DOI: 10.5285/c0c56e6d-d13b-4480-bbd3-cd613ab57b33
title: Geochemical and geochronological data from the LeMay Group, Antarctic Peninsula

description: The files include full analytical details and datasets from the laboratories used for the acquisition of U-Pb zircon geochronology, Lu-Hf isotope geochemistry and 40Ar/39Ar analysis of detrital white mica. Also included are a list of all the published datasets used in the construction of the MDS and ridge plots for detailed regional comparisons. The data were collected in the interval January 2021 to March 2022 across a number of laboratories: Stockholm, University College London, British Geological Survey, Trinity College Dublin, Australian National University (U-Pb zircon geochronology); Open University (40Ar/39Ar analysis) and British Geological Survey (Lu-Hf isotopes). The analyses were conducted by Teal Riley (Stockholm, British Geological Survey), Ian Millar (Australian National University), Andrew Carter (University College London), Joaquin Bastias (Trinity College Dublin), Craig Storey (Open University). The analyses were conducted to examine the provenance and depositional history of the accretionary LeMay Group complex of Alexander Island.
description: 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 &lt;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 207Pb/206Pb ages. The results are presented in the supplementary file (Table S2). British Geological Survey Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) analysis were carried out at the British Geological Survey and zircons were separated using standard crushing and mineral separation techniques. Analyses were carried out prior to the advent of chemical abrasion techniques (Mattinson, 2005) therefore grains were selected from non-magnetic fractions, and subjected to air abrasion prior to analysis. Grains were spiked with a mixed 205Pb/235U tracer (Krogh & Davis, 1985; Corfu & Ayres, 1984) and dissolved in Teflon microcapsules. U and Pb were separated following Corfu & Noble (1992) and references therein. U and Pb were loaded together onto single rhenium filaments using silica gel, and measured by peak-jumping using a Daly detector on a VG354 thermal ionization mass spectrometer. Data reduction was carried out using the PBDAT (Ludwig, 1989). Ages were calculated using Isoplot (Ludwig, 2012), and ages were calculated using the decay constants recommended by Steiger & Jager (1977). Trinity College Dublin U-Pb isotope ratios were determined using a Photon Machines Iridia 193 nm ArF Excimer coupled to an Agilent 7900 ICP-MS using an Aerosol Rapid Introduction System (ARIS; Teledyne Photon Machines) at the Department of Geology, Trinity College Dublin. The ICP-MS instrument was tuned using NIST 612 standard glass to yield Th/U ratios of unity and low oxide production rates (ThO+/Th+ typically less than 0.15%). 0.46 l/min He carrier gas was fed into the cell body and 0.04 l/min He was fed into the cup, and the aerosol was subsequently mixed with 0.7 l/ min Ar make-up gas and a small volume of N2 (ca. 9 ml/min). The following 12 isotopes were measured (with their respective dwell times in milliseconds listed in parentheses): 49Ti(5), 91Zr(7), 175Lu(7.5), 178Hf(7.5), 202Hg(10), 204Pb(18), 206Pb(45), 207Pb(90), 208Pb(18), 232Th(18), 235U(45) and 238U(45). For all analyses the laser fluence was 2.25 J/cm2, with a repetition rate of 11 Hz, a 25 microm spot size and an analysis time of 27 s, followed by a 9 s background measurement. Blocks of eight standards and two NIST612 standard glass analyses were followed by 20 unknown samples. The 91500 zircons (206Pb/238U TIMS age of 1065.4 ± 0.6 Ma; Wiedenbeck et al., 1995) were used as the primary standard. Secondary standards used to monitor consistency in the measured U-Pb dates were Plesovice zircon (206Pb/238U ID-TIMS age of 337.13 ± 0.37 Ma; Sláma et al., 2008), WRS 1348 zircon (206Pb/238U TIMS age of 526.26 ± 0.70 Ma; Pointon et al., 2012) and GZ7 zircon (206Pb/238U SIMS age of 530.26 Ma ± 0.05 Ma; Nasdala et al., 2018). Dates were calculated using the software Iolite4 and IsoplotR (Vermeesch, 2018). Australia National University Detrital zircon ages were determined using a Sensitive High Resolution Ion Microprobe (SHRIMP) at the Australian National University. Zircons were mounted in epoxy resin, polished to expose the centers of the grains, and gold-coated. Internal zoning and grain characteristics were mapped by microphotography and cathodoluminescence imaging. Clean areas, free of cracks, inclusions and radiation damage were analyzed. Analysis was carried out with a primary O2- beam; secondary ion beam intensities were measured using an ion-counting detector. Calibration was carried out using zircon standards mounted together with the samples (mostly AS-3; Paces & Miller, 1993). Data were reduced for background, mass bias, calibration and common-Pb corrections using Isoplot (Ludwig, 2012). All errors on ages are at the 2sigma level. Lu-Hf isotope analysis Isotope analyses were carried out at NIGL using a Thermo Scientific Neptune Plus MC-ICP-MS coupled to a New Wave Research UP193UC Excimer laser ablation system. Helium was used as the carrier gas through the ablation cell with Ar make-up gas being connected via a T-piece and sourced from a Cetac Aridus II desolvating nebulizer. 0.01 l/min of nitrogen were introduced via the nebulizer in addition to Ar in order to minimize oxide formation. Lutetium (175Lu), Ytterbium (172Yb, 173Yb), and Hafnium (176Hf, 178Hf, 179Hf and 180Hf) isotopes were measured simultaneously during static 30 second ablation analyses. The spot size used was 25 µm; fluence = 8 J/cm2. Hf reference solution JMC475 was analyzed during the analytical session and sample 176Hf/177Hf ratios are reported relative to a value of 0.282160 for this standard. Correction for 176Yb on the 176Hf peak was made using reverse-mass-bias correction of the 176Yb/173Yb ratio empirically derived using Hf mass bias corrected Yb-doped JMC475 solutions (Nowell & Parrish, 2001). 176Lu interference on the 176Hf peak was corrected by using the measured 175Lu and assuming 176Lu/175Lu = 0.02653. Hf-isotope data was processed using the Iolite data reduction package (Paton et al., 2011). Three zircon reference materials (91500, Mud Tank, GJ1) were analyzed throughout the analytical session. The 91500 zircon reference material was used as the primary standard in Iolite, and was used to normalize the 176Lu/177Hf ratio assuming a value of 0.000311 (Woodhead & Hergt, 2005). Analytical uncertainties for unknowns were propagated by quadratic addition to include the standard error of the mean of the analysis and the reproducibility of the 91500 reference material. εHf values were calculated using a 176Lu decay constant of 1.867 x 10-11y-1 (Söderlund et al., 2004), the present-day chondritic 176Lu/177Hf value of 0.0336 and 176Hf/177Hf ratio of 0.282785 (Bouvier et al., 2008). Detrital white mica 40Ar-39Ar analysis Detrital white micas were analyzed at the Open University, Milton Keynes. Micas with grainsizes between 250µm and 500µm were extracted from two LeMay Group siltstones. Cleaned grains were wrapped in aluminum foil and irradiated at the McMaster reactor, Canada together with the international biotite standard GA1550 (98.79 ± 0.96 Ma; Renne et al., 1998), to monitor the fast-neutron flux. Individual mica grains were fused using short (c. 10 second) laser pulses from a focused CW Nd-YAG infrared laser with a computer-controlled external shutter. The released gases were cleaned by Zr-Al getters. Argon isotopes were measured in a MAP 215-50 noble gas mass spectrometer. Analyses were corrected for blanks, 37Ar decay and neutron-induced interference reactions. Errors on individual grains are quoted at the 2sigma level.