Incremental growth of a large volume,chemically zoned magma body: a study of the tephra sequence beneath the Rainier Mesa ash flow sheet of the Timber Mountain Tuff |
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| Authors: | Kristin T Huysken Thomas A Vogel Paul W Layer |
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| Institution: | (1) Department of Geological Sciences, Michigan State University, 48824-1115 East Lansing, MI, USA;(2) Geophysical Institute, University of Alaska, 99775-0800 Fairbanks, AK, USA |
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| Abstract: | The Rainier Mesa ash-flow is a large (1200 km3), 11.6 My old, chemically zoned unit that ranges in composition from 55 to 76% SiO2 — one of the largest chemical ranges ever observed in a large volume ash-flow sheet. Two chemical trends occur in this sheet, a low silica (55–66% SiO2) and a high silica (>66% SiO2) trend. Ninety per cent of the Rainier Mesa sheet occurs in the high silica trend. Immediately beneath the Rainier Mesa sheet is a thick tephra sequence. The chemical variation of this sequence is nearly equivalent to the high silica portion of the Rainier Mesa ash-flow sheet (about 66–78% SiO2). Throughout the tephra sequence numerous small ash-flow layers occur, and each ash-flow layer is chemically zoned from more evolved at the base to less evolved at the top. This is consistent with having been erupted from a zoned magma body. The lowest silica tephra units are at the base of the sequence and the highest silica units are at the top — that is, the large-scale chemical trend of the entire sequence is opposite to that of the individual ash-flow layers. These ash-flow layers are of very small volume. The tephra sequence provides a unique record of the incremental development of the zoned, high silica portion of the Rainier Mesa magma body. |
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| Keywords: | Zoned magma body Chemical variation ash-flow sheets Tephra sequence Differentiation time constraints Evolution |
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