John and all,
   Metamophology?  a "fist" indeed!  Does that contrast with Ignology?   I
am also forwarding this message to JC's favorite website, that on ignology.
   Personally I think that the term "dehydration melting" is misleading as
it could refer either to the vapor absent or the vapor present case.  Of
course, with enough dilution of H2O by CO2 one may reach the locus of the
vapor-absent reaction that is more or less equivalent to the vapor present
analogue. The melt temperatures reported by Kaszuba and Wendlandt are
similar to those obtained for vapor absent melting of hornblende.
Eric




>Dear Metamophologists,
>	Occasionally I have been known to stray into your "territory",
>where my contributions are generally unwelcome, I suppose.
>Nevertheless, I would appreciate your views on the following paper
>(abstract and reference below).
>	In discussing partial melting reactions I have always used the
>term "fluid-absent" for reactions such as Bt + Qtz + Pl = Opx + Kfs +
>melt. This is because, in Shreinemakers bundles, we conventionally
>label equilibria according to the absent phase. I use the word
>"fluid" in Holloway's sense - the low-density volatile phase, usually
>in the COHS system. Hence these reactions are fluid-absent. I believe
>that C. Wayne Burnham or Dave Eggler may have been the fist to use
>this terminology. More recently, but still a good while ago, Alan
>Thompson coined the phrase "dehydration melting reactions" to mean
>precisely the same thing.
>	One can debate the relative merits of the two terms, and I have
>done so. However, the fact remains that they both refer to melting
>reactions that involve the breakdown of a hydrous phase, with
>transfer of the OH in the crystals to "H2O" dissolved in the melt,
>without the presence of a free volatile fluid phase at any stage.
>This, naturally, contrasts with fluid-present melting reactions in
>which hydrous phases also break down (e.g. Bt + Qtz + Pl + fluid =
>Opx + melt). Therein lies the rub! The experiments reported below
>seem to have all been carried out in the presence of fluids in the
>COH system. Thus I am puzzled by the writers' use of the phrases
>"Dehydration melting in the system alkali basalt-H2O" and
>"Dehydration melting in the system alkali basalt-H2O-CO2". Have the
>writers redefined Alan's term? Have they perhaps misunderstood the
>term? Either way this is not a trivial problem because misuse of
>terminology has led many a researcher up a blind alley. Any
>enlightenment would be appreciated.
>Cheers,
>JC
>
>
>Effect of carbon dioxide on dehydration melting reactions and melt
>compositions in the lower crust and the origin of alkaline rocks
>Kaszuba JP, Wendlandt RF
>JOURNAL OF PETROLOGY
>41: (3) 363-386 MAR 2000
>Abstract:
>Dehydration melting experiments of alkali basalt associated with the
>Kenya Rift were performed at 0.7 and 1.0 GPa, 850-1100 degrees C, 3-5
>wt % H2O, and f(O2) near nickel-nickel oxide. Carbon dioxide [X-CO2 =
>molar CO2/(H2O + CO2) = 0.2-0.9] was added to experiments at 1025 and
>1050 degrees C. Dehydration melting in the system alkali basalt-H2O
>produces quartz- and corundum-normative trachyandesite (6-7.5 wt %
>total alkalis) at 1000 and 1025 degrees C by the incongruent melting
>of amphibole (paragasite-magnesiohastingsite). Dehydration melting in
>the system alkali basalt-H2O-CO2 produces nepheline-normative
>tephriphonolite, trachyandesite, and trachyte (10.5-12 wt % total
>alkalis). In the latter case, the solidus is raised relative to the
>hydrous system, less melt is produced and the incongruent melting
>reaction involves kaersutite. The role of carbon dioxide in alkaline
>magma genesis is well documented for mantle systems. This study shows
>that carbon dioxide is also important to the petrogenesis of alkaline
>magmas at the lower pressures of crustal systems. Select suites of
>continental alkaline rocks, including those containing phonolite, may
>be derived by low-pressure dehydration melting of an alkali
>basalt-carbon dioxide crustal system.
>--
>
>
>
>John D. Clemens
>Professor of Geosciences, Director - Centre for Earth and Environmental
>Science Research,
>Editor in Chief - Electronic Geosciences
>http://link.springer.de/link/service/journals/10069/index.htm
>School of Earth Sciences and Geography, CEESR, Kingston University,
>Penrhyn Rd, Kingston-upon-Thames, Surrey, KT1 2EE, UK
>phone: +44 (0)20 8547-7023       fax: +44 (0)20 8547-7497
>e-mail: [log in to unmask]
>personal web page:
>http://www.kingston.ac.uk/geolsci/staff/clemens/jdclemens.htm
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>Our passions cannot alter the facts, only hide them from us.
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