Zircon dating

The isotopic age range of the Tyrone Volcanic Group is c. 473 - 464 Ma and is based on eight U-Pb zircon dates. 

Pb / U vs. Pb / U plot, where denotes the radiogenic portion of Pb isotopes. This diagram also finds wide applications in zircon U-Pb geochronology. For U-Pb zircon dating, the daughter-parent ratios are given by... 

With these caveats, one can deduce the following. Early single grains appear to have recorded hafnium isotopic compositions that provide evidence for chondritic or enriched reservoirs. There is no evidence of depleted reservoirs in the earliest (Hadean) zircons dated thus far (Amelin et al., 1999). Use of alternative values for the decay constants or values for the primitive mantle parameters increases the proportion of hafnium with an enriched signature (Amelin et al., 2000), but does not provide evidence for early mantle depletion events. Therefore, there is little doubt that the Hadean mantle was extremely well mixed. Why this should be is unclear, but it probably relates in some way to the lack of preserved continental material from prior to 4.0 Ga. 

Konzett J., Armstrong R. A., and Gunther D. (2000) Modal metasomatism in the Kaapvaal craton lithosphere constraints on timing and genesis from U-Pb zircon dating of... 

Schmitz M. D. and Bowring S. A. (2000) The significance of U—Pb zircon dates in lower crustal xenohths from the southwestern margin of the Kaapvaal craton, southern Africa. Chem. Geol. 172, 59—76. 

Bruguier O., Telouk P., Cocherie A., Fouillac A. M., and Albarede F. (2001) Evaluation of Pb-Pb and U-Pb laser ablation ICP-MS zircon dating using matrix-matched calibration samples with a frequency quadrupled (266 nm) Nd-YAG laser. Geostand. Newslett. J. Geostand. Geoanal. 25(2-3), 361-373. 

Kober B., Pidgeon R. T., and Lippolt H. J. (1989) Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack HiUs, Western Austrdia. Earth Planet. Sci. Lett. 91, 286-296. 

Kroner A., Compston W., and WiUiams 1. S. (1989) Growth of early Archaean crust in the Ancient Gneiss Complex of Swaziland as revealed by single zircon dating. Tectonophy-sics 161, 271-298. 

Williams, I. S. Collins, W. J. 1990. Granite-greenstone terranes in the Pilbara Block, Australia, as coeval volcano-plutonic complexes evidence from U-Pb zircon dating of the Mount Edgar Batholith. Earth and Planetary Science Letters, 97, 41-53. 

Zegers, T. E., Nelson, D. R., Wijbrans, J. R. White, S. H. 2001. SHRIMP U-Pb zircon dating of Archean core complex formation and pancra-tonic strike-slip deformation in the East Pilbara granite-greenstone terrain. Tectonics, 20, 883-908. 

Dirks, P. H. G. M., Kroner, A., Jelsma, H. A., SiTHOLE, T. A. ViNYU, M. L. 1999. Pb-Pb zircon dates from the Makuti gneisses evidence for a crustal-scale Pan-African shear zone in the Zambezi belt, NW Zimbabwe. Journal of African Earth Sciences, 28, 427-442. 

Oberh F, Fischer H, Meier M (1990) High-resolution U- ° Pb zircon dating of Tertiary bentonites and the Fish Canyon Tuff a test for age concordance by single-crystal analysis. Geol Soc Australia 27 74 (abstr)... 

Bouman, C., Cocherie, A., Robert, M., Schwieters, J.B., Wiser, M. (2003) In situ U-Pb zircon dating using laser ablation multi ion counting ICP-MS (LA-MIC-ICPMS). Thirteenth Annual Goldschmidt Conference, September 7-12,2003, Kurashiki, Japan. 

Cocherie, A., Rossi, R, Fanning, C.M., Guerrot, C. (2005) Comparative use of TIMS and SHRIMR for U-Rb zircon dating of A-type granites and mafic tholeiitic layered complexes and dykes from Corsican Batholith (France). Lithos,82,185-219. 

Since the middle of the 1980s, the traditional method of zircon geochronology was complemented by the use of the sensitive high-resolution ion microprobe (SHRIMP) [17], with which 15 pm diameter spots on zircons can be dated, and metamict or mixed-age domains avoided. Since minimal amounts of material are used, this method is still preferred in cases where the material to be analyzed is scarce, such as zircons from extraterrestrial samples, or the early Archean ( 4.4 Ga) [18]. Unfortunately, the number of SHRIMP instmments is limited, and in cases where scarcity of material is not a problem, and a precision on the level of a few million years is acceptable, SHRIMP and TIMS techniques for zircon dating have been largely replaced by LA-ICP-MS analyses, pioneered in the early 1990s [19,20]. Here a laser beam is used to excavate a small (15-90 pm diameter) pit in the mineral, from which the ablated material is transferred to the plasma of an ICP-MS instrument, where ionization and analysis take place (see also Chapters 2 and 4). 

Klotzli, U., Klotzli, E., Gunes, Z., and Koslar, J. (2009) Accuracy of laser ablation U-Pb zircon dating results from a test using five different reference zircons. Geostand. Geoanal. Res., 33 (1), 5-15. 

Hartlaub, R.P., Greaser, R.A., MacHattie, T.G., and Bohm, C. (2005) U-Pb zircon dating by laser ablation-MC-ICP-MS using a new multiple ion counting Faraday collector array. 

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