Christian Lacasse, Martine Paterne, Reinhard Werner, Hans-Joachim Wallrabe-Adams, Haraldur Sigurdsson, Steven Carey, and Guy Pinte


  The upper 90 m of sediments from Ocean Drilling Program Leg 151 Site 907 on the Iceland Plateau contain numerous well-preserved volcanic ash layers that provide an excellent record of the source and timing of major Pliocene and Pleistocene explosive eruptions that have occurred in this region. A total of 23 tephra layers and six ash zones were analyzed for major and trace element chemistry and grain size characteristics. Relative ages of the tephra layers were estimated based on paleomagnetic and oxygen isotope stratigraphy. Thicknesses of the ash layers range from less than 1 to 18 cm. It is inferred on the basis of their sorting coefficient and grain size that the majority of the tephra layers are the result of ash fallout from large explosive eruptions. Most of the tephra layers are crystal-poor, with less than 10% total crystal content. Colorless shards dominate over sideromelane (brown glass) and tachylite. Platy bubble wall shards represent the dominant morphological type of glass, with minor amounts of pumice and vesicular shards.
  The major element composition of glasses indicates four compositional groups: basalt, basaltic andesite, trachyte and rhyolite. All of the compositionally bimodal tephra layers and the rhyolitic layers have a tholeiitic affinity, compatible with a source from volcanoes in the Pliocene-Pleistocene and upper Pleistocene volcanic rift zones in Iceland. Tephra with alkaline and per-alkaline rhyolitic (comendite) glass composition were likely erupted from the only two regions in Iceland having produced transitional and alkali rock series of similar composition: the Snæfellsnes Peninsula and the Eastern Volcanic Zone. Three tephra layers are found to have a trachytic composition, high total alkali concentration, and affinity with the trachy-basaltic rock series of Jan Mayen. All of the basaltic glasses show high potassium concentration (>0.2 wt%), which excludes their origin from the Kolbeinsey Ridge. Rare earth element (REE) patterns from trace element analyses of bulk ash layers indicate various degrees of enrichment in light rare earth elements (LREE) for the silicic tephra, a finding that confirms sources from Iceland and Jan Mayen.

Date of initial receipt: 30 June 1995
Date of acceptance: 3 December 1995

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