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キーワード:
abyssal peridotites; mantle melting; garnet
要旨:
The role of residual garnet during melting beneath mid-ocean ridges has been the subject of many recent investigations. To address this issue from the perspective of melting residues, we obtained major and trace element mineral chemistry of residual abyssal peridotites from the Central Indian Ridge. Many clinopyroxenes have ratios of middle to heavy rare earth elements (MREE/HREE) that are too low to be explained by melting in the stability field of spinel peridotite alone. Several percent of melting must have occurred at higher pressures in the garnet peridotite stability field. Application of new trace element partitioning models, which predict that HREE are compatible in high-pressure clinopyroxene, cannot fully explain the fractionation of the MREE from the HREE. Further, many samples show textural and chemical evidence for refertilization, such as relative enrichments of highly incompatible trace elements with respect to moderately incompatible trace elements. Therefore, highly incompatible elements, which are decoupled from major and moderately incompatible trace elements, are useful to assess late-stage processes, such as melt entrapment, melt-rock reaction and veining. Moderately incompatible trace elements are less affected by such late-stage processes and thus useful to infer the melting history of abyssal peridotites.
