Electronic appendix A
Analytical method
SIMS U-Th-Pb zircon
Zircon grains were separated from the rock samples, mounted in epoxy resin, polished to approximately half grain thickness, imaged by a variety of techniques (cathodoluminescence – CL; secondary electron; backscatter electrons – BSD; transmitted and reflected light) and analysed using the Nordsim Cameca IMS1270 ion microprobe at the Swedish Museum of Natural History, Stockholm. Operating conditions for the instrument were routine (e.g. Whitehouse et al. 1999), namely a ca. 15 µm analytical spot size, primary beam current of 5-10 nA at -13 kV (total energy 23 kV) which illuminated a 150 µm aperture and a mass resolution of 4500. After 120 seconds of pre-sputtering to remove the gold coating, beam centring, mass calibration and energy adjustments were performed automatically and 12 cycles through the mass stations were used for data collection. Correction of isotope ratios for common Pb was based on the measured 204Pb (corrected for average background) and is considered to represent surface laboratory contamination (e.g. Zeck and Whitehouse 1999). Common Pb correction uses an assumed present-day average crustal composition (Stacey and Kramers 1975). Isotope ratios and corresponding ages (calculated using standard decay constants) are listed in Table 1 (available online in electronic supplementary material), together with 1s uncertainties. All ages discussed in the text are given at 2s uncertainties. U/Pb calibration is based on repeat analysis of 91500 zircon and uses a best fit power law relationship between Pb/U and UO2/U for each analytical session. Phanerozoic aged crystals have generally more reliable 206Pb/238U ages than 207Pb/206Pb ages as error magnification on low count rates of 207Pb increases rapidly towards younger times. However, 206Pb/238U ages are dependent on the appropriateness of the U/Pb calibration as determined from repeat analysis of the standard throughout the analytical session. The UOx/U value of the unknowns is within the range defined by the standards and there is no indication of matix matching effects. Within run ratios were examined and for zircon rim analyses confirm that homogeneous material was sampled throughout the ablation depth.
Oxygen isotopes in zircon
Oxygen isotopic analysis was performed in situ on selected zircons previously dated using the ion probe. Analytical procedures followed Nemchin et al. (2006) and are briefly summarised below. A 20 keV Cs+ primary beam (+10 kV primary, −10 kV secondary) of c. 3-4 nA was used in aperture illumination mode to sputter a c. 15 μm sample area. Charge compensation was provided by a normal incidence electron gun. Analytical data was acquired under fully automated runs which comprised a c. 2 minute pre-sputter with a raster of 25 μm, followed by field aperture, entrance slit and mass centring, using the 16O signal. After centring, 4 minutes of peak counting in two Faraday detectors, operating at a common mass resolution of c. 2500, measured the 16O and 18O isotopes. Data were normalised to 91500 zircon assuming a δ18O value of +9.86‰ (Wiedenbeck et al. 2004). Results are presented in table 7 (available online in electronic supplementary material) with external uncertainties at the 1σ level, including propagated external reproducibility on the standard, which is illustrated in Figure 9.
REE in zircon
Abundances of all 14 REE were determined on a Cameca IMS1270 ion microprobe operating at relatively low mass resolution and with high-energy offsets to reduce molecular interferences and minimise matrix effects. Samples were selected from the previously dated grains. Full analytical details are given in Whitehouse and Platt 2003. Concentrations (Table 6; available online in electronic supplementary material) were determined relative to NIST SRM610 glass using recommended values (‘preferred average’ of Pearce et al., 1996). During each analytical session, analyses of the Geostandards 91500 zircon were made in order to monitor the reproducibility of measurements. Analytical errors on individual REE determinations vary, depending upon the element concentration, abundance of the chosen isotope and instrument sensitivity. For this dataset the errors are comparable to that reported in Whitehouse and Kamber (2002) (i.e. average analytical error ranges from c. ±10–15% (1σ) for most of the LREE to c. ±5% for the other REE). Plots use the C1 chondrite values of Anders and Grevesse (1989).
LA-ICP-MS (zircon and monazite)
The LA–ICP–MS analyses of zircon and monazite were conducted at the Geological Survey of Norway using a Finnigan 266 nm laser coupled to a Finnigan element 1 single collector high-resolution sector ICP–MS. The zircons were mounted on double-sided adhesive tape and analysis of each grain was performed in two-steps. First, ablation of the zircon surface was performed using low laser energy in an attempt to ablate only the outermost, thin rims (denoted by a “p” in their identification in table 1; available online in electronic supplementary material). The second, main ablation was performed at higher laser energy and sampled remaining rim material as well as deeper parts of the grains. The laser was operated at a frequency of 10 Hz with energies varying between 0.005 to 0.5 mJ. To optimise counting statistics, the laser energy was tuned as a function of the U content of the zircon. Monazite analyses were conducted on the same epoxy-mounted grains as were analysed during SIMS and SEM analysis. The analytical protocols for monazite and zircon were similar, except that the laser energies during monazite analysis were kept significantly lower (between 0.005 and 0.006 mJ) due to the much higher U content of monazite, which compromises counting statistics at higher energies. Results from the monazite unknowns are presented in table 4 with 1 s uncertainties (available online in electronic supplementary material).
