Dear All
Bruce made a few comments that inspired me to continue with the
metasomatism blog.
Bruce made the comment that an indication for extensive fluid flow and
fluid-rock reaction is a decreasing number of minerals in a rock leading
ultimately to monomineralic rocks.
I agree. Each externally controlled chemical potential reduces the
number of phases in the assemblage by one until only one (solid) phase
is left over.
An excellent example for this behavior can be visited in the Tromsø
region (where I spent two month in the "summer" this year) on Vannøya
where old Precambrian mafic volcanics have been metasomatized by various
processes until finally pure albite has been left over. The entire
process is called albitization and it is a classic.
On the other hand: In addition to my tremolite example (note that
dolomite = tremolite + calcite in the case of the Bergell example does
not change the Ca/Mg bulk ratio in contrast to Bruce's assumption) there
are other interesting examples where the general trend of multi-phase
assemblages (closed system) to a single-phase assemblage (metasomatic)
seems to be contradicted.
Dunite is a good starting material for this exercise: Fo + CO2 = En +
Mgs is a wunderfull metasomatic reaction (and recently has been
re-invented by the CO2 sequastration community). It produces a two phase
Opx and Mgs assemblage from olivine (single phase starting material). If
you don't belief this, have a look at the attached jpeg from the Ørnes
Body in the Svartisen Area of Norway (the picture is in my book,
however, I problably shoud publish this properly). The process is known
as sagvandite formation, the rock as sagvandite, after the type locality
at Sagelvvannet near Balsfjord close to Tromsø in Norway.
An even more common case is soapstone formation from serpentinite: Serp
+ CO2 = Tlc + Mgs + H2O. Formation of a metasomatic two-mineral
metasomatic reaction zone along fractures in monomineralic serpentinite.
Still all this does not contradict Korzinski. And I totally acknowledge
his fundamental contribution to petrology. The Korzinski publication
cited below has been crucial to my own scientific career.
Cheers
Kurt
Korzinski, D. S.: Physicochemical Basis of the Analisis of the
Paragenesis of Minerals, pp 142. London: Chapman & Hall 1959
Bruce Yardley wrote:
>Dear All
>
>It is always a pleasure to be able to debate with Kurt! Of course he is are right, it is possible to come up with scenarios which seem exceptional - in the case Kurt puts forward my first point is that you will only have a monomineralic dolomite starting material as a result of open system behaviour during diagenesis! But lets look in more detail: the pure dolomite can be viewed as a 2 component (CaMgO2 and CO2) 2 phase (dolomite, fluid) system if you allow for a CO2 pore fluid. Add silica in aqueous solution and recognise Ca and Mg are separate components because the ratio between them is no longer constant once tremolite and calcite develop, and you end up with a calcite-tremolite system of 5 components (CaO, MgO, CO2, SiO2, H2O) but only 3 phases (tremolite, calcite, fluid) so the apparent variance HAS actually increased as a result of infiltration, which is the underlying message of Korzhinsky's analysis.
>
>As an aside, although I have seen both tremolite and talc in veins cutting pure dolomitic marbles, I don't recall there being much calcite with them. Not surprising since under metamorphic conditions most fluids carry a lot more Ca than Si. Tremolite + calcite is a common assemblage where original quartz-bearing dolomite layers are infiltrated by small amounts of water, but then they follow bedding, not fractures.
>
>So in my view, despite the possibility of anomalies, the rules do work if you use them with understanding. Too complex for the average geologist? Maybe, but if you are attempting to understand the chemical effects of fluid flow, you are not an average geologist! Korzhinsky's approach is unique and allows you to evaluate what is happening independently from other approaches that might be used to identify zones of fluid flow. Never go into the field without it.
>
>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: 09 October 2012 18:20
>To: [log in to unmask]
>Subject: [geo-metamorphism] Metasomatism
>
>Dear all,
>
>Your lively discussion on metasomatism entertained me a lot. However,
>please consider the following not uncommon situation: Starting material
>is a pure monomineralic dolomite marble. The marble reacts with an
>SiO2-saturated fluid advecting on fractures cutting across the marble.
>The resulting metasomatic rock consists of about 70 vol.% tremolite and
>30 vol.% calcite. This means that the original rock consists of one
>phase (dolomite) and the metasomatic rock consists of two phases (Tr +
>Cal). This is a 100% increase in the number of phases from the source
>rock to the metasomatic rock. Say hello to Mr. Kozinski. Korzinski, of
>course, was not wrong. But the counting of components, phases, mobile
>components, independent variables and all that is to complex for the
>average geologist to be of any practical use.
>
>Cheers
>Kurt
>
>Bucher-Nurminen, K., 1981, The formation of metasomatic reaction veins
>in dolomitic marble roof pendants in the Bergell intrusion (Province
>Sondrio, Northern Italy). Am. Jour. Sci., 281, 1197-1222.
>
>Bucher-Nurminen, K., 1989, Reaction veins in marbles formed by a
>fracture-reaction-seal mechanism. European J. Mineral., 1, 701-714.
>
>Bucher, K., 1998, Growth mechanisms of metasomatic reaction veins in
>dolomite marbles from the Bergell Alps. Mineralogy and Petrology, 63,
>151-171.
>
>
>
--
Kurt Bucher (Prof. Dr.)
Institute of Mineralogy and Geochemistry
University of Freiburg
Albertstrasse 23b D-79104 Freiburg Germany
Phone 49-761-203-6395 (direct) 6396 (general office) 6407 (FAX)
http://www.minpet.uni-freiburg.de
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