ダイナミクスセミナー-次回予定-

要　旨
Subduction zones are located at the junction of two converging tectonic
plates, where one sinks (subducts) beneath the other into the Earth's
mantle. The central role for subduction zones in the evolution of the
solid Earth has been repeatedly emphasized. Raw materials, such as
pelagic/terrigenous sediments, oceanic crust, and mantle lithosphere,
are fed into the subduction factory. Output products, such as arc
magmas emerge from the factory and may form continental crusts. The
remainder of the material, which is processed in the subduction
factory, sinks into the deep mantle and may have contributed to the
geochemical evolution of the Earthユs mantle.

The continental crust possesses an andesitic composition, although
basaltic magmas dominate the magmatism on the modern Earth. This fact
is probably the greatest dilemma facing those interested in theories of
the origin of the continental crust. Further, at least four geochemical
reservoirs, in addition to the primitive mantle, would be required
within the Earthユs mantle to explain the isotopic variations observed
for ocean island and mid-oceanic ridge basalts; EMI (Enriched Mantle
I), EMII (Enriched Mantle II), HIMU (high-m; m=238U/204Pb), and DMM
(Depleted MORB Mantle). Among these, the origin of the enriched
components is essential for understanding the dynamics and evolution of
the deep mantle, because such components typify magmas rising from
deep-seated hotspots.

Geochemical formulation of dehydration, partial melting, and
fluid-solid reactions suggests that a process, which includes
slab-dehydration-induced mantle melting, basalt magma generation, and
remelting of the initially created basaltic crust, can reasonably
explain the major and trace element compositions of the andesitic bulk
continental crust. It should be further stressed that the mechanism
requires delamination of mafic melting residues from the initial crust
and recycling of such メanti-crustalモ materials into the mantle.
Isotopic modeling suggests that a pyroxenitic delaminated component
formed at 3-4 Ga possesses Sr-Nd-Pb isotopic compositions typical of
the EMI component, one of the enriched geochemical endmembers in the
mantle.

Mobilization of elements during dehydration processes in the subducting
plate was examined by high-pressure experiments on a pelite and an
amphibolite, both typifying subducting plate materials. The results
confirm selective transport of particular elements such as Pb and Rb,
and document effective element fractionation during dehydration
processes within the subduction factory. Isotopic modeling based on
such experimental results suggests that subducting sediments and
oceanic crusts, which have been stored in the deeper part of the
mantle, may form the EMII and HIMU endmember components, respectively.

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第22回ジオダイナミクスセミナー Geodynamics Seminar

第22回ジオダイナミクスセミナー
　　　Geodynamics　Seminar