2009 Annual Science Report
University of Hawaii, Manoa Reporting | JUL 2008 – AUG 2009
Chemistry, Origin and Evolution of Subduction Zone Fluids Rising Beneath the Mariana Forearc
Ultramafic rocks make up the mantle of most rocky bodies in the Solar System. When ultramafic rocks come in contact with liquid water they are altered to serpentinite over a wide range of temperatures, from freezing to about 500ºC. On Earth, plate tectonics provides ample opportunity for this contact to occur, especially in subduction zones. In extraterrestrial environments, serpentinization should occur wherever liquid water comes into contact with mantle-type rocks, such as on Mars and on the parent bodies of asteroids. We have collected waters upwelling through serpentinite mud volcanoes in the forearc region of the Mariana subduction zone in the NW Pacific Ocean. These waters are rich in methane produced inorganically during serpentinization. The methane supports chemosynthetic communites of extremophilic Archaea that thrive at an in-situ pH of 13.1.
In the past year we have made significant progress in understanding the origin of the methane and the exceptionally high pH in this setting, which is much too high to result from serpentinization alone. The high alkalinity at the distal sites apparently results from decarbonation at the top of the subducting plate, triggered by the phase transition at depth from lawsonite to epidote as the principal hydrous Ca-silicate mineral, which occurs at a temperature of ~220ºC. Unlike lawsonite, epidote consumes most of the dissolved Ca, causing CaCO3 to dissolve in response. The resulting dissolved carbonate is inorganically reduced to methane at elevated temperature by reaction with H2 generated during serpentinization. Serpentinization also generates high pH, such that carbonate alkalinity is replaced by hydroxyl alkalinity according to the reaction 4H2 + CO3= = CH4 + H2O + 2OH-.
The excess hydroxyl produced in this reaction accounts for the exceptionally high pH of 12.5, the highest ever measured in the deep sea.
PROJECT MEMBERS:Michael Mottl
RELATED OBJECTIVES:Objective 5.3
Biochemical adaptation to extreme environments
Biosignatures to be sought in Solar System materials