Peridotites of the upper mantle provide us with evidence of magmatic processes that affect the evolution of both oceanic and suprasubduction-zone lithosphere and are the primary material from which we can determine the processes that are involved in evolution of the crust. Exposures of upper mantle in oceanic settings are primarily associated with major fault structures formed in response to several different types of regional processes, such as transition from continental to oceanic crust, off-ridge extensional faults in the deep ocean basins, development of transform faults on mid-ocean ridges, and extension at amagmatic mid-ocean-ridge segments or in forearc areas of convergent plate margins. In these localities the mantle peridotites generally form extensive bodies of serpentinite as a consequence of interaction with seawater making its way through fault zones into the upper mantle or interaction with fluids derived from an underlying subducting plate. Exposures of such bodies occur world-wide in subaerial settings where the relationships of these rocks to the regional structural setting have been determined.
Serpentinite bodies have been targeted at numerous Ocean Drilling Program (ODP) sites over the last two decades. A summary of the results of these drilling efforts and a comparison of the nature of upper mantle peridotites from these settings was given in Fryer (2002). In this paper we focus on a synthesis of studies of the Izu-Bonin/Mariana (IBM) forearc (Fig. F1), highlighting work done during Leg 195 at Site 1200 on the southern Mariana forearc (Fig F2) at the summit of South Chamorro Seamount, a serpentinite mud volcano. We present a summary of the results from drilling Site 1200 and sites drilled during ODP Leg 125, during which the summits and flanks of two serpentinite seamounts farther north on the forearc of this convergent margin system were drilled (Conical Seamount Sites 778–780; Fig. F2) and Torishima Forearc Seamount Sites 783 and 784; Fig. F3). In our discussion section we suggest how these may fit in a broader context of terrestrial exposures. Research on these drill sites shows us that fundamental processes associated with subduction produce mantle effects that differ significantly from most abyssal peridotites and that the suprasubduction-zone mantle varies in composition regionally and probably also in time. It is a far more complex environment than most previous work has suggested.