Alaska Volcano Observatory - Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America

CHANGING IDEAS ON THE IDENTITY AND STRATIGRAPHIC SIGNIFICANCE OF THE SHEEP CREEK TEPHRA BEDS IN ALASKA AND THE YUKON TERRITORY, NORTHWESTERN NORTH AMERICA

Westgate, J.A., Preece, S.J., Froese, D.G., Pearce, N.J.G., Roberts, R.G., Demuro, M., Hart, W.K., and Perkins, W., 2008, Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America: Quaternary International, v. 178, n. 1, p. 183-209, doi: 10.1016/j.quaint.2007.03.009.

DATA TABLES *Not all tables in a publication may be listed. Not all tables from a publication have data entered in the database.

3921: Average major-element composition of glass in Sheep Creek tephra, Sheep Creek tephra at Canyon Creek, and Ash Bend tephra

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* Notes: Analyses were done on a Cameca SX-50 wave-length dispersive electron microprobe operating at 15 keV accelerating voltage, 6 nA beam current and 10 micon beam diameter. Standardization achieved by use of mineral and glass standards. Analyses recast to 100% on a water-free basis. Standard deviation in parentheses following each oxide abundance. n is the number of analyses. FeOt is total Fe as FeO, H2Od is water by difference. Sheep Creek tephra samples are UT734 and UT1052, Sheep Creek tephra at Canyon Creek is UT250, and Ash Bend tephra is UT1051. Concerning the accuracy of Na2O values, when using tugtupite to find the Na peak position and UA5831(obsidian) to calibrate the intensity, we get Na2O values that are statistically indistinguishable from values on pure glass separates by other methods. For example, Na2O content of glass shards in Old Crow tephra (Westgate et al., 1985) by Flame Photometry is 3.54 +/- 0.18wt% (4 analyses), 3.68 +/- 0.12 by INAA (14 analyses), and 3.55 +/- 0.23wt% by microprobe (542 analyses, unnormalized). H2Od values show that some glass analyses in a tephra bed total <95 wt%, but on normalization to anhydrous basis, these analyses compare well with normalized values of analyses that total >95 wt%, as can be seen in the low standard deviations for SiO2. *

3931: Average major-element composition of glass in Sheep Creek tephra beds

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* Notes: Most analyses were done on a Cameca SX-50 wave-length dispersive electron microprobe operating at 15 keV accelerating voltage, 6 nA beam current and 10 micron beam diameter. Some additional analyses of SCt-F and SCT-K were done on an ETEC energy dispersive microprobe operating at 20 keV, 50–60nA beam current and 1–2 micron beam diameter. Standardization achieved by use of mineral and glass standards. Analyses recast to 100% on a water-free basis. Prior to anhydrous normalization, SCt-F ETEC analyses had 0.8wt% and SCt-K ETEC analyses had 0.57wt% added to the total to account for unanalysed MgO, MnO and Cl. 0.8 and 0.57wt% is the concentration for these elements from selected analyses of SCt-F and SCt-K done on the Cameca SX-50. (#) is standard deviation, n is the number of analyses, FeOt is total Fe as FeO, H2Od is water by difference. Average composition is based on non-zero values for the following samples: SCt-F (UA207, UA372, UT389, UT491, UT734, UT735, UT746, UT845), Sct-CC (UT250), SCt-C (UT2096, UT2109), SCt-K (UA42, UA44, UT104, UT1052, UT1058, UT1060, UT1064, UT1065, UT1459, UT1460, UT1461, UT1462, UT1463, UT1474, UT1613, UT1646, UT1771, UT1772, UT1773, UT1774, UT1775, UT1784, UT1794, UT1908, UT1920, UT1931, UT2019, UT2021, UT2022, UT2023, UT2026, UT2097, UT366, UT40), SCt-A (UT1051). *

3941: Average major- and trace-element composition of glass shards from proximal tephra units

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* Notes: Major-element analyses (wt%) were done on a Cameca SX-50 wave-length dispersive electron microprobe operating at 15 keV accelerating voltage, 6 nA beam current and 10 micron beam diameter. Standardization achieved by use of mineral and glass standards. Analyses recast to 100% on a water-free basis. (#) is standard deviation, n is the number of analyses, FeOt is total Fe as FeO, H2Od is water by difference. Trace-element analyses (ppm) for Mount Churchill were performed on purified glass separates by the instrumental neutron activation technique following the methods of Barnes and Gorton (1984). Trace-element analyses for Mount Drum and Hayes volcano were performed on purified glass separates by solution nebulisation inductively coupled plasma mass spectrometry using a VG Plasma-Quad PQ II. Further details of the methods are in Pearce et al. (2004). (#) is the standard deviation, n is the number of analyses, n.d. is not determined. WRA-E is White River Ash, eastern lobe; WRA-N is White River Ash, northern lobe. *

5751: Average trace-element composition of glass from Sheep Creek tephra beds

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