<p>There is a limit to how much of a given salt can be dissolved into water, this is known as it's <a href="https://en.wikipedia.org/wiki/Solubility" rel="nofollow noreferrer">solubility</a> and it's dynamic based upon how much of any particular <a href="https://en.wikipedia.org/wiki/Ion" rel="nofollow noreferrer">ion</a> is already present. Once concentrations exceed the local solubility of a given compound precipitation occurs, this can be seen in the modern Mediterranean basin with the precipitation of <a href="https://en.wikipedia.org/wiki/Calcium_carbonate" rel="nofollow noreferrer">calcium carbonate</a> onto the seabed. There are also older examples of this response such as chemically precipitated chert in New Zealand.</p>

<p>That's one way in which dissolved chemicals can be re-integrated into the rock cycle, biological processes can also play an important role. Plankton take up many elements from sea water during their life cycle, notably silica and calcite for forming rigid structures these are then deposited in the deep ocean when they and the creatures that predate them die and form the basis for many sedimentary rocks.</p>

<p>So while there is a steady input of mineral salts into the ocean there's only so much of it that can accumulate and in fact a lot of them are removed at far lower concentrations due to biological use and subsequent deposition.</p>

<p>A measure of any deposited material may eventually be subducted with the oceanic crust on which it has settled, at which point it will be integrated into any subductive-melt sourced minerals. There is a trend towards the chemical depletion  of the mantle when it comes to certain elements which appears to be a one way street but the ocean does eventually give back a large percentage of what is dissolved into it.</p>

CC BY-SA 4.0
2018-10-29T14:18:35.860Z
Thanks for your answer, though i fear you got my question wrong: I'd like to know whether the the road from (e.g.)(Ba,Ca,Na,K,NH4)(Al,B,Si)4O8(Fieldspar) to Salt, i.e. ionically bound metal+halogen is mainly one-way, or whether the non-salt minerals get re-formed after the precipitated salt is subducted again. It is not just about dissolved salt, but about salt in any configuration
bukwyrm
CC BY-SA 4.0
2018-10-29T14:28:02.297Z
@bukwyrm Sorry I hadn't quite said what I thought I had, try that one for size.
Ash
CC BY-SA 4.0
2018-10-29T15:19:38.017Z
Thanks! Could you expand on the depletion of the mantle, e.g what elements tend to go, and at what rate?
bukwyrm
CC BY-SA 4.0
2018-10-29T15:54:35.400Z
@bukwyrm Not really, that's a large answer in it's own right, the concept of the [Primitive Mantle](https://en.wikipedia.org/wiki/Primitive_mantle) comes into play as does [Planetary Differentiation](https://en.wikipedia.org/wiki/Planetary_differentiation) and the [Goldschmidt series](https://en.wikipedia.org/wiki/Goldschmidt_classification). From memory the main elements that tend to go disproportionately in modern melts are iron, cobalt, and nickel, and of course historically the carbon, hydrogen, sulfur, and light alkali metals that form the bulk of the continental crust.
Ash
CC BY-SA 4.0
2018-11-10T21:46:09.853Z
@Ash no, nickel is extremely compatible in the mantle and iron weakly incompatible. Cobalt is controlled by sulfides.
Gimelist
CC BY-SA 4.0
2018-11-11T10:11:11.493Z
@Gimelist Mantle compatibility is not the issue, mineral formation in basalt melts is.
Ash
CC BY-SA 4.0
2018-11-11T10:39:51.573Z
@Ash since the minerals that exist in basalt (olivine and pyroxene) are the same that are in the mantle, it is a very similar issue.
Gimelist
CC BY-SA 4.0
2018-11-11T10:43:26.323Z
@Gimelist Except that Olivine and Pyroxene are what melts _out_ of the mantle not what the raw mantle is made of.
Ash
CC BY-SA 4.0
2018-11-11T11:43:27.167Z
@Ash olivine is present in both primitive mantle (unmelted) and depleted (melted out) mantle. Olivine is also a major early crystallising mineral in basaltic melts that result from melting of the mantle. If you're interested, I'm more than happy to thoroughly answer a question on the the mantle is made of, what remains after it's melted, and what crystallises out of the melt, and how it affects certain minor elements such as nickel.
Gimelist
CC BY-SA 4.0
2018-11-11T12:00:43.947Z
@Gimelist Except no-one has ever seen the primitive mantle, it hasn't existed since the late Hadean. Estimates on the make up of the modern mantle are debated, we think the upper mantle at least is peridotite but we have never yet sampled the mantle directly. You can give the same best guess answer I could if I cared to dig up my mineralogy notes, but I don't.
Ash
CC BY-SA 4.0
2018-11-11T12:07:46.203Z
Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/85613/discussion-between-gimelist-and-ash).
Gimelist
CC BY-SA 4.0
2018-10-29T14:03:02.810Z
2018-10-29T14:27:28.527Z
2018-10-29T14:27:28.527Z
Ash
<p>The rainfall leaches salts (Mg,Na,Ca,K,.., Cl) from weathering rocks, where those ions were freed from chemical bonds by sunlight, acidic water etc. The runoff eventually goes into the oceans, where evaporation of water leads to accumulation of salts. Over geological timeframes the oceans fall dry and the salts deposit as huge layers. </p>

<p>My question: Is there a net accumulation of salts, or do subduction or other effects reintroduce these ions into mineralic (and non-salt) chemical bonds? -my question is about whether this is a primarily one-way street from minerals to salts, or whether the salts get 'recycled' by, e.g. being subducted and then re-bound in the heat of the mantle.</p>

CC BY-SA 4.0
2018-10-29T13:33:38.000Z
Sometime the salt deposits can get covered by rock & locked away for long periods of time. Such deposits have been & still are [mined](https://en.wikipedia.org/wiki/Salt_mining) in some countries.
Fred
CC BY-SA 4.0
2018-10-29T14:13:32.570Z
@Fred : Yes, the Halide deposits were all created that way. my question is about whether this is a primarily one-way street from minerals to salts, or whether the salts get 'recycled' by, e.g. being subducted and then re-bound in the heat of the mantle.
bukwyrm
<water><crust><salinity>
Is salt accumulating over time?
bukwyrm
# Is salt accumulating over time?

