Online Supplementary Publication. Geochronological procedures and results




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Online Supplementary Publication. Geochronological procedures and results


A-1. Analytical procedures

SHRIMP-II U-Pb dating procedure

Zircon grains were hand selected and mounted in epoxy resin together with chips of the TEMORA (Middledale Gabbroic Diorite, New South Wales, Australia) and reference zircons (91500 Geostandard zircon). The grains were sectioned approximately in half and polished. Reflected and transmitted light photomicrographs and cathodoluminescence (CL) SEM images were prepared for all zircons. The CL images were used to decipher the internal structures of the sectioned grains and to target specific areas within these zircons. The U-Pb analyses of the zircons were made using SHRIMP-II ion microprobe (Center of Isotopic Research, VSEGEI, St.Petersburg, Russia). Each analysis consisted of 5 scans through the mass range. The spot diameter was about 25 microns and the primary beam intensity was about 2 nA. The data have been reduced in a manner similar to that described by Williams (1998, and references therein), using the SQUID Excel Macro of Ludwig (2000). The Pb/U ratios have been normalized relative to a value of 0.0668 for the 206Pb/238U ratio of the TEMORA reference zircons, equivalent to an age of 416.75 Ma (Black et al., 2003). Uncertainties given for individual analyses (ratios and ages) are at the one s level, however, the uncertainties in calculated concordant ages are reported at the two s level. The Ahrens-Wetherill concordia plot (Wetherill, 1956) has been prepared using ISOPLOT/EX (Ludwig, 1999).

40Ar-39Ar determinations

Ar-Ar determinations were conducted at the University of Alaska Geochronology Laboratory. The monitor mineral MMhb-1 (Samson & Alexander, 1987) with an age of 513.9 Ma (Lanphere & Dalrymple, 2000) was used to monitor neutron flux (and calculate the irradiation parameter, J). The samples and standards were wrapped in aluminium foil and loaded into aluminium holders of 2.5 cm diameter and 6 cm height. The samples were irradiated in position 5c of the uranium enriched research reactor of McMaster University in Hamilton, Ontario, Canada for 20 megawatt-hours.

Upon their return from the reactor, the samples and monitors were loaded into 2 mm diameter holes in a copper tray that was then loaded in an ultra-high vacuum extraction line. The monitors were fused, and samples heated, using a 6-watt argon-ion laser following the technique described in York et al. (1981), Layer et al. (1987) and Layer (2000). Argon purification was achieved using a liquid nitrogen cold trap and a SAES Zr-Al getter at 400 ºC. The samples were analysed in a VG-3600 mass spectrometer at the Geophysical Institute, University of Alaska Fairbanks. The argon isotopes measured were corrected for system blank and mass discrimination, as well as calcium, potassium and chlorine interference reactions following procedures outlined in McDougall & Harrison (1999). System blanks generally were 2.10-16 mol 40Ar and 2.10-18 mol 36Ar, which are 10 to 50 times smaller than the fraction volumes. Mass discrimination was monitored by running both calibrated air shots and a zero-age glass sample. These measurements were made on a weekly to monthly basis to check for changes in mass discrimination.


K–Ar dating of muscovite samples

Mica separation was performed by the standard techniques such as crushing, sieving, Frantz magnetic separation and selection by hand. The pure micas were ground in alcohol and sieved to remove altered rims which might have suffered argon loss. The argon isotopic composition was measured in a Pyrex glass extraction and purification line coupled to a VG 1200 C noble gas mass spectrometer operating in static mode. The amount of radiogenic 40Ar was determined by the isotopic dilution method using a highly enriched 38Ar spike from Schumacher, Bern (Schumacher 1975). The spike is calibrated against the biotite standard HD-B1 (Fuhrmann et al., 1987). The age calculations are based on the constants recommended by the IUGS quoted in Steiger & Jäger (1977). Potassium was determined in duplicate by flame photometry using an Eppendorf Elex 63/61. The samples were dissolved in a mixture of Hf and HNO3 according to the technique of Heinrichs & Herrmann (1990). CsCl and LiCl were added as an ionisation buffer and internal standard, respectively. The analytical error for the K–Ar age calculations has a 95% confidence level of 2σ. The procedural details for argon and potassium analyses at the laboratory in Göttingen are given in Wemmer (1991).

