Abstract
Serpentinite diapiric seamounts have been reported exclusively from the forearc region of the Izu-Ogasawara-Mariana (hereafter, IOM) arc-trench system. Petrological characteristics of mantle peridotites constituting these seamounts are summarized in comparison with other trench region peridotites. Mantle peridotites drilled from the Conical seamounts during ODP Leg 125 (Site 779) have distinctive compositions both in bulk rock chemistry and mineral chemistry. Their compositions suggest that they underwent a high degree of partialmelting (more than 30 %), which is thought to be related to island arc volcanism in the mantle wedge. The compositions of mantle peridotites collected by submersibles from other serpentinite seamounts in the IOM forearc are similar to those from the Conical seamount. This indicates that most mantle peridotites from these seamounts are residues derived from similar high degrees of partial melting related to the island arc volcanism.
In contrast, mantle peridotites recovered from the southern Mariana exhibit wider range of compositions including more fertile peridotites, suggesting that they are residues of relatively lower degrees of partial melting. It is probable that they are related to volcanism that occurred during the formation of the back arc basin. Furthermore, mantle peridotites found in the Tonga forearc have wider compositional range than peridotites found in the IOM forearc region. These peridotites are considered to be derived from a layered sequence from upper crust through lower crust to the upper mantle rather than serpentinite diapiric seamounts. The wide range of mantle peridotite compositions from the Tonga forearcregion indicate a wider range of partial melting than found in the IOM region. The “remnant mantle diapir (RMD)” hypothesis was previously proposed for the origin of the mantle peridotites constituting serpentinite diapiric seamounts. It is plausible that part of the RMDs constitutes the serpentinite seamounts.
