アデン湾，東経４６°近傍のＥタイプ中央海嶺玄武岩類の成因

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タイトル別名
  • Petrogenesis of E-type Mid-Ocean Ridge Basalts near Longitude 46.DEG.E, the Gulf of Aden
  • アデンワン トウケイ 46 キンボウ ノ Eタイプ チュウオウカイレイゲンブガンルイ ノ セイイン

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We report major and trace element compositions for 45 fresh basalts from 13 dredge sites between 45.5° E and 49° E along the Gulf of Aden obtained during the Aden New Century cruise (KH-005 : RN Hakuho-maru, ORI, University of Tokyo). The basalts from Aden New Century (ANC) Seamount at around 45.5° E, discovered by bathymetric mapping during the cruise, have the highest Nb/Zr (0.22-0.27) and La/SmN (3.1-3.7) and show the strongest OIB signature among those in the Gulf of Aden. The longitudinal variation of Nb/Zr and La/SmN of the dredged basalts shows two enrichment peaks, i.e., first “keen” enrichment peak with OIB signature near ANC Seamount (45.5°E) and second “broad” enrichment peak with E-MORB signature along the ridge axis from 46° E to 48° E, and at 49° E, N-MORB is dominant. In spite of variable Nb/Zr and La/SmN, the estimated pressure of magma segregation and degree of partial melting from the dredged basalts, using reported results of experimental melting on spinel lherzolite, are relatively constant and almost similar to those of N- and T-MORBs in Pacific, Atlantic, and Indian Oceans. This suggests that the longitudinal variation of the dredged basalts reflects mantle compositions equilibrated with their primitive magmas. Nb/Zr, Nb/Y, and La/SmN systematics for the dredged basalts suggest binary magma mixing of the NMORB source with the OIB source, composed of FOZO (or C) mantle component with the incorporation of minor, if any, HIMU components.<BR>Recent studies (George et al., 1998; Orihashi et al., 1998) have suggested that two mantle plumes might be impinged beneath the Afar province in Eocene to Oligocene (45-30 Ma) and Miocene (19-15 Ma) times. Considering the above, two enrichment peaks of the mantle in the Gulf of Aden must be produced by two mantle plumes impinged in the Afar province and spread sideways sequentially, with the front of spreading first plume head corresponding to the broad enrichment peak. This broad peak is formed because the plume head emplaced beneath the lithosphere, producing a Large Igneous Province (30-26Ma), was progressively defused into and diluted by the upper mantle component (N-MORB source) over time.

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  • 地学雑誌

    地学雑誌 112 (5), 732-749, 2003

    公益社団法人 東京地学協会

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