A Synthesis of Leg 125 Drilling of Serpentinite Seamounts on the Mariana and Izu-Bonin Forearcs

The forearc region of the Mariana and Izu-Bonin convergent margin systems, the area between the trench axis and the active volcanic fronts, contains numerous large serpentine seamounts. These include either mud volcanoes, composed of unconsolidated serpentine mud flows that have entrained serpentinized ultramafic and other rock types, or horst blocks of serpentinized ultramafics, partially draped with unconsolidated serpentine slump deposits. Composite edifices form partially by uplift of blocks of metamorphosed lower crust or upper mantle, partially by subsequent or concomitant debris flows of serpentinized ultramafics, and typically display later serpentine mud flows. Tectonic processes on the forearc, particularly faulting, exert important controls over the distribution of serpentinite seamounts. Drilling at Sites 778 and 779 on the flanks of Conical Seamount on the Mariana forearc retrieved siltto clay-sized serpentine mud with blocks to small clasts of metamorphosed ultramafic and mafic rocks. The rheological properties of the Conical Seamount summit muds are consistent with mobilization of the serpentine at depth and surficial emplacement of the muds as fluid-charged flows. Mineralogy and bulk compositions of the mud samples from Conical Seamount suggest that three to six flow units are located on the flank of the seamount with possibly two at the summit. Similar studies of Torishima Forearc Seamount show one unit on the north flank (Site 783) and at most two on the west flank (Site 784). Variations in the lithology, mineralogy, and structure of the muds suggest differences in source material and possibly a combination of both debris deposits and mud flows at the two seamounts. Unique pore fluids, both at the flank sites and at the summit on Conical Seamount, support the earlier findings from ALVIN submersible work of unusual vent and pore fluids suggested to derive from the subducted slab. The compositions of the pore waters from Torishima Forearc Seamount indicate fluids resulting almost solely from seawater interaction with peridotite at low temperatures. Peridotite clasts entrained in the serpentine muds of the seamounts trace a sequence of tectonic and magma genesis events: (1) initial formation of the peridotite under high-temperature asthenospheric conditions, such as are prevalent in the upper mantle of most ophiolite complexes (either at mid-ocean ridges or in island-arc regimes) followed by remelting and possible infusion of the peridotite with a fluid phase accompanied by a period of high-temperature, low-stress deformation of the peridotite, either precursory to initiation of convergence, or during proto-arc formation; (2) development of ductile shear zones coeval with a protracted period of retrograde metamorphism that accompanied subduction; and (3) brittle failure of the metamorphosed peridotite. Mafic clasts recovered in the serpentine muds of Conical Seamount comprise five geochemical groups: island-arc tholeiite, boninite, mid-ocean-ridge-like basalt, and basalt intermediate between island-arc tholeiite and mid-ocean-ridge basalt. The presence of basalt with mid-ocean-ridge basalt composition requires a reevaluation of existing theories of the formation of this part of the Mariana forearc. Metamorphism of the mafic samples took place principally under low-temperature and low-pressure conditions. The mafic rocks from Conical Seamount that contain blueschist-facies metamorphic minerals are considered to have derived from a low-temperature (150°-250°C) but deep (13-18 km) portion of the forearc. The grade of metamorphism of the Torishima Forearc Seamount samples is prehnite-actinolite facies, which requires a low-temperature/low-pressure regime (200°300° C and 0-6 km deep). Numerous examples of sedimentary serpentinite deposits exposed on land are similar to the deposits forming on Conical and represented by the older deposits at Torishima Forearc Seamount. Conical Seamount may represent a type locality for the study ofin-situ formation of sedimentary serpentinite bodies that originated by a combination of protrusion and seafloor mud volcanism.