1. Leg 195 Synthesis: Site 1200—Serpentinite Seamounts of the Izu-Bonin/Mariana Convergent Plate Margin (ODP Leg 125 and 195 Drilling Results)1

Patricia B. Fryer2 and Matthew H. Salisbury3


The Izu-Bonin/Mariana convergent plate margin is characterized by a nonaccretionary forearc with numerous serpentinite seamounts that occur along a 50-km-wide ridge in the Izu-Bonin system and are distributed over a zone as wide as 90 km in the Mariana system. The seamounts are formed primarily by mud volcanism; however, seamounts in the two systems differ in tectonic setting and the materials that form them differ in compositional characteristics. These compositional characteristics suggest that there may be regional differences in the suprasubduction-zone mantle within the Izu-Bonin/Mariana system. In the Izu-Bonin forearc, the seamounts and the ridge on which they formed lie at the top of a wedge of low-velocity material that underlies the outer half of the forearc toe. This wedge is interpreted to be serpentinized forearc peridotite remobilized in response to subduction processes. In the Mariana forearc, seamounts form adjacent to major fault traces scattered over a broad zone in the outer half of the forearc. Studies of pore fluids from serpentinite seamounts show systematic variation in composition related to depth to the slab and to various devolatilization reactions that are controlled by pressure and temperature conditions at the décollement. The serpentinized peridotite materials erupted in mud volcanism indicate that the forearc mantle of the Izu-Bonin system has experienced a different history of both magmatic and melt-fluid interaction than those of the Mariana system. The mud flows at some of these seamounts bring up altered mafic rocks with compositional characteristics of both oceanic plate and island arc origin and fragments of metabasites that reflect the high-pressure and low-temperature conditions of the subduction zone. Some of these rock fragments are likely derived from the subducted slab. The three seamounts drilled during Ocean Drilling Program Legs 125 and 195 thus provide a window into suprasubduction-zone mantle processes, as well as those at work within the décollement itself.

1Fryer, P.B., and Salisbury, M.H., 2006. Leg 195 synthesis: Site 1200—Serpentinite seamounts of the Izu-Bonin/Mariana convergent plate margin (ODP Leg 125 and 195 drilling results). In Shinohara, M., Salisbury, M.H., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 195 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/195_SR/synth1/synth1.htm>. [Cited YYYY-MM-DD]

2Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Road, Honolulu HI 96821, USA. pfryer@hawaii.edu

3Geological Survey of Canada Atlantic, Bedford Institute of Oceanography, PO Box 1006, Dartmouth NS B2Y-4A2, Canada.

Initial receipt: 31 March 2005
Acceptance: 21 December 2005
Web publication: 7 April 2006
Ms 195SR-112