Electronprobe microanalysis

Constituent mineral phases in the samples were analysed for major and minor elements using a Cameca SX-50 WDS electronprobe microanalyser at Macquarie University. The accelerating voltage was 15 kV and beam current was 20 nA. Count times varied from 20 s on major elements to 60 s on minor elements. Calibration was performed on natural mineral standards. Representative data is presented in Table 1.

X-ray fluorescence analysis

Whole rock major and minor element abundances were determined on powdered samples using X-ray fluorescence (XRF) microscopy, at Macquarie University (Table 2).

Solution – ICP-MS analysis

Trace element abundances in the chilled margins were measured by solution – ICP-MS at the Australian National University (ANU) following procedures similar to those described by Norman et al.24 (Table 2).

Isotopic analyses

Rb–Sr and Sm–Nd isotope analyses were performed on leachate–residue pairs. Sample 77063 (105 mg, fine crush) was leached with cold 2M HCl (12 h), whereas rock powders for 77081A and 77082A were leached with cold 6M HCl. After removal of the clear solution, the residues were dried and re-weighed to determine weight loss during acid leaching. The residues were dissolved at low pressure using HF/HNO 3 and HCl. Following preliminary semi-quantitative trace element analysis, all fractions were spiked with 85Rb–84Sr and 149Sm–150Nd tracers, followed by extraction of Rb, Sr and LREE using conventional cation exchange and EICHROM Sr and LN resins25. Total analytical blanks (<0.1 ng) were negligible. Isotopic analyses were carried out on a NU Plasma multi-collector ICP-MS coupled to a CETAC Aridus desolvator. As part of an online iterative spike removal/mass bias correction procedure, instrumental mass bias was corrected by internal normalization to 88Sr/86Sr=8.37521 and 146Nd/145Nd=2.0719425 (equivalent to 146Nd/144Nd=0.7219 (ref. 26)); data are reported relative to SRM987=0.710230 and La Jolla Nd=0.511860. Typical in-run precisions (two standard errors) are ±0.000020 (Sr) and ≤±0.000010 (Nd), whereas external (2σ) precisions – based on rock standards – are ±0.000020 (Nd) and ±0.000040 (Sr). External precision (2σ) for 87Rb/86Sr and 147Sm/144Nd is ±0.5% and ±0.2%, respectively. 147Sm/144Nd and 143Nd/144Nd in modern CHUR is 0.1967 and 0.512638. Initial 87Sr/86Sr(t) and ε Nd (t) were calculated for t=120 Ma. Results for standards (±2 s.d.) are as follows: BCR-2 (n=2); Rb 46.8 ppm, Sr 339 ppm, 87Rb/86Sr 0.400, 87Sr/86Sr 0.70502, Sm 6.48 ppm, Nd 28.32 ppm, 147Sm/144Nd 0.1382, 143Nd/144Nd 0.512629; 0.512640±17 (mean of 21 runs of BCR-2 in 2010), 0.705015±34 (mean of 8 runs of BCR-2 in 2010–2011: JNd-1; 0.512114±23 (mean of 9 runs in 2010); BHVO-2 0.512992±17 (mean of 6 runs in 2010). Results are consistent with Thermal Ionisation Mass Spectrometry (TIMS) reference values. Uncertainties in initial ratios are: 87Sr/86Sr ±0.00010; ε Nd ±0.5 units. The decay constants are: 87Rb 1.42 10−11/yr, 147Sm 6.54 10−12/yr.

Rb–Sr dating of megacrystic phlogopite from sample 77063 was done on euhedral (1–3 mm) mineral flakes. Two handpicked fractions (2–4 mg) were cleaned in 0.1M HNO 3 , spiked with 87Rb–84Sr tracer and dissolved on a hotplate. Sr was extracted and purified using two passes over a small (0.1 ml) column of EICHROM Sr-resin (50–100 μm). Rb was extracted using cation resin chromatography (4 ml of AG50-X8, 200–400 mesh). Sr isotope data were measured as described above, whereas Rb isotope dilution analyses were done using the Zr-doping method28. Rb–Sr mica ages were derived by combining each phlogopite analysis with the Rb–Sr data for 77063L and R; calculations were done in ISOPLOT29. External precision (2 s.d.) for 87Rb/86Sr is 0.5%, and for 87Sr/86Sr is 0.01%. 87Sr/86Sr is reported relative to SRM987=0.710230. Rb–Sr model ages (±2 s.d.) for glauconite standard GLO-1 (89.3±0.8 Ma, n=16 over 7 years, assumed initial 87Sr/86Sr 0.7074) and SRM607 feldspar (1424±7 Ma, n=11 over 7 years, assumed initial 87Sr/86Sr 0.705) obtained using the same spike and techniques as those used for the phlogopites are consistent with Ar-Ar reference ages. Biotite GA1550 from Mount Dromedary (SE Australia) yields a Rb–Sr isochron age of 98.0±0.3 Ma (n=9, 0.70445±6, MSWD (mean square weighted deviation) 1.6), consistent with Ar–Ar ages of 98.5±0.8 Ma32 and 98.8±0.5 Ma33. Results are given in Table 4.

