U-Pb-Hf zircon data from Bruce Rise and Naturaliste Plateau
These dataset files (2 tables) are supplementary material to: Halpin, J.A., Daczko, N.R., Direen, N.G., Mulder, J.A., Murphy, R.C., Ishihara, T., 2020. Provenance of rifted continental crust at the nexus of East Gondwana breakup. Lithos 354-355. https://www.sciencedirect.com/science/article/pii/S002...
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Australian Antarctic Data Centre
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| Online Access: | https://researchdata.edu.au/u-pb-hf-naturaliste-plateau/1468137 https://doi.org/10.26179/5b9205cde1613 https://data.aad.gov.au/metadata/records/AAS_4355_U-Pb-Hf_Bruce_Naturaliste http://nla.gov.au/nla.party-617536 |
| Summary: | These dataset files (2 tables) are supplementary material to: Halpin, J.A., Daczko, N.R., Direen, N.G., Mulder, J.A., Murphy, R.C., Ishihara, T., 2020. Provenance of rifted continental crust at the nexus of East Gondwana breakup. Lithos 354-355. https://www.sciencedirect.com/science/article/pii/S0024493719305237?via%3Dihub https://doi.org/10.1016/j.lithos.2019.105363 They include: Supplementary Table 1. Zircon U-Pb datasets Supplementary Table 2. Zircon Lu-Hf datasets Sample details from Halpin et al. (2020): Ishihara et al. (1996) made initial reports of the dredge sites from which our analyses have been made. Dredging at four sites on the eastern margin of the Bruce Rise was undertaken during cruise TH-94 of the R/V Hakurei-Maru during the austral summer of 1994/5, along with other geophysical data acquisition reported in Ishihara et al. (1996). Of the four sites dredged, sites D1502, D1503 and D1504 all recorded hauls of basement rocks, including crystalline basement fragments. These dredge sites and hauls are summarised in Table 1. The recovered dredge samples have variable shapes, but the samples analysed here (two granites from D1502; Fig. 3) contain sharp and unweathered faces consistent with dredging from in situ basement. The interpretation of composite seismic profiles TH94/21 and GA-229/19 from the eastern flank of the Bruce Rise (Fig. 1b) shows a folded and faulted syn-rift sequence that thickens towards the south and is separated from flat-lying post-rift sediments by a prominent erosional unconformity (Stagg et al., 2006). The outermost ridge, flanked by the Vincennes Fracture Zone, is interpreted to comprise exposed crystalline basement, and we interpret the granitic samples studied here to represent parts of this basement complex. Sample D1502-A is a medium to coarse-grained (~2–4 mm) red granite, whereas sample D1502-B is a fine to medium-grained (~1–1.5 mm) cream-grey granite (Fig. 3a). Both samples comprise quartz, plagioclase, alkali feldspars (including microcline), biotite and accessory magnetite and zircon. Sample D1502-B additionally contains minor biotite-amphibole-rich schlieren. Biotite is variably orientated suggesting weak magmatic foliation. The low-strain character of the samples is supported by very limited undulose extinction of some grains, minor development of sub grains in quartz, and preserved igneous microstructures that include subhedral feldspar grains presenting some crystal faces (red lines, Fig. 3b), quartz-feldspar interstitial textures (blue ‘i’, Fig. 3b), low dihedral angles (double blue arrow heads, Fig. 3b), elongate mineral films along grain boundaries that are inferred to have pseudomorphed former melt, and growth twinning in plagioclase. Minor low-temperature alteration of feldspar and biotite to sericite ± chlorite is observed in sample D1502-A. High resolution whole thin section photomicrographs are available at https:// imagematrix.science.mq.edu.au/. Details of analytical methods from Halpin et al. (2020): Initial sample preparation including zircon separation and mounting was performed at Curtin University. Zircon grains were imaged via cathodoluminescence (CL) on a FEI Quanta 600 SEM at the Central Science Laboratory, University of Tasmania, to reveal internal structure in order to optimise and contextualise U-Pb analyses. U-Pb zircon analyses were performed on an Agilent 7500cs quadrupole ICPMS with a 193nm Coherent Ar-F gas laser and the Resonetics S155 ablation cell at the Discipline of Earth Sciences, University of Tasmania. Each analysis was pre-ablated with 5 laser pulses to remove the surface contamination then the blank gas was analysed for 30 s followed by 30 s of zircon ablation at 5 Hz and ~2 J/cm2 using a spot size of 29 μm. Isotopes measured include 49Ti, 56Fe, 91Zr, 178Hf, 202Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th and 238U. The down hole