40Ar/39Ar Geochronology of Legacy Samples Heard Island

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Summary of 40Ar/39Ar geochronology analytical results published in Fox, Jodi M., et al. "Construction of an intraplate island volcano: The volcanic history of Heard Island." Bulletin of Volcanology 83.5 (2021): 37.

Fourteen volcanic samples were selected for 40Ar/39Ar dating from collections housed at the University of Tasmania. Thirteen samples are from the 1986/1987 ANARE Wheller collection and one sample from the 2016 Cordell collection. These collections were the most accessible, include many specimens that have been collected insitu and have well documented sample locations. Sample KP-C78 is the only sample selected that was not collected insitu; it was collected from the Stephenson glacier moraine and is presumed to have been transported from the upper slopes of Big Ben. Due to the expected young age of the lavas less than 20 000 ka), sample selection within the collection was based on the freshness of the groundmass and the highest percentage of K2O measured in whole rock X-ray fluorescence spectrometry (XRF) analysis. Selected samples were predominantly from Laurens Peninsula and from the coastal, basaltic volcanic cones. Seven samples were analysed at the Western Australian Argon Isotope Facility (WAAIF), Curtin University, Australia. Seven samples were analysed at the Oregon State University Argon Geochronology Lab (OSUAGL), United States of America. Sample preparation was completed by the first author at Curtin University and by laboratory staff at Oregon State University.

40Ar/39Ar Methodology, West Australian Argon Isotope Facility, Curtin University, Australia
Weathered outer layers of the sample were removed and the rock crushed and sieved to 350-500 µm-size. The crushate was washed in water and fresh groundmass separated by careful hand picking under binocular microscope. The selected groundmass material were further leached in diluted HF for one minute and then thoroughly rinsed with distilled water in an ultrasonic cleaner.

Samples were loaded into five large wells of one 1.9 cm diameter and 0.3 cm depth aluminium disc. These wells were bracketed by small wells that included Fish Canyon sanidine (FCs) used as a neutron fluence monitor for which an age of 28.294 ± 0.036 Ma (1σ) was adopted (Renne et al., 2011). The discs were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated for 20 minutes in the US Geological Survey nuclear reactor (Denver, USA) in central position. The mean J-values computed from standard grains within the small pits range from 0.00009337 ± 0.00000011 (0.12%) to 0.00009545 ± 0.00000019 (0.20%) determined as the average and standard deviation of J-values of the small wells for each irradiation disc. Mass discrimination was monitored using an automated air pipette and provided a mean values of 0.994047 ± 0.004 to 0.995193 ± 0.003 per dalton (atomic mass unit) relative to an air ratio of 298.56 ± 0.31 (Lee et al., 2006). The correction factors for interfering isotopes were (39Ar/37Ar)Ca = 7.30x10-4 (± 11%), (36Ar/37Ar)Ca = 2.82x10-4 (± 1%) and (40Ar/39Ar)K = 6.76x10-4 (± 32%).

The 40Ar/39Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University. The samples (150 mg each) were step-heated using a continuous 100 W PhotonMachine© CO2 (IR, 10.4 µm) laser fired on the crystals during 60 seconds. Each of the FCs standard crystals was fused in a single step.

The gas was purified in an extra low-volume stainless steel extraction line of 240cc and using two SAES AP10 and one GP50 getter. Ar isotopes were measured in static mode using a low volume (600 cc) ARGUS VI mass spectrometer from Thermofisher© (Phillips and Matchan, 2013) set with a permanent resolution of ~200. Measurements were carried out in multi-collection mode using four faradays to measure mass 40 to 37 and a 0-background compact discrete dynode ion counter to measure mass 36. We measured the relative abundance of each mass simultaneously using 10 cycles of peak-hopping and 33 seconds of integration time for each mass. Detectors were calibrated to each other electronically and using air shot beam signals. The raw data were processed using the ArArCALC software (Koppers, 2002) and the ages have been calculated using the decay constants recommended by (Renne et al., 2011). Blanks were monitored every 3 to 4 steps and typical 40Ar blanks range from 1 x 10-16 to 2 x 10-16 mol. Ar isotopic data corrected for blank, mass discrimination and radioactive decay. Individual errors are given at the 1σ level.

Our criteria for the determination of plateau are as follows: plateaus must include at least 50% of 39Ar. The plateau should be distributed over a minimum of 3 consecutive steps agreeing at 95% confidence level (Baksi 2007) and satisfying a probability of fit (p) (Jourdan et al. 2009) of at least 0.05. The probability of fit is used in conjunction with the MSWD to assess if the clustering of data is consistent with measurement errors (Jourdan et al. 2007). Plateau ages are given at the 2σ level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their individual analytical error Inverse isochrons include the maximum number of steps with a probability of fit ≥ 0.05. All sources of uncertainties are included in the calculation.

References
Baksi, A.K. 2007. A quantitative tool for detecting alteration in undisturbed rocks and mineral — I: water, chemical weathering and atmospheric argon. In: Foulger G.R. and Jurdy D.M. (eds) Plates, Plumes and Planetary Processes, Geological Society of America Special Paper, vol 430, 285-304.

Jourdan F, Renne P.R., and Reimold, W.U. 2009. An appraisal of the ages of terrestrial impact structures. Earth and Planetary Science Letters, vol 286, 1-13.

Koppers, A.A.P., 2002. ArArCALC-software for 40Ar/39Ar age calculations. Computers and Geosciences, vol 28, 605–619.

