ALBERT

Description: Forty-five volcanic ash horizons cored at Sites 642, 643, and 644 on the Vøring Plateau and ranging in age from early Miocene to Pleistocene, are discussed in terms of their magmatic features as well as their diagenetic evolution. Most of these layers, some centimeters thick, are mainly made of fresh rhyolitic glass. Twenty-five percent of the ash layers, however, contain variable amounts of more basic glass shards, ranging in composition from Mg-rich tholeiites to icelandites through Mg-poor basalts, ferrobasalts, and tholeiitic andesites, and are commonly associated with rhyolitic shards. Many chemically heterogeneous ash layers show bimodal acidic (rhyolites to icelandites) - basic (Mg-rich basalts to ferrobasalts) frequency distributions of the glass shards; intermediate compositions are not simple mixtures between acidic and basic endmembers. We suggest these ash layers result from the ejection of the upper (rhyolitic) to intermediate (ferrobasaltic) levels of density-stratified magma chambers intruded by ascending basaltic magmas, as exemplified by several Quaternary Icelandic explosive eruptions. The overall characteristics of the chemical trends of glass shards from heterogeneous ash layers are typically tholeiitic, with a strong increase of total iron and TiO2 at the level of intermediate compositions. Major and trace element data on bulk ash layers indicate that all the tephra of bimodal composition, as well as most of Neogene rhyolitic ash levels (low-K type), belong to LREE-enriched tholeiitic series. However, some Miocene rhyolitic ash levels (high-K type) show distinct geochemical features and are probably derived from other sources. The volcanic glass alteration patterns have no relationship with the ages of the deposits, and are different for acid and basic glasses. The most common alteration process leads to the formation of iron-beidellite-type smectites through loss of Si and alkalis from silicic glasses and loss of Si, Mg, Fe from basic glasses. Another kind of alteration, observed in lower Miocene ash layers, leads to glauconite formation through the development of iron-rich smectites. The frequency of ash layer occurrence with time indicates two apparent maxima of volcanic activity, an early one at the early-middle Miocene boundary (16 to 14 Ma) and another one in the late Miocene (8 to 7 Ma). Iceland is the most likely source for all the chemically heterogeneous ash layers as well as the low-K rhyolitic ash levels belonging to these two major episodes. Other subaerial sources, possibly at rifted margins (East Greenland, Jan Mayen Ridge), are required for the high-K rhyolitic ash horizons.