The volcanogenic component of 54 ash layers and ash-bearing sediments recovered
at Sites 907, 908, and 913 (Ocean Drilling Program Leg 151) in the
Greenland-Iceland-Norwegian Seas (Iceland Plateau, Greenland Basin, Hovgård 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%, respectively).
The rhyolitic and trachytic tephra deposits can be attributed to highly explosive Plinian eruptions. Some mafic or intermediate ash deposits of Hole 907A 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 eruptions 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.
Date of initial receipt: 5 July 1995
Date of acceptance: 20 December 1995
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