(Asked by: username_0 on 2018-10-29)

The rainfall leaches salts (Mg,Na,Ca,K,.., Cl) from weathering rocks, where those ions were freed from chemical bonds by sunlight, acidic water etc. The runoff eventually goes into the oceans, where evaporation of water leads to accumulation of salts. Over geological timeframes the oceans fall dry and the salts deposit as huge layers.

My question: Is there a net accumulation of salts, or do subduction or other effects reintroduce these ions into mineralic (and non-salt) chemical bonds? -my question is about whether this is a primarily one-way street from minerals to salts, or whether the salts get 'recycled' by, e.g. being subducted and then re-bound in the heat of the mantle.

**Tags:** water, crust, salinity

> **Comments:**
> **username_1**: Sometime the salt deposits can get covered by rock & locked away for long periods of time. Such deposits have been & still are [mined](https://en.wikipedia.org/wiki/Salt_mining) in some countries.  
> **username_0**: @username_1 : Yes, the Halide deposits were all created that way. my question is about whether this is a primarily one-way street from minerals to salts, or whether the salts get 'recycled' by, e.g. being subducted and then re-bound in the heat of the mantle.  

---

## 1 answers

### Answer by username_2 on 2018-10-29. Score: 4

There is a limit to how much of a given salt can be dissolved into water, this is known as it's [solubility](https://en.wikipedia.org/wiki/Solubility) and it's dynamic based upon how much of any particular [ion](https://en.wikipedia.org/wiki/Ion) is already present. Once concentrations exceed the local solubility of a given compound precipitation occurs, this can be seen in the modern Mediterranean basin with the precipitation of [calcium carbonate](https://en.wikipedia.org/wiki/Calcium_carbonate) onto the seabed. There are also older examples of this response such as chemically precipitated chert in New Zealand.

That's one way in which dissolved chemicals can be re-integrated into the rock cycle, biological processes can also play an important role. Plankton take up many elements from sea water during their life cycle, notably silica and calcite for forming rigid structures these are then deposited in the deep ocean when they and the creatures that predate them die and form the basis for many sedimentary rocks.

So while there is a steady input of mineral salts into the ocean there's only so much of it that can accumulate and in fact a lot of them are removed at far lower concentrations due to biological use and subsequent deposition.

A measure of any deposited material may eventually be subducted with the oceanic crust on which it has settled, at which point it will be integrated into any subductive-melt sourced minerals. There is a trend towards the chemical depletion of the mantle when it comes to certain elements which appears to be a one way street but the ocean does eventually give back a large percentage of what is dissolved into it.

> **Comments:**
> **username_0**: Thanks for your answer, though i fear you got my question wrong: I'd like to know whether the the road from (e.g.)(Ba,Ca,Na,K,NH4)(Al,B,Si)4O8(Fieldspar) to Salt, i.e. ionically bound metal+halogen is mainly one-way, or whether the non-salt minerals get re-formed after the precipitated salt is subducted again. It is not just about dissolved salt, but about salt in any configuration  
> **username_2**: @username_0 Sorry I hadn't quite said what I thought I had, try that one for size.  
> **username_0**: Thanks! Could you expand on the depletion of the mantle, e.g what elements tend to go, and at what rate?  
> **username_2**: @username_0 Not really, that's a large answer in it's own right, the concept of the [Primitive Mantle](https://en.wikipedia.org/wiki/Primitive_mantle) comes into play as does [Planetary Differentiation](https://en.wikipedia.org/wiki/Planetary_differentiation) and the [Goldschmidt series](https://en.wikipedia.org/wiki/Goldschmidt_classification). From memory the main elements that tend to go disproportionately in modern melts are iron, cobalt, and nickel, and of course historically the carbon, hydrogen, sulfur, and light alkali metals that form the bulk of the continental crust.  
> **username_3**: @username_2 no, nickel is extremely compatible in the mantle and iron weakly incompatible. Cobalt is controlled by sulfides.  
> **username_2**: @username_3 Mantle compatibility is not the issue, mineral formation in basalt melts is.  
> **username_3**: @username_2 since the minerals that exist in basalt (olivine and pyroxene) are the same that are in the mantle, it is a very similar issue.  
> **username_2**: @username_3 Except that Olivine and Pyroxene are what melts _out_ of the mantle not what the raw mantle is made of.  
> **username_3**: @username_2 olivine is present in both primitive mantle (unmelted) and depleted (melted out) mantle. Olivine is also a major early crystallising mineral in basaltic melts that result from melting of the mantle. If you're interested, I'm more than happy to thoroughly answer a question on the the mantle is made of, what remains after it's melted, and what crystallises out of the melt, and how it affects certain minor elements such as nickel.  
> **username_2**: @username_3 Except no-one has ever seen the primitive mantle, it hasn't existed since the late Hadean. Estimates on the make up of the modern mantle are debated, we think the upper mantle at least is peridotite but we have never yet sampled the mantle directly. You can give the same best guess answer I could if I cared to dig up my mineralogy notes, but I don't.  
> **username_3**: Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/85613/discussion-between-username_3-and-ash).  

---

