A synthesis of Leg 125 drilling of serpentine seamounts on the Mariana and Izu-Bonin Forearcs

Abstract

The forearc region of the Mariana and Izu-Bonin convergent margin systems contains numerous large serpentine seamounts. These include either mud volcanoes or horst blocks of serpentinized ultramafics, partially draped with unconsolidated serpentine slump deposits. Tectonic processes on the forearc, particularly faulting, exert important controls over the distribution of serpentinite seamounts. 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. 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. 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; 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 basalt. -from Author