ABSTRACT: Fluid Seeps Atop Serpentinite Seamounts in the Mariana Forearc

J. A. Haggerty

The Mariana fore arc is a nonaccretionary fore arc with numerous seamounts. Dredge hauls from these seamounts are composed primarily of serpentinite, indicating a nonvolcanic origin. In addition, carbonate samples reveal (1) the presence of aragonite in water depths greater than the aragonite compensation depth, (2) unusual porosity and cement types, (3) light carbon and heavy oxygen isotope compositions, and (4) an unusual trace element geochemistry of aragonite and calcite. These mineralogic, petrologic, and geochemical characteristics led to a prediction that fluids associated with methane vent from these seamounts. Subsequent to this prediction, submersible dives sampled chimneys seeping cold water containing dissolved methane from the summit of a Mariana serpentinite seamount.

Fluid inclusion contents of chimney samples also reveal an earlier history of methane as well as higher hydrocarbons in these fluids. The presence of aromatic compounds and organic acids, if derived from the maturation of organic compounds, indicates a thermogenic origin for these volatiles. Strontium isotope ratios of the chimney aragonite indicate an igneous component in the precipitating fluids, inferring a deep source. The lack of a large accretionary prism in the Mariana fore arc removes the complication of fluid contribution from dewatering of the prism by compression during convergence, in contrast to the Oregon-Washington or Barbados margins. The organic compounds in the fluid inclusions, as well as the isotopic and geochemical composition of the carbonates may indicate that f uids are derived by thermal maturation of organics in subducted sediments and dehydration of the subducted plate. These fluids may serpentinize the overriding plate, which subsequently forms the seamounts.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990