マントル起源超塩基性岩の希ガス同位体研究 : 希ガス同位体から見たマントルプロセス  [in Japanese] Nobl gas studies on mantle-derived ultramafic rocks: things that noble gas can tell us about mantle processes  [in Japanese]

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Author(s)

    • 松本 拓也 MATSUMOTO Takuya
    • 大阪大学大学院理学研究科宇宙地球科学専攻 Department of Earth and Space Science, Graduate School of Science, Osaka University

Abstract

Elemental and isotopic compositions of noble gases trapped in ultramafic rocks (xenoliths and orogenic peridotites) provide valuable information regarding the processes that affected the terrestrial mantle. Here I present some of the examples we found in fresh suites of xenoliths from southeastern Australia and in orogenic peridotites from Horoman complex in northern Japan. The great majority of SE Australian xenoliths yielded MORB-like <sup>3</sup>He/<sup>4</sup>He ratios by crushing gas extraction, indicating that fluids derived from the convecting upper mantle (= MORB source mantle) clearly dominate the continental lithospheric mantle. In contrast, noble gas extraction by stepheating revealed the presence of significantly radiogenic <sup>21</sup>Ne (and <sup>4</sup>He to a lesser extend) in crystal structures of U-Th-bearing minerals such as apatite, amphibole and clinopyroxene. The presence of radiogenic noble gas component is clearly a consequence of mantle metasomatism by fluid/melt enriched in incompatible elements. Separation of CO<sub>2</sub> + noble gas-rich fluid from the incompatible-element-rich melt during the ascent would be a plausible explanation for the preservation of mantle source signature in fluid inclusions. This in turn suggests that noble gases in fluid inclusions of ultramafic rocks can be used to infer the source of metasomatic agent which sometimes is difficult to speculate upon. This notion can further be confirmed by the occurrence of plume-like neon in metasomatic apatite in the xenoliths in which the coexisting minerals showed MORB-like noble gases signatures. It was also found in the orogenic peridotites from Horoman complex that they were affected by metasomatic fluids from the dehydrating slab which introduced recycled-atmospheric argon into the mantle wedge. We note that this recycled component could have been stored in mantle wedge for a significant period of time to become part of continental lithospheric mantle. Somewhat lower <sup>40</sup>Ar/<sup>36</sup>Ar ratios in xenoliths from the subcontinental mantle might be due to an involvement of a recycled component during the formation of the metasomatizing melt. These observations suggest that the continental lithospheric mantle have multiple noble gas components of distinct origins; a mantle component (both from degassed- and less-degassed-sources), an in situ radiogenic component and a recycled atmospheric component. It was also noted that Kola carbonatites of supposed lower mantle origin also showed some indication of a recycled volatile component. This issue certainly has important implications for bettering our understandings on the origin of noble gas heterogeneity observed in samples from the deeper mantle origin.

Journal

  • Chikyukagaku

    Chikyukagaku 36(2), 89-106, 2002

    The Geochemical Society of Japan

References:  80

Codes

  • NII Article ID (NAID)
    110008680036
  • NII NACSIS-CAT ID (NCID)
    AN00141280
  • Text Lang
    JPN
  • Article Type
    ART
  • ISSN
    0386-4073
  • NDL Article ID
    6182859
  • NDL Source Classification
    ZM41(科学技術--地球科学)
  • NDL Call No.
    Z15-645
  • Data Source
    CJP  NDL  NII-ELS  J-STAGE 
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