How do we know that the water that goes down with the subducting plate can be the same water that comes up with the

Question: How do we know that the water that goes down with the subducting plate can be the same water that comes up with the explosive eruptions at these destructive margins?
- Keywords:
  eruption,
  subduction,
  water
Asked by anon-257313 on 16 Jul 2020.
- Emma Thorpe answered on 16 Jul 2020:
  
  I’m not a geologist or vulcanologist, so I can’t comment on the plates and the eruptions. But I can comment on the water. The chemicals in with the water can give it a ‘fingerprint’ which is mostly unique (but not completely). Things such as pH, metal ion content, alkalinity tell you if it is hard or soft (hard water leaves lots of limescale on taps and in kettles). Then more unique identifiers – organic compounds such as pesticides, pharmaceuticals or industry byproducts. And even more unique – you could look at the different isotope ratios for different elements. By comparing all these in the water going up and down, it would give you an idea if it was from the same source – but not 100% certainty I don’t think.

anon-257313 commented on 16 Jul 2020:

Thank you very much Emma. Most interesting. I suppose sea water is different to groundwater and the chemical fingerprint in the volatiles in the magma could be the clue to where the water came from.

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Question: How do we know that the water that goes down with the subducting plate can be the same water that comes up with the explosive eruptions at these destructive margins?