Appendix B
Zircon methodology
Mineral separation
Zircons from well Habanero 3 (12,830-12,840 ft depth) were separated from ~200g sample. The sample was crushed and sieved to separate the 50-300 μm size fraction. The remainder of the zircon separation was conducted at the Research School of Earth Sciences, Australian National University (RSES, ANU). The less-dense fraction was removed by tetrabromoethane, and the denser fraction was divided into magnetic and non-magnetic subsets by a Franz magnetic separator. The non-magnetic samples were then placed in methylene iodide to separate the dense, zircon-bearing fraction. Around 40 grains were hand picked under a binocular microscope and mounted on a glass slide. Standards for natural zircons (TEM and R33) and NIST 610 glass (homogeneous element concentrations) were mounted in separate windows on the glass slide, and epoxy resin poured into the mould. The resin block was polished using diamond pads to remove ~20 μm of material and expose a cross section of the zircon’s interior. Due to the variations in crystal size and orientation to the grain mount, features defined as ‘cores’ are displayed at different depths in individual zircon grains.
CL-SEM
Zircons were imaged by cathodoluminescence (CL) on a JEOL JSM-6610A scanning electron microscope (SEM) using a Robinson Detector at the Australian National University to identify growth zoning and recrystallisation textures. The type of CL response (bright or dark) relates to trace element abundances; high U can result in metamictisation (structural disorder resulting from radiation damage) of the grain, and suppresses the CL response resulting in a dark CL zone (e.g. Timms and Reddy, 2009). Consequently, the location of U-rich zones can be identified as within the core or rim and allow insight into the chemical evolution of the magma, as well as revealing those areas with the least damage and therefore greatest likelihood of providing concordant ages.
Imaging in reflected light on a petrographic microscope was conducted at The University of Queensland following LA-ICP-MS analyses to check whether the analytical spot had hit a fracture or inclusion which could adversely affect the reliability of the analysis.
Data-reduction procedure for zircon analyses
Geochronological data reduction was done using the Iolite add-on to Igor Pro ( et al., 2008; Paton et al., 2010; Paton et al., 2011). Using this software, time intervals were selected during the laser-off period for spots throughout the day, creating an accurate background value to subtract from the laser-on element acquisitions (spline smooth auto was selected for all the data reduction). After acquiring a background level, the standards (610, TEM, R33) were selected and reduced individually, to ensure that the elemental ratios were averaged over the analytical day. The initial part of the spectra (~3 s) was excluded from the data reduction to allow time for the laser signal to stabilise (Bryan et al., 2004; Harris et al., 2004). 207Pb/206Pb, 206Pb/238U and 208Pb/232Th ratios were reduced relative to TEMORA 2 (natural zircon with an ID-TIMS 206Pb/238U age of 416.78 ± 0.33 Ma; Black et al., 2004), with 232Th/238U and elements of geochemical interest reduced against the homogeneous glass NIST 610 (values used as in Jochum at el., 2011). The third standard, R33, is a natural zircon (ID-TIMS 206Pb/238U age 419.26 ± 0.39; Black et al., 2004) used as an unknown to help monitor machine drift and correct for depth-dependent elemental and isotopic fractionation (Hirata and Nesbitt, 1995; Horn et al., 2000). All data for the unknowns was captured initially. Minor peaks of elements such as Ti, La and P are indicative of an inclusion (e.g. titanite, apatite), and where possible the selected time slice avoided these peaks. In other instances where an inclusion had been targeted for the majority of the laser-on period, the time slice was not changed during the data reduction, but the analysis was instead removed from final age and geochemical results. Uncertainties reported for individual spots are a combination of the measured (206Pb/238U age for spots <1000 Ma, or 207Pb/206Pb age for spots >1000 Ma) and propagated uncertainties (the uncertainty across the standards). Uncertainties reported are to 2 standard errors.
After initial data reduction using Iolite, the data was run through the VizualAge data reduction scheme (DRS) of Petrus and Kamber (2012), another add-on to Igor Pro which can graphically display large data sets. This DRS allows for calculation of the 207Pb/206Pb age in addition to the U/Pb and Th/Pb ages calculated in Iolite’s U/Pb geochronology DRS.
Data selected for the final age calculation were imported into Excel and run through the plotting and age regression add-on Isoplot, developed by Ludwig (1991), generating weighted mean ages and concordia plots.
References
Black, L. P., Kamo, S. L., Allen, C. M., Davis, D. W., Aleinikoff, J. N., Valley, J. W., Mundil, R., Campbell, I. H., Korsch, R. J., Williams, I. S., and Foudoulis, C., 2004, Improved 206Pb/238U microprobe geochronology by the monitoring of trace-elements-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards: Chemical Geology, v. 205, p. 115-140.
Bryan, S. E., Allen, C. M., Holcombe, R. J., and Fielding, C. R., 2004, U-Pb zircon geochronology of Late Devonian to Early Carboniferous extension-related silicic volcanism in the northern New England Fold Belt: Australian Journal of Earth Sciences, v. 51, no. 5, p. 645-664.
Harris, A. C., Allen, C. M., Bryan, S. E., Campbell, I. A., Holcombe, R. J., and Palin, J. M., 2004, ELA-ICP-MS U–Pb zircon geochronology of regional volcanism hosting the Bajo de la Alumbrera Cu–Au deposit: implications for porphyry-related mineralization: Mineralium Deposita, v. 39, p. 46-67.
Hellstrom, J. C., Paton, C., Woodhead, J. D., and Hergt, J. M., 2008, Iolite: Software for spatially resolved LA-(quad and MC) ICP-MS analysis, in Sylvester, P., ed., Laser ablation ICP-MS in the Earth sciences: Current practices and outstanding issues: Canada, Mineralogical Association of Canada, p. 343-348.
Hirata, T., and Nesbitt, R. W., 1995, U-Pb isotope geochronolgy of zircon: evaluation of the laser probe-inductively coupled plasma mass spectrometry technique: Geochimica Cosmochimica Acta, v. 59, p. 2491-2500.
Horn, I., Rudnick, R. L., and McDonough, W. E., 2000, Precise elemental and isotope ratio determination by simultaneous solution nebulization and laser ablation ICP-MS: Application to U-Pb geochronology: Chemical Geology, v. 164, no. 281-301.
Ludwig, K. R., 1991, Isoplot - a plotting and regression program for radiogenic isotope data: USGS Open-File Report.
Paton, C., Hellstrom, J. C., Paul, B., Woodhead, J. D., and Hergt, J. M., 2011, Iolite: Freeware for the visualization and processing of mass spectrometric data: Journal of Analytical Atomic Spectrometry, v. 26, no. 2508-2518.
Paton, C., Woodhead, J. D., Hellstrom, J. C., Hergt, J. M., Greig, A., and Maas, R., 2010, Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation correction: Geochemistry Geophysics Geosystems, v. 11, p. 1-36.
Petrus, J. A., and Kamber, B. S., 2012, VizualAge: A novel approach to laster ablation ICP-MS U-Pb geochronology data reduction: Geostandards and Geoanalytical Research, p. 1-24.