NOBLE-GASES IN PILLOW BASALT GLASSES FROM THE NORTHERN MARIANA TROUGHBACK-ARC BASIN

Noble gas concentrations and isotopic compositions have been measured in eight samples of pillow basalt glasses collected from seven differe nt localities along 250 km of the Mariana Trough spreading and rifting axis. The samples have uniform and mid-ocean ridge basalt (MORB)-like He-3/He-4 values of 9-12x10(-6) (6.4-8.6 times atmospheric) despite l arge variations in He-4. Concentrations of the noble gases Ne, Ar, Kr, and Xe show much smaller variations between samples, but larger varia tions in isotopic compositions of Ne, Ar, and Xe. Excess radiogenic Ne -21 is observed in some samples. Ar-40/Ar-36 varies widely (atmospheri c to 1880). Kr is atmospheric in composition for all samples. Some sam ples show a clear excess Xe-129, which is a well-known MORE signature. Isotopic compositions of the heavier noble gases (Ar, Kr, and Xe) in some samples, however, show more atmospheric components. These data re flect the interaction of a MORE-like magma with an atmospheric compone nt such as seawater or of a depleted mantle source with a water-rich c omponent that was probably derived. from the subducting slab.