Geochemistry of ash layers from the Bonin Forearc

Many ash-rich layers, varying from a few millimeters to several centimeters thick, were identified in the sedimentary sequences penetrated during Ocean Drilling Program Leg 125 at Sites 782, 784, and 786, located about 400 to 500 km south of Tokyo in the Bonin forearc. The total age range of the ash layers is from Eocene to Pleistocene, although not all sites cover this full span. 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. The major-element compositions of individual vitric shards collected from selected layers of Holes 782A, 784A, and 786A were determined by electron microprobe analyses; particular care was taken to ensure that the analytical results were not compromised by electron beam damage to the glasses. 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 alkali compositions with time. In Holes 782A and 784A, some high-K rhyolite compositions of late Miocene and Pleistocene age are present. A clear chemical distinction has existed since arc inception between the source(s) of these ashes and the upper mantle source(s) tapped during construction of the igneous basement that formed the forearc.

(Table 2) Concentrations of dissolved aluminum in interstitial water from sediments of the Indian Ocean

Results are presented of application of laser stepwise photoionization of atoms in combination with thermal atomization of matter in vacuum for direct determination of aluminum dissolved in sea and interstitial waters. Dry residue from evaporation of 40 ?l sea water was atomized in a crucible at 1800°C, and aluminum atoms in the beam thus formed were energized into Rydberg state in two steps by two tunable dye laser beams; the atoms were then ionized by an electric pulse and resulting ions were recorded by secondary emission electron multiplier (ion detector). Ionic signal dependence on sample vaporization time was studied. The procedure is suggested for separating out a selective signal in a single measurement. Dissolved aluminum concentrations in interstitial waters of the Indian Ocean and in waters of the river-sea zone were determined using preliminarily plotted calibration characteristics for aluminum solutions in deionized and sea waters. The minimum detectable Al concentration in seawater was 1 ?g/l that corresponds to 40 pg of Al in a sample.