(Table 1) Composition of selected ash layers in ODP Site 126-792

Glass shards from 26 megascopic tephras (1-29 cm thick) were separated and analyzed for 26 elements and 87Sr/86Sr and 143Nd/144Nd isotope ratios. The isotope data allowed us to assess the degree of alteration of the samples. The separates contain less than 10% of palagonitized glass or secondary alteration minerals. Correlation diagrams and multivariate analyses have been used to characterize the glass shards in a tectonic setting and group the tephras according to their magmatic affinity. Four tholeiitic groups, two calc-alkaline groups, and one group with glass of shoshonitic affinity can be distinguished. The tholeiitic groups form a time series; however, judging from the rare-earth-element patterns and the temporal variations of the Ce/Yb ratios, the evolution is not unidirectional from strong to weak tholeiitic affinity.
The tephras have been correlated with volcanism along the Izu-Ogasawara and Ryukyu arcs by means of trace element distribution, 87Sr/86Sr ratios, and their magmatic affinity. The tholeiitic layers were derived from volcanoes on the Izu-Ogasawara Arc, most probably from the volcanic island Hachijojima. Only one group of the calc-alkaline layers can have derived from the Izu-Ogasawara Arc. The other calc-alkaline layers and the two layers with shoshonitic affinity probably derive from volcanism along the Ryukyu Arc.