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Volcanoes and Their Products
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Geologists have identified a small number of other supervolcano eruptions that were larger than Toba, such as the largest known eruption, the Wah Wah Springs eruption, that occurred in Utah 30 million years ago, and an eruption that created the Fish Canyon Tuff in southwest Colorado 28 million years ago. Figure 7.4 shows several different layers of the Fish Canyon Tuff—thick layers of consolidate volcanic ash and coarser debris. The ash layers vary in color due to slightly different compositions, and are easily eroded, as seen in this photograph. The eruption, centered at La Garita Caldera in southwest Colorado, produced an estimated 5000 cubic kilometers of volcanic ash.
Extensional deformation along the Footwall Fault below the Hyde-Macraes Shear Zone, Otago Schist, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
Uwe Ring, Nick Mortimer, Christoph Butz, Matthias Bernet
Zircons aliquots were mounted in Teflon® sheets, polished and etched at 228 °C for 10–20 h. Using the external detector method, muscovite sheets were attached to each sample mount. All samples were irradiated together with CN1 and IRMM540R dosimeter glasses and Fish Canyon Tuff and Buluk Tuff age standards at the FRM II reactor in Garching, Germany. Central ages (Galbraith and Laslett 1993), quoted with 2σ uncertainties, were calculated using the IUGS recommended Zeta-calibration approach of Hurford and Green (1983), which allows for non-Poissonian variation within a population of single-grain ages belonging to an individual sample. For all samples fission-tracks were counted dry at ×1250 magnification using an Olympus BH2 microscope at the ISTerre Thermochronology laboratory at Université Grenoble Alpes, France.
Basanite cobbles in Pleistocene sediments in Central Otago and their implications for intraplate volcanism and Clutha River paleo-drainage
Published in New Zealand Journal of Geology and Geophysics, 2023
James M. Scott, Alan F. Cooper, Dave Craw, Petrus J. le Roux, Hayden B. Dalton, Marshall C. Palmer
The groundmass separated for 39Ar-40Ar dating was ultrasonically cleaned with 5% HNO3 (10 min), 2% HF (1 min), demineralised water (10 min) and then rinsed in acetone. The cleaned grains were weighed and wrapped in aluminium foil envelopes, and then placed into quartz glass vials together with interspersed aliquots of the flux monitor Fish Canyon Tuff sanidine (Age = 21.176 ± 0.005 Ma; Phillips et al. 2022). The package (UM#98) was then encapsulated in an outer sealed glass vial and irradiated in the CLICIT facility of the Oregon State University TRIGA Reactor for 20 MWh. Following irradiation and cooling, 40Ar-39Ar analyses were undertaken in the Noble Gas laboratory at the University of Melbourne. Step-heating analyses were conducted on single weighed aliquots of groundmass grains placed into the sample chamber of a gas-handling system equipped with a Photon Machines Fusions 10.6 CO2 laser and connected to a Thermo Fisher Scientific ARGUSVI mass spectrometer at the University of Melbourne. Apparatus details are given in Phillips and Matchan (2013) with updated Faraday detectors now equipped with 1 × 1013 Ω resistors as described in Heath et al. (2018). Analytical methods follow those described by Matchan and Phillips (2014) and Heath et al. (2018). For incremental heating a 6 mm laser beam-size was utilised with laser power varied (2–30%), dependent on number of heating steps (1-13) for each aliquot. All results are corrected for system blanks, mass discrimination, radioactive decay and reactor-induced interference reactions. Correction factors vary for each irradiation are supplied in the Supplementary File. Mass discrimination was monitored by analysis of standard air volumes, assuming the air argon isotopic composition of Lee et al. (2006). Inclusion of uncertainties in the J-value and age of Fish Canyon Tuff sanidine have a negligible impact on uncertainties. Decay constants are those of Steiger and Jäger (1977). The 40Ar-39Ar dating technique is described in detail by McDougall and Harrison (1999). Age spectra were generated using ISOPLOT (Ludwig 2003). Age uncertainties reported in the results section are 2σ.