SUPPLEMENTARY MATERIAL

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SUPPLEMENTARY MATERIAL Powered By Docstoc
					Supplementary Methods


1. U-Pb isotope SIMS analysis of zircons
The U-Th-Pb data (Supplementary Table 2) was obtained from polished, gold-coated
zircon grain mounts (epoxy resin) with a SHRIMP II housed at the Curtin University of
Technology, Perth. Operating conditions and analytical protocols are essentially the same
as described by ref. 1. The Pb/U calibration was performed relative to the in-house
standard zircon CZ3 (564 Ma), which was analysed repeatedly throughout each session.
Apparent magmatic ages in Supplementary Table 2 include uncertainty in the Pb/U
calibration. All errors are quoted at 1 precision.


2. Oxygen isotope analysis of zircon
Oxygen isotope data (Supplementary Table 3) were acquired with a Cameca ims 1270,
using a 6 nA primary 133Cs+ beam and charge compensation by normal-incidence electron
gun. The method closely followed that outlined by ref. 19. Secondary ions were extracted
                18
at 10 kV, and        O- and   16
                                   O- were monitored simultaneously on dual Faraday cups. Each
analysis involved a pre-sputtering time of 45 seconds, followed by data collection in two
blocks of five cycles, amounting to a total count time of 40 seconds. Data were collected
over two separate analytical periods using slightly different analytical protocols. In the first
period (15-20 November 2004), a fixed primary beam was focussed directly onto the
sample, sputtering material from an oval area measuring ~25 m in the longest dimension.
The secondary yield of 18O under these conditions was typically between 2.5 x106 and 3.0
x 106 counts. Kohler illumination was employed in the second period (25-27 April 2005),
which produced a slightly smaller and more regular, flat-bottomed pit and improved
sensitivity for 18O (average 4.0 x106 counts). To correct for instrumental mass fractionation
(IMF), all data were normalised to an internal standard, Geostandards zircon 91500 (18O
10.07 ± 0.03‰ VSMOW, 0.66 wt% HfO2, ref. 16), which was assumed to be
homogeneous under the analytical conditions employed. Chips of this zircon were
embedded into each grain mount and analysed (typically in blocks of ten) to bracket every
10-15 measurements on the sample zircons. For the November 2004 analytical period, the
internal precision on each standard analysis based on counting statistics was typically
between ±0.3‰ and ±0.8‰ (2 s.e.) and spot-to-spot reproducibility ~0.25-0.40 ‰ (1 s.d.).
The internal precision was marginally improved for the April 2005 period (average 0.4 ‰,
mostly < 0.5 ‰ at 2 s.e.), but the external precision based on replicate analysis of the same
standard chip remained ~ 0.3 ‰ (1 s.d.).
A potential source of error with in situ oxygen isotope analysis by ion microprobe
concerns a matrix-induced 18O/16O fractionation resulting from different zircon Hf contents
(Peck et al. 2001, and see summary in ref. 16). For the Cameca 4f, operating at high
voltage offset, this can reach 1 ‰ for every 1 wt % HfO2 (Peck et al. 2001), although this
effect has yet to be rigorously appraised for multi-collector ion microprobes using zero off-
set. For this reason, in both the November 2004 and May 2005 periods, analyses of the
KIM-5 zircon standard (1.23 % HfO2, 18O = 5.09 ± 0.06 ‰, ref. 16) were interposed
throughout each session as an independent monitor on data quality. Analyses during
November 2004 and April 2005 yielded mean values of 5.14 ± 0.28 ‰ (1 s.d., n = 68) and
5.15 ± 0.28 ‰ (1 s.d., n = 34) respectively, both indistinguishable from the laser
fluorination value (18O = 5.09 ± 0.06 ‰) quoted by ref. 16. This demonstrates that
matrix-related fractionation is negligible over this HfO2 range, which, fortunately, brackets
most of the spread of HfO2 values observed within the detrital and inherited zircons
analysed in this study. One grain mount analysed in April 2005 also contained the
multigrain zircon standard Temora 2 (1 % HfO2). The average 18O of 23 analyses from
these grains was 8.25 ± 0.20 ‰ (1 s.d.), indistinguishable from the laser fluorination value
quoted by ref. 16 (8.20 ± 0.01 ‰).


