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
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