Dear All,
Bruce made a very good point. The externally controlled chemical
potentials are the key to understanding metasomatism, not the number of
phases. But didn't I say that too in one of my earlier contributions?
A few extra comments regarding my earlier examples follow:
a) the Dunite example: Ignore the Fe-component and magnetite for
simplicity. The starting rock has one component and one phase namely
forsterite. Adding CO2 to the system means adding a new component and
thus a new phase. Now we have two solid phases En + Mgs. If you prefer
to consider CO2 as a phase present in the rock, then we have Fo + CO2 in
the starting rock and En + Mgs + CO2 in the final rock. The chemical
potential of CO2 controls if Fo is present or En + Mgs. The provided
texture picture shows the rock progressing from one (Fo) to the other
(En + Mgs) state. By definition this is not "truly" a metasomatic
reaction and the product is not "truly" a metasomatic rock because the
states are controlled by the chemical potential of a volatile component
(CO2). Metasomatism should change the volatile-free bulk rock
composition. I think.
b) Therfore let us return to the tremolite rock example I started with.
The Bergell tremolite + calcite veins are compositionaly clearly
different from the starting dolomite rock. The externally controlled
chemical potential is µSiO2. Still the game is the same. Starting rock
has one phase and one component (dolomite)(+ a CO2-H2O fluid if you
wish). Adding two components (to the dolomite), SiO2 (to make tremolite)
, CaO (to make independent Cal and Dol) gives us three components and
thus three solid phases (in addition to the volatiles) should be present
in the general case. However, we find only two solid phases, temolite +
calcite in the new rock, because the chemical potental of SiO2 is
externally controlled . So the Tr+Cal veins are true metasomatic rocks.
If the vein forming fluid is understaurated with respect to calcite,
then the resulting material is monomineralic tremolite (which, I think,
was the original starting point for the discussion). In this case an
additional component is externally controlled (e.g. µCaO). The remaining
component, in this case µMgO, is still internally controlled (by the
presence of tremolite). I think.
If also the last component is externally controlled (eg. below the
saturation level of tremolite) the metasomatic rock contains no minerals
(this is a good one !!). Examples for this situation are e.g. karst
caves, leaching cavities, central open zones in reaction veins and
similar transport and reaction (dissolution) features (porosity).
But I probably should stop thinking about metasomatism for a while.
Cheers
Kurt
Bruce Yardley wrote:
>Dear All
>
>All Kurt says is of course true, but, because you have to define components differently for the altered rock rather than the initial dunite, none of his examples involve a decrease in the number of degrees of freedom. In both cases, if more fluid flow occurs, one of the secondary phases will disappear and the number of degrees of freedom will increase. There are plenty of field examples that show this in marbles and ultramafic rocks. So really, we should say that infiltration metasomatism leads to an increase in the number of degrees of freedom, not a decrease in the number of phases.
>
>If you are baffled by that: the altered dunite started off with just olivine having constant ratios of Fe:Mg:Si (so they combine as 1 component). As soon as a trace of CO2-H2O fluid arrives (and some H2 is lost) it makes a phase which concentrates the Si (enstatite), a phase with just Mg (magnesite) and a phase with the Fe (magnetite). Now these 3 elements must be treated as separate components because their relative proportions are different in each mineral. So we have gone from a rock with 1 immobile component and 1 solid phase to a rock with 3 immobile components and 3 solid phases. For sure, it may have begun to experience metasomatism but at present it still has reserves of minerals available to react and modify the fluid composition to something defined by the rock. Eventually, if the rock is fortunate enough to experience even more fluid infiltration, all the olivine will go and the enstatite may then become carbonated. When the carbonation of enstatite is complete, th
>
>
>e number of phases has been reduced by 1 because we now have another component acting as mobile (the Si is being leached out).
>
>And to get back to the original query which we lost sight of some time ago - there is some really nice recent work about alteration of sea floor rocks from Ron Frost (B.R. Frost if you are searching) and his collaborators.
>
>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: 12 October 2012 13:06
>To: [log in to unmask]
>Subject: [geo-metamorphism] Metasomatism
>
>Dear All,
>
>The metasomatism topic seems to be attractive. It has, at least, attracted sevral responses that have been adressed directly to me personally. This is of course not the purpose of a mailbase discussion.
>
>Anyhow: I have labeled the photomicrograph that was attached to my last mail. On this picture you can see two extremely resorbed olivine grains (olivine 1 and olivine 2) from the original dunite. The metasomatic reaction products are Mg-carbonate (magnesite) and Mg-silicate (enstatite). The dark opaque phase is magnetite (Fe-oxyde). It does not participate in the reaction. It is irrelevant for Korzinsky games.
>
>The frozen-in reaction recorded by the texture is: Olivine + CO2 = Enstatite + Magnesite.
>The reaction, as shown, is an efficient natural CO2-sequestration reaction. The CO2 from the gas/fluid phase is transferred to a solid ( magnesite).
>
>However, this was not my major point. The point is, that the metasomatic rock contains more minerals than the starting material.
>
>Cheers
>Kurt
>
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>
--
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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