At 04:10 PM 2000/05/18 +0600, Nina Volkova wrote:
>...
>Help me with references concerning the examples of diffusion metasomatism...
>...
Shohei Banno responded:
>...
>Theoretical part is well written in Korzhinskii. ...
The prevailing theory of metasomatic zoning is flawed by a usually tacit
assumption that pressure is constant and uniform regardless of whatever
local volume-changes may be dictated by the metasomatic reactions. In rock
systems this assumption is not tenable, because rocks and minerals resist
deformation. Any metasomatic reaction that would tend to change the volume
is resisted by an induced stress field, and the stress field induces
secondary chemical-potential gradients that drive secondary mass transfer.
Provided that the secondary mass transfer is not the rate-limiting step in
the overall metasomatic process, the induced stress field tends to relax
towards constant and uniform pressure and the metasomatism tends to take
place at virtually constant volume. Thus nature neatly circumvents the
strength of rocks and minimizes the mechanical work of metasomatic
alteration (Carmichael 1986, Induced stress and secondary mass transfer:
Thermodynamic basis for the tendency toward constant-volume constraint in
diffusion metasomatism. in Helgeson HC editor, Chemical Transport in
Metasomatic Processes, NATO ASI Series C 218, 239-264).
This natural tendency toward constant-volume constraint in metasomatism was
understood by Lindgren as early as 1912 (Econ Geol 8, 521-535). In 1947
Ramberg provided a sound thermodynamic explanation for the induced-stress
part of the story (Geol Foren Forhandl 69, 189-194), and in his 1952
textbook the secondary-mass-transfer part is also clearly explained. But
despite additional explanations by Maliva & Siever (1988 Geology 16,
688-691) and Merino et al (1993 Am J Sci 293, 135-155), respectable journals
still promulgate a flawed theory of metasomatic zoning that is capable of
predicting only primary mass transfer with uninhibited changes of volume at
constant and uniform pressure (e.g. Joesten 1991 Am Mineral 76, 743-755; Guy
1993 Eur J Mineral 5, 317-399; Lichtner & Balashov 1993 Geochim Cosmochim
Acta 57, 369-387; Ashworth & Sheplev 1997 ibid 61, 3671-3689; Rusinov &
Zhukov 1998 Geochem Internat 36, 934-947).
Dugald
Dugald M Carmichael Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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