Oxygen isotope data for both standards and unknowns are listed chronologically in
Supplementary Table 3. The session divisions variously reflect primary beam restarts,
specimen exchanges, or major adjustments to magnet calibration, electrostatic lens settings
or the primary beam steering or intensity. Note that only analyses relevant to this paper are
tabulated. Fractionation-corrected oxygen isotope data are quoted at 2 standard errors and
include the propagated standard deviation of the 91500 standard analyses to account for
any instrumental drift. For unknowns, the fractionation factors used to correct raw 18O/16O
data were calculated from bracketing analyses of the 91500 standard. When the instrument
was stable, assessed by comparison between standard datasets acquired over the course of
the session, the fractionation factor was derived from pooled standards. In cases where
standard analyses showed significant drift, unknowns were corrected to the average
fractionation factor derived from the immediate bracketing standard set. For each block of
91500 standard analyses, the table also lists the per mil deviation of the average measured
18
     O/16O from the accepted ratio reported by ref. 16. This gives a convenient measure of the
magnitude of the IMF and its variation during and between sessions.
3. Lu-Hf isotope analysis of zircons by laser ablation
All laser ablation Lu-Hf isotope analyses (supplementary Table 4) were conducted in the
Department of Earth Sciences, University of Bristol, using a 193 nm ArF laser and
Finnigan Neptune, with the same Faraday cup configuration employed by Woodhead et al.
(2004). The method is outlined by Hawkesworth & Kemp (2005). Data were acquired over
two analytical sessions (27-30 November 2004 and 2-3 May 2005) using either a 40 m,
50 m (predominantly) or 65 m beam size and 4 Hz laser pulse repetition rate, producing
total Hf beams of ~8-28 V. To counteract ArF degradation the power density at the sample
was maintained at around 6-7 J/cm2, which translated into an estimated drilling rate of ca.
0.5 m/sec. Ablation was conducted in He (flow rate ~1.3 l/min, optimised daily), this
being combined with argon (~0.9 l/min) in a small glass mixing chamber prior to transport
into the ICP. A small (~0.005 l/min) N2 flow was introduced into the Ar carrier gas
upstream of the mixing chamber, which suppressed oxide formation in the plasma
associated with the use of X-skimmer cones and enhanced sensitivity by a factor of two.
Tests on pure REE solutions established that formation of nitride molecules is negligible.


The 176Hf/177Hf and 176Lu/177Hf ratios quoted in supplementary Table 4 were derived from
a 60 second ablation period, comprising 60 cycles of 1 s integration time. In the case of
isotopic zoning or with intersecting cracks/inclusions, only the flattest, most stable portions
of the time-resolved signal were selected for integration, performed off-line using a
customised Excel spreadsheet. The correction for the isobaric interference of Lu and Yb on
176
      Hf was performed in 'real time' as advocated by Woodhead et al. (2004). For Yb, this
                                                171
involved monitoring the interference-free             Yb during the analysis and then calculating
the magnitude of the 176Yb interference using the 176Yb/171Yb ratio determined by Segal et
al. (2003) (176Yb/171Yb = 0.897145). The much smaller Lu correction is performed in the
same fashion by measuring 175Lu, and using 176Lu/175Lu = 0.02669 (De Bievre and Taylor,
1993). To correct for instrumental mass fractionation, Yb isotope ratios were normalised to
173
      Yb/171Yb = 1.130172 (Segal et al. 2003) and Hf isotope ratios to      179
                                                                                  Hf/177Hf = 0.7325.
The mass bias behaviour of Lu was assumed to follow that of Yb. The veracity of this
protocol was checked by replicate analysis of a solution of known Hf isotope composition
(JMC 475) that was spiked with variable amounts of pure Yb solution. Analyses of pure
JMC 475 (diluted to 80 ppb Hf) were interspersed with the Yb-doped JMC 475 solutions,
                       176
yielding an average          Hf/177Hf of 0.282152 ± 6 (2s.d., n = 16), slightly lower than the
commonly cited value of 0.282160 (e.g. Söderlund et al. 2004) but within the range quoted
for other laboratories (e.g. 0.282144 ± 14, Vervoort et al. 2004; 0.28216 ± 1, ref. 31). The
results are summarised in Supplementary Figure 1, where the interference-corrected
176
      Hf/177Hf values were calculated using three recently published estimates of Yb isotope
composition (Chu et al. 2002, Segal et al. 2003 and Vervoort et al. 2004) and the Yb
isotope abundance ratios quoted by IUPAC. This shows that the interference correction
using the Yb isotope ratios of Segal et al. (2003) accurately reproduce the Hf isotope
composition of pure JMC 475 over a large range of Yb/Hf ratios that greatly exceeds that
encountered in zircons of this study (Yb/Hf < 0.077).




