Samples and preparation

Two samples from the Yarrabubba impact structure were selected for analysis (Fig. 1): a sample of the shocked Yarrabubba monzogranite (14YB07) and a sample from the Barlangi granophyre (14YB03). The Yarrabubba monzogranite was sampled from a small outcrop approximately 1.2 km WSW of Barlangi Rock. The Barlangi granophyre was sampled 2.7 km NNW of Barlangi Rock from an apophyse intruding the Yarrabubba monzogranite along what appears to be a shallowly dipping fault plane or fracture.

Thin sections were prepared from each hand sample to allow petrographic characterisation of the lithological fabric and identification of shock features. Zircon and monazite grains were separated from each sample. To separate zircon and monazite grains ~1 kg splits of each sample were processed with a Selfrag electric pulse disaggregator in the John de Laeter Centre (JdLC), Curtin University, Western Australia. The heavy mineral fraction was then separated using the heavy liquid methylene iodide. Further concentration of zircon and monazite was achieved with a Frantz isodynamic magnetic separator. Grains were then handpicked and mounted in a 25.4 mm epoxy round. The epoxy rounds were given a mechanical polish to 1 µm with diamond paste before a final chemical–mechanical polish with a colloidal dispersion of 5 nm silica in NaOH.

After polishing, monazite and zircon grains were imaged using backscatter electron (BSE) atomic contrast imaging and cathodoluminescence (CL) imaging; images can be found in Supplementary Figs. 1–4. All scanning electron microscope (SEM) analyses were undertaken on the Tescan Mira3 field emission-gun (FEG) SEM at the Electron Microscopy Facility, within the JdLC. BSE photomicrographs were collected using an accelerating voltage of 15 kV, and CL images were collected with an accelerating voltage of 10 kV.

Electron backscatter diffraction microstructural analyses

Shock-deformed monazite and zircon grains were mapped by electron backscatter diffraction (EBSD). Electron backscatter patterns (EBSPs) were collected from the monazite and zircon in orthogonal grids using a Nordlys Nano high-resolution detector and Oxford Instruments Aztec 2.4 acquisition software package on the Mira3 FEG-SEM. EBSD analyses were collected with a 20 kV accelerating voltage, 70° sample tilt, ~20 mm working distance and 18 nA beam current. EBSPs were collected with the following parameters; an acquisition speed of ~40 Hz, 64 frames were collected for a background noise subtraction, 4 × 4 binning, high gain, a Hough resolution of 60 and band detection min/max of 6/8. Maps were collected with a step size between 1.0 and 0.12 µm. Mean angular deviation values of the electron backscatter patterns for the maps ranged between 0.81 and 0.29. Individual zircon grains were mapped using the match unit Zircon 5260 based on the unit cell parameters of Hazen et al.58 after the methods of Reddy et al.59. Monazite grains were mapped with the match unit described in ref. 60, which originates from crystallographic data of ref. 61. From the Yarrabubba monzogranite (14YB07) seven zircon and four monazite grains were analysed, while eight zircon and seven monazite grains were analysed from the Barlangi granophyre (14YB03).

Post-processing the EBSD data was undertaken with Oxford Instruments Channel 5.11 software suite. All EBSD data were given a wild-spike noise reduction and a six nearest neighbour zero-solution correction. EBSD maps were produced using the Tango suite of Channel 5, while pole figures were processed in the Mambo suite of Channel5. EBSD maps and pole figures (as equal area, lower hemisphere projections) of the shocked monazite and zircons can be found in Supplementary Figs. 1–4. Using Tango the following maps were produced for the shocked zircon and monazite grains:

(1) Inverse pole figure (IPF) maps of crystallographic orientations of zircon (Fig. 2a, c). (2) All Euler crystallographic orientation map of shocked monazite (Fig. 2b, d). (3) Grain misorientation map, using the grain rotation orientation direction (GROD)-angle function of Channel5, which helps visualise the substructure of the grains by plotting the deviation angle of each pixel from the mean grain orientation, grain boundaries are defined as >10°. Blue domains are low strains, while warm colours represent higher degrees of misorientation (Supplementary Figs. 1–4).

