Building combined subsolidus-suprasolidus phase diagrams

Dave Pattison, University of Calgary

Building combined subsolidus-suprasolidus phase diagrams(with implications for how you think about the behaviour of H2O in phase diagrams)

All assemblages +Bt+Qtz+Pl+Ilm+Mt, except where indicated by “-Qtz”, “-Bt”, “-Ilm”.H₂O present below wet solidus;H₂O absent above wet solidus.

MsChlGrt Ms GrtSt

MsGrtSil

MsSil

MsGrtLSil

MsGrtKyL

MsGrtKy

MsChl

KfsGrtLSil

MsLSil

MsCrdCrdKfs

CrdKfsL

GrtCrdKfsL

GrtCrdKfsL (-Bt)

KfsGrtLSil (-Bt)

KfsGrtRtLSil (-Ilