Williams, Trevor; van de Flierdt, Tina; Hemming, Sidney R; Chung, Elena; Roy, Martin; Goldstein, Steven L (2010): (Appendix A2) Complete results of 40Ar/39Ar dating of hornblende grains from DSDP Hole 28-268 [dataset]. PANGAEA, https://doi.org/10.1594/PANGAEA.786663,

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In supplement to: Williams, T et al. (2010): Evidence for iceberg armadas from East Antarctica in the Southern Ocean during the late Miocene and early Pliocene. Earth and Planetary Science Letters, 290(3-4), 351-361, https://doi.org/10.1016/j.epsl.2009.12.031

Method: Samples were co-irradiated with Mmhb hornblende monitor standard and Cima sanidine (internal standard) for 10 hours in the Cd lined facility at the Oregon State University Reactor, and are referenced to an Mmhb age of 523 Ma (Renne et al., 1998). Single-step CO2 laser fusion 40Ar/39Ar analyses for individual irradiated grains were processed at the Ar geochronology laboratory at Lamont-Doherty Earth Observatory. Reported data are corrected for background based on frequent analysis of the blank and mass discrimination based on frequent measurements of the air pipette. They are also corrected for nuclear interferences using the parameters provided in Renne et al. (1998) for the Oregon State reactor. The "J" values were calculated for the entire irradiation package, as horizontal and vertical gradients were less than the 1% variation of the Mmhb monitor standard measurements. Errors reported are based on the internal precision of the measurement and the 1% variation from measurements of Mmhb (1 sigma). LDEO Ar lab facilities: The LDEO Ar lab consists of a VG5400 noble gas mass spectrometer equipped with a modified Neir ion-source and a 90° sector extended-geometry. The mass spectrometer is operated in static mode and measurements are made on a Balzars multiplier in analogue mode using a high voltage of 1580 V on the multiplier (At these conditions, the 40Ar signal is about 1e-9 amps for 1e-11 moles of atmospheric Ar. A typical background 40Ar signal is approximately 2e-11 Amps). The LDEO Ar lab is entirely automated. Automation of the vacuum valves is achieved by using pneumatically operated valves that are activated by electro-pneumatic solenoid valves. Gasses released from the heating of samples are scrubbed of reactive gases such as H2 CO2, CO and N2 by exposure to Zr-Al sintered metal alloy getters. The remaining inert gasses, principally Ar, are then admitted to the mass spectrometer and the Ar-isotopic ratios are determined using "Mass Spec", an automated data collection software developed by Al Deino of the Berkeley Geochronology Center (http://www.bgc.org/facilities/other_facil.html). Both labs used a CO2 laser for fusing the individual grains.