4 ms·

Deepest water found 1000km down, a third of way to Earth’s core

24 points by happy-go-lucky 10y ago

wycx 10y ago
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.
tzs 10y ago
> 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:Plates_tect2_en.svg https://en.wikipedia.org/wiki/Plate_tectonics#/media/File:Pl... 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?
- stouset 10y ago
  I find it helpful to imagine the plates are flexible and not rigid. They have forces acting on them that cause them to want to move in a direction, whether or not they're capable of actual doing so. When they can't, pressure builds up until something gives and… you have an earthquake. This, plus plates deforming, I believe should solve your confusion. They are locked up against one another (in the case of strike-slip faults, moving past one another). There are also have other fault types, like subduction zones where one plate gets pushed beneath another (plate tectonics are 3D, not just 2D!). All this twisting and squeezing and pushing with nowhere to go until something fails. If this is a bad mental model, someone please correct me.
- ponyous 10y ago
  I have been taught that because of moving plates mountains can rise, besides that one plate may be slowly moving under another - confirming your hypothesis that plates do change shape, but shape change is not required to move.
- wycx 10y ago
  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.

wycx 10y ago

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.

tzs 10y ago

> 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:Plates_tect2_en.svg https://en.wikipedia.org/wiki/Plate_tectonics#/media/File:Pl... 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?

stouset 10y ago
I find it helpful to imagine the plates are flexible and not rigid. They have forces acting on them that cause them to want to move in a direction, whether or not they're capable of actual doing so. When they can't, pressure builds up until something gives and… you have an earthquake. This, plus plates deforming, I believe should solve your confusion. They are locked up against one another (in the case of strike-slip faults, moving past one another). There are also have other fault types, like subduction zones where one plate gets pushed beneath another (plate tectonics are 3D, not just 2D!). All this twisting and squeezing and pushing with nowhere to go until something fails. If this is a bad mental model, someone please correct me.
ponyous 10y ago
I have been taught that because of moving plates mountains can rise, besides that one plate may be slowly moving under another - confirming your hypothesis that plates do change shape, but shape change is not required to move.
wycx 10y ago
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.