References

Black, L. P., Kamo S. L., Allen C. M., Aleinikoff, J. N., Davis D. W., Korsch, R. J. & Foudoulis, C. 2003. TEMORA 1: a new zircon standard for U-Pb geochronology. Chemical Geology, 200, 155-170.

Fuhrmann, U., Lippolt, H. J. & Hess, J. C. 1987. Examination of some proposed K–Ar standards: 40Ar/39Ar analyses and conventional K–Ar-Data. Chemical Geology (Isotope Geoscience Section), 66, 41–51.

Heinrichs, H. & Herrmann, A. G. 1990. Praktikum der Analytischen Geochemie. Springer Verlag. 669 pp.

Lanphere, M. A., & Dalrymple, G. B. 2000. First-principles calibration of 38Ar tracers: Implications for the ages of 40Ar/39Ar fluence monitors. United States Geological Survey Professional Paper, 1621, 10 p.

Layer, P. W., Hall, C. M. & York, D. 1987. The derivation of 40Ar/39Ar age spectra of single grains of hornblende and biotite by laser step heating. Geophysical Research Letters, 14, 757-760.

Layer, P.W. 2000. Argon-40/argon-39 age of the El’gygytgyn impact event, Chukotka, Russia. Meteoritics and Planetary Science, 35, 591-599.

Ludwig, K. R. 1999. User’s manual for Isoplot/Ex, Version 2.10, A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication, 1a, 2455, Ridge Road, Berkeley CA 94709, USA.

Ludwig, K. R. 1998. On the treatment of concordant uranium-lead ages. Geochimica et Cosmochimica Acta, 62, 665-676.

McDougall, I. & Harrison, T. M. 1999. Geochronology and Thermochronology by the 40Ar/39Ar method-2nd ed., Oxford University Press, New York, 269 pp.

Samson, S. D. & Alexander, E. C. 1987. Calibration of the interlaboratory 40Ar/39Ar dating standard, MMhb1. Chemical Geology, 66, 27-34.

Schumacher, E. 1975. Herstellung von 99,9997% 38Ar für die 40K/40Ar Geochronologie. Geochronologia Chimia, 24, 441–442.

Steiger, R. H. & Jaeger, E. 1977. Subcommission on geochronology: Convention on the use of decay constants in geo and cosmochronology. Earth and Planetary Science Letters, 36, 359-362.

Wemmer, K. 1991. K/Ar-Altersdatierungsmöglichkeiten für retrograde Deformationsprozesse im spröden und duktilen Bereich - Beispiele aus der KTB Vorbohrung (Oberpfalz) und dem Bereich der Insubrischen Linie (N-Italien). Göttinger Arbeiten Geologie und Paläontologie, 51, 1-61.

Wetherill, G. W. 1956. Discordant uranium–lead ages. Transactions, American Geophysical Union, 37, 320–326.

Williams, I. S. 1998. U-Th-Pb Geochronology by Ion Microprobe. In: McKibben, M. A., Shanks III, W. C. & Ridley, W. I. (eds.) Applications of microanalytical techniques to understanding mineralizing processes. Reviews in Economic Geology, 7 , 1-35.

York, D., Hall, C.M., Yanase, Y., Hanes, J.A. & Kenyon, W.J., 1981. 40Ar/39Ar dating of terrestrial minerals with a continuous laser. Geophysical Research Letters, 8, 1136-1138.


A.2. Results

Table A-1. SHRIMP U-Pb data.

Table A-2. Ar-Ar isotopic data from sample UY-7-04

Table A-3. K-Ar isotopic data.

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