SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION doi: /nature11021 Sample Description Tuff beds and granular iron formation Tuff beds were identified in the basal Frere Formation in diamond drill-core from drill hole TDH26 (Supplementary Fig. 1) in the Earaheedy Basin, Australia (see Figure 1) at depths of m, m and m. The basal tuff bed was collected from immediately above the first bed of granular iron formation, which occurs between m and m (Supplementary Fig. 1). The iron formations comprise sub-angular to rounded grains of hematite, magnetite and ironsilicates (see Supplementary Fig. 2a, c, d). The granules are coarse Supplementary Fig. 1. Stratigraphic section intersected in drill hole TDH26, showing the location of the three dated tuff beds at the base of the Frere Formation (from Dörling, 1998). 1

2 RESEARCH SUPPLEMENTARY INFORMATION to very coarse-grained and appear to float in a pre-compaction cement of silica (Supplementary Fig. 2c, d) or carbonate. Fine-grained layers lack early diagenetic, pore-filling cement and commonly form densely packed anastomosing layers of ironoxides (Supplementary Fig. 2b). Supplementary Fig. 2. Granular iron formations in the Frere Formation immediately above the basal tuff layer. a, Transmitted light image of sample of granular iron formation (TDH26, m) ~1.5 meters above the lowermost tuff bed dated in this study (TDH26, m). The bed contains coarse- to very coarse-grained, subangular to rounded particles (~1 mm in diameter; red) in porefilling chert and quartz. b, Transmitted light image of granular iron formation showing chert cemented layers with granules (red) (top and bottom) and fine-grained layers with anastomosing hematite and magnetite (opaque) (TDH26, m). c, Transmitted light of chert bed with coarsegrained granules (see inset in 2b) showing numerous sub-angular to rounded grains composed of hematite (opaque and red-brown), silica and carbonate in a matrix of chert. d, Transmitted light image of sample TDH26, m showing a rounded, elongate granule composed of chlorite (green), hematite (opaque) and silica (clear) in a matrix of chert (clear). 2

3 SUPPLEMENTARY INFORMATION RESEARCH Zircon crystals in tuff beds Zircon crystals in polished thin sections (PTSs) from the Frere Formation were identified by transmitted and reflected light microscopy using their characteristic optical properties (see Deer et al., 1983). Most of the PTSs contain up to zircon crystals, ranging in size from <5 µm to 50 µm, although most are between µm in size. The zircons occur in a matrix of chlorite, with minor quartz, monazite, apatite and rutile. Zircon and monazite are surrounded by pleochroic haloes (Supplementary Fig. 3a). The zircons are typically clear to light brown and display oscillatory zoning (Supplementary Fig. 3b). The crystals vary in shape from equant (Supplementary Fig. 3b) to stubby and prismatic (Supplementary Fig. 3c and 3d). In places, minute outgrowths of zircon and xenotime (see Rasmussen, 2005) line zircon crystals. Supplementary Fig. 3: Zircons in tuffs from basal Frere Formation. a, Transmitted light (TL) image of zircon surrounded by a pleochroic halo (brown) in a chlorite matrix. b, TL image of small, equant zircon with oscillatory zoning. c, TL image of euhedral stubby zircon in chlorite matrix. d, Reflected light image of 3c showing zircon crystal with two oval SHRIMP pits surrounded by raster areas. e, Polished thin section of tuff band (TDH26, m) with numerous holes after zircon-bearing plugs were drilled removed. f, SHRIMP mount containing circular plugs (2-3 mm in diameter) containing euhedral zircons from tuff band (TDH26, m). a-d are from tuff bed TDH26, m. Zircon crystals in the tuff beds were examined by scanning electron microscope (SEM) using back-scattered electron (BSE) and cathodoluminescent (CL) imaging techniques following SHRIMP U-Pb geochronology. Imaging by BSE and CL shows that the zircon crystals display a combination of fine-scale oscillatory zoning and concentric growth zoning (Supplementary Figure 4a-h). The various growth textures 3

