Supplementary publication to: “Terrane analysis along a Neoproterozoic active margin of Gondwana: insights from u-pb zircon geochronology”




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НазваниеSupplementary publication to: “Terrane analysis along a Neoproterozoic active margin of Gondwana: insights from u-pb zircon geochronology”
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Supplementary publication to: “Terrane analysis along a Neoproterozoic active margin of Gondwana: insights from U-Pb zircon geochronology”

Strachan et al.

Analytical Techniques

Zircons were separated from crushed rock samples by conventional magnetic and methylene iodide liquid separation. Grains were handpicked and mounted in epoxy resin discs that were coated with a thin membrane of gold that produced a resistively of 10-20 ohms across the disc. The mounts were then imaged using a CL detector fitted to a Phillips XL30 scanning electron microscope at a working distance of 15 mm and using an accelerating voltage of 10 Kv. The resulting images highlight distortions in the crystal lattice (Stevens Kalceff et al. 2000) that are related to trace-element distribution and/or radiation damage (e.g. Rubatto & Gebauer 2000; Nasdala et al. 2003).

Zircon U-Th-Pb isotopic data was collected using the Sensitive High Resolution Ion Microprobe Mass Spectrometer (SHRIMP II) based in the John de Laeter Centre of Mass Spectrometry, Perth, Western Australia. The sensitivity for Pb isotopes in zircon using SHRIMP II was ~18 cps/ppm/nA, the primary beam current was 2.5-3.0 nA and mass resolution was ~5000. Correction of measured isotopic ratios for common Pb was based on the measured 204Pb in each sample and often represented a <1% correction to the 206Pb counts (see %common 206Pb in Table 1). Any common Pb component in the standard was interpreted as surface contaminant and modelled on the composition of Broken Hill ore Pb, additional common Pb in the unknowns was modelled on the approximate age of the grain using the Pb isotope evolution model of Stacey and Kramers (1975).

Pb/U isotopic ratios were corrected for instrumental inter-element discrimination using the observed covariation between Pb+/U+ and UO+/U+ (Hinthorne et al. 1979; Compston et al. 1984) determined from interspersed analyses of the Perth standard zircon CZ3. CZ3 is a single zircon megacryst from Sri Lanka with an age of 564 Ma and a 206Pb/238U=0.0914 (Nelson 1997). The calibration constant, spot external 2σ error and standard 2σ mean error were, for COE1, 0.00504, 5.49%, and 1.88% respectively; for Gu1 these were, 0.00528, 5.64% and 1.91% respectively. The standard 2σ mean error was incorporated into the quoted spot errors to allow comparison between analytical techniques.


Table Heading

Table 1: SIMS U-Pb isotopic data.

References


Compston, W., Williams, I.S. & Meyer, C. 1984. U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe. Journal of Geophysical Research, 89(Supplement), B525-B534.

Hinthorne, J.R., Anderson, C.A., Conrad, R.L. & Lovering, J.F. 1979. Single-grain 207Pb/206Pb and U/Pb age determinations with a 10 µm spatial resolution using the ion microprobe mass analyser (IMMA). Chemical Geology, 25, 271-303.

Nasdala, L. Zhang, M., Kempe, U., Panczer, M., Gaft, M., Andrut, M. & Plötze, M. 2003. Spectroscopic methods applied to zircon. In: J.M. Hanchar and P.W.O. Hoskin (eds), Zircon. Mineralogical Society of America, Reviews in Mineralogy & Geochemistry, Volume 53, Washington, D.C., 427-467.

Nelson, D.R. 1997. Compilation of SHRIMP U-Pb zircon geochronology data, 1996. Geological Survey of Western Australia, Record 1997/2, 189 pp.

Rubatto, D. & Gebauer, D. 2000. Use of cathodoluminescence for U-Pb zircon dating by ion microprobe: some examples from the western Alps. In: M. Pagel, V. Barbin, P. Blanc and D. Ohnenstetter (eds), Cathodoluminescence in Geosciences. Springer-Verlag, Berlin, 373-400.

Stacey, F.S. & Kramers, J.D. 1975. Approximation of terrestrial lead isotope evolution by two-stage model. Earth and Planetary Science Letters, 26, 207-221.

Stevens Kalceff, M.A., Phillips, M.R., Moon, A.R. & Kalceff, W. 2000. Cathodoluminescence microcharacterisation of silicon dioxide polymorphs. In: M. Pagel, V. Barbin, P. Blanc and D. Ohnenstetter (eds), Cathodoluminescence in Geosciences. Springer-Verlag, Berlin, 193-224.



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