RARE-GASES IN SAMOAN XENOLITHS

The rare gas isotopic compositions of residual harzburgite xenoliths from Savai'i (SAV locality) and an unnamed seamount south of the Samoan chain (PPT locality) provide important constraints on the rare gas evolution of the mantle and atmosphere. Despite heterogeneous trace element compositions, the rare gas characteristics of the xenoliths from each of the two localities are strikingly similar. SAV and PPT xenoliths have He-3/He-4 ratios of 11.1 +/- 0.5 R(A) and 21.6 +/- 1 R(A), respectively; this range is comparable to the He-3/He-4 ratios in Samoan lavas and clearly demonstrates that they have trapped gases from a relatively undegassed reservoir. The neon results are not consistent with mixing between MORB and a plume source with an atmospheric signature. Rather, the neon isotopes reflect either a variably degassed mantle (with a relative order of degassing of Loihi < PPT < Reunion < SAV < MORB), or mixing between the Loihi source and MORB. The data supports the conclusions of Honda et al. that the Ne-20/Ne-22 ratio in the mantle more closely resembles the solar ratio than the atmospheric one. Ar-40/Ar-36 ratios in the least contaminated samples range from 4,000 to 12,000 with the highest values in the 22 R(A) PPT xenoliths. There is no evidence for atmospheric Ar-40/Ar-36 ratios in the mantle Source of these samples, which indicates that the lower mantle may have Ar-40/Ar-36 ratios in excess of 5,000. Xenon isotopic anomalies in Xe-129 and Xe-136 are as high as 6%, or about half of the maximum MORB excess and are consistent with the less degassed nature of the Samoan mantle source. These results contradict previous suggestions that the high He-3/He-4 mantle has a near-atmospheric heavy rare gas isotopic composition.