Major-element geochemistry of ashes from Site 782, 784, and 786 in the Bonin Forearc

Abstract

Many ash-rich layers were identified in the sedimentary sequences penetrated during ODP Leg 125 at Sites 782, 784, and 786. The total age range of the ash layers is from Eocene to Pleistocene. The ashes consist of vitric, microlite-bearing, and crystal-rich components; the glassy shards are typically highly vesicular, with elongate, flattened bubbles. The dominant crystalline phases are orthopyroxene, clinopyroxene, and plagioclase. Compositions range from basalt through andesite and dacite to rhyolite and generally belong to a tholeiitic, low-K suite. There is no indication of any regular secular change during the evolution of the Bonin arc from tholeiitic through calc-alkalic to alkalic compositions with time. -from Authors