Methodology:

Analytical methods:

Zircon U-Pb geochronology was conducted at five separate laboratories:

NORDSIM analytical facility (Stockholm)

Zircons were separated and concentrated from rock samples at the British Antarctic Survey, Cambridge. From the zircon concentrate, hand-picked grains were mounted in epoxy resin along with the Geostandards zircon 91500 (207Pb/206Pb age of 1065.4 ± 0.3 Ma), which has reported U and Pb concentrations of 80 ppm and 15 ppm respectively (Wiedenbeck et al., 1995). The mount was polished to expose the centers of the grains and imaged by SEM using a cathodoluminescence (CL) detector in order to reveal their internal structure and guide analysis location. U-Pb ion-microprobe zircon geochronology was carried out using a CAMECA 1280 ion microprobe at the NordSIM facility housed at the Swedish Museum of Natural History in Stockholm. The analytical method closely followed Whitehouse & Kamber (2005), but differs insomuch that the oxygen ion primary beam was generated using a high-brightness, radiofrequency (RF) plasma ion source (Oregon Physics, Hyperion II, rather than a duoplasmatron) and a focused beam instead of illuminated aperture. The 10 nA O2- beam was rastered over 5x5 µm to homogenize beam density, the final analytical spot size being ca. 15 µm in diameter. Sputtered secondary ions introduced into the mass spectrometer were analyzed using a single ion counting electron multiplier over 10 cycles of data. Data were reduced using in-house developed software. The power law relationship between 206Pb/238U16O and 238U16O2/238U16O measured from the 91500 standard was used to calibrate U/Pb ratios following the recommendations of Jeon & Whitehouse (2015). Common-Pb corrections were applied to analyses where statistically significant 204Pb was detected, using the present-day terrestrial common Pb estimate of Stacey & Kramers (1975). Terra-Wasserburg U-Pb concordia diagrams were drawn using Isoplot v. 4.15 (Ludwig, 2012). 207Pb corrected ages were calculated assuming non-radiogenic Pb was from surface contamination and had an isotopic composition of modern-day average terrestrial common Pb (207Pb/206Pb = 0.836; Stacey & Kramers 1975). Representative cathodoluminescence images are presented in Supplementary Figure 4.

University College London

Zircon U-Pb geochronology used laser ablation inductively coupled mass spectrometry (LA-ICP-MS) facilities (Agilent 7700 coupled to a New Wave Research 193 nm excimer laser) at the London Geochronology Centre based in University College London. Heavy minerals were separated from bulk sediment samples using standard density liquid and magnetic separation procedures. Zircon-enriched extracts were mounted in hard epoxy resin on glass slides and polished for analysis. Typical laser spot sizes of 25 µm were used with a 7-10 Hz repetition rate and a fluence of 2.5 J/cm2.

Background measurement before ablation lasted 15 seconds and laser ablation dwell time was 25 seconds. The external zircon standard was Plesovice, which has a TIMS reference age 337.13 ± 0.37 Ma (Slama et al., 2008). Standard errors on isotope ratios and ages include the standard deviation of 206Pb/238U ages of the Plesovice standard zircon. Time-resolved signals that record isotopic ratios with depth in each crystal were processed using GLITTER 4.5, data reduction software, developed by the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC) at Macquarie University and CSIRO Exploration and Mining. Processing enabled filtering to remove spurious signals owing to overgrowth boundaries, weathering, inclusions, or fractures. Ages were calculated using the 206Pb/238U ratios for samples dated as <1.1 Ga, and the 207Pb/206Pb ratios was used for older grains. Discordance was determined using (207Pb/235U - 206Pb/238U) / 206Pb/238U) and similar for 20...(83)

Data collection:

Data was processed using software by Ludwig (2012) and Vermeesch (2021)