Processed 40Ar/39Ar age data from rock samples collected offshore Heard and McDonald Islands.

Processed 40Ar/39Ar age data from rock samples collected offshore Heard and McDonald Islands.40Ar/39Ar geochronology summary spreadsheet and seven analytical results tables for samples analysed from submarine edifices around Heard and McDonald Islands, Kerguelen Plateau. The Oregon State University Geochronology Laboratory was used for these analyses.Sample PreparationConcentrations of groundmass, glass, and biotite were prepared for this study. We obtained high-purity separates (greater than 99% purity) by crushing and sieving (150-250 um size), then removing phenocryst phases by Frantz magnetic separator and hand-picking under binocular microscope. We treated separates in a series of acid leaching steps: 1N and 6N HCl, 1N and 3N HNO3, followed by ultrasonic washing in triple distilled water (3X) to remove any remaining fine particles. We encapsulated between 40 and 20 mg of high purity groundmass and minerals in aluminium foil and interspersed packets with a standard of known age (FCT-NM-Fish Canyon Tuff sanidine (FCT-NM-Fish Canyon Tuff sanidine, 28.201 ± 0.023 Ma, 1σ; Kuiper et al., 2008) and vacuum sealed the samples with standards in quartz vials. The samples were irradiated for 30 minutes at 1 MW power in the TRIGA (CLICIT-position) nuclear reactor at Oregon State University.Instrument Description and Analytical Procedure40Ar/39Ar incremental heating age determinations are performed at Oregon State University on a multi-collector ARGUS-VI mass spectrometer with 5 Faraday collectors (all fitted with 1012 Ohm resistors) and 1 ion-counting CuBe electron multiplier (located in a position next to the lowest mass Faraday collector). This allows simultaneous measurement of all argon isotopes, with mass 36 on the multiplier and masses 37 through 40 on the four adjacent Faradays. This configuration provides the advantages of running in a full multi-collector mode while measuring the lowest peak (on mass 36) on the highly sensitive electron multiplier (which has an extremely low dark-noise and a very high peak/noise ratio). Procedure blanks and air shots are measured daily to correct for system contributions and mass discrimination. Following irradiation in the CLICIT position in the Oregon State University TRIGA nuclear reactor, and decay of short-lived radionuclides, we analyse the standard sanidines (FCT-NM-Fish Canyon Tuff sanidine neutron flux monitors) to create a neutron flux gradient (J-curve) for the age calculations. We calculate individual sample J-values (0.2-0.3% uncertainties, 1σ) by parabolic fitting of the measured flux gradient against irradiation height. We load irradiated samples into Cu-planchettes in an ultra-high vacuum sample chamber and incrementally heat by scanning a defocused 25W CO2 laser beam in preset patterns across the sample, in order to release the argon evenly in increasing power steps (up to 22% full power for basaltic groundmass) to fusion. After heating, reactive gases are removed using a SAES Zr-Al ST101 getter operated at 400°C for ~10 minutes and two SAES Fe-V-Zr ST172 getters operated at 200°C and room temperature, respectively. Heating ExperimentsWe acquired 40Ar/39Ar age data by incremental heating experiments using an ARGUS-VI mass spectrometer with 25W CO2 laser and associated high vacuum system. Sample heating occurred in steps (by increasing laser power), following an initial surface degassing, up to fusion. The heating schedule, based on experience with basaltic compositions and sample weights, produced 26-36 gas release steps, which is sufficient to resolve reliable age information from potential alteration (Ar-loss) and experimental artifacts (Ar-recoil) without sacrificing step age precision. We calculated ages using the corrected Steiger and Jäger (1977) decay constant of 5.530 ± 0.097 x 10-10 1/yr (2σ) as reported by Min et al. (2000) and the Lee et al. (2006) value for trapped atmospheric 40Ar/36Ar (298.56 ± 0.31). For all other constants used in the age calculations we refer to Table 2 in Koppers et al. (2003).Interpretation of ResultsPlateau age refers to the weighted mean age (Taylor, 1997) of multiple, contiguous temperature steps with apparent ages that are indistinguishable at the 95% confidence interval and represent 50% or more of the total 39Ar released (Fleck et al., 1977). We used isochron analyses (York, 1969) to assess if non-atmospheric argon components were trapped in submarine-erupted samples, and to confirm the plateau ages. An integrated gas age (total fusion) was calculated by summing all gas fractions for each sample. All calculations used the ArArCALC software (Koppers, 2002) v2.5.2, available from the http://earthref.org/ArArCALC/ website. ReferencesFleck, R. J., Sutter, J. F., and Elliot, D. H. (1977). Interpretation of discordant 40Ar/39Ar age-spectra of Mesozoic tholeiites from Antarctica. Geochemica et Cosmochimica Acta, 41, 15-32. Koppers, A. (2002). ArArCALC-software for 40Ar/39Ar age calculations. Computers and Geosciences, 28, 605-619. Koppers, A. A. P., Staudigel, H., Pringle, M. S., and Wijbrans, J. R. (2003). Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism? Geochemistry, Geophysics, Geosystems, 4(10). https://doi.org/10.1029/2003gc000533 Kuiper, K. 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