Water heavier than stone

[HydrogenBond]

There is a way for water to sink into the mantle, inspite of crust. This affect is not due to gravity, but uses the chemical potentials associated with hydrothermal water, i.e., water above its critical temperature and critical pressue. This is based on a technique that is used to make crystals, such as quartz, with hydrothermal water. Minerals become far more solubile in water, when it reaches it critical point. At the critical point, water becomes a dense fluid that is no longer a liquid. The best way to describe it, is a dense gas that behaves like it is fluid.

 

The way the technique works, the raw quartz is on the bottom of the appartus. A seed crystal is hung at the top with a platinum string. The apparatus is filled with water and heated until the water reaches the critical point and becomes a dense fluid that is not a liquid. The bottom is the heat source, which causes the hydrothermal water to dissolve the raw quartz. The water rises up to the top due to thermal convection. The top of the appartus is slightly cooler. Because the hotter bottom can dissolve more, because it is hotter, when the hot saturated liquid reaches the top, it cools and has to release some of the quartz so it will not supersaturate. The release goes onto the seed crystal, causing the crystal to grow.

 

 

Let is take this technique into the crust of the earth. We will start with a good sized pocket of water. This pocket it is hotter at the bottom, due to direction of the mantle heat. If this water was trapped by high pressure, and was induced above its critical point, i.e., hydrothermal, it would dissolve downward, since the hotter bottom is more soluble. The thermal convection would cause this enriched water to float up and deposit rock onto the rock crystals at the cooler top of the pocket. The water would just keep eating downward, sealing the top behind itself, with the upper boundry moving downward, also as its thickens with rock deposit. So what we theoretically have are water pockets falling into the mantle.

 

This can be demonstrated in the lab using the above apparatus.

[HydrogenBond]

One of the more contemporary theories of earth's formation has the earth's water coming from asteroids. This scenario is ideally suited to forming water pockets. If one assumes the asteroids are laden with water, when they hit the forming earth, they would send water deep into the crust. As long as there is a way for the hot steam to vent, it will head to the surface. But the next asteroid impact, shifts the crust causing vents to seal. Most of the water reaches the surface, but some is trapped.

 

If we had a pocket of steam, it is not very reactive. But gravity will cause the crust to compact, causing the steam to lower volume until it becomes critical water. The critical water will now dissolve until is saturates. It will just sit there until there is a thermal gradient. As the surface cools, the cooling allows a way for part of the hydrothermal pocket saturaton to release material, as crystalization at the top of the pocket. There is now room for more dissovling in the critical water. It will start to move toward the heat source of the mantle and keep dissolving downward.

 

If critical water reached the mantle, its impact on the binding forces between the solid crust and the fluid mantle, would be to weaken this relatively weak binding force, In other words, if it can dissolve rock, the rock-fluid mantle interface is much easier to overcome. The net affect is the critical grease needed to make the plates slide.

[CraigD]

A neat – and, in my experience, novel – idea. Having been demonstrated with a benchtop device adds to its plausibility. :)

 

I have a few questions:

:)

Must there be a specific heat difference between the hot bottom and the cooler top of the water chamber? If so, how do we know this heat gradient exists all the way to the crust-mantle boundary?

 

Will the effect continue as long as it has heat, or is it in some way self-limiting. For example, will the reformed crystal at the top, “***” end of the effect be less dense that what’s being dissolved at the bottom, filling the chamber? Will the superheated water electrolysize, and the free hydrogen and oxygen be bound into the formed crytal?

 

Can evidence of the effect be found in rock formation reachable by drilling and mining? Can it be found in archived samples? Do you, HydrogenBond, or anyone else, have plans to search for such evidence?

 

And, on a historic note,

Is this, as it appears to me, an original idea, or has it to anyone’s knowledge been proposed before by others?

 

Finally, a moderator-ly question:

As it’s more an applied geochemistry subject, shouldn’t we move the thread to the Earth science forum?

[HydrogenBond]

It should have been started in the earth science section. At that time, there was a good thread going there and I didn't want to cut in. Maybe it can be moved there.