4 RESEARCH SUPPLEMENTARY INFORMATION in zircons from the Frere Formation (Supplementary Figure 4a-h) are consistent with the range of textures displayed by zircons that form during magmatic crystallization (e.g., Corfu et al., 2003) indicating an igneous origin for the Frere zircons. The euhedral shape of the zircon crystals, combined with their igneous growth zoning and their occurrence in thin, massive bands containing tuffaceous material are consistent with their derivation from a magma and their deposition associated with a volcanic eruption. Supplementary Fig. 4: SEM images of zircons in tuffs from the Frere Formation (drill-hole TDH26, m). a, BSE image of an equant zircon with oscillatory zoning. b, SEM-CL image of zircon in Figure 4a with growth zoning. c, SEM-BSE image of two euhedral zircons with a dark core and light rim. d, SEM-CL image of zircons in Figure 4c showing regular growth zoning. e, SEM-BSE of a broken euhedral zircon with regular growth zoning. f, SEM-CL of zircon in Figure 4e showing concentric growth zoning. g, SEM-CL image of euhedral prismatic zircon displaying regular growth zoning. Two oval outlines define SHRIMP pits ( 207 Pb/ 206 Pb dates are for grain 11-32J1 see Supplementary Table 1). h, SEM-CL image of an equant zircon with oscillatory zoning. Two oval outlines define SHRIMP pits ( 207 Pb/ 206 Pb dates are for grain 11-32E2 see Supplementary Table 1). i, SEM-CL image of an euhedral zircon with oscillatory zoning. The oval outline defines a SHRIMP pit ( 207 Pb/ 206 Pb date is for grain 11-32M see Supplementary Table 1) Zircon sample preparation Zircon crystals that were sufficiently large for analysis by ion microprobe (>10 µm) were drilled out of polished thin sections (Supplementary Fig. 3e). These were removed by core-drilling ~3 mm diameter plugs. The plugs were cast into 25 mm diameter epoxy resin mounts (Supplementary Fig. 3f). Multiple mounts were prepared for sample m (mounts BR11-03, BR11-04 and BR11-32) and sample 4

5 SUPPLEMENTARY INFORMATION RESEARCH m (mounts BR11-05 and BR11-34), and a single mount (BR11-36) for m. The BR266 reference zircon (fragments of a gem-quality zircon megacryst with a TIMS 206 Pb/ 238 U age of ± 0.3 Ma and 207 Pb/ 206 Pb age of ± 0.5 Ma; U = 903 ppm; Th = 201 ppm; Hf = 8220; Stern, 2001) was cast into two of the mounts (BR11-03 and BR11-05) and used for one of the analytical sessions. For the other session, BR266 was in a separate mount that was cleaned and Au-coated with the sample mount. SHRIMP Analytical Methods The SHRIMP analytical procedures closely follow those of Rasmussen & Fletcher (2010) and references therein. All grains that were mounted and were large enough to accommodate the SHRIMP primary ion beam (12-15 µm) were analysed. Thus there is no analytical bias due to sample separation procedures or grain selection. However, multiple analyses were made on some larger grains. All data were recorded in two 2-day analytical sessions. In the first session (for mounts BR11-03, BR11-04 and BR11-05), the primary ion beam was ~0.5 na on a ~15 µm spot and Pb/U external precision was 1.1% (n = 19 [of 20]). In the second session (mounts BR11-32, BR11-34 and BR11-36), the primary ion beam was ~0.3 na on a ~12 µm spot and Pb/U external precision was 1.3% (n = 34). No corrections were required for instrumental mass fractionation of Pb isotopes. Data reduction was by Squid-2 software (Ludwig, 2009), using spot-average values for all ratios and a fixed exponent of 2.0 for the 206 Pb/ 238 U calibration. SHRIMP Analytical Data All the weighted mean 207 Pb/ 206 Pb values in following paragraphs are quoted with 95% confidence limits. Plots show data with 1σ precision. Sample TDH26, m Seventy-three analyses were taken from 50 grains (Supplementary Table 1). There is one distinct old outlier, interpreted to be a xenocryst, and forty analyses that are >5% discordant (and several of these have >1% common 206 Pb) (Supplementary Table 1). The discordant data show a dominant Pb-loss trajectory towards the origin (Supplementary Fig. 5) implying predominantly recent Pb loss, but there is also spread around this, mainly toward lower 207 Pb/ 206 Pb, suggesting some early Pb loss. The 32 analyses in the main concordant group (Supplementary Fig. 6) have significant excess scatter in 207 Pb/ 206 Pb (MSWD = 3.2), which is clearly asymmetric (Supplementary Fig. 7). There are two distinct (~4σ) young outliers, further indicating early Pb loss. Omitting four analyses that are >2σ young outliers leaves a symmetric distribution with MSWD = 1.5 (Supplementary Fig. 8). These 28 analyses, on 25 grains, give a weighted mean age of 1891 ± 8 Ma. 5

6 RESEARCH SUPPLEMENTARY INFORMATION Pb/ 238 U Pb/ U Supplementary Fig. 5: U Pb data, sample m. Concordia plot of all data. White main group used for chronology; light grey >5% discordant; mid-grey 10% discordant or >1% common 206 Pb; dark grey concordant outliers Pb/ 238 U Pb/ 235 U Supplementary Fig. 6: U Pb data, sample m. Concordia plot of the most concordant data near 1890 Ma. White main group used for chronology; light grey >5% discordant; mid-grey 10% discordant or >1% common 206 Pb; dark grey concordant outliers. 6

