Dear all,
just a short addition to what Bruce mentioned:
The infiltrated rock will act as a chromatographic column. That is, not
only the rockīs composition will change (eventually giving raise to pure
albite in Bruceīs case) but also the fluid, by traveling through the
rock and reacting with the mineral assemblage, will change its
composition. Depending on mass ratio, fluidīs composition will evolve
towards the composition defined by the rockīs mineral assemblage (in the
case given by Bruce the composition at the triple point). Once the fluid
composition is adjusted to the triple point it may move through rock
without leaving any further chemical fingerprint. On the other hand, any
new fluid pulse will first enter a rock that has already been albitized
and thus will pass this part of the rock without any change in
composition of neither the rock nor the fluid itself.
Best regards
Axel
Am 09.10.2012 09:53, schrieb Bruce Yardley:
> Dear Trisrota and all
>
> As far as I am aware the rules of thermodynamics have not changed since Sumit and I were young, and the Korzhinsky phase rule still stands. We ought to be able to make more of it nowadays because we understand a lot more about which elements can reach high concentrations in solution, but it has been largely forgotten. Put very simply, the argument is that the larger the fluid flux through a rock the fewer the minerals that will be present. If the mineral that is present is a solid solution, it will not show the chemical variation you might expect.
>
> A simple way of thinking about it is to imagine an activity diagram for a quartz-saturated system with axes that are, for example, log (aK+/aH+) and log (aNa+/aH+). There will be fields for the stability of K-feldspar, albite and muscovite (among others) and they will meet at a triple point. If a rock contains those 3 minerals and a small amount of fluid, then the fluid composition will be adjusted to that point. However if fluid is introduced from a different rock type, or even from the same lithology at a different temperature, it will have a different composition which will lie within the stability field of one of the minerals. Suppose the incoming fluid is higher in Na and has a higher pH than the fluid at the triple point; it will gain protons by converting muscovite to feldspar, and swap Na for K by albitising K-feldspar. Eventually, if enough fluid comes through, the rock will be composed only of albite, perhaps also with quartz. This reflects the relatively low mobility of Al and Si compared to Na and K.
>
> Personally, when I read accounts of rocks that have supposedly experienced large fluid fluxes but still have low variance assemblages, I am supicious of their veracity. If I can be forgiven some advertising, Springer are about to produce a book dealing with aspects of metasomatism, edited by Dan Harlov and Hakon Austreheim and in my chapter I have tried to remind readers of Korzhinsk's contributions.
>
> Bruce
>
> Professor Bruce Yardley
> School of Earth and Environment
> University of Leeds
> Leeds LS2 9JT
> UK
> Tel. +44 (0)113 343 5227
>
> ________________________________________
> From: Sumit Chakraborty [[log in to unmask]]
> Sent: 07 October 2012 12:28
> To: Metamorphic Studies Group; Bruce Yardley
> Subject: Re: [geo-metamorphism]
>
> Dear Trisrota and others,
>
> As Bruce points out, pure tremolite rocks are most likely derived from
> marble, and they are white....beautiful samples come from Romania, for
> example....should be very easy to check if your hand specimens are like
> this.
>
> This brings me, however, to another question that I have been thinking
> about for a while. When I was a student, admittedly quite some time
> back, we were taught that a monomineralic rock was a tell tale sign of
> open system behavior.
>
> This was used as a simple example of the use of the phase rule.....the
> general idea being (very loosely paraphrasing Korzhinskii, Thompson et
> al.) that there are obviously many elements in natural rocks and fluids,
> and so no matter how you define things, you tend to end up with many
> "components" (in the phase rule sense). Therefore, to get only one
> phase, one would have to fix several intensive variables in addition to
> P and T, and this was most readily accomplished by fixing / defining /
> constraining some chemical potentials by contact with an external
> reservoir ("fluid bath"). I am staying away here from using the somewhat
> more loaded term - "buffering", and from using the names of the variety
> of "components" that have been used in the literature (e.g. K-component
> etc.). That general thinking seemed like sound logic to me, and was used
> to explain all kinds of things from quartz veins in crustal metamorphic
> rocks to dunites in mantle rocks. "Tremolite rock" reminded me of this
> because these Rumanian samples always seemed to me to be a prime
> illustration of this - with all these elements available (Ca, Mg, Si,
> H,....), one can think of many minerals with simple structures that
> could have formed....instead, nature chose to make a monomineralic rock
> with one, single mineral with a relatively complex structure....because,
> several chemical potentials were constrained?
>
> Now we deal a lot more with fluid fluxes and open system behavior, but
> that line of reasoning appears to have disappeared, and I often find
> that people are not even aware of this kind of reasoning using the phase
> rule. Newer textbooks / notes do not deal with this either. I am
> wondering if there is a reason for this, and if some of you who have
> been more continuously involved with the evolution of the field have
> some insights about why / how thinking has evolved in this regard? I
> have followed the evolution only sporadically myself, more for teaching
> than for my own research.
>
> Sumit
>
>
>
>
> On 07.10.2012 12:26, Bruce Yardley wrote:
>> Dear Trisrota
>>
>> Are you sure it is tremolite? Pure tremolite rocks are normally derived from dolomitic marble, which seems unlikely with greywackes. In particular I think you need to have some idea of the Al-content, but just as important is a clear picture of the field relationships and whether there are veins or any metasomatic effects in other units nearby.
>>
>> Bruce Yardley
>>
>> Professor Bruce Yardley
>> School of Earth and Environment
>> University of Leeds
>> Leeds LS2 9JT
>> UK
>> Tel. +44 (0)113 343 5227
>>
>>
>> ________________________________
>> From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Trisrota Chaudhuri [[log in to unmask]]
>> Sent: 07 October 2012 06:46
>> To: [log in to unmask]
>> Subject: [geo-metamorphism]
>>
>> Dear all,
>> I've found some Archean, sheet like basic dykes which, when observed under thin section, is seen that they consist of 100% tremolite. They are associated with grewacke. Is there aby possibility that these mafics could be a part of metasomatized (rodingitized?) oceanic crust? Please enlighten me that how to explain the formation of such 100% tremolite bearing rocks.
>>
>> --
>> Trisrota Chaudhuri,
>> JRF of Indian Statistical Institute and Jadavpur University,
>> Kolkata, India.
>
> --
>
>
> ***************** Sumit Chakraborty ****************************************
> http://www.gmg.ruhr-uni-bochum.de/petrologie
>
> Institut fuer Geologie, Mineralogie und Geophysik;
> Ruhr-Universitaet Bochum;
> D-44780 Bochum; Germany
>
> Email: [log in to unmask]
> Tel: +49-(0)234-322 -4395 / -8521 / -8155
> Fax: +49-(0)234-321 4433
> ****************************************************************************
--
Dr. Axel Liebscher
Head of Centre for CO2 Storage
Phone: +49 (0)331/288-1553
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