3
$\begingroup$

Recently I was reading 'How to build a habitable planet' by Langmuir and Broecker and in chapter 15 they speak about how Life has led to a difference in oxidation potential in the atmosphere, essentially leading to the earth becoming a fuel cell. They also spoke about how this led to certain ions being oxidized - ie uraninite to uranyl oxide, iron to hematite.

I was wondering if this could mean that as atmospheric composition would affect the sea floor, when this subducted it would affect the melt of the mantle - as far as I'm aware, it is the dehydration of the subducting slab which allows for the hydration, and therefore melt, of the mantle in regions such as Japan. If the subducting slab's composition is affected by the atmosphere, could this have any effect on the composition of the igneous rocks formed? Would this be detectable?

Thank you.

$\endgroup$
  • $\begingroup$ Related: What, if any, paleoclimate data can be derived from igneous rocks? $\endgroup$ – Gimelist Jul 13 '17 at 9:50
  • $\begingroup$ Also, the answer is yes. Carbonatites are rare igneous rocks formed by melting of carbonated mantle. That carbon can be introduced by subduction of marine sediments. Oxidation state is another thing. Magmas formed by melting of oxidised rocks tend to be more oxidised, and this affects the composition and fractionation trends of minerals like magnetite, ilmenite and other mafic minerals. $\endgroup$ – Gimelist Jul 13 '17 at 9:59
  • $\begingroup$ Thanks! With regard to the oxidation state, could something like the Fe/S ratio also be affected? While Fe and S would both get oxidised in the mantle, their precipitation out of the ocean depends on the oxygen present - would this have a notable effect? $\endgroup$ – user1150512 Jul 13 '17 at 10:14
  • $\begingroup$ you're opening a can of worms here - the oxidation state of Fe in the mantle is still a highly debated topic (mostly Fe2+ though, Fe3+ is minor). Sulfur is usually reduced as sulfide, and not sulfate. This is much more complicated and long than can be written down in a comment here. $\endgroup$ – Gimelist Jul 13 '17 at 10:34
  • $\begingroup$ Thank you! Do you know of any relevant papers I could look through? Cheers. $\endgroup$ – user1150512 Jul 13 '17 at 14:20
1
$\begingroup$

I figure you're looking for more exotic examples, and some geologists may well have some offhand, but I want to say "yes" because of the example of carbonatite rocks described in this geology page from San Diego State University:

Carbonatites are perhaps the most unusual of all lavas. They are defined, when crystalline, by having more than 50% carbonate (CO3-bearing) minerals, and typically they are composed of less than 10% SiO2.

Which would rely on the carbon cycle and the deposition of atmospheric CO2 onto the seafloor by biological processes.

However:

Some scientists believe that the Ca-carbonatites are generated by fractional melting of crustal carbonate rocks. However, others believe that the Ca-carbonatites are not primary magmas at all, but rather derived from the alteration of Na-rich natrocarbonatite lavas.

There is an interesting paper in GeoScienceWorld titled Atmospheric K-feldspar as a potential climate modulating agent through geologic time that addresses your topic as the author tries to estimate K-feldspar (Kfs) flux through geologic time -- which addresses your question from the other direction: assessing atmospheric content (in some cases simple dust deposition) over billions of years by evaluating its traces in the geological record.

$\endgroup$
  • 1
    $\begingroup$ Carbonatites are my favourite rock! And the topic of my PhD research... $\endgroup$ – Gimelist Jul 13 '17 at 10:00
  • $\begingroup$ @Michael Knowing things is like cheating! Any suggestions to improve, withdraw, or replace my answer are welcome. $\endgroup$ – jeffronicus Jul 13 '17 at 15:25
  • $\begingroup$ That's the problem - there are so many things we don't know about carbonatites :) $\endgroup$ – Gimelist Jul 13 '17 at 23:06

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.