NOBLE-GAS SYSTEMATICS OF DEEP RIFT-ZONE GLASSES FROM LOIHI SEAMOUNT, HAWAII

We report new noble gas fusion and crushing data for six pillow rim gl asses, recovered between 3 and 5 km water depth on the south rift zone of Loihi Seamount, Hawaii. Helium abundances of the glasses vary from 0.3 to 2.3 mu cc/g, with He-4/He-3 ratios between 30000 and 27000 (24 -27 R-A), similar to previously reported values. The neon data form a correlation line which is similar to the Loihi-Kilauea line reported b y Honda et al. [1], but extends to much higher ratios, up to 12.9 and 0.0382 for the Ne-20/Ne-22 and Ne-21/Ne-22 ratios, respectively. This provides conclusive evidence for the suggestion that the Hawaiian plum e, thought to originate in the lower mantle, has a solar-like Ne-20/Ne -22 composition [1], but a slightly higher Ne-21/Ne-22 ratio. Ar-40/Ar -36 ratios of the deep rift-zone glasses are as high as 2600, and show a positive correlation with neon isotopic ratios. In contrast to neon and argon, all xenon isotopic compositions are isotopically indisting uishable from air, which either suggests preferential atmospheric cont amination of xenon, or could indicate an atmospheric xenon isotopic co mposition for the lower mantle. (C) 1997 Elsevier Science B.V.