Geochemical Investigation of Serpentinized Oceanic Lithospheric Mantle in the Feather River Ophiolite, California: Implications for the Recycling Flux of Water by Subduction

The petrology and geochemistry of serpentinized harzburgites within the Feather River Ophiolite in northern California were investigated to constrain the origin of serpentinization. Trace-element systematics (high Cs, Rb, Ba, U and Sr) indicate that serpentinization was associated almost solely with relatively low temperature hydrothermal addition of seawater and not with the addition of metamorphic fluids associated with subduction or accretion. Major element systematics show almost negligible disturbance, suggesting low water/rock ratios rather than the very high water/rock ratios characteristic of serpentinization of exhumed mantle at the seafloor. These observations were taken to indicate that serpentinization occurred by the addition of seawater to ultramafic protoliths while they were structurally within the oceanic lithosphere prior to tectonic accretion. Major and trace element systematics were used with simple melting models and assumptions about the thermal state of the mantle to put constraints on the paleo-depths at which the ultramafic protoliths were last melted. Assuming that these peridotites were formed beneath a passive upwelling system, these paleo melting depths were assumed to equate with paleo-serpentinization depths. For a model mantle potential temperature of 1350 oC, the paleo-serpentinization depth was estimated to be ~40 km based on the minimum melting degree of Feather River Ophiolite peridotites. This depth constraint on serpentinization was then combined with model estimates of the lateral extent of serpentinization assuming that serpentinization is associated with infiltration of water through faults and fractures. It is shown here that recycling rates of serpentine-bound water in subduction zones is roughly one order of magnitude higher than that associated with mineralogically bound water in subducting oceanic crust and sediment. After accounting for serpentine recycling, it is shown that the recycling rate of water via subduction is roughly equal to the global volcanic dewatering rate, suggesting that the present water budget of the ocean and Earth's interior may presently be close to steady state.