Supplementary Figure 1. Analyses of JMC 475 solutions spiked with variable amounts of Yb,
where the interference corrected 176Hf/177Hf ratios are calculated using the Yb isotope compositions
of IUPAC (De Bievre and Taylor, 1993; grey diamonds), Chu et al. (2002) (open circles), Segal et
al. (2003) (black squares) and Vervoort et al. (2004) (open triangles). The shaded band bound by
dashed lines depicts the Hf isotope composition of pure JMC 475 solutions measured during the
same analytical session (176Hf/177Hf = 0.282152 ± 6, 2 s.d. n = 16), and the solid bar shows the
range of Yb/Hf ratios encountered within zircons of this study (Yb/Hf 0.01-0.077).


                                                                             176
Supplementary Table 5 summarises the mean interference-corrected                   Hf/177Hf ratios of
the zircon standards utilised at Bristol, measured between August 2004 and May 2005.
Internal precision routinely varies between ± 0.000010 and ± 0.000030 (at 2 s.e. level),
being consistently the best for Sri Lankan zircon CZ3, reflecting the greater signal
intensity and higher Hf concentration. External precision was ± 0.000019 for 91500, ±
0.000016 for Mud Tank (66 and 58 ppm, respectively) and ± 0.000024 for Temora 2 (86
ppm, all at 2 s.d.). The laser ablation data on these three zircon standards are
                                          176
indistinguishable to the solution               Hf/177Hf data of Woodhead et al. (2004). Solution data
are as yet unavailable for CZ3, but our data are similar to the laser ablation data quoted by
Ping et al. (2004) using the same instrumentation. To monitor data quality, analyses of
standards (especially Temora 2, which has the highest Yb/Hf ratio) were regularly
interposed with 'unknowns' during the course of the analytical session. Mass bias, peak
shape/centre characteristics and Hf sensitivity showed little variation throughout each
analytical session.



                                176
           Standard zircon            Hf/177Hf meas (laser)       176
                                                                        Hf/177Hf meas (solution)
           91500                0.282299 ± 19 (n = 71)            0.282306 ± 4
           Temora 2             0.282687 ± 24 (n = 95)            0.282686 ± 4
           Mud Tank             0.282496 ± 16 (n = 49)            0.282507 ± 3
           CZ3                  0.281697 ± 12 (n=15)
                                0.281703 ± 24 (n=16)#


Supplementary Table 5. Summary of Hf isotope data of standard zircons using the
                                                                           176
instrumentation at the University of Bristol, and corrected for                  Yb interference using the Yb
isotope compositions of Segal et al. (2003). All data were acquired over 60 seconds ablation, using
a 50 mm spot and 4 Hz laser pulse rate. Solution data are from Woodhead et al. (2004). Data are
quoted at 2 standard deviations.
#
    laser ablation data from Ping et al. (2004)


References


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