
Deepest water found 1000km down, a third of way to Earth’s core - happy-go-lucky
https://www.newscientist.com/article/mg23231014-700-deepest-water-found-1000km-down-a-third-of-way-to-earths-core/
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wycx
It would be more accurate to say: deepest minerals with hydroxyl groups found
1000 km down.

Never at any stage will there be a fluid that can be described as free water
at these depths. Breakdown of minerals with OH- such as hydrous ringwoodite at
the top of the lower mantle results in dehydration melting. The melts/fluids
produced will have many 10s of weight percent solutes, but in a mol. percent
sense these melts/fluids are dominantly H2O, given how low the molar mass of
H2O is relative to the other components. If some of this melt gets trapped
inside a growing diamond, it freezes during ascent and forms minerals with
structural OH-, e.g Mg(OH)2.

There is ambiguity regarding what to call melts/fluids at these pressure and
temperature conditions as many bulk compositions will be above the second
critical end point where there is complete miscibility between H2O and 'molten
rock'.

Last paragraph of the paper:

Our data support the idea that dehydration melting generates volatile-rich
fluids/melts at the top of the lower mantle, yet being a key process for
diamond genesis and for transportation of water across the 660 km
discontinuity.

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tzs
> Jacobsen thinks that this water may help explain why Earth is the only
> planet we know to have plate tectonics. “Water mixes with ocean crust and
> gets subducted at convergent plate boundaries,” he says. “Introducing water
> into the mantle promotes melting and weakens rock, likely helping out the
> motions of plates like grease.”

I am completely befuddled by plate tectonics. In particular, every drawing
I've seen of the plates shows the surface of the Earth as kind of like a
jigsaw puzzle, with the pieces being the various plates, and everyplace being
part of some plate.

I don't see how there can be any significant relative motion of the plates.
For instance, take this plate diagram, found on Wikipedia:
[https://en.wikipedia.org/wiki/Plate_tectonics#/media/File:Pl...](https://en.wikipedia.org/wiki/Plate_tectonics#/media/File:Plates_tect2_en.svg)

That looks like it should be pretty much locked up. For anything to move,
something else would have to give way, and for that to move, something else
would have to give way, and so on, and you'd end up at most just being able to
make the whole assembly move as a unit.

For there to be significant movement, it would seem that the plates would need
to be changing shape in addition to moving. Is that something they do that is
simply left out of the popular science explanations?

~~~
wycx
Do an image search for "subduction zone", you will get some images that show
you what is going on, schematically in cross section.

A simplified summary: New oceanic crust is created at mid ocean ridges, but
since the earth is not expanding, when you create new crust, subduction must
occur elsewhere. For instance, new crust is created in the pacific, and
subduction is occurring beneath Japan in the east and the west coast of north
america in the west. Once the oceanic crust subducts far enough into the
mantle, minerals that make the oceanic crust change to more dense forms in
response to the increased pressure, (basalt to eclogite transition) and the
resulting rock is more dense than the surrounding mantle, meaning it wants to
sink further and further, providing a "slab pull" component to subduction (as
opposed to "ridge push" due to new crust formation at the mid ocean ridge).
Oceanic crust will almost universally subduct beneath continental crust due to
density (~2.3 g/cc for continental, 2.7 for oceanic, >3 for mantle).
Continental crust is too low density to subduct into the mantle, so you make
mountain ranges like the Himalaya when continents colide.

After new oceanic crust is erupted at mid ocean ridges, it remains hot, and
experiences sustained interaction with seawater forming new minerals with
structural water/OH- (i.e. chlorite), as well as accumulating a veneer of
sediment that includes clay minerals also with structural water. When that
oceanic crust then subducts back into the mantle, it heats up, the hydrous
minerals are no longer stable, and they release their H2O into the overlying
mantle. This addition of water lowers the melting point of the mantle such
that it will melt under the ambient temperature conditions, this melting is
ultimately manifest as volcanic arcs, such as that that extends from the
andes, north through central america, mexico, the cascades, the aleutians ,
kuriles, japan, philippines, indonesia, new guinea, solomons, fiji, down to
new zealand. These volcanoes are characterised by high H2O contents, leading
to explosive eruptions, in contrast to the style of eruption seen in places
like Hawaii.

Not all the water (structural OH-) is released when from the subducting slab,
and makes its way deeper into the mantle, and aspects of the ultimate fate of
this is the subject of the article.

