Guillaume Richard 氏（JSPS Postdoctoral Fellow, University of Tokyo）

「Deep Mantle Slabs and the Deep Earth's Water Cycle」

2007年1月24日

Water enters the Earth's mantle at trenches by subduction of oceanic
lithosphere. Most of water immediately returns to the atmosphere through
island arc volcanism, but part of it is expected as deep as the Earth's
mantle transition zone(410-660km depth). Water carried by subducted slabs
is the only source of water of the deep mantle. It makes the understanding
of the processes of water transfer from a deep slab to the mantle a key
element to estimate the water content and distribution in the Earth's
mantle, especially in the transition zone. Here I present results from
numerical modeling of two possible processes for slab dehydration and
discuss them in regard to the Transition Zone water content: i) Within
the Earth's mantle transition zone by diffusion of water out of
the slab, and ii) at the base of the upper mantle (660km depth) by
percolation of hydrous fluids formed by water exsolution from the slab.
Regarding stagnant slab dehydration by diffusion, we consider temperature
dependent water solubility in transition zone olivine polymorphs
(Wadsleyite, Ringwoodite). When slab eventually reaches the lower mantle
(660km depth), due to its low solubility in lower mantle minerals, the
remaining water is likely to be released as anhydrous fluid, during the
spinel-postspinel phase change. The dynamics of this fluid phase is
investigated through a 1-D model using two-phases flow theory. Results
show that those two processes are likely to make the transition zone
rather wet in the vicinity of a subducting slab.