7
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ Sometime the salt deposits can get covered by rock & locked away for long periods of time. Such deposits have been & still are mined in some countries. $\endgroup$ – Fred Oct 29 '18 at 13:33
  • 1
    $\begingroup$ @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. $\endgroup$ – bukwyrm Oct 29 '18 at 14:13
4
$\begingroup$

There is a limit to how much of a given salt can be dissolved into water, this is known as it's solubility and it's dynamic based upon how much of any particular 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 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.

$\endgroup$
  • $\begingroup$ 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 $\endgroup$ – bukwyrm Oct 29 '18 at 14:18
  • $\begingroup$ @bukwyrm Sorry I hadn't quite said what I thought I had, try that one for size. $\endgroup$ – Ash Oct 29 '18 at 14:28
  • $\begingroup$ Thanks! Could you expand on the depletion of the mantle, e.g what elements tend to go, and at what rate? $\endgroup$ – bukwyrm Oct 29 '18 at 15:19
  • 1
    $\begingroup$ @bukwyrm Not really, that's a large answer in it's own right, the concept of the Primitive Mantle comes into play as does Planetary Differentiation and the Goldschmidt series. 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. $\endgroup$ – Ash Oct 29 '18 at 15:54
  • $\begingroup$ @Ash no, nickel is extremely compatible in the mantle and iron weakly incompatible. Cobalt is controlled by sulfides. $\endgroup$ – Gimelist Nov 10 '18 at 21:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.