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Interpretations of SHRIMP and isotope dilution zircon ages for the geological time-scale: I. The early Ordovician and late Cambrian

Published online by Cambridge University Press:  25 June 2018

W. Compston
W. Compston*
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra 0200, A.C.T., Australia
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Abstract

Ion probe data for zircons from tuffs within the Llfynant flags (Arenig) and the Serw Formation (lower Llanvirn) of north Wales have been revised using better statistical methods for separating detrital ages, making allowance for recently-found variability in radiogenic 206Pb/238U in the reference zircon SL13, and testing the sensitivity of the ages to the secondary ion discrimination slope. The revised ages are options of 469.2 ± 2.1 (σ) or 472.9 ± 2.9 Ma for the Llfynant flags dependent on mixture modelling, and 465.3 ± 1.4 Ma for the Serw Formation. All ages are within error of previous SHRIMP results and the Serw age now has the same numerical value as a previous MSID age for the same sample. It is shown that an MSID age of 483 ± 0.5 Ma with interpreted Pb loss for a late Tremadoc bentonite is dependent on the correction for common Pb, and that a slightly more radiogenic choice for the common Pb composition places nearly all data on Concordia. The latter would indicate that the bentonite might contain two zircon populations: inherited grains at 482 Ma and tuff magmatic grains at 473 Ma, which is more compatible with the SHRIMP Arenig result. Interpretations of other MSID zircon ages from the Ordovician are also sensitive to choice of common Pb, and raise the likelihood that many multigrain ages might be too old owing to admixture with slightly older inherited zircon. A supposed 1–2% technical bias of SHRIMP 206Pb/238U ages relative to MSID is refuted.

Keywords

SHRIMPisotope dilution zircon agesgeological time-scaleOrdovicianCambrian
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 1 , February 2000 , pp. 43 - s14
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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References

Claoué-Long, J.C., Compston, W., Roberts, J. and Fanning, C.M. (1995) Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40Ar/39Ar analysis. SEPM Special Publication 3, Geochronology Time Scales and Global Stratigraphic Correlation. 54, 3–21.CrossRefGoogle Scholar
Compston, W. (1999) Geological age by instrumental analysis: the 29th Hallimond Lecture. Mineral. Mag., 63, 297–311.CrossRefGoogle Scholar
Compston, W. and Williams, I.S. (1992) Ion probe ages from the British Ordovician and Silurian stratotypes. Pp. 59–67 in: Global Perspectives on Ordovician Geology (Webby, B.D. and Laurie, J.R., editors). Balkema, Rotterdam.Google Scholar
Davidek, K., Landing, E., Bowring, S.A., Westrop, S.R., Rushton, A.W.A., Fortey, R.A. and Adrain, J.M. (1998) New uppermost Cambrian U-Pb date from Avalonian Wales and age of the Cambrian-Ordovician boundary. Geol. Mag., 135, 305–9.CrossRefGoogle Scholar
Dunning, G.R. and Krogh, T.E. (1991) Stratigraphic correlation of the Appalachian Ordovician using advanced U-Pb zircon geochronology techniques. Geol. Surv. Canada, Paper 90-9, 85–92.Google Scholar
Keay, S., Steele, D. and Compston, W. (1999) Identifying granite sources by SHRIMP U-Pb zircon geochronology: an application to the Lachlan fold belt. Contrib. Mineral. Petrol., 137, 323–41.CrossRefGoogle Scholar
Mortimer, G. E. (1984) Early to Middle Proterozoic granitoids, basaltic dykes and associated layered rocks of S.E. Eyre Peninsula, South Australia. Ph.D. thesis, University of Adelaide.Google Scholar
Landing, E., Bowring, S.A., Fortey, R.A. and Davidek, K. (1997) U-Pb zircon date from Avalonian Cape Breton Island and geochronologic calibration of the Early Ordovician. Canad. J. Earth. Sci., 34, 724–30.CrossRefGoogle Scholar
Landing, E., Bowring, S.A., Davidek, K.L., Westrop, S.R., Geyer, G. and Heldmaier, W. (1998) Duration of the Early Cambrian: U-Pb ages of volcanic ashes from Avalon and Gondwana. Canad. J. Earth. Sci., 35, 329–38.CrossRefGoogle Scholar
Ludwig, K. (1998) On the treatment of concordant uranium-lead ages. Geochim. Cosmochim. Acta, 62, 665–76.CrossRefGoogle Scholar
McClaren, A.C., Fitz Gerald, J.D. and Williams, I.S. (1995) The microstructure of zircon and its influence on the age determination from Pb//U isotopic ratios measured by ion microprobe. Geochim. Cosmochim. Acta, 58 993–1005.CrossRefGoogle Scholar
Perkins, C. and Walshe, J.L. (1993) Geochronology of the Mount Read Volcanics, Tasmania, Australia. Econ. Geol., 88, 1176–97.CrossRefGoogle Scholar
Roddick, J.C. and Bevier, M.L. (1995) U-Pb dating of granites with inherited zircon: Conventional and ion microprobe results from two Paleozoic plutons, Canadian Appalachians. Chem. Geol. (Isotope Geoscience Section), 119, 307–29.Google Scholar
Ross, R.J. Jr, Naeser, C.W., Izett, G.A., Obradovich, J.D., Bassett, M.G., Hughes, C.P., Cocks, L.R., Dean, W.T., Ingham, J.K., Jenkins, C.J., Richards, R.B., Sheldon, P.R., Toghill, P., Whittington, B.H. and Zalasiewicz, J. (1982) Fission-track dating of British Ordovician and Silurian stratotypes. Geol. Mag., 119, 135–53.CrossRefGoogle Scholar
Sambridge, M.S., Compston, W. (1994) Mixture modelling of multi-component data sets with application to ion-probe zircon ages. Earth Planet. Sci. Letters, 128, 373–90.CrossRefGoogle Scholar
Stacey, J.S. and Kramers, J.D. (1975) Approximation of terrestrial lead isotope evolution by a two stage model. Earth Planet Sci Letters, 27, 201–21.Google Scholar
Tucker, R.D. and McKerrow, W.S. (1995) Early Paleozoic chronology: a review in light of new U-Pb zircon ages from Newfoundland and Britain. Canad. J. Earth Sci., 32 368–79.CrossRefGoogle Scholar
Tucker, R.D., Krogh, T.E., Ross, R.J. and Williams, S.H. (1990) Time-scale calibration by high-precision U-Pb zircon dating of interstratified volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain. Earth Planet Sci Letters, 100 51–8.CrossRefGoogle Scholar