Аннотация:Although there is a consensus that dehydration of downgoing oceanic lithosphere can release
tremendous volumes of fluid, there are no decisive arguments to favor its porous or focused
flow and related element transfer through the mantle wedge. We present the results of pistoncylinder
experiments with natural rocks and mineral separates, carried out at 750-900o
C and
2.9 GPa - conditions relevant to hot subduction zones. The experiments illustrate the
mechanisms of metasomatic alteration of the mantle-wedge rocks such as dunite and lherzolite
and transfer of trace elements by infiltration of СОH fluids and/or rhyolitic melts (at T=850o
C)
released from the carbonate-bearing amphibolite during its eclogitization.
Element transfer from the slab to the mantle lithologies in the capsules was implemented by
porous, focused and diffusion flow regimes that totally obliterate melt and carbon, and partially
water, from the metabasite layer. The porous flow is recorded by growth of
OpxrGrtrMgsrChl along the grain boundaries and the dissolution of clinopyroxene in the
peridotite layers. The porous flow of the same fluids/melts produces harzburgite mineralogy in
the both dunite and lherzolite layers. The focused flow is developed along the side walls of the
capsules and is manifested by the development of melt segregations, separated from the host
peridotite layers by newly formed OmprGrtrPhl+Opx+Mgs. The diffusion flow leads to the
formation of OpxrMgsrGrt reaction zones at the metabasite-peridotite interface.
Trace-element compositions of the glasses agree well with high-silica adakites and TTGs,
though enriched in K2O compared to TTGs. Trace-element profiling reveals the development
of a Eu anomaly in the peridotite layers and diffusion of many trace elements out of the
respective layers toward the contact zone. Since the contact zone itself actually is low in these
elements, we assume their transfer by fluid/melt migration to the sides. The documented
processes might applicable to both Phanerozoic and Precambrian subduction zones.
This study was supported by RFBR grant 16-05-00495.