To minimise elemental fractionation, the zircon and monazite crystals were ablated in a raster area mode. A typical raster area is about 60 x 37 mm. The sample aerosol was transported from the sample chamber in He gas, and introduced into the ICP–MS instrument in a mixture of He and Ar gas. The data were acquired in a time-resolved counting mode for 60 sec with masses 202Hg, 204(Hg+Pb), 206Pb, 207Pb, 208Pb, 232Th and 238U measured. 235U was recalculated from the natural 238U/235U ratio. The interference between 204Pb and 204Hg contained in the Ar gas was corrected by monitoring 202Hg and assuming a 204Hg/202Hg ratio of 0.2293. A 60 sec gas blank analysis was performed at regular intervals.
To monitor the accuracy of the analyses, zircon and monazite from well characterised references were analysed together with the unknowns. The measured isotope ratios were corrected for element- and mass-bias effects using the 91500 zircon standard (Wiedenbeck et al. 2004).
Chemical dating of monazite
Chemical dating of epoxy-mounted and in situ monazite was carried out on a Carl Zeiss 1450VP electron microscope, equipped with both Oxford Instruments Wave 500 and Energy 400 X-ray detection systems, at the Norwegian Geological Survey. The X-ray detection system includes an energy dispersive spectrometer (EDS) and a single wavelength dispersive spectrometer (WDS) for trace and critical element analysis. During monazite analysis, the EDS is used for qualitative determination of P, Ca, La, Ce, Pr, Nd and Sm, and the WDS is used for quantitative determination of Y, Th, U and Pb concentrations. The monazite analyses were conducted using the analytical protocol described in detail in Slagstad (2006). Age calculation was solved by iteration using the equation given in Montel et al. (1996). Monazite standard B9952 was analysed repeatedly during analysis to check for instrument drift (Appendix figure A). Results from the unknowns are reported in table 5 (available online in electronic supplementary material) with 1 s errors.
Monazite commonly displays distinct compositional domains that, in many cases, coincide with discrete age domains (e.g., Dahl et al. 2005; Williams et al. 1999). The compositional domains may not be discernible in BSE or CL images, but are identifiable on compositional maps (most commonly Y, Th, U, Pb). Chemical mapping of the analysed grains was also undertaken on the SEM, with a beam current of 3 nA and an accelerating voltage of 20 kV.
Appendix figure A. Age plot for sequential analysis of monazite reference B9952 (B. Bingen pers com, see Slagstad 2006) during each of three analytical sessions. The vertical bars represent the timing of analysis of an unknown. The weighted mean age for each set of monazite references is given.
Additional references in appendix A
Anders E, Grevesse N (1989) Abundances of the elements: meteoritic and solar. Geochimica et Cosmochimica Acta 53(1):197-214
Dahl PS, Hamilton MA, Jercinovic MJ, Terry MP, Williams ML, Frei R (2005) Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.). American Mineralogist 90:619-638
Montel J-M, Foret S, Veschambre M, Nicollet C, Provost A (1996) Electron microprobe dating of monazite. Chemical Geology 131:37-53
Nemchin AA, Pidgeon RT, Whitehouse MJ (2006) Re-evaluation of the origin and evolution of > 4.2 Ga zircons from the Jack Hills metasedimentary rocks. Earth and Planetary Science Letters 244(1-2):218-233
Pearce NJG, Perkins WT, Westgate JA, Gorton MP, Jackson SE, Neal CR, Chenery SP (1996) A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21:115–144
Slagstad T (2006) Chemical (U-Th-Pb) dating of monazite:Analytical protocol for a LEO 1450VP scanning electron microscope and examples from Rogaland and Finnmark, Norway. NGU Bulletin 446:11-18
Stacey JS, Kramers JD (1975) Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci.Lett 26:207-221
Whitehouse MJ, Kamber BS (2002) On the overabundance of light rare earth elements in terrestrial zircons and its implication for Earth's earliest magmatic differentiation. Earth and Planetary Science Letters 204(3-4):333-346
Whitehouse MJ, Kamber BS, Moorbath S (1999) Age significance of U-Th-Pb zircon data from Early Archaean rocks of west Greenland: a reassessment based on combined ion-microprobe and imaging studies. Chemical Geology (Isotope Geoscience Section) 160:201-224
Whitehouse MJ, Platt JP (2003) Dating high-grade metamorphism - Constraints from rare-earth elements in zircon and garnet. Contributions to Mineralogy and Petrology 145(1):61-74
Wiedenbeck M, Hanchar J, Peck WH, Sylvester P, Valley J, Whitehouse MJ, Kronz A, Morishita Y, Nasdala L (2004) Further characterization of the 91500 zircon crystal. Geostand. Geoanalytical Res. 28:9–39
Williams ML, Jercinovic MJ, Terry MP (1999) Age mapping and dating of monazite on the electron microprobe: Deconvoluting multistage tectonic histories. Geology 27:1023-1026
Zeck HP, Whitehouse MJ (1999) Hercynian, Pan-African, Proterozoic and Archean ion microprobe zircon ages for a Betic-Rif core complex, Alpine belt, W Mediterranean consequences for its P-T-t path. Contributions to Mineralogy and Petrology 134:134-149
- 1 -