Poreda, R. J. and Farley, K. A. (1992) Rare gases in Samoan xenoliths. Earth and Planetary Science Letters, 113 (1-2). pp. 129-144. ISSN 0012-821X. http://resolver.caltech.edu/CaltechAUTHORS:20121113-101918346
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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 ^3He/^4He ratios of 11.1 ± 0.5 R_A and 21.6 ± 1 R_A, respectively; this range is comparable to the ^3He/^4He 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 ^(20)Ne/^(22)Ne ratio in the mantle more closely resembles the solar ratio than the atmospheric one. ^(40)Ar/^(36)Ar 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 ^(40)Ar/^(36)Ar ratios in the mantle source of these samples, which indicates that the lower mantle may have ^(40)Ar/^(36)Ar ratios in excess of 5,000. Xenon isotopic anomalies in ^(129)Xe and ^(136)Xe 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 ^3He/^4He mantle has a near-atmospheric heavy rare gas isotopic composition.
|Additional Information:||© 1992 Elsevier Science Publishers B.V. Received February 24, 1992; revision accepted July 13, 1992. The authors wish to thank J. Natland and J. Hawkins for providing the xenoliths and for useful discussions. Reviews by D. Fisher, D. Graham and Ph. Sarda improved the quality of the manuscript. This work was funded by the Petrology and Geochemistry Program, Earth Sciences Division, NSF, EAR 9105032 to RJP at the University of Rochester.|
|Official Citation:||R.J. Poreda, K.A. Farley, Rare gases in Samoan xenoliths, Earth and Planetary Science Letters, Volume 113, Issues 1–2, September 1992, Pages 129-144, ISSN 0012-821X, 10.1016/0012-821X(92)90215-H. (http://www.sciencedirect.com/science/article/pii/0012821X9290215H)|
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|Deposited By:||Ruth Sustaita|
|Deposited On:||13 Nov 2012 18:46|
|Last Modified:||13 Nov 2012 18:46|
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