The Architecture of Oceanic Plateaus Revealed by the Volcanic Stratigraphy of the Accreted Wrangellia Oceanic Plateau [Geosphere]. Andrew R. Greene, James S. Scoates, Dominique Weis, Erik C. Katvala, Steve Israel, and Graham T. Nixon

SUPPLEMENTARY METHODS FILE.

40Ar/39Ar analytical methods

Twenty mineral separates (6 hornblende, 1 biotite, and 13 plagioclase) from 19 samples were processed for 40Ar/39Ar geochronology. The least altered samples were selected for 40Ar/39Ar dating based on thorough petrographic inspection of minerals in the basaltic groundmass. For processing plagioclase separates, the samples were first crushed in a Rocklabs hydraulic piston crusher between WC-plates. During crushing, approximately 400 g of each sample was sieved to isolate the fraction of grains between 250 and 450 µm. These grains were then rinsed in deionized water (~10 times). After drying, a hand magnet was used to separate the most magnetic grains. A Frantz magnetic separator was then used to isolate the non-magnetic plagioclase from the groundmass. The remaining grains were then hand-picked to select the least altered, inclusion-free plagioclase grains. A 2 mm-diameter circle of the freshest plagioclase was leached with 3 N cold HCl in an ultrasonic bath for 30 min, followed by rinsing with deionized water. Samples were finally rinsed with 1 N HNO3 and deionized water and allowed to dry. For hornblende and biotite, samples were crushed, sieved, washed in deionized water and dried at room temperature. Mineral separates from the 0.25 mm to 0.15 mm size fraction were hand-picked, washed in acetone, and dried.

Plagioclase, hornblende, and biotite grains were wrapped in aluminum foil and stacked in an irradiation capsule with similar-aged samples and neutron flux monitors (Fish Canyon Tuff sanidine, 28.02 Ma (Renne et al., 1998)). The samples were irradiated at the McMaster Nuclear Reactor in Hamilton, Ontario, for 90 MWH, with a neutron flux of approximately 6x1013 neutrons/cm2/s. Analyses (n=57) of 19 neutron flux monitor positions produced errors of <0.5% in the J value.

Samples were analyzed at the Noble Gas Laboratory in the Pacific Centre for Isotopic and Geochemical Research at University of British Columbia. The mineral separates were step-heated at incrementally higher powers in the defocused beam of a 10W CO2 laser (New Wave Research MIR10) until fused. The gas evolved from each step was analyzed by a VG5400 mass spectrometer equipped with an ion-counting electron multiplier. All measurements were corrected for total system blank, mass spectrometer sensitivity, mass discrimination, radioactive decay during and subsequent to irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl and K (Isotope production ratios: (40Ar/39Ar)K=0.0302±0.00006, (37Ar/39Ar)Ca=1416.4±0.5, (36Ar/39Ar)Ca=0.3952±0.0004,Ca/K=1.83±0.01(37ArCa/39ArK).

The analytical data are presented in Supplementary data file 4 and are summarized in Table 4. Age spectra and isochron diagrams for each of the samples are shown in Supplementary data file 4. The plateau and correlation ages were calculated using Isoplot ver.3.09 (Ludwig, 2003). Errors are quoted at the 95% confidence level (2σ) and are propagated from all sources except mass spectrometer sensitivity and age of the flux monitor. Following Ludwig (2003), the best statistically-justified plateau and plateau age were picked based on: (1) a well-defined plateau with at least three or more contiguous steps comprising more than 50% of the 39Ar released; (2) probability of fit of the weighted mean age greater than 5%; (3) slope of the error-weighted line through the plateau ages equals zero at 95% confidence; (4) ages of the two outermost steps on a plateau are not significantly different from the weighted-mean plateau age (at 1.8σ, six or more steps only); (5) outermost two steps on either side of a plateau must not have nonzero slopes with the same sign (at 1.8σ, nine or more steps only).

REFERENCES CITED

Ludwig, K. R. (2003). Isoplot 3.09, A Geochronological Toolkit for Microsoft Excel. BerkeleyGeochronologyCenter. Special Publication No. 4.

Renne, P. R., Swisher, C. C., III, Deino, A. L., Karner, D. B., Owens, T. & DePaolo, D. J. (1998). Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology 145(1-2), 117-152.

1