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11
Hi,
I am new to MELTS and am trying to model viscosity variations in a 'wet' granitic magma. I am using the MS Excel based GUI for Rhyolite MELTS. I cannot find a tab in the results which shows the computed viscosity after equilibration. Could anyone tell me what I am doing wrong?
Regards,
Ritabrata
12
Dear Anna,

Negative values of end-members is normal and not a matter of concern. You should only be concerned if the quantity of some actual element were negative. Let's look at why you get negative end-members so you can understand my answer. The pyroxene components are:

diopside                CaMgSi2O6
clinoenstatite        Mg2Si2O6
hedenbergite        Ca(Fe2+)Si2O6
alumino-buffonite  Ca(Ti0.5Mg0.5)(AlSi)O6
buffonite              Ca(Ti0.5Mg0.5)(Si(Fe3+))O6
essenite               CaFe3+SiAlO6
jadeite                 NaAlSi2O6

So, for starters, consider how to make the perfectly legal and legitimate pyroxene ferrosilite, (Fe2+)2Si2O6. It isn't one of the end-members, but you can make it by a linear combination of them: ferrosilite = clinoenstatite + 2 hedenbergite – 2 diopside. So that means, in normalized percent terms, ferrosilite is

diopside clinoenstatit  hedenbergite alumino-buffo     buffonite      essenite       jadeite
    -200              100               200                   0                0                0               0

In general, any low-Ca, high-Fe2+ pyroxene is going to have negative diopside (geometrically, this happens if it plots below the diagonal from enstatite to hedenbergite in the pyroxene quadrilateral and so is outside the enstatite-hedenbergite-diopside triangle).

For the non-quadrilateral components, you will note that Al-Bf has Al and Ti. Bf has Fe3+ and Ti. Ess has Al and Fe3+. By linear combinations of these three, you can make whatever amount of Al, Ti, and Fe3+ you need. But the abundances of the end-members may well be negative. Consider a pyroxene that contains tetrahedral Al but is free of either Ti or Fe3+. To make that, you have to have positive Al-Bf and Ess and negative Bf (to cancel out the Ti and Fe3+ that come along with the Al in Al-Bf and Ess).

Hope that helps!

-- Paul
13
Greetings colleagues!

I know it's a lot massages from me on the forum last weeks.
I've read the topic about nagative poroptions of end memebers in spinel that is published here a time ago, but I observe the large values of negative propotions and it's confusing.
What is the right way to interpretate this data?

Quote
Title: osyb9

T = 1205,00 (C)  P = 9,000 (kbars)  log(10) f O2 = -7,58  delta HM = -4,92  NNO = -0,07  QFM = 0,00  COH = 1,83  IW = 4,11

Constraint Flags: fO2 path = QFM 

Liquid           mass = 27,95 (gm)  density = 3,09 (gm/cc)  viscosity = 1,49 (log 10 poise)     (analysis in wt %)
       G = -388332,13 (J)  H = -280978,92 (J)  S = 72,63 (J/K)  V = 9,05 (cc)  Cp = 38,42 (J/K) 
        SiO2   TiO2  Al2O3  Fe2O3  Cr2O3    FeO    MnO    MgO    NiO    CoO    CaO   Na2O    K2O   P2O5    H2O    CO2    SO3 Cl2O-1  F2O-1
       41,29   5,16  13,04   2,97   0,00  23,16   1,04   3,10   0,00   0,00   7,64   2,61   0,00   0,00   0,00   0,00   0,00   0,00   0,00

clinopyroxene    mass = 30,84 (gm)  density = 3,39 (gm/cc)     (analysis in mole %)
                 cpx Na0,06Ca0,63Fe''0,41Mg0,61Fe'''0,12Ti0,05Al0,41Si1,72O6
                 G = -463007,38 (J)  H = -356018,61 (J)  S = 72,38 (J/K)  V = 9,09 (cc)  Cp = 35,52 (J/K) 
           diopside clinoenstatit  hedenbergite alumino-buffo     buffonite      essenite       jadeite
              -6,76         31,38         41,26         16,54         -7,09         18,89          5,77

clinopyroxene    mass = 14,56 (gm)  density = 3,42 (gm/cc)     (analysis in mole %)
                 cpx Na0,03Ca0,32Fe''0,62Mg0,79Fe'''0,09Ti0,02Al0,35Si1,77O6
                 G = -212375,73 (J)  H = -161371,57 (J)  S = 34,51 (J/K)  V = 4,25 (cc)  Cp = 16,95 (J/K) 
           diopside clinoenstatit  hedenbergite alumino-buffo     buffonite      essenite       jadeite
             -53,03         64,95         62,06         13,29         -9,29         18,66          3,35

feldspar         mass = 26,90 (gm)  density = 2,65 (gm/cc)     (analysis in mole %)
                 K0,00Na0,47Ca0,53Al1,53Si2,47O8
                 G = -459947,06 (J)  H = -361093,18 (J)  S = 66,88 (J/K)  V = 10,14 (cc)  Cp = 33,22 (J/K) 
             albite     anorthite      sanidine
              46,73         53,27          0,00

Total solids     mass = 72,31 (gm)  density = 3,08 (gm/cc)
                 G = -1135330,17 (J)  H = -878483,36 (J)  S = 173,76 (J/K)  V = 23,49 (cc)  Cp = 85,69 (J/K) 

Viscosity of the System cannot be computed.

