Hi,
this is a very interesting discussion and I've really enjoyed it.
I would welcome comments on something - I haven't seen it stated explicitly anywhere but suspect that either somebody must have worked it out before me, or that it is wrong. Either way, I'd like to know.
It relates to the minimum number of phases that can occur at equilibrium in a rock in which all compositional variables are internally controlled. i.e. without metasomatism.
If you consider any system that has no solid solutions in any of the minerals, and that is not compositionally degenerate (i.e. the composition of the rock does not lie on a tieline, or tie plane or volume, in larger dimensional space) then the maximum variance is two, i.e. the minimum number of phases is C. You can easily show this graphically for C=1, 2 and 3. If C=4 or higher, it gets harder to demonstrate but there is no reason the relationship would break down.
The minimum number of phases decreases by 1 for every exchange vector, e.g. FeMg-1, Tschermak's, and so on. This is how come we get garnet-omphacite in eclogites, there are just a lot of exchange vectors (a kind of internal degree of freedom). Again, you can see this graphically if you draw it up on a ternary plot. One exchange vector gives you two phase fields, two exchange vectors gives you one phase fields.
So, to generalise, the minimum number of phases is C minus the number of exchange components.
If P is smaller than this then at least one component must have been externally controlled. So what we have is a useful preliminary check for metasomatism.
cheers, Katy
Katy Evans
Applied Geology
Curtin University of Technology
GPO Box U1987
Bentley, WA 6845, Australia
+61 8 92664682
On 13/10/2012, at 12:50 AM, Sumit Chakraborty wrote:
> 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
>>
>
>
> --
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>
> ***************** Sumit Chakraborty ****************************************
> http://www.gmg.ruhr-uni-bochum.de/petrologie
>
> Institut fuer Geologie, Mineralogie und Geophysik;
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