A polished section (1.2 × 1 cm2) of NWA 4485 was prepared from a chip of the meteorite, and it was carbon coated. The section was observed in backscatter electron (BSE) images using a field emission scanning electron microscope NOVA NANOSEM450 equipped with a Gatan cathodoluminescence (CL) detector at Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The operating conditions were 15 kV accelerating voltage and 3.2 nA beam current. Quantitative analysis of zirconolite was determined using a Cameca SXFiveFE electron probe microanalyzer (EPMA) equipped with wavelength-dispersive X-ray spectrometers (WDS) at IGGCAS. The accelerating voltage of 20 kV and a beam current of 70 nA (~ 1 μm in diameter) were used. During data acquisition, peak counting time for each element is the same as their background counting time: Ca (Kα, 10 s), Ti (Kα, 10 s), Fe (Kα, 20 s), Y(Lα, 20 s), Zr (Lα, 20 s); Mg (Kα, 30 s), Al (Kα, 30 s), Si (Kα, 30 s), La (Lα, 30 s), Ce (Lα, 30 s), Nd (Lα, 30 s), Sm (Lβ, 30 s), Gd (Lα, 30 s), Pb (Mα, 30 s), Th (Mα, 100 s), and U (Mβ, 100 s). Additionally, Pb, Th, and U lines are all measured on LPET crystals. Overlapping correction method was used for interference correction according to the method reported by Pyle et al. (2005). Corrections for the interference of YLγ2,3 on PbMα, SmLβ7 on UMβ, and ThMζ1 on PbMα were precisely carried out. Due to the small correction factor of PbMα/ThMζ1 and low Th concentration in NWA 4485 zirconolite, the interference of ThMζ1 on PbMα was too small to affect the Pb concentration and can be negligible. The full range WDS spectra acquired with TAP, LPEP, and LiF diffractors are shown in Additional file 1: Fig. S1. The standard materials used for U, Th were both synthesized glasses: 7.40 wt% Al, 27.25 wt% Si, 15.76 wt% Ca, 5.18 wt% Th, and 44.63 wt% O for Th; 7.42 wt% Al, 27.52 wt% Si, 16.07 wt% Ca, 3.85 wt% U, and 44.90 wt% O for U. Natural crocoite (PbCrO4) was used as Pb standard material. Other synthetic and natural standards were periclase (MgO) for Mg, corundum (Al2O3) for Al, wollastonite (CaSiO3) for Si and Ca, rutile (TiO2) for Ti, hematite (Fe2O3) for Fe, yttrium aluminum garnet (Y3Al5O12) for Y, and 10% REE Si–Al–Ca–O glasses for La, Ce, Nd, Sm and Gd, respectively. Detection limits are 30–50 ppm for Mg, Al, Si, Ca and Ti, 70–90 ppm for Fe, Th and U, 110–120 ppm for Y and Gd, 130 ppm for Pb, 220 ppm for Zr, 150–170 ppm for La, Ce and Nd, 300 ppm for Sm. The Phi-Rho-Z matrix correction were applied for all data (Merlet 1994). In addition, Monte Carlo simulation of electron scattering in zirconolite was generated using Casino (Drouin et al. 2007) at accelerating voltage of 20 kV. The density of zirconolite is defined as 4.8 g/cm3. 1 × 104 electron trajectories were calculated and used for displaying images. The resultant spatial resolution for our EPMA analyses is shown in Additional file 1: Fig. S2. We acquired the smallest interaction volume of electrons and generation volume of X-rays of 1.3 μm diameter when the focused beam size is used. The corresponding YZ energy distribution shows 95% of energy are distributed less than 1.5 μm in depth.
Following the method of Montel et al. (1996) for monazite, we calculated the chemical age of zirconolite, assuming that all the Pb is radiogenic. This assumption is verified by the negligible Pb204/Pb206 ratio in our NanoSIMS analyses in Sect. 3.3. Given this assumption, the amount of Pb produced is the sum of the Pb from the decay of both U and Th:
$$\begin{aligned} {\text{Pb}} &= \frac{Th}{{232}}\left( {e^{{^{232} t}} - 1} \right)208 + 0.9928\frac{U}{238.04}\left( {e^{{^{238} t}} - 1} \right)206 \\ &\quad+ \,0.0072\frac{U}{238.04}\left( {e^{{^{235} t}} - 1} \right)207\end{aligned}$$
where Pb, Th, U are in ppm, and λ232 (4.9475 × 10–11 year−1; Steiger and Jäger 1977), λ235 (9.8485 × 10–10 year−1; Steiger and Jäger 1977), and λ238 (1.55125 × 10–10 year−1; Steiger and Jäger 1977) are the radioactive decay constants for 232Th, 235U and 238U, respectively.
Because of the lack of zirconolite references, the fractionation between U and Pb during the SIMS analysis cannot be calibrated. Instead, since the instrumental mass fractionation (IMF) of Pb isotopes is negligible (Yang et al. 2012), Pb–Pb ages of the zirconolite can be measured without U–Pb calibration. Therefore, Pb–Pb dating was carried out on the same zirconolite grain by a CAMECA NanoSIMS 50L at IGGCAS, following the method for Pb–Pb dating (Yang et al. 2012). The O− primary beam was accelerated at 16 kV with a current of ~ 500 pA and a beam size of ~ 1.7 μm in diameter. In order to achieve the maximum spatial resolution, the scanning off mode was used (Yang et al. 2012). Magnetic peak-switching mode was applied for the Pb–Pb dating, and four magnetic fields were used in the following sequence: B1 for Pb background (203.5), B2 for 204Pb, B3 for 96Zr, 206Pb, 238U, 232Th16O, 238U16O2, and B4 for 89Y and 207Pb, among which lead background (203.5) and Pb isotopes (204Pb, 206Pb, 207Pb) were measured with the same EM#4 detector. Before data acquisition, the sample surface was pre-sputtered by rastering the beam over 3 × 3 μm2 area for 3 min in order to remove the surface contamination of terrestrial common Pb. M257 zircon (206Pb/238U age = 561.3 Ma) and Phalaborwa baddeleyite (207Pb/206Pb age = 2059.6 Ma) references (Yang et al. 2012) were measured to assess the potential drift of the instrument during the analytical session. Measured 204Pb/206Pb ratios were used for the correction of common Pb based on the two-stage evolution model of Stacey and Kramers (1975). Uncertainties of individual analysis are reported at 1 sigma level. Weighted average Pb–Pb age was processed using Isoplot program (Ludwig 2003) and quoted at 2 sigma level.