Conical Seamount, the edifice drilled at Ocean Drilling Program (ODP) Sites 778 through 780, is a large seamount on the outer half of the 200-km-wide Mariana forearc in the western Pacific. It is forming by the protrusion of cold, unconsolidated serpentine mudflows and debris flows and by vertical tectonic activity. Dredging on the flanks of this seamount recovered rocks and serpentine muds that are similar to sedimentary serpentinite deposits found in subaerially exposed convergent margin terranes worldwide. The dredged samples were formed by different mechanisms from those previously proposed for sedimentary serpentinite deposits. The dredged samples from Conical Seamount are primarily serpentinized harzburgite. However, serpentinized dunite, metamorphosed gabbro, and basalts have been retrieved from similar seamounts on the Mariana forearc. SeaMARC II imagery and bathymetry of Conical Seamount revealed sinuous flow forms on the flanks of the seamount. Conical Seamount also has both concentric ridges and radial fractures indicative of tumescence. Alvin submersible studies showed these flows to be composed of unconsolidated serpentine muds, containing clasts of serpentinized ultramafic and metamorphosed mafic rocks and authigenic carbonate and silicate minerals. Near the summit of one of the seamounts, chimney structures less than 150 yr old and composed of carbonate and silicate were sampled using Alvin. During sampling of a silicate chimney, cold fluids seeped from numerous orifices in the chimney. The fluids associated with the chimney are unique in composition among fluids collected in the oceans and point to a deep source, probably the subducted Pacific lithospheric slab. Small limpets and gastropods and bacterial mats were collected from the chimneys. Faulting of the forearc region partially controls the distribution of the Mariana forearc serpentinite seamounts. Seismic-reflection profiles show that Conical Seamount is located at the intersection of at least two fault zones. The extent of the exposure of serpentinized ultramafic rocks on the Mariana forearc and the pervasive normal faulting of the region argue for considerable extension. Dredged samples from the 2-km-high wall of a deep graben adjacent to Conical Seamount include a variety of mafic rock types of arc tholeiite, boninite, alkalic basalts, and basalts of mid-ocean ridge (MORB) composition. The presence of MORBs in this part of the forearc suggests either exposure of a fragment of entrapped or accreted oceanic plate or a period of rifting with associated magma generation in the forearc. The serpentine mud volcanoes and associated egress of fluids generated at great depth in the forearc provide a mechanism for the flux of fluids from the subducted oceanic lithosphere through the outer forearc region. The escape of circulating fluids would alleviate problems of mass balance of constituents derived from subduction in the convergent margin environment. In particular, the disparity between the observed volatile effluent from arc volcanoes and the ...

The ultramafic rock types present in the seamount are primarily harzburgite with subordinant dunite that show evidence of melt extraction in an island arc environment (Fryer, Pearce, Stokking, et al., 1990; Parkinson et al., this volume; Ishii et al., this volume). The purpose of this paper is to describe the mineralogic relationships observed in the ultramafic clasts and to present an interpretation of the metamorphic history of the rocks.

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