Variability in serpentinite mudflow mechanisms and sources: ODP drilling results on Mariana forearc seamounts

Abstract

Samples of metamorphic rock were collected from drilled holes on ODP Leg 195 and in piston /gravity cores collected with the Jason 2 remotely operated vehicle from S. Chamorro Seamount, a serpentinite mud volcano on the Mariana forearc. The recovered muds are approximately 90% serpentinite, including grit- to boulder-sized clasts of serpentiniz e d peridotites, but also contain a wide variety of small fragments of metabasic rocks. These metabasic fragments include high-pressure, low-temperature rocks derived from the subduction zone. Other serpentinite seamounts also have yielded metabasic rock fr a gments as small clasts in the serpentinite mudflows, but none have as wide a variety of rock types as S. Chamorro Seamount. The sources of the rock clasts, both serpentinized peridotites and metabasic schists, vary with the eruptive episodes of the mud vo l canoes. Swath mapping of S. Chamorro Seamount shows that a sector collapse of its southeastern flank has resulted in debris flows from the summit region of the seamount that have traveled more than 70 km eastward toward the trench. These debris flows, how e ver, have a very different morphology from mudflows observed at the summit. High-resolution seafloor mapping of the summit shows both thin, presumably highly fluid- (or gas-) charged serpentinite mudflows and a relatively viscous protrusion that has forme d the main summit knoll. By comparison, the summit of Conical Seamount drilled on ODP Leg 125 lacks a distinct summit knoll and has numerous, relatively thin and widespread mudflows covering the flanks of the edifice. The style of eruption at a given seamo u nt probably varies with time and with the amount of fluid or gas incorporated in a given pulse of mud. The greater diversity of metabasic rocks at S. Chamorro Seamount may be a consequence of recycling of forearc materials through tectonic erosion and sub duction in the southern part of the forearc.