Dear Doug, and all,
as far as I understand it, because of the difficulties to set up a good
plagioclase model, using pure ab is an approximation to represent
ordered albitic plagioclase below the low/high ab transition. But at
least, used with the plagioclase model it allows to model the
peristerite gap. In my experience, as far as I remember, the plagioclase
coexisting with pure albite is commonly low-An oligoclase, as it should
be in the peristerite solvus.
Also, I'm not sure it's clear to everyone following the discussion,
"MTlow" / "MThi" does not refer to this problem, but the different
models of magnetite.
All the best,
Pavel
Le 07/05/2018 à 23:54, Dugald Carmichael a écrit :
> Hmmm... seems thermodynamically inconsistent, unless perhaps the HP2003 plagioclase-solution-model by itself does predict a small T-X field of pure ab coexisting with albitic plagioclase?
>
> -----Original Message-----
> From: Doug Tinkham [mailto:[log in to unmask]]
> Sent: May 7, 2018 2:52 PM
> To: Dugald Carmichael <[log in to unmask]>; [log in to unmask]; 'Metamorphic Studies Group' <[log in to unmask]>
> Subject: Re: Phase diagrams with THERIAK-DOMINO
>
> Not formally, no. At low T for many pelite/psammitic compositions, going up temperature you do predict the sequence pure ab at low T, a small pure ab field coexisting with albitiic plagioclase, followed by plagioclase solid solution at higher T. The HP 2003 plagioclase solution model by itself does not predict a peristerite gap.
>
> Doug
>
> On 2018-05-07, 2:45 PM, "Dugald Carmichael" <[log in to unmask]> wrote:
>
> May I ask does the "MTlow" solution-model take adequate account of the field-postulated "peristerite-solvus" between albite and oligoclase in metamorphic plagioclase?
>
> Dugald
>
> -----Original Message-----
> From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Doug Tinkham
> Sent: May 7, 2018 1:35 PM
> To: [log in to unmask]
> Subject: Re: Phase diagrams with THERIAK-DOMINO
>
> Hi Gisela
>
> You might also try a slightly older version of Theriak-Domino from my website (if you are using Windows) from http://dtinkham.net/peq.html. I haven't had time to put up the mac binaries yet. That version does a slightly better job for some of the problematic mode zero boundaries. See attached. The 2017 version sometimes has more of a problem, and all of those horizontal lines trying to connect different boundaries is a sign of the problem.
>
> You will rarely remove all of the squiggles or incomplete lines when working in a 9-11 component system over a large P-T range like this. A good approach is to use both Theriak-Domino (TD) and thermocalc in combination, using TD to get the majority of the fields determined, and then use thermocalc to construct the final diagram. In fact, you could just use Theriak to calculate the equilibrium assemblages at individual P-T points, and then use thermocalc to calculate the diagram with just limited knowledge of the equilibrium assemblage and phase compositions at a few points. Manual construction in thermocalc is an excellent way to learn phase equilibria even though it is more time consuming.
>
> Finally, if you are going to calculate down to low temperatures like you are, you should activate the ab pure end-member, which might have inadvertently been deactivated in that file.
>
>
> Doug
>
>
>
> On 2018-05-07, 8:06 AM, "Metamorphic Studies Group on behalf of Erik Duesterhoeft" <[log in to unmask] on behalf of [log in to unmask]> wrote:
>
> Hi Gisela,
>
> you are using the wrong solution model. MTlow should be used at
> greenschist up to amphibolite grade. If you are looking at suprasolidus
> conditions use the MThi model. I hope this will solve your problem.
>
> Cheers,
>
> Erik
>
> --
> Erik Duesterhoeft
> Institute of Geosciences,
> University of Kiel
> Ludewig-Meyn-Str. 10
> D-24118 Kiel
> Room 454
>
>
> Am 5/7/18 um 11:50 AM schrieb Gisela Leoz Munte:
> > Dear colleagues,
> >
> > I'm having some trouble with the generation of some phase diagrams
> > with the program THERIAK-DOMINO. I'm trying to plot the composition of
> > a High-K mica rich metapelitic schist and of a qz rich semipelitic
> > gneiss in aims to constrain their P-T metamorphic conditions. After
> > calculating the local bulk composition of my samples, the outputed
> > diagrams show some problematic areas with some strange instabilities
> > (see those lines interrupting the diagram; see attached images) of
> > wich I cannot establish its origin, as I find myself unable to locate
> > the problem that causes them.
> >
> > Has anyone encountered this before? If so, anyone knows how can I fix
> > the problem? I'm using the td-tcds62-6axmn-03.txt database, and I've
> > been able to plot the T-X diagram with it, but not the P-T diagrams.
> >
> >
> > Thanks in advance! Any help would be highly appreciated
> >
> >
> > Gisela Leoz
> >
> >
>
>
>
>
>
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