7 SUPPLEMENTARY INFORMATION RESEARCH Number Relative probability t[ 207 Pb/ 206 Pb] (Ma) Supplementary Fig. 7: Probability density plot of 207 Pb/ 206 Pb, sample m. All data with 5% discordance and <1% common 206 Pb, except data for one xenocryst Number Relative probability t[ 207 Pb/ 206 Pb] (Ma) Supplementary Fig. 8: Probability density plot of 207 Pb/ 206 Pb, sample m. Data used for age determination. 7

8 RESEARCH SUPPLEMENTARY INFORMATION Sample TDH26, m Forty-seven analyses were made on 27 grains (Supplementary Table 2). Almost 70% of the analyses are >5% discordant (Supplementary Fig. 9) but these conform closely to a single recent Pb-loss trend, with a well-defined lower intercept that is indistinguishable from the origin. The 15 analyses in the main concordant group have no significant excess scatter in 207 Pb/ 206 Pb (MSWD = 1.3). The youngest 207 Pb/ 206 Pb value is from a low-u grain and has correspondingly poor precision in 207 Pb/ 206 Pb, so the weighted mean 207 Pb/ 206 Pb age of 1890 ± 10 Ma is, in effect, determined by 14 analyses from 10 grains. A discordia regression, constrained to a lower intercept of 0 ± 20 Ma and using all data with <1% common 206 Pb except one strong outlier gives 1889 ± 9 Ma but with higher MSWD (1.6). These results are indistinguishable, and we use the result from the weighted mean 207 Pb/ 206 Pb for concordant data as the age of the tuff Pb/ 238 U Intercepts at 24 ± 36 & 1890 ± 13 Ma MSWD = Pb/ 235 U Supplementary Fig. 9: U Pb data, sample m. Concordia plot of all data, with discordia regression. White main group considered for 207 Pb/ 206 Pb age; light grey >5% discordant; mid-grey 10% discordant or >1% common 206 Pb. 8

9 SUPPLEMENTARY INFORMATION RESEARCH Sample TDH26, m Only nine analyses were obtained from five grains in this sample (Supplementary Table 3, Supplementary Fig. 10). Three results are >5% discordant, leaving six (from 4 grains) that were considered for chronology. These give a weighted mean 207 Pb/ 206 Pb age of 1889 ± 32 (MSWD = 1.4). It is questionable whether the uncertainty should be expanded (as in this case) to account for apparent excess scatter with this small number of points and relatively low MSWD. The 2σ uncertainty determined from internal precision is ± 22 Ma. Furthermore, if the two analyses that are between 5% and 10% discordant are included, the weighted mean becomes 1885 ± 18 Ma with MSWD = 1.1. This is not a definitive age determination but the result is clearly consistent with the ages of the two lower tuffs Pb/ 238 U Pb/ 235 U Supplementary Fig. 10: U Pb data, sample m. Concordia plot of all data. White concordant; light grey >5% discordant; mid-grey >10% discordant. 9

10 RESEARCH SUPPLEMENTARY INFORMATION Supplementary References Corfu, F., Hanchar, J. M., Hoskin, P. W. O. & Kinny, P. Atlas of zircon textures. In Hanchar, J. M. & Hoskin, P. W. O. (Eds) Zircon. Reviews in Mineralogy and Geochemistry, Volume 53, p (2003). Deer, W. A., Howie, R. A. & Zussman, J. An introduction to the rock-forming minerals. Longman Group Ltd, Essex, 528p (1983). Dörling, S. Teague Project: Exploration activities during the period to , RGC Exploration Pty Ltd Annual Report 75p (1998). Ludwig, K. R. Squid 2; a user s manual. Berkeley Geochronology Center, 100p (2009). Rasmussen, B. Zircon growth in very low grade metasedimentary rocks: evidence for zirconium mobility at ~250 C. Contrib. Mineral. Petrol. 150, (2005). Rasmussen, B. & Fletcher, I. R. Dating sedimentary rocks using in situ U Pb geochronology of syneruptive zircon in ash-fall tuffs <1 mm thick. Geology 38, (2010). Stacey, J. S. & Kramers, J. D. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207 (1975). Stern, R. A. A new isotopic and trace-element standard for the ion microprobe: preliminary thermal ionization mass spectrometry (TIMS) U Pb and electronmicroprobe data. Radiogenic Age and Isotopic Studies: Report 14. Geol. Survey Canada, Current Research 2001-F1, 11p (2001). 10

