standard deviation of the gas background signal divided by the element sensitivity (Longerich et al. 1996).

All rocks were also analyzed for Pb, Sr and Nd isotopic compositions at the Department of Mineralogy, University of Geneva. Between 100 and 150 mg of powdered rock fractions (<70 µm) were leached overnight at room temperature with 3M HCl to remove possible surface contamination. The leachate was discarded and the residue, after being rinsed with deionized water twice, centrifuged and separated from the supernatant, was dissolved on a hot plate at 140°C in closed Teflon vials during 7 days with a mixture of 4 ml conc. HF and 1 ml HNO3 15 M. The sample was then dried on a hot plate, and redissolved in 3 ml of 15M HNO3 in closed Teflon vials at 140°C and dried down again. Sr, Nd and Pb separation was carried out using cascade columns with Sr-spec, TRU-spec and Ln-spec resins following a modified method after Pin et al. (1996). Pb was further purified with a AG-MP1-M anion exchange resin in hydrobromic medium.

Pb, Sr and Nd isotope ratios were measured on a Thermo Finnigan TRITON mass spectrometer on Faraday cups in static mode. Pb was loaded on Re filaments using the silica gel technique and all samples (and standards) were measured at a pyrometer controlled temperature of 1220°C. Pb isotope ratios were corrected for instrumental fractionation by a factor of 0.07% per amu based on more than 90 measurements of the SRM981 standard and using the standard values of Todt et al. (1996). External reproducibility (2) of the standard ratios are 0.05% for 206Pb/204Pb, 0.08% for 207Pb/204Pb and 0.10% for 208Pb/204Pb.

Sr was loaded on single Re filaments with a Ta oxide solution and measured at a pyrometer-controlled temperature of 1490°C. 87Sr/86Sr values were internally corrected for fractionation using a 88Sr/86Sr value of 8.375209. Raw values were further corrected for external fractionation by a value of +0.03‰, determined by repeated measurements of the SRM987 standard (87Sr/86Sr = 0.710250). External reproducibility (2) of the SRM987 standard is <7 ppm. Nd was loaded on double filaments with 1M HNO3. 143Nd/144Nd values were internally corrected for fractionation using a 146Nd/144Nd value of 0.7219 and the 144Sm interference on 144Nd was monitored on the mass 147Sm and corrected by using a 144Sm/147Sm value of 0.206700. External reproducibility (2) of the JNdi-1 standard (Tanaka et al., 2000) is 4 ppm.

References

Longerich HP, Jackson SE, Günter D (1996) Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. J Anal At Spectrom 11: 899–904

Pin C, Santos Zalduegui JF (1997) Sequential Separation of Rare-earth Elements, Thorium and Uranium by Miniaturized Extraction Chromatography: Application to Isotopic Analyses of Silicate Rocks. Analytica Chim Acta 339: 79-89

Tanaka T, Togashi S, Kamioka H, Amakawa H, Kagami H, Hamamoto T, Yuhara M, Orihashi Y, Yoneda S, Shimizu H, Kunimaru T, Takahashi K, Yanagi Y, Nakano T, Fujimaki H, Shinjo R, Asahara Y, Tanimizu M, Dragusanu C (2000) JNdi-1: a neodymium isotopic reference in consistency with La Jolla neodymium. Chem Geol 168: 279–281

Todt W, Cliff RA, Hanser A, Hofmann AW (1996) Evaluation of a 202Pb–205Pb double spike for high-precision lead isotope analysis. In A Basu, S Hart (eds) 1996. Earth Processes: Reading the Isotopic Code, Geophys Monogr 95, American Geophys Union, Washington, pp 429–437