Volcanic glasses from sediments from Sites 453 and 454 in the Marian trough ( DSDP, Leg 60).

Abstract

Pliocene and Pleistocene volcanic glass fragments from Mariana Trough sediments at Sites 453 (16 samples) and 454 (4 samples), located near the W edge of the trough and just W of the spreading axis, respectively, have been analyzed for major elements with an electron microprobe. They derive from volcanic activity on the present Mariana active arc. The glasses from Site 453 are all tholeiitic with a wide range of SiO2 contents. Those less than 2Ma old have slightly lower TiO2 and higher K2O contents than the older ones. The glasses from Site 454 are all Pleistocene and resemble the younger glasses at Site 453. Major element conpositions of the older basaltic glasses at Site 453 are similar to those of the Mariana Trough basalts drilled on Leg 60. Both older and younger suites of glasses differ from the composition of rocks exposed on the active arc, which are assumed to be younger than any of the samples studied (i.e. about 200 000yr). A third suite is represented by the arc rocks exposed on the volcanic islands. These have a smaller range of SiO2 contents and contain more Al2O3 but less K2O, TiO2, and FeO2 (total Fe as FeO) than the sediment glasses studied. Further, a plot of FeO1 against MgO for the arc rocks does not follow the island arc tholeiite trend of the trough sediment glasses. -Authors Dept of Geol & Geophys, Univ of Sydney, Sydney, Australia.