Noble gas studies on mantle-derived ultramafic rocks: things that noble gas can tell ns about mantle processes

摘要

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 maiority of SE Australian xenoliths yielded MORB-like ~3He/~4He ratios by crushing gas extraction, in6icating 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 ~(21)Ne (and ~4He 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_2~+ 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 m field inclusions of climatic rocks can be used to infer the source of metrisomatic agent which sometimes is dutiful to speculate upon. This notion can further be confronted by the occurrence ofplume4ke neon in metasomatic apatite in the xenoliths in which the coexisting minerals showed MORB-like noble gases signatures. It was also found in the organic 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 receded component could have been stored in mantle wedge for a significant period of time to become part of continental lithosphere mantle. Somewhat lower ~(40)Ar/~(36)Ar ratios in xenoliths from the sub continental mantle might be due to an involvement of a recycle component during the for nation office 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 argentites 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.