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Noble gases in deformed xenoliths from an ocean island: characterization of a metasomatic fluid

Farley, K. A. and Poreda, R. J. and Onstott, T. C. (1994) Noble gases in deformed xenoliths from an ocean island: characterization of a metasomatic fluid. In: Noble Gas Geochemistry and Cosmochemistry. Terra Scientific Publishing Company , Tokyo, pp. 159-178. ISBN 9784887041141 http://resolver.caltech.edu/CaltechAUTHORS:20121108-152313030

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Abstract

New noble gas measurements have been made on Samoan ultramafic xenoliths in order to characterize the composition and nature of entrapment of a postulated mantle metasomatic agent. The new measurements were performed on gases extracted from severely tectonized harzburgites and dunites by both bulk crushing and laser microprobe. The tectonized specimens have the highest noble gas concentrations yet reported from mantle materials and attest to deformation in a highly gas-charged environment. The noble gas isotopic systematics are similar to those observed in undeformed specimens from the same locality, and are consistent with mixing between a mantle component (e.g. ^3He/^4He = 12 R_A, ^(40)Ar/^(36) Ar > 10,000) and an atmospheric contaminant. Within the xenoliths, the mantle component is spatially associated with features previously attributed to metasomatism (e.g. HAURI et al., 1993). Although this metasomatic component has many characteristics suggesting derivation from material returned to the mantle by subduction, its relatively high ^3He/^4He ratio is enigmatic. Whatever its source, this fluid appears to have existed within the Samoan mantle over fairy large temporal and spatial scales, and plays an important role in the geochemistry of Samoan basalts. Just as with the mantle component, the deformed xenoliths are also enriched in the atmospheric contaminant. This enrichment suggests pervasive penetration of air into the ubiquitous micro fractures and decrepitated fluid inclusions of the deformed specimens. In addition to source and contamination effects, the noble gases within these xenoliths record variable degrees of elemental fractionation. While the gas-rich (deformed) xenoliths have ^4He-^(21)Ne*-^(40)Ar* systematics close to long-term closed-system behavior, the comparatively gas-poor samples have lost up to 90% of their helium without concomitant loss of neon and argon. This likely represents diffusive loss of helium after fluid inclusion entrapment.


Item Type:Book Section
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http://www.terrapub.co.jp/e-library/ng/PublisherUNSPECIFIED
Additional Information:© 1994 by Terra Scientific Publishing Company. We thank James H. Natland for supplying the samples and for providing insights to their composition and origin and the Yamada Foundation for their financial support of the Noble Gas Conference. Discussions with Julie Dieu and comments from an anonymous reviewer were helpful. This work was supported by the NSF (EAR 92-04822).
Funders:
Funding AgencyGrant Number
Yamada Foundation UNSPECIFIED
NSFEAR 92-04822
Record Number:CaltechAUTHORS:20121108-152313030
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121108-152313030
Official Citation:Noble Gases in Deformed Xenoliths from an Ocean Island: Characterization of a Metasomatic Fluid Farley K. A., Poreda R. J., and Onstott T. C. Noble Gas Geochemistry and Cosmochemistry, ed. by J. Matsuda, pp. 159-178. Terra Scientific Publishing Company (TERRAPUB), Tokyo, 1994.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35372
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:12 Nov 2012 23:20
Last Modified:12 Nov 2012 23:20

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