Serpentinites are one of the major components of the oceanic lithosphere
and are stable in the slab and the mantle wedge up to 100-150 km depth
in subduction zones. During oceanic mantle hydration and alteration,
they trap trace and fluid mobile (FME: B, Li, As, Sb, Rb, Ba, Cs, Sr, U
and Pb) elements that participate to elemental transfer occurring
between the dehydrating slab and the mantle wedge in subduction context.
The Lanzo massif is an eclogitized oceanic lithosphere that preserved
its oceanic structure and recorded different steps of
serpentinization/de-serpentinization from oceanic lizardite to prograde
antigorite in subduction context, up to its dehydration and secondary
olivine crystallization, and finally retrograde antigorite during massif
exhumation. It constitutes a suitable place to study trace element
behavior during serpentinization/de-serpentinization processes and
associated chemical transfers between the different envelopes of the
oceanic lithosphere and the mantle wedge.
Geochemical analyses of serpentine and associated minerals show that the
serpentinization/de-serpentinization of the Lanzo massif took place in a
relatively closed system without significant trace element transfer
between the different parts of the oceanic lithosphere. In the deeper
part of the lithosphere, from the slightly serpentinized mantle
peridotites (SSP, <20% serpentinization) to the paleo-Moho, composed of
massive serpentinites (MS, 80% serpentinization), the trace elements
mobility is reduced. The chemical composition of lizardite and
antigorite is homogenized with the local degree of serpentinization: in
SSP, serpentine veins composition is inherited from the host mineral
while, in MS, their composition is homogenous between destabilized
phases at the scale of the outcrop (similar to 5 m). In the shallowest
part of the oceanic lithosphere, from the paleo-Moho to the oceanic
paleo-seafloor, the serpentinites are foliated (FS, >90%
serpentinization). In that zone, the alpine deformation enhances the
mobility of trace elements and permits their redistribution and the
homogenization of antigorite composition at massif scale. Locally, in
the SSP and MS, the crystallization of metamorphic veins of similar to
1-2 m corresponds to channelized fluid flows that allowed fluid
transfers - and thereby trace elements - to longer distance. The
successive crystallizations of antigorite and then olivine are
accompanied by a diminution of some FME (B, Li, As, Sb, Ba, Rb) and Eu
contents attesting that these elements are removed from slab to mantle
wedge during subduction. (C) 2013 Elsevier B. V. All rights reserved.