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, 13, 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 (luring 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-are
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-evaluated.