SHRIMP U-Th-Pb Analytical Techniques

SHRIMP U-Th-Pb analytical techniques

Analyses for major, minor and trace elements were determined by fusion-ICP/MS at the Acme Analytical Laboratories Ltd., Vancouver (Canada). 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 along with several pieces of Temora zircon standard (Black et al. 2003) in an epoxy resin disc. The disc was ground-down and polished so as to effectively section the zircons in half. Zircon grains were imaged optically using reflected and transmitted light. The mount was carbon coated for cathodoluminescence (CL) imaging and then cleaned and gold coated in readiness for ion microprobe analysis. 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 Centre, Chinese Academy of Geological Sciences. Instrumental conditions and data acquisition were generally as described by Compston et al. (1992) and Williams (1998). 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. The zircon standards used were M257 (561.3 ± 0.3 Ma, U ~ 840 ppm, Th/U ~ 0.27; Nasdala et al. 2008) for U concentrations and Temora (416.8 ± 1.1 Ma; Black et al. 2003) for interelement fractionation. 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).

Cited references

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 London 149:171-184.

Nasdala L, Hofmeister WG, Norberg N, Mattinson JM, Corfu F, Dorr W, Kamo SL, Kennedy AK,

Kronz A, Reiners PW, Frei D, Kosler J, Wan YS, Gotze J, Hager 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 costants for geochronology using ID-TIMS U-Pb data. Geochim Cosmochim Acta 70: 426-

445.

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 MA, Shanks III

WC, Ridley WI (eds) Applications of microanalytical techniques to understanding mineralizing

processes. Rev Econ Geology 7: 1-35.