Chemical characteristics of ash layers from ODP Leg 104

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.