Dear Trisrota
In a word, yes. No possible protolith has the Ca:Mg ratio of tremolite, so monomineralic tremolite rocks must be a) metasomatic and b) have undergone metasomatism at the time that the tremolite grew.
If the ultimate precursor was igneous you would hope to find a little Fe left and also low mobility "igneous" elements like Cr, whereas if it was originally dolomite it could be very pure tremolite.
Bruce
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: 13 October 2012 12:48
To: [log in to unmask]
Subject: [geo-metamorphism]
Dear all,
Kindly forgive me for questioning again.
So can I take it as granted that sent percent tremolite bearing rock is unlikely to be a product of close system equilibrium assemblage except the cases of skerns? Because fortunately or unfortunately I didn't get any calcite and/or dolomite. So, I have to discard this possibility. And about the possible protolith even if we consider even it was pyroxenite/diopsidite, at least water has to be added to the system (?).
Diopside, reacting to H2O and CO2 can produce Tremolite and calcite. If I'm not getting calcite/dolomite, again this extra Ca theoritically has to leave the system (open system process again?).
On Oct 9, 2012 1:47 PM, "Axel Liebscher" <[log in to unmask]<mailto:[log in to unmask]>> wrote:
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]<mailto:[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]<mailto:[log in to unmask]>] On Behalf Of Trisrota Chaudhuri [[log in to unmask]<mailto:[log in to unmask]>]
Sent: 07 October 2012 06:46
To: [log in to unmask]<mailto:[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
FAX: +49 (0)331/288-1529
Email: [log in to unmask]<mailto:[log in to unmask]>
Helmholtz Centre Potsdam
GFZ German Research Centre For Geosciences
Public Law Foundation State of Brandenburg
Telegrafenberg, 14473 Potsdam
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