This is good fun, folks. Now that I have triggered this off with a
comment that included "text books", I figured I can get in again and
point people to the first few pages of Ch. 19 of Spear. Would make good
reading in this context. While you are at it, pieces of Winter, Clarke
and Vernon, and Philpotts and Ague, would add fun to the deal too. And
just in case you were about to, don't forget Bruce (I mean the book now)
either.
To finish cooking my spicy curry, here is some thought for the weekend:
Korzhinskii's rules says that if a ch. pot is constrained, the number of
phases get reduced by one. WE *interpret* constrained ch. pot to mean
open system, fluid transport etc. So it follows that if fluids flow, but
ch. pots do not become pinned by that, Korzhinskii has told us nothing
about the number of phases. I could not help noticing that in all of
Kurt's examples, the fluid was CO2. Not a great "constraint-er" of ch.
pot of Mg, Fe, Si and the like...?
I could not let such fun die out, could I?
Have a
On 10/12/2012 6:27 PM, Bruce Yardley wrote:
> Dear All
>
> All Kurt says is of course true, but, because you have to define components differently for the altered rock rather than the initial dunite, none of his examples involve a decrease in the number of degrees of freedom. In both cases, if more fluid flow occurs, one of the secondary phases will disappear and the number of degrees of freedom will increase. There are plenty of field examples that show this in marbles and ultramafic rocks. So really, we should say that infiltration metasomatism leads to an increase in the number of degrees of freedom, not a decrease in the number of phases.
>
> If you are baffled by that: the altered dunite started off with just olivine having constant ratios of Fe:Mg:Si (so they combine as 1 component). As soon as a trace of CO2-H2O fluid arrives (and some H2 is lost) it makes a phase which concentrates the Si (enstatite), a phase with just Mg (magnesite) and a phase with the Fe (magnetite). Now these 3 elements must be treated as separate components because their relative proportions are different in each mineral. So we have gone from a rock with 1 immobile component and 1 solid phase to a rock with 3 immobile components and 3 solid phases. For sure, it may have begun to experience metasomatism but at present it still has reserves of minerals available to react and modify the fluid composition to something defined by the rock. Eventually, if the rock is fortunate enough to experience even more fluid infiltration, all the olivine will go and the enstatite may then become carbonated. When the carbonation of enstatite is complete, the number of phases has been reduced by 1 because we now have another component acting as mobile (the Si is being leached out).
>
> And to get back to the original query which we lost sight of some time ago - there is some really nice recent work about alteration of sea floor rocks from Ron Frost (B.R. Frost if you are searching) and his collaborators.
>
> Bruce
>
> Professor Bruce Yardley
> School of Earth and Environment
> University of Leeds
> Leeds LS2 9JT, UK
>
> Tel: +44 (0)113 3435227
> Fax: +44 (0)113 3435259
>
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Kurt Bucher
> Sent: 12 October 2012 13:06
> To: [log in to unmask]
> Subject: [geo-metamorphism] Metasomatism
>
> Dear All,
>
> The metasomatism topic seems to be attractive. It has, at least, attracted sevral responses that have been adressed directly to me personally. This is of course not the purpose of a mailbase discussion.
>
> Anyhow: I have labeled the photomicrograph that was attached to my last mail. On this picture you can see two extremely resorbed olivine grains (olivine 1 and olivine 2) from the original dunite. The metasomatic reaction products are Mg-carbonate (magnesite) and Mg-silicate (enstatite). The dark opaque phase is magnetite (Fe-oxyde). It does not participate in the reaction. It is irrelevant for Korzinsky games.
>
> The frozen-in reaction recorded by the texture is: Olivine + CO2 = Enstatite + Magnesite.
> The reaction, as shown, is an efficient natural CO2-sequestration reaction. The CO2 from the gas/fluid phase is transferred to a solid ( magnesite).
>
> However, this was not my major point. The point is, that the metasomatic rock contains more minerals than the starting material.
>
> Cheers
> Kurt
>
--
***************** Sumit Chakraborty ****************************************
http://www.gmg.ruhr-uni-bochum.de/petrologie
Institut fuer Geologie, Mineralogie und Geophysik;
Ruhr-Universität Bochum; D-44780 Bochum; Germany
Email: [log in to unmask]
Tel: +49-(0)234-322 4395 / 8155
Fax: +49-(0)234-321 4433
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