 

I came up with this idea independantly and am not sure if it was already proposed before. I reverse engineered the theory, starting with some equipment and technique that is widely used and has tons of data. It is was sort of free preliminary research, that only required holding onto the coat-tails of others.

 

Hydrothermal research was big in the late 1950's and 1960's, when critical water was investigated as a possible way to make synthetic rubies for lasars. Other techniques were more cost effective. Quartz crystals were ideally suited to the hydrothermal technique to make quartz for high temperature applications, such as the windows on furnaces. Typically they only use about 10 degree C gradient. It will work with less, but the goal is usually production so they can make money with the process.

 

The profile from the mantle to the surface defines a thermal gradient, with the gradient highest at the poles. Whether this is maintained locally would be hard to determine. But if for the sake of argument, water did reach the crustal-mantle interface, it would make a excellent grease that would allow the crustal plates to slide. There needs to be energy input to make crustal movement possible. Water gives more bang for the buck.

 

Will the effect continue as long as it has heat, or is it in some way self-limiting. For example, will the reformed crystal at the top, “***” end of the effect be less dense that what’s being dissolved at the bottom, filling the chamber? Will the superheated water electrolysize, and the free hydrogen and oxygen be bound into the formed crytal?

 

It is not so much density that is important, as phase diagrams, where combinations of minerals have an impact of each other. For example, one thing in solution can help another things dissolve. If you add small cations like sodium, the water gets more aggressive. Another complication is, if one can have many distinct crystals needing to dissolve at the bottom, once they get into solution, they can all form a crystal together. Beryllium aluminium silicate with chromium, Be3Al2(SiO3) is emerald.

 

As far as the water electrolyzing, the minerals in the crust are relatively inert such that there is little energy to push this reaction. As far as inclusion of H and O, the H will lower the MP of a mineral so it will not be able to solidify as easily. If it was to absorb O, that could be helpful, but that would leave acid H+ to help eat downward faster.

[HydrogenBond]

If you compare the crust to the mantle, the crustal material is typically crystaline, while the mantle is a visco-plastic fluid. Silica exists in two useful states, in our day-to-days lives, the first is glass, which is a dense fluid-solid and the other is quartz, which is solid crystaline. The quartz has a much higher melting point and is in the crust, while glass is more analogous to the visco-plastic within the mantle. The main difference is quartz is highly orderred whereas glass has no internal order and will contain more potential energy within the entropy of its disorder.

 

The reason I bring this up, the movement of the crustal plates causes an subversion as crustal material needs to sink back into the mantle. What we essentially get is the higher MP crystaline material being pushed into the lower melting point visco-plastic molten glass of the mantle. Under those conditions there is an energy consideration with respect to getting this material to dissolve at a fast rate, i.e., the heat of fusion. What compounds the problem, as the pressure increases, the melting point of the crystal material will increase. This pressure-MP effect is why a solid iron core can exist thousands of degree above its normal ambient melting point.

 

Water at the crust-mantle interface would lower the energy requirments for the subversion recycle. It provides a chemical way to add dissolving power to lower the energy requirements needed to get the crustal inversion materials into the mantle's visco-plastic state.

 

One possible mechanism by which the water can play a role in both the formation and recycle of crustal materials is if the mantle material was deficient in the oxygen needed to form crystals such that it remains amorphous sort of by default. For example instead of bulk SiO2, say it was bulk SiO1.89. If the extra O came from the interface water to boost some of the bulk SiO1.89 to SiO2, for example, this material can now crystalize, taking some of the O of water out of the game. The result is more H. The subversion material is higher in O. The H goes after the extra O causing the material to dissolve into the more amorphous state. This regenerates water that is then recycled. This mechanism would require some type of subcrustal convection within the acid-neutral water.

 

The alternative is that the mantle is higher in O instead of deficient. Say instead of SiO2 we had SiO2.1. This can also cause a problem. In this case, the O of the water would have to go after the subversion. This would cause the H to be used to tie up some of the O so we get SiO2.