18. DISTRIBUTION, CHEMICAL COMPOSITION, AND SOURCES OF OLIGOCENE TO MIOCENE VOLCANIC ASHES FROM SITES 907,908, AND 9131
The volcanogenic component of 54 ash layers and ash-bearing sediments recovered at Sites 907, 908, and 913 (Ocean Drill-ing Program Leg 151) in the Greenland-Iceland-Norwegian Seas (Iceland Plateau, Greenland Basin, Hovgàrd Ridge) consists mainly of colorless and/or light to dark brown glass particl...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.590.4030 http://www-odp.tamu.edu/publications/151_SR/VOLUME/CHAPTERS/sr151_18.pdf |
| Summary: | The volcanogenic component of 54 ash layers and ash-bearing sediments recovered at Sites 907, 908, and 913 (Ocean Drill-ing Program Leg 151) in the Greenland-Iceland-Norwegian Seas (Iceland Plateau, Greenland Basin, Hovgàrd Ridge) consists mainly of colorless and/or light to dark brown glass particles with an average medium grain size of 100 µm (±60 µm); crystals and lithics are minor constituents. Colorless glass shards are primarily bubble wall shards and pumice fragments. Most light to dark-brown glass shards are slightly vesicular or dense blocky shards, but highly vesicular or pumice-like fragments also occur. The compositions of glass shards comprise subalkalic and low-K subalkalic basaltic (27 % of all analyzed shards), basaltic-andesitic (9%), andesitic (6%), dacitic (<1%), and trachytic (4%), as well as low-K and high-K rhyolitic (47 % and 7%, respec-tively). The rhyolitic and trachytic tephra deposits can be attributed to highly explosive Plinian eruptions. Some mafic or intermedi-ate ash deposits of Hole 907 A show the influence of hydroclastic fragmentation processes, suggesting submarine or subglacial/ sublacustrine eruption environments. Major element compositions suggest that most ash layers are derived from Iceland and to a lesser extent from the Jan Mayen System. The oldest basaltic ashes of Hole 907A possibly originate from submarine erup-tions at the ancient Kolbeinsey Ridge as indicated by their particle shape and the low degree of degassing. The well-preserved record of middle to late Miocene ash deposits in Hole 907A indicates a short but strong increase in explosive volcanic activity during the late middle Miocene but relatively low activity rates for early middle Miocene and late Miocene. |
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