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  • 館藏編號:NTUG010-T700086
  • 題名:岩石地球化學及地球的演化
  • 作者:孫賢鉥
  • 藏品形式:稿本
  • 尺寸:297X210(mm)
  • 簡述:第六講 島弧玄武岩, 板塊隱沒及大陸地殼的形成
  • 內容:

    7. If over-abundances of Sr and Ba in the primary arc basalts are due to contribution from the subducted oceanic crust, either from the altered oceanic crust or from sediment, depending on the relative 87Sr/86Sr of the ""uncontaminated"" mantle and the contaminant, positive or inverse correlatlon between 87Sr/86Sr values and the size of Sr and Ba over- abundances is expected. Lack of correlation suggests more complicated situation. Size of the Ba and Sr over-abundances seem to decrease as decreasing of light REE enrichment increase away from the trench suggesting decreasing relative amount of ""contaminant"" further away from the trench or relatively fixed amount of crustal contamination.
    8. Comparison of Sr and Nd isotope data of island arc volcanics with oceanic volcanics (MORB and ocean island basalts) shows that the arc volcanics generally have higher 87Sr/86sr ratios suggesting addition of Sr from sea water altered subducted oceanic crust and/or input from mainly upper crustal component. Some arc volcanices fall within the field for oceanic volcanics (how to explain this?).
    9. Correlations among Pb, Sr, Nd and 0 isotope compositions for Banda arc volcanics strongly suggest mixing of the subducted continental sediments component with mantle peridotite under Banda arc.
    10. Pb isotope data for circumpacific island arc (Aleutian, cascades, Taiwan, Japan) are confined by the sediments and MORB end members suggesting mixing of these two components. For Aleutian samples, there is no distinction between those islands built on the oceanic crust and those on the continental crust. For Japanese data, about 2% of sediment involvement could explain the Pb isotope composition, but only for para of the over-abundances of Ba, Rb, Cs and K and small amount of Sr. Involvement of subducted oceanic crust seems to be important.