Serpentinite subduction: Implications for fluid processes and trace-element recycling

Serpentinites are important components of the oceanic lithosphere and are viewed as major water sources in subduction zones. Study of a set of ultramafic rocks equilibrated at increasing subduction depths (oceanic and high-pressure antigorite serpentinites, olivine-orthopyroxene rocks), shows that these rocks can act as carriers of Cl, B, Sr, Rb, Cs, and alkalis. Serpentinization of the oceanic mantle produces enrichment in Sr, Cl, B, and alkalis. The subduction path within the stability field of antigorite serpentine is accompanied recycling of oceanic Cl, B, Sr, and alkalis into variably saline fluids (4-8 wt% NaClequiv based on mass balance calculations; 10 to 50 wt% NaClequiv based on fluid inclusion analysis). Fluids are produced during a first (olivine-in) dehydration reaction releasing a maximum amount of 2 wt% H2O. The oxygen isotope compositions of the high-pressure antigorite serpentinites largely mirror those of the pre-subduction protoliths. This is interpreted as a lack of re-equilibration due to limited fluid production and mobility during high-pressure metamorphism Breakdown of the antigorite serpentinites to olivine-orthopyroxene rocks releases 6.5 wt% H2O with formation of a low salinity fluid (0.4-2 wt% NaClequiv based on mass balance calculations). The salt contents of subduction fluids appear to decrease with increasing depth: a "differentiation" in the Cl content of fluids can thus be expected, with shallower saline fluids being followed by deeper, less saline, solutions. Primary fluid inclusions in olivine-orthopyroxene rocks represent remnants of the antigorite-breakdown fluid. Their trace-element compositions are enriched in several LILE (Rb, Sr, Cs, Pb), Li, B, and alkalis and are depleted in HFSE; their trace-element patterns are similar to those of many present-day arc lavas. Oxygen isotope compositions of olivine-orthopyroxene rocks are rather homogeneous, and are suggestive of larger fluid mobility during dehydration. The antigorite breakdown reaction appears to produce a mobile fluid that can play a role in the metasomatism of sub-arc mantle. Serpentinized oceanic mantle thus represents a valuable candidate reservoir for fluids and incompatible elements: consequently, its role in the overall element cycle in subduction settings should be re-evaluate