The Tertiary dike magmatism in the Southern Alps:
Geochronological data and geodynamic significance
Bergomi MA, Zanchetta S, Tunesi A
Department of Earth and Environmental Sciences
University of Milano-Bicocca
Piazza della Scienza 4
20126 Milano - Italy
1. Analytical methods
1.1 Whole rock geochemical analyses
Analyses for major, minor and trace elements were determined by at the ACME Analytical Laboratories Ltd. in Vancouver (Canada). Total abundances of the major oxides were obtained by ICP-ES (Inductively Coupled Plasma Emission Spectroscopy), whereas REE (Rare Earth Element), refractory elements and precious and base metals by ICP-MS (Mass Spectroscopy). Sample preparations follow a LiBO2 fusion and dilute nitric digestion for major oxides, REE and refractory elements, whereas precious and base metals were digested in acquaregia. Analytical errors are within 2% for major elements and in the 5% - 10% range for trace elements.
1.2 SHRIMP U-Pb zircon dating
Zircon separation was carried out at laboratories of the Department of Geological Sciences and Geotechnologies of the University of Milano-Bicocca. After crushing and sieving, zircon grains were separated from the rock powders using a combination of magnetic and heavy-liquid separation techniques. Individual zircon crystals were hand picked and mounted in epoxy resin together with chips of the Temora zircon standard (Black et al. 2003). The mount was then polished, cleaned and photographed in reflected and transmitted light and under cathodoluminescence (CL). CL images were obtained on a Hitachi S-3000N scanning electron microscope equipped with a Gatan ChromaCL cathodoluminescence, and operating conditions were 15kV accelerating voltage and 12 nA beam current.
U-Th-Pb analyses were carried out at the new SHRIMP Remote Operation System (SROS) lab at the University of Milano-Bicocca (Italy) using the SHRIMP II ion microprobe located at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences. Instrumental conditions and data acquisition were generally as described by Compston et al. (1992) and Williams (1998 and reference therein). An average mass resolution of > 5000 (1%) was obtained during measurement of Pb/Pb and Pb/U isotopic ratios. Five scans through the masses Zr2O+, 204Pb+, 206Pb+, 207Pb+, 208Pb+, 238U+, 248ThO+, and 254UO+ were made for each age determination. Reference zircon from Temora granodiorite (416.8 ± 1.1 Ma; Black et al. 2003) was used to correct the measured 206Pb/238U ratios of our unknown samples. A zircon of known composition (M257, 561.3 ± 0.3 Ma, U ~ 840 ppm, Th/U ~ 0.27; Nasdala et al. 2008) was used to determine the U content. . The beam size was ~20-25 μm and each analysis spot was rastered over 120–200 s prior to analysis, in order to remove any common Pb on the surface or contamination from gold coating. The 235U decay constant used for age calculation is after Schoene et al. (2006), whereas the 238U one is after the IUGS Subcommission on Geochronology (Steiger and Jäger 1977).
The analyses were corrected for common Pb using three different methods based on 204Pb, 207Pb and 208Pb measurements following Williams (1998). All three corrections returned results identical within error. The common Pb composition was obtained according to the Stacey and Kramers (1975) model. Additionally, weighted average 206Pb/238U ages, given in Online Resource 3 and discussed in the paper, are always within error of the Concordia ages. U-Pb data were collected over three analytical sessions using the same standard, with the different sessions having calibration errors between 0.44% and 2.11% (2σ), which were propagated to single analyses. Additionally, a 1% external error was propagated to the final average age. Data evaluation and age calculation were done using Squid 1.02 (Ludwig 2003a) and Isoplot/Ex3 (Ludwig 2003b), respectively.
Black LP, Kamo SL, Allen CM, Aleinikoff JN, Davis DW, Korsch RJ, Foudoulis C (2003) Temora 1: a new zircon standard for Phanerozoic U-Pb geochronology. Chem Geol 200: 155–170.
Compston W, Williams IS, Kirschvink JL, Zhang Z, Ma G (1992) Zircon U-Pb ages for the Early Cambrian time-scale. J Geol Soc 149: 171–184.
Ludwig KR (2003a) SQUID 1.02: A User’s Manual. Berkeley Geochronological Center, Special Publication 2.
Ludwig KR (2003b) A User’s Manual for Isoplot/Ex 3: A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronological Center, Special Publication 4.
Nasdala L, Hofmeister W, Norberg N, Mattinson JM, Corfu F, Dörr W, Kamo SL, Kennedy AK, Kronz A, Reiners PW, Frei D, Kosler J, Wan Y, Götze J, Häger T, Kröner A, Valley JW (2008) Zircon M257 - a homogeneous natural reference material for the ion microprobe U-Pb analysis of zircon. Geostand Geoanal Res 32: 247–265.
Schoene B, Crowley JL, Condon DJ, Schmitz MD, Bowring SA (2006) Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data. Geochim Cosmochim Acta 70: 426–445.
Stacey JS Kramers JD (1975) Approximation of terrestrial lead evolution by a two-stage model. Earth Planet Sci Lett 26: 207–221.
Steiger RH, Jäger E (1977) Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet Sci Lett 36: 359–362.
Williams IS (1998) U-Th-Pb geochronology by ion microprobe. In: McKibben, M.A., Shanks, W.C. III, Ridley, W.I. (Eds): Applications of microanalytical techniques to understanding mineralizing processes. Rev Econ Geol 7: 1–35.