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Elemental and isotopic compositions of noble gases
trapped in ultramafic rocks provide valuable information@ regarding the processes that affected the terrestrial mantle. Here I present some of such examples we found in fresh suites of xenoliths from SE Australia (e.g., Matsumoto et al., 2000) and in orogenic peridotites from Horoman (Matsumoto et al., 2001) and from Finero complexes (Matsumoto et al., 2005, in press) with a particular emphasis on mantle metasomatism. Based on analyses on these suites of rocks, we identified at least four kinds of characteristic noble gas components: (1) A MORB-type isotopic component (2) A Plume-like component (3) A hybrid component with mantle-He and air-Ar (4) A highly radiogenic component Irrespective of their isotope signatures, these components are preserved in fluid inclusions of mantle minerals without being significantly disturbed by ingrowths of radio- and nucleogenic noble gas isotopes. Therefore, these noble gas signatures should reflect those of mantle domains from which respective metasomatic agents had been derived. A MORB-like component is the most widespread in variably metasomatised mantle xenoliths from SE Australia, whereas the plume-like neon is rare and only found in metasomatic apatite, indicating an involvement of mantle plume to this particular metasopmatic event (e.g., Matsumoto et al., 1997 and 2004). A Hybrid of mantle-He and air-Ar can be regarded as a signature characteristic of the subduction zone settings, and is often found in orogenic lherzolites. We also found that this component is highly concentrated in thin apatite-rich layer of Finero lherzolite that hosts LREE- enrichment (Matsumoto et al., 2005). This can be taken as rather direct evidence for metasomatic introduction of incompatible elements and noble gases with the same agent. Also, it is possible identify more than two different noble gas components coexist in a single specimen, suggesting multiple episodes of metasomatism. These demonstrate the usefulness of noble gas isotopes to constrain tectonic setting of the metasomatism and source of its agent. References Matsumoto et al., (1997) Nature 388, 162-164. Matsumoto et al., (2000) Chem. Geol. 168, 49-73. Matsumoto et al., (2001) Earth Planet. Sci. Lett. 185, 35-47. Matsumoto et al., (2004) Terra Nova 16, 23-26. Matsumoto et al., (2005, in press) Earth Planet. Sci. Lett. |