System           mass = 100,26 (gm)  density = 3,08 (gm/cc)
                 G = -1523662,30 (J)  H = -1159462,28 (J)  S = 246,39 (J/K)  V = 32,54 (cc)  Cp = 124,11 (J/K) 

Oxygen           delta moles = 0,00145398  delta grams = 0,0465258
                 G = -495,59 (J)  H = 57,85 (J)  S = 0,37 (J/K)  V = 178,69 (cc)  Cp = 0,05 (J/K) 
14
Thank you for answers!

Quote
What is the aegirine content of the pyroxene that MELTS predicts?

Now MELTs predicts no aegerine.
As I understand from the answers It's more about features of Cpx solid solution model is used in MELTs code.

I've attached here the Cpx compositions are predicted by MELTs and initial parameters.

Quote
Initial parameters:
Initial Composition: SiO2 46,8800
Initial Composition: TiO2 2,0500
Initial Composition: Al2O3 15,3600
Initial Composition: FeO 15,6000
Initial Composition: MgO 6,1800
Initial Composition: CaO 10,7900
Initial Composition: Na2O 2,4700
Initial Temperature: 1400,00
Final Temperature: 900,00
Initial Pressure: 9000,00
Final Pressure: 9000,00
Increment Temperature: 5,00
Increment Pressure: 0,00
dp/dt: 0,00
log fo2 Path: FMQ

Here is a link to file with Cpx compositions: https://dropmefiles.com/xkaHh
15
Some versions of the MELTS code, including the rhyolite-MELTS GUI, have a separate "aegirine" phase. But this is just a dependent end member (jadeite + buffonite – alumino-buffonite) that was used in testing during the development of the pyroxene solid solution model (see Sack & Ghiorso 1994c, though note that it is referred to as "acmite" there). Aegirine will never crystallize, unless you suppress pyroxene, because the pyroxene solid solution should be more stable for natural compositions.

[There is also sometimes a "fayalite" phase, that should also be ignored.]

What is the aegirine content of the pyroxene that MELTS predicts?

Paula
16
Anna,

I cannot think of any way to do that. You can turn off cpx, but you can't turn off components of cpx except by leaving their ingredients out of the bulk composition. But even if you remove all the Al and Ti, MELTS can still make a Fe3+-bearing clinopyroxene by linear combination of Buffonite + Essenite – Al-Buffonite. Your best bet is to work around the tendency of MELTS to incorporate a lot of Fe3+ in clinopyroxene by artificially raising the Fe3+ content of your bulk composition until you saturate with an additional phase, perhaps it will be aegirine. You might also artificially inflate the Na content. I realize you aren't then really predicting phase equilibria in your actual target bulk composition, but the resulting model might still give you a guide to trends that are interesting and useful.

-- Paul
17
Greetings colleagues!

Is it possible to turn off buffonite and essenite end members of clinopyroxene in rhyolite-MELTs?
In my samples I have a significant amount of Fe3+ that provides aegerine existence (it's approved by recalculation of composition). But MELTs places all of the ferric into buffonite and essenite so aegerine is absent.
Aegerine containig is really important for our study.
18
Thanks for your reply.
I will try the procedure as your advice.
If there's progress, I'll show it.
19
You are right that in the paper and also in Ghiorso and Gualda (2013), which is cited, it is a(TiO2)liquid-rutile that they are estimating i.e. everything is referenced to the standard state of rutile. a(TiO2)liquid-rutile is independent of the liquid standard state and is related to affinity of rutile as you mention in your reply.

It turns out that there is a mistake in the standard state of TiO2 in the liquid in rhyolite-MELTS (inherited in the MELTS liquid from The Oxide Handbook [Samsonov 1982] where Tfus is listed as 1870 K when it should be 1870 oC). Fortunately this doesn't affect the logic of the Ti-in-zircon paper as mu0(TiO2)liquid ends up cancelling out. It does mean that the activity of TiO2 in the rhyolite-MELTS output file may be slightly off but the mu(TiO2)liquid should be fine. The chemical potentials were calibrated on a wide range of mineral-liquid constraints for natural composition liquids.

Note that in pMELTS the standard state of TiO2, and some of the other end members in the liquid, are deliberately tweaked by adjusting Sfus so that the model behaves better in the compositional range of interest (i.e. for partial melting of peridotite and mafic pyroxenite). So the melting point of rutile is not reproduced there either, but this time by design. Likewise the melting point of quartz is not matched in rhyolite-MELTS because the standard state properties of the solid quartz and sanidine are adjusted in order to reproduce the granite ternary minimum. There's no such thing as a free lunch!

I think Ghiorso and Gualda (2013) used MELTS-batch (which is the MELTS implementation used in the Magma Chamber Simulator package). It can be used to output mu(TiO2)liquid and mu(TiO2)rutile in a relatively automated way. You could also try MELTS for Excel, which does record the affinities, though the traffic on the MELTS for Excel server is quite heavy at the moment with more people working from home than normal. Or you can take the affinities displayed in the bottom panel of the graphical user interface.

Alternatively, alphaMELTS for MATLAB/Python can be used for this calculation, and alphaMELTS 2 will be able to do it soon (just a bit hectic at the moment so I haven't had time to hook up the "thermodynamic output file" option).

Paula

20
Thanks a lot.
I know the difference between affinity and activity.
There is an equation that can transform affinity to activity.
a(TiO2)=e^(-A(TiO2)/R*T)
Anyway, thank you very much for your answer.



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