11 SUPPLEMENTARY INFORMATION RESEARCH Supplementary Table 1: SHRIMP U Pb data for zircons in the lower tuff bed r tuff bed (drill-hole TDH26, m). Analysis 207 Pb*/ (ppm) (ppm) 208 Pb*/ Th/U (%) 206 Disc. Pb* ± t[ 207 Pb*/ 238 U 206 Pb*] ± ± 235 U ± 232 Th ± (%) (Ma) ± Main group, in 207 Pb/ 206 Pb sequence J D H K I L A J K C J B B H G I C A E E M E C E.1-1 Concordant young outliers E L E B H Concordant 5.49 old 0.11 outlier J >5% 1132E discordant 0.08 or >1% common Pb F H Supplementary Table 1: SHRIMP U Pb data for zircons in the lower tuff bed (drill-hole TDH26, m). U Th f Pb*/ 206 Pb*/ 207 Pb*/ 208 Pb*/ Disc. t[ 207 Pb*/ 206 Pb*] 235 U ± 232 Th ± (%) (Ma) ± 1104F K B H D I G I G L D C C D L F L B B A C C D L C E C I

12 RESEARCH SUPPLEMENTARY INFORMATION 1103J E B N D N D D G G L A F Analysis identification is is nnnna.p-q where where nnnn nnnn is the is the mount mount number, number, A is A a polished a polished thin section thin section disc in disc the mount, in the mount, p is the p zircon is the grain zircon within grain that within disc, that and disc q is the analysis spot within that grain. f206 is the proportion of 206 Pb [ 208 Pb] calculated to be common Pb on the basis of measured 204 Pb/ 206 Pb and the model common Pb composition (Stacey and Kramers, 1975) at the approximate sample age. All listed Pb isotope data are corrected for common Pb, based on measured 204 Pb/ 206 Pb. Disc. is apparent discordance, defined as 100*(1-t[ 206 Pb/ 238 U]/t[ 207 Pb/ 206 Pb]). Listed uncertainties are 1σ and include all components of statistical precision. Supplementary Table 2: SHRIMP U Pb data for zircons in the middle tuff bed (drill-hole TDH26, m). U Th f Pb*/ 206 Pb*/ 207 Pb*/ 208 Pb*/ Disc. t[ 207 Pb*/ 206 Pb*] Analysis (ppm) (ppm) Th/U (%) 206 Pb* ± 238 U ± 235 U ± 232 Th ± (%) (Ma) ± Main group, in 207 Pb/ 206 Pb sequence 1105C G F G N I J B B A H M G A G >5% discordant or >1% common 206 Pb 1134D D B G K K M C N J L B M

13 SUPPLEMENTARY INFORMATION RESEARCH 1105K B L K G D E J A A I I L J L F B H O Analysis identification is nnnna.p-q where nnnn is the mount number, A is a polished thin section disc in the mount, p is the zircon grain within that disc, and q Analysis identification is nnnna.p-q where nnnn is the mount number, A is a polished thin section disc in the mount, p is the zircon grain within that disc, and q is the analysis spot within that grain. f206 is the proportion of 206 Pb [ 208 Pb] calculated to be common Pb on the basis of measured 204 Pb/ 206 Pb and the model common Pb composition (Stacey and Kramers, 1975) at the approximate sample age. All listed Pb isotope data are corrected for common Pb, based on measured 204 Pb/ 206 Pb. Disc. is apparent discordance, defined as 100*(1-t[ 206 Pb/ 238 U]/t[ 207 Pb/ 206 Pb]). Listed uncertainties are 1σ and include all components of statistical precision. 13

14 RESEARCH SUPPLEMENTARY INFORMATION Supplementary Table 3: SHRIMP U Pb data for zircons in the upper tuff bed (drill-hole TDH26, m). U Th f Pb*/ 206 Pb*/ 207 Pb*/ 208 Pb*/ Disc. t[ 207 Pb*/ 206 Pb*] Analysis (ppm) (ppm) Th/U (%) 206 Pb* ± 238 U ± 235 U ± 232 Th ± (%) (Ma) ± Main group, in 207 Pb/ 206 Pb sequence 1136E G C G B E >5% discordant 1136C B A Analysis identification is nnnna.p-q where nnnn is the mount number, A is a polished thin section disc in the mount, p is the zircon grain within that disc Analysis identification is nnnna.p-q where nnnn is the mount number, A is a polished thin section disc in the mount, p is the zircon grain within that disc, and q is the analysis spot within that grain. f206 is the proportion of 206 Pb [ 208 Pb] calculated to be common Pb on the basis of measured 204 Pb/ 206 Pb and the model common Pb composition (Stacey and Kramers, 1975) at the approximate sample age. All listed Pb isotope data are corrected for common Pb, based on measured 204 Pb/ 206 Pb. Disc. is apparent discordance, defined as 100*(1-t[ 206 Pb/ 238 U]/t[ 207 Pb/ 206 Pb]). Listed uncertainties are 1σ and include all components of statistical precision. 14