中央海嶺下のマントル多様性を形成した古融解イベント

Abstract

Abyssal peridotites recovered directly from mid-ocean ridges (mid-ocean ridge peridotites hereafter) are generally interpreted to be formed as a residue after partial melting and melt extraction in the adiabatically upwelling mantle beneath mid-ocean ridges. Osmium and some other isotopic characteristics of some mid-ocean ridge peridotites, which were formed by partial melting events, are much older than present-day mid-ocean ridge systems (ancient melting residual peridotites), however, suggesting ancient origins. Ancient melting residual peridotites might have been incorporated into the oceanic asthenosphere beneath mid-ocean ridges. If this is the case, the nature of the oceanic plate and its formation processes need to be reconsidered because the asthenospheric mantle, which has abundant ancient melting residual peridotites, is not sufficient to create basaltic oceanic crusts, followed by formation of the serpentinized peridotite crust by hydrothermal circulation along faults developed around the mid-ocean ridges by continuous spreading. Comprehensive geochemical studies on depleted mid-ocean ridge peridotites are required to detect and verify ancient melting residual peridotites from mid-ocean ridge peridotites. It is crucial to examine the effects of minor Os-rich phases on 187Os/188Os isotopic compositions to understand the meaning of their model ages. Precise chemical analyses of fluid mobile elements including light elements, such as H, Li, Be, and B, on clinopyroxene, as well as orthopyroxene, provide clues about differences in geochemical signatures and thermal histories between present-day mid-ocean ridge residual peridotites and ancient melting residual peridotites.