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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