Semiquantitative trace element data for leachate–residue pairs (see above) indicate that most of the Hf resides in the residue (>90%), whereas Lu favours the leachates, consistent with their mineralogy and Bizimis et al.34. Overall, Lu–Hf budgets are dominated by the residue. Lu–Hf isotope analyses were therefore done on unleached whole rock powders. After dissolution at high pressure, a ~90% split of the solutions was spiked with a 176Lu–180Hf tracer. Lu and Hf were isolated using a single-column technique35 and measured by Multi-Collector-ICP-MS. The smaller split was equilibrated with a 233U–205Pb tracer, followed by extraction of Pb and U on small columns of AG1-X8 (100–200 mesh, HBr-HCl) and EICHROM TRU resin (100–150 μm), respectively. Pb and U isotope ratios were measured by MC-ICP-MS25. Isotope dilution calculations for U–Pb were done with the EarthTime Excel U–Pb data reduction module36. 176Hf/177Hf was normalized to 179Hf/177Hf=0.7325 and reported relative to JMC475=0.282160. Internal precision (2se) for 176Hf/177Hf is ≤±0.000008, external precision ±0.000015 (2 s.d.). External precision for 176Lu/177Hf is ±1%. USGS standards BCR-2 and BHVO-2, analysed together with the kimberlite samples, yielded the following results: BCR-2, 0.50 ppm Lu, 4.94 ppm, 176Lu/177Hf 0.01439, 176Hf/177Hf 0.282868, 0.282875 and 0.282883; BHVO-2, 0.274 ppm Lu, 4.45 ppm Hf, 176Lu/177Hf 0.00877 and 176Hf/177Hf 0.283104. Modern CHUR has 176Lu/177Hf = 0.0332 and 176Hf/177Hf = 0.282772. The decay constant for 176Lu is 1.865 10−11/year. Mass bias during U and Pb runs was corrected using internal normalisation to the natural U ratio and standard bracketing, respectively. External precision for 238U/204Pb is ±1% (2 s.d.). 232Th/204Pb was derived from 238U/204Pb measured by isotope dilution and Th/U from quadrupole ICP-MS trace element data on different splits of the same powders; expected errors ±10%. External precision for Pb isotope data is ≈±0.1−0.2% (2s.d.); data reported are corrected for mass bias and blank (20±10 pg Pb). Uncertainties for calculated initial Pb isotope ratios, obtained by Monte Carlo simulation with the stated analytical uncertainties and an assumed age uncertainty of ±5 Ma: 206Pb/204Pb ±0.02, 207Pb/204Pb ±0.01, 208Pb/204Pb ≤±0.17. The decay constants are: 238U 0.155125 × 10−9/year, 235U 0.98485 × 10−9/year and 232Th 0.049485 × 10−9/year. Lu-Hf and U-Pb isotope results are given in Table 5.

U–Pb LA-ICP-MS dating of perovskite

U–Pb dating work on perovskite was performed during two analytical sessions: the first was done on polished grains in situ in blocks of fresh kimberlite samples 77063, 77081 and 77082; the second analytical session on the sample 77063 was performed on separated perovskite grains mounted in an epoxy block.

The U–Pb analyses were performed at Macquarie University using a NewWave UP-213 laser ablation microprobe attached to an Agilent 7700 ICP-MS. The analytical technique was described in ref. 37. Ablation occurs in He, permitting efficient sample transport, signal stability and reproducibility of U/Pb fractionation. The spot size varied between 30 μm for perovskite from the polished blocks and 40 μm for separated grains. Because of the smaller spot sizes used on the grains from the polished blocks, the analytical uncertainties are much higher for these grains.

Samples were analysed in runs of 16 analyses comprising 12 analyses of unknowns bracketed by two analyses of a standard zircon (GEMOC GJ1, age 609 Ma) at the beginning and end of each run. The ‘unknowns’ include two near-concordant reference zircons, 91500 (ref. 38) and Mud Tank39, which are analysed before the samples and are used as independent control on reproducibility and instrument stability.

The major problem of the U–Pb analysis of perovskite is the presence of high contents of common Pb. Instead of applying a common-Pb correction, we have used a regression technique, treating population of analyses from a single sample as representing mixtures between the common-Pb and radiogenic-Pb components. The intercepts of the regression line through the raw data on an inverse-Concordia (Tera-Wasserburg) plot thus provide both an estimate of the 207Pb/206Pb of the common-Pb component (upper intercept) and the inferred crystallization age (lower intercept).

A total of 40 perovskites from sample 77063 were analysed during two sessions in April and June 2010. After rejection of 12 analyses with the largest analytical errors, the lower intercept of the regression line through the rest of the data (n=28) gave an age of 113±13 Ma, which we consider as the best estimate for the crystallization age of this kimberlite (Supplementary Fig. S1).

Only 12 perovskites were found and analysed from sample 77081. These produce a high MSWD of 2.7 due to a scatter about the regression line. The rejection of four analyses had reduced this scatter (MSWD=0.63), giving the lower intercept age at 125±8 Ma and the best estimate for the crystallization age for this sample (Supplementary Fig. S2).

Sixteen out of 17 analyses done for sample 77082 produced a lower intercept age of 121±13 Ma, interpreted as crystallization age for this sample (Supplementary Fig. S3).