What Is the Primary Source of Chloride in Seawater?

[DrDu]

Most of the salt in seawater is sodium chloride. From my understanding, the primary source of sodium is the errosion of magmatic rocks, which contain lots of feldspars, the most abundant mineral in Earth crust. On the other hand, I don't know any abundant magmatic mineral which contains chloride. So what is the primary source of chloride in the oceans?

 

[trurle]

Most of the salt in seawater is sodium chloride. From my understanding, the primary source of sodium is the errosion of magmatic rocks, which contain lots of feldspars, the most abundant mineral in Earth crust. On the other hand, I don't know any abundant magmatic mineral which contains chloride. So what is the primary source of chloride in the oceans?

Chloride ions are slightly soluble in molten feldspar, especially if alcali, aluminum and fluorine are also present. Some volcanoes also deposits FeCl2 at vents, although it is secondary mineral (formed when magma pass hydrothermal layers). It is difficult to say which chloride mineral was most common in magma before oceans have formed on ancient Earth. Anyway, the reservoir of that mineral was depleted billions years ago.

In the Moon, devoid of oceans, the chlorine is concentrated in Mg-rich feldspars and apatite-rich (KREEP) magmas. Same may be true for ancient Earth.

 

[DrDu]

Well, this is very interesting.
Meanwhile, I found an interesting publication:
http://publications.iupac.org/pac/1996/pdf/6809x1689.pdf
On page 1696 there is a diagram with the relevant reservoirs of chlorine.
The ocean contains 26 Zg of Chlorine and the only relevant source is the Earth's crust, from which 158 Tg/ a are transported into the ocean.
Trying to get an estimate how long it would have taken to reach the actual Cl mass found in the ocean I get
26 Zg / (158 Tg/a) = 165 Ma
This seems way to low, especially since the Cl content of the crust was probably even lower in former times. They also state that the chloride content of the oceans stayed almost constant during the last 600 Ma. I don't understand how this is compartible with their model.

 

[trurle]

Well, this is very interesting.
Meanwhile, I found an interesting publication:
http://publications.iupac.org/pac/1996/pdf/6809x1689.pdf
On page 1696 there is a diagram with the relevant reservoirs of chlorine.
The ocean contains 26 Zg of Chlorine and the only relevant source is the Earth's crust, from which 158 Tg/ a are transported into the ocean.
Trying to get an estimate how long it would have taken to reach the actual Cl mass found in the ocean I get
26 Zg / (158 Tg/a) = 165 Ma
This seems way to low, especially since the Cl content of the crust was probably even lower in former times. They also state that the chloride content of the oceans stayed almost constant during the last 600 Ma. I don't understand how this is compartible with their model.

165 MY is the approximately timescale of full recycling of oceanic crust. The reservoirs in mantle+crust and ocean are approximnately in equilibrum currently - chlorine lost in subduction is compensated by chlorine received through spreading zones.

 

[Ophiolite]

Building on @trurle's post, this paper (abstract only) suggests the bulk of chlorine is extracted from seawater during hydrothermal alteration of peridotite to serpentinite. The authors conclude " Serpentinite may be the major conduit for surficial chlorine transfer to mantle depths and an important part of the global chlorine cycle. The Cl flux in serpentinites into the mantle is larger than all other previously identified fluxes."

As a historical aside, relating to @DrDu 's post, prior to dating via radioactive decay attempts were made to estimate the age of the Earth on the basis of the salt content of the oceans. Even taking into account the salt removed in evaporites the calculated age was less than expected. Plate tectonics resolved the paradox, as trurle noted. Recycling of several elements - e.g. carbon dioxide - are influenced in analagous ways.

 

[DrDu]

Ah, interesting paper!
But even assuming an equilibrium between ocean and crust, (which contradicts the article I cited first), it remains astonishing, that this equilibrium corresponds to an approximately equimolar mixture of sodium and chlorine and not to an excess of either chlorine or sodium.

 

[DrDu]

I try a partial answer to the last point: An excess of chlorine would either mean a very acidic solution which increases weathering of Feldspars or alternative counter ions like Ca or Mg which form less soluble compounds than sodium, and therefore get exchanged.

 

[Ophiolite]

But even assuming an equilibrium between ocean and crust, (which contradicts the article I cited first), it remains astonishing, that this equilibrium corresponds to an approximately equimolar mixture of sodium and chlorine and not to an excess of either chlorine or sodium.

Perhaps you can explain why you find this astonishing. Chlorine will preferentially associate with sodium. Excess sodium is readily taken up by a host of minerals.

 

[DrDu]

You are right, I started from the wrong assumption, that the sodium and chloride content of the oceans is due to mainly weathering.

 

[Ophiolite]

You are right, I started from the wrong assumption, that the sodium and chloride content of the oceans is due to mainly weathering.

I am still perplexed. The sodium and chloride ion content of the ocean, in the sense of the ions that are present, is mainly due to weathering. The ion content that is no longer there is due to other factors, but I still don't see how that would create the problem you suggested in post #6.

The point is that far more sodium goes into the ocean than chlorine. However, you do not require to weather equal amounts, which is what I suspect you are suggesting, in order to get an "approximately equimolar mixture" . Any excess sodium is taken up by other means.

 

[DrDu]

I think that weathering is important only for sodium.
The main source of sodium from the lithosphere is probably the weathering of Albite Na AlSi3O8,

CO2 + Albite + H2O -> NaHCO3 + Clay + Quarz

The NaHCO3 ends up in the ocean while the clay minerals like Kaolinite and Quarz depleted in sodium get metasomatized.

On the other hand, NaCl from the oceans gets metasomatized, too, via binding to serpentinite.
Now, I suspect that in the mantle, the following reaction takes place:

NaCl+ metamorphized Clay+ Quarz -> Albite + HCl

which restores Albite.
Finally, hydrochloric acid returns to the ocean mainly via mid oceanic ridge fumaroles
where it reacts with NaHCO3:

NaHCO3 + HCl -> NaCl + H2O+CO2
which closes the cycle.

On the long term, these cycles keep the ratio of Na to Cl constant. The rest is buffered by the Carbonate cycle.