Where did all the water in the Earth’s oceans come from?
One theory is it came from ice comets crashing into the Earth.
However, scientists have discovered that a diamond from Brazil that was that formed deep inside the Earth’s mantle (410-660 km) contains a rare mineral called ringwoodite.
This ringwoodite contains a relatively large amount of water.
Since the mantle is so big and contains quite a lot of ringwoodite, this means there could be huge amounts of water down there.
Therefore, the water in our oceans could have come from deep inside the Earth and not from comets.
That is, the water was mixed in with the rocks when the Earth was originally formed, and some of it is still down there.
Future isotopic analysis of water from the oceans, mantle and comets could sort this out conclusively.
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Background
Where did all the water in the Earth’s oceans come from?
One theory is it came from ice comets crashing into the Earth.
Another theory is it mixed with the rocks when the Earth first formed, and some of it still remains inside the Earth.
The crust is very thin (5-30 km thick).
Below that, the mantle is ~3,000 km thick.
Within the mantle is a layer called the transition zone at depths between 410-660 km.
Very little is known about the transition zone because samples from this region are extremely rare.
Materials and Methods
Diamond Juc29 from the Juina district of Brazil is 5mm across and weighs 0.09g. Scientists from Canada analysed its chemical composition using various forms of spectroscopy (X-ray diffraction, Raman, infrared).
Results
The diamond was formed in the transition zone (410-660 km) under huge pressures (>15GPa) and high temperatures.
Inside the diamond they discovered a tiny speck (0.04 mm across) called ringwoodite.
This mineral (MgSiO4) is only formed by ultra-high pressures at great depths. It is common in the mantle, but rarely seen at the surface because it is destroyed at lower pressures.
Since ringwoodite is present in the diamond, it means that the diamond must have reached the surface very quickly (e.g. volcanic eruption).
Importantly, the ringwoodite contains a relatively high amount of water.
Discussion
Considering the large amount of ringwoodite and related minerals (olivine, wadsleyite) in the mantle, this means there could be a lot of water down there, perhaps as much as all of the surface oceans combined.
There are 2 theories for where the mantle’s water came from:
1) Surface oceans, carried underground by subduction/plate tectonics.
2) The water remained in the mantle from when the Earth originally formed.
Future Directions
Future isotopic analysis (deuterium to normal hydrogen) of water from the oceans, mantle and comets will sort out where the oceans water came from.
Need to determine if the whole mantle uniformly contains water, or whether there are a few isolated ‘wet spots’.
Article
Hydrous mantle transition zone indicated by ringwoodite included within diamond
Pearson et al., 2014 Nature 507:221-4
Keywords
Ocean, water, Earth, mantle, crust, comet, transition zone, rock, geology, plate, tectonics, volcano, lava, magma, ringwoodite, olivine, spectroscopy, diamond, mineral
Subject
Science, Earth and Space, Geology, ST1-8ES, ACSSU019, ST1-9ES, ACSSU032, ST1-11LW, ACSSU211, ST2-8ES, ACSSU075, ST2-9ES, ACSSU048, ST2-12MW, ACSSU046, ST3-8ES, ACSSU078, ST3-9ES, ACSSU096, ST3-12MW, ACSSU077, SC4-12ES, ACSSU153, SC4-12ES, ACSSU115, SC4-13ES, ACSSU116, ACSSU222, SC5-13ES, ACSSU180, ACSSU189