fractionation, instrument drift and mass bias correction factors for Pb/U and Pb/Th ratios on zircons were calculated using the primary standard (91500, Wiedenbeck et al., 1995) and secondary standards (TEMORA 1, Black et al., 2003; Plešovice, Sláma et al., 2008) analysed at the beginning of the session and every 15–20 unknowns using the same spot size and conditions as used on the samples to provide an independent control to assess accuracy and precision. The correction factor for the 207Pb/206Pb ratio was calculated using 17 analyses of the international glass standard NIST610 analysed throughout the analytical session and corrected using the recommended values (Baker et al., 2004). All data reduction calculations and error propagations were done within Microsoft Excel® via macros designed at the University of Tasmania (see Halpin et al., 2014; Sack et al., 2011). No common Pb corrections were applied. However, time-resolved isotopic ratios for each analysis were scrutinised on concordia diagrams to investigate the presence of common Pb and/or ancient Pb-loss and/or mixing of age zones, and analyses (or parts of analyses) were excluded from the dataset where a combination of these trends was detected. Uncertainties quoted in Supplementary Tables and in figures are the internal measured uncertainty only (i.e., those from random based sources of error, e.g., counting statistics). External sources of uncertainty (i.e., from systematic sources of uncertainty, e.g., decay constant uncertainty, uncertainty in the age of the primary zircon standard) calculated after Horstwood et al. (2016) and Thompson et al. (2018) are quoted in parentheses for the standard data below (see also Supplementary Table 1). 206Pb/238U ages for the secondary zircon standards Plešovice and TEMORA 1 over the course of this study (at 95% confidence) are 333.6 ± 2.7 (4.3) Ma (n = 7, MSWD = 1.3) and 415.1 ± 3.2 (5.3) Ma (n = 6, MSWD = 0.4), compared to the published TIMS zircon ages of 337.13 ± 0.37 Ma (Sláma et al., 2008) and 416.8 ± 1.1 Ma (Black et al., 2003), respectively. Although Plešovice is slightly outside 2σ of the internal uncertainties, it is well within the published values when considering the external uncertainties. The primary zircon standard 91500 yields a 207Pb/206Pb weighted mean age of 1065.3 ± 8.8 (10.5) Ma (n = 27, MSWD = 0.71) within error of the recommended value of 1065.4 ± 0.3 Ma (Wiedenbeck et al., 1995). Tera-Wasserburg diagrams and age calculations were made using Isoplot v4.11 (Ludwig, 2003). Uncertainties for individual analyses as quoted in text and as error bars on U\\Pb plots have been calculated to the two-sigma level. Weighted mean and intercept ages are reported at 95% confidence limits. Hf isotope analyses were performed in situ on a subset of the same grains analysed for U-Pb using a Photon Machines Excimer 193 nm Ar-F laser ablation micro-probe attached to a Nu Plasma multi- collector (MC)-ICPMS system at Macquarie University GeoAnalytical (MQGA)(see Griffin et al., 2004 for a detailed methodology). A gas blank was analysed for 30 s followed by up to 120 s of ablation at a beam diameter of 40–50 μm, 5 Hz and ~7.5 J/cm2. Zircon CL images were used to ensure that Hf isotope analyses overlapped the same do- main analysed for U-Pb. The Mud Tank and Temora-2 zircon standards were used as a reference standard for Hf analysis; our weighted average 176Hf/177Hf values for these standards are 0.282526 ± 41 (n = 17, MSWD = 1.6) and 0.282678 ± 10 (n = 7, MSWD = 2.1), respectively, within error of the published values of 0.282523 ± 43 (Mud Tank; Griffin et al., 2006) and 0.282680 ± 24 (Temora-2; Woodhead et al., 2004). Uncertainties quoted are the internal measured uncertainty and do not include any propagation of error from the reference standard. The initial 176Hf/177Hf value (Hfi) in zircon is calculated using the measured 176Lu/177Hf, 176Hf/177Hf and apparent 207Pb/206Pb age and the 176Lu decay constant of Scherer et al. (2001) of 1.865 x 10-11. Model age calculations (TDM) are based on a depleted-mantle source with Hfi = 0.279718 and 176Lu/177Hf = 0.0384. This provides a value of 176Hf/177Hf (0.28325) similar to that of average mid-ocean ridge basalt over 4.56 Ga. The calculated TDM ages use the measured 176Lu/177Hf of the zircon and give a minimum age for the source material of the magma from which the zircon crystallised. Two-stage model ages (TDM2) are calculated assuming that the parental magma was derived from the average continental crust (176Lu/177Hf = 0.015), which in turn was originally derived from the depleted mantle. |
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