Secondary ion mass spectrometry U–Pb age analyses

Following EBSD mapping of monazite and zircon, in situ U–Th –Pb isotopic measurements targeting specific shock microstructural domains were carried out using the SHRIMP-II secondary ion mass spectrometer (SIMS) at the JdLC. Operating procedures for uranium, thorium and lead isotopic measurements on zircon are based on those described by Compston et al.62 and Claoué-Long et al.63, with modifications summarised by Williams64. SHRIMP U–Pb zircon and monazite data are reduced using SQUID 2.50 and Isoplot 3.71 (add-ins for Microsoft Excel65,66) with decay constants recommended by Steiger and Jäger67. Ratios of 206Pb+/238U+ in zircon are calibrated to the known 206Pb/238U of the zircon standard, using a power-law relationship between 206Pb+/238U+ and UO+/U+, with a fixed exponent of 2.0 (determined empirically from measurements of zircon standards over several years63). All zircon analyses were run during one session and were standardised with primary zircon reference material BR266 (559 Ma, 206Pb/238U = 0.09059 (ref. 68)) and concentration reference Temora (416.8 Ma, 206Pb/238U = 0.06683 (ref. 69)). Archaean zircon reference OGC (3465 Ma, 207Pb/206Pb = 0.29907 (ref. 70)) was analysed during the session to check for 207Pb/206Pb fractionation. No 207Pb/206Pb fractionation correction was deemed necessary. Eight analyses of the BR266 standard were obtained during the session which indicated an external spot-to-spot (reproducibility) uncertainty of 1.42% (1σ) and a 238U/206Pb* calibration uncertainty of 0.54% (1σ). Calibration uncertainties are included in the errors of 238U/206Pb* ratios and dates listed in Supplementary Table 1. Common Pb corrections were applied to all analyses using contemporaneous isotopic compositions determined according to the model of Stacey and Kramers71.

Detailed SHRIMP operating procedures for monazite are outlined in Foster et al.72 and Wingate and Kirkland73. A ~10 μm diameter primary beam was employed with an intensity of ~0.5 nA. Ion microprobe analyses of monazite are affected by an uneven background spectrum of scattered ions74, which can be reduced effectively by use of the SHRIMP retardation lens system, which is set at ~10 kV. This discriminates against low-energy ions entering the collector. Each analysis consists of six cycles through the isotopic masses in the following sequence: 202 (species [139La31P16O 2 ]+, count time 2 s), 203 ([140Ce31P16O 2 ]+, 2 s), 204 (204Pb+, 10 s), 204.1 (background, 10 s), 206 (206Pb+, 10 s), 207 (207Pb+, 30 s), 208 (208Pb+, 5 s), 232 (232Th+, 5 s), 254 ([238U16O 2 ]+, 5 s), 264 ([232Th16O 2 ]+, 2 s) and 270 ([238U16O 2 ]+, 3 s). The monazite standard “India” was used for concentration calibration (509 Ma 2890 ppm 238U74) and also U–Pb calibration. Ratios of 206Pb+/238U+ in monazite are calibrated to the known 206Pb/238U of the monazite standard using a linear relationship between 206Pb+/UO 2 + and UO+/UO 2 +74. Monazite generates an unresolvable isobaric interference on 204Pb+, which may be (232Th144Nd16O 2 )++75,76. This interference has been observed to correlate with thorium content74. Excess 204Pb+ counts are corrected against the India monazite standard assuming 206Pb/238U–207Pb/235U age-concordance of the standard at a known thorium concentration. Fractionation of the 207Pb/206Pb ratio is typically observed when the retardation lens system is at operating voltage during monazite analysis. Fractionation of the 207Pb/206Pb ratio is monitored and corrections were applied, if necessary, by reference to the GM3 monazite standard77 which was run as an unknown. Uncertainties associated with this correction are added in quadrature to the uncertainties of 207Pb*/206Pb* ratios and dates. The common-Pb correction was based on measured 204Pb and Stacey and Kramers71 crustal Pb composition appropriate for the age of the sample. Data were reduced using SQUID 2 software and plotted using Isoplot 3.66 (ref. 66).

Nine SHRIMP analyses from 7 zircon grains and 16 SHRIMP analyses from four monazite grains were collected from the sample of the Yarrabubba monzogranite 14YB07. Nineteen SHIRMP analyses from 8 zircon grains and 19 SHRIMP analyses from 7 monazite grains were collected from the sample of the Barlangi granophyre 14YB03. Selection criteria for the analytical spots were based on the EBSD data, and both the strained domains and the strain-free neoblastic domains of the shocked monazite and zircon were targeted.

U–Pb isotopic data are provided in Supplementary Tables 1 and 2; uncertainties given for individual analyses in the tables (ratios and ages) are at the 1σ level. Terra–Wasserburg concordia plots with 2σ error ellipses for all analyses are shown in Fig. 2. Age uncertainties cited in the text are at the 2σ level. SHRIMP analytical pit locations are documented in Supplementary Figs. 1–4.

Hydrocode impact simulations