Lee, J.-Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.-S., Lee, J.B., Kim, J.S. 2006. A redetermination of the isotopic abundance of atmospheric Ar. Geochimica et Cosmochimica Acta, vol 70, p. 4507-4512.

Renne, P.R., Balco, G., Ludwig, K.R., Mundil, R., Min, K., 2011. Response to the comment by W.H. Schwarz et al. on "Joint determination of K-40 decay constants and Ar-40*/K-40 for the Fish Canyon sanidine standard, and improved accuracy for Ar-40/Ar-39 geochronology" by PR Renne et al. (2010). Geochimica et Cosmochimica Acta, vol 75, 5097-5100.

40Ar/39Ar Methodology, Oregon State University Argon Geochronology Lab, United States of America

ARGUS VI – Methodology 40Ar/39Ar Geochronology
Sample Preparation:
Both groundmass concentrations of basalts and one glass separate were prepared for this study. High-purity basalt groundmass concentrates ( greater than 99% purity); were obtained using standard separation techniques. All groundmass and glass separates were rigorously put through a series of acid leaching procedures. Each sample was treated with 1N and 6N HCl, 1N and 3N HNO3. Glass separates were treated in a dilute bath of HF (~5%) for approximately 5-10 minutes. Final mineral separates were hand picked under a binocular microscope to a purity of greater than 99% with particular attention to excluding grains with abundant inclusions, adhering material, carbonate, or alteration. All groundmass concentrates range in size between 60-100 mesh (250-150 µm). Visible phenocrysts were removed using a magnetic separator and detailed hand picking. Both glass and groundmass concentrates were washed in triple distilled water (3X) to dissolve any remaining fine particles and possible acid.

Brief Methods:
Between 40 and 20 mg of high purity groundmass and glass were hand picked using a binocular microscope. They were then encapsulated in aluminium and loaded with a standard of known age (FCT-NM-Fish Canyon Tuff sanidine standard produced from the New Mexico Geochronology Research Laboratory in Socorro, New Mexico) and vacuum sealed in quartz vials. The samples geometries (sample heights) were determined using a vernier caliper. After irradiation, the samples were separated from the flux monitors. Prior to analysing the basalt samples, the flux monitors (FCT-NM sanidines) were analysed in order to create a J-curve for the age calculation.

The 40Ar/39Ar ages were obtained by incremental heating using the ARGUS-VI mass spectrometer. 4 groundmass splits and one glass sample were irradiated for 6 hours at 1 Megawatt power (Irradiation 16-OSU-05) in the TRIGA (CLICIT-position) nuclear reactor at Oregon State University, along with the FCT sanidine (28.201 ± 0.023 Ma, 1σ) flux monitor (Kuiper et al. 2008). Individual J-values for each sample were calculated by parabolic extrapolation of the measured flux gradient against irradiation height and typically give 0.2-0.3% uncertainties (1σ).

The term “plateau” refers to two or more contiguous temperature steps with apparent dates that are indistinguishable at the 95% confidence interval and represent 50% of the total 39ArK released (Fleck et al., 1977). Isochron analysis (York, 1969) of all samples was used to assess if non-atmospheric argon components were trapped in any samples, and in some cases, confirm the Plateau ages for each sample. A total gas age (Total Fusion Age), analogous to conventional K-Ar age, is calculated for each sample by weight averaging all ages of all gas fractions for the sample.

The 40Ar/39Ar incremental heating age determinations were performed on a multi-collector ARGUS-VI mass spectrometer at Oregon State University that has 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 us to measure simultaneously 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). Irradiated samples were loaded into Cu-planchettes in an ultra-high vacuum sample chamber and incrementally heated by scanning a defocused 25 W CO2 laser beam in preset patterns across the sample, in order to release the argon evenly. After heating, reactive gases were cleaned up using an 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.

All ages were calculated 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). For all other constants used in the age calculations we refer to Table 2 in Koppers et al. (2003). Incremental heating plateau ages and isochron ages were calculated as weighted means with 1/σ2 as weighting factor (Taylor 1997) and as YORK2 least-square fits with correlated errors (York 1969) using the ArArCALC v2.6.2 software from Koppers (2002) available from the http://earthref.org/ArArCALC/ website.

References
Koppers, A. A. P., 2002, ArArCALC—Software for 40Ar/39Ar age calculations, Comput. Geosci., 28, 605–619. (Available at http://earthref.org/tools/ararcalc.htm.)

Koppers, A., H. Staudigel, J. R. Wijbrans, and M. Pringle, 2003, Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism?, Geochem. Geophys. Geosyst., 4, doi:10.1029/2003GC000533.

K. F. Kuiper, A. Deino, F. J. Hilgen, W. Krijgsman, P. R. Renne, J. R. Wijbrans, 2008, Synchronizing Rock Clocks of Earth History, Science 320, 500 (2008); DOI: 10.1126/science.1154339

Min, K., R. Mundil, P. R. Renne, and K. R. Ludwig, 2000, A test for systematic errors in 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite, Geochim. Cosmochim. Acta, 64, 73–98.

Steiger, R. H., and E. Ja¨ger, 1977, Subcommission on geochronology: Convention on the use of decay constant in geo- and cosmochronology, Earth Planet. Sci. Lett., 36, 359–362.

Taylor, J. R., An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements, 327 pp., Univ. Sci. Books, Mill Valley, Calif., 1997.

York, D., 1969, Least squares fitting of a straight line with correlated errors, Earth Planet. Sci. Lett., 5, 320–324.

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