ATMOSPHERIC NOBLE-GASES IN VOLCANIC GLASSES FROM THE SOUTHERN LAU BASIN - ORIGIN FROM THE SUBDUCTING SLAB

Noble gas concentrations and isotopic compositions have been determine d for four submarine volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin. The samples are the least differentiated ones from this area, and they display enrichments in fluid-mobile elements similar to the nearby island are. He-3/He-4 ratios are slightly below average MORE (6.8-7.8 times atmospheric), whereas Ne, Ar, Kr, and Xe h ave isotopic compositions very similar to air. Together with previousl y published data from the Valu Fa Ridge and other spreading segments i n the Lau Basin, our data show a systematic latitudinal variation of i ncreasing Ne, Ar, Kr, and Xe abundances from north to south as well as Ne and Ar isotopic compositions changing from MORB-like to atmosphere -like in the same direction. Moreover, isotopic compositions and noble gas abundances of the lavas correlate strongly with Ba/Nb ratios and H2O concentrations. Based on these observations and mass balance argum ents, we propose that the atmospheric noble gases come from the subduc ting oceanic crust and are not due to shallow contamination with air d issolved in seawater or assimilation of old crust. Our data suggest th at the noble gases released from the subducting slab are atmospheric a nd thus contain little or no solar He and Ne. in addition to the fact that ratios of He to heavy noble gases are small in aged ocean crust, He has possibly fractionated from the other noble gases due to its hig her diffusivity, and thus He transport from the subducting slab into t he mantle wedge is probably insignificant. We propose that the 3He/4He ratios lower than MORE observed in the VFR lavas result from radiogen ic ingrowth of He in a highly depleted, and hence degassed, mantle wed ge after the enrichment of U and Th released from the downgoing slab. (C) 1998 Elsevier Science B.V. All rights reserved.