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New form of ice found


jerseyscum

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from NYT

The scientists started considering what happens to tectonic plates after they are pushed back down into Earth's interior. At about 100 miles down, the temperature of these descending plates is 300 to 400 degrees — well above the boiling point of water at the surface — but cool compared with that of surrounding rocks. The pressure of 700,000 pounds per square inch at this depth, Dr. Bina and Dr. Navrotsky calculated, could be great enough to transform any water that was there into a solid phase known as Ice VII.

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I wonder what subduction model their evidence is based on. Most subduction zone models put the 600 deg C isotherm at a max depth of 120km on the cool, descending slab. Much warmer in the mantle wedge and descending oceanic lithosphere adjacent to it. It could get cooler at the same pressure for a more rapidly or steeply descending slab, but not that much cooler. By that depth in most arcs, amphiboles have dewatered. Hard to picture that water turning to ice, since it is what we attribute to be a primary cause of partial melting in the mantle wedge.

 

Where's the link to the whole NYT article? I wonder if it is causing debate among earth scientists, or if some think that its a crock...

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Dylan: The quote isn't from main thrust of article, which is "why is ice slippery" or something like that. Excerpt is late in story and I gather on closer reading theory is mere speculation...though various ices are believed more certainly to exist on other planets...BTW thanks for guiding me on Triumph some years back and humoring me with a little geology....

 

http://www.nytimes.com/2006/02/21/science/21ice.html

 

Article adds caveat: "No one knows whether ice can be found inside Earth, because no one has yet figured out a way to look 100 miles underground. Just as salt melts ice at the surface, other molecules mixing with the water could impede the freezing that Dr. Bina and Dr. Navrotsky have predicted."

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Hey John! What memories! I was just looking at old slides of Triumph...

 

Anyway, Google is an amazing thing. This satisfied some curiousity:

 

 

Bina, C. R., and A. Navrotsky, Till Hell freezes over - Ice in subducting slabs?, Eos, Transactions of the American Geophysical Union, Western Pacific Geophysics Meeting Supplement, T22B-05, 2000.

 

T22B-05

 

During subduction of oceanic lithosphere, a series of progressive dehydration reactions occurs. The water liberated participates in geologically important processes. It has been invoked in melting point depression responsible for island-arc volcanism, in enhanced pore pressures responsible for intermediate-depth seismicity, and in transport of soluble chemical constituents from slab to mantle. While supercritical fluid water is liberated during dehydration of most slabs, the stable phase of H2O is solid ice VII along portions of the geotherm for the coldest slabs. The substitution of ice VII for fluid water as a product of dehydration reactions has significant implications for the physical processes by which H2O is generated, stored, transported, and released within cold subduction zones. The locations and slopes in P-T space of dehydration reactions change, potentially affecting depths of seismogenesis and magmagenesis. Significant amounts of ice VII can be accumulated during progressive dehydration of mineral solid solutions during subduction. As the sinking slab warms, melting of this pure ice will occur at a single temperature and pressure, releasing large amounts of water in a small spatial region over a short time. This univariant melting reaction has a significant positive volume change and may trigger seismicity. The initially chemically pure liquid water that is formed, being undersaturated with respect to all dissolved constituents, will react strongly with the surrounding rock, with possible implications for trace element distributions and metal transport. Accumulation of ice VII in a cooling planetary interior (e.g., Mars) may eventually lead to a decline in or cessation of tectonic activity.

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I wonder what subduction model their evidence is based on. Most subduction zone models put the 600 deg C isotherm at a max depth of 120km on the cool, descending slab. Much warmer in the mantle wedge and descending oceanic lithosphere adjacent to it. It could get cooler at the same pressure for a more rapidly or steeply descending slab, but not that much cooler. By that depth in most arcs, amphiboles have dewatered. Hard to picture that water turning to ice, since it is what we attribute to be a primary cause of partial melting in the mantle wedge.

 

Where's the link to the whole NYT article? I wonder if it is causing debate among earth scientists, or if some think that its a crock...

 

Ack. You're causing me to have grad school flashbacks....

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As the sinking slab warms, melting of this pure ice will occur at a single temperature and pressure, releasing large amounts of water in a small spatial region over a short time. This univariant melting reaction has a significant positive volume change and may trigger seismicity.
Allow me to translate for you. When the Water VII changes to a different crystalline phase, it's density decreases, and therefore its volume increases- it takes more space and therefore displaces rock. This may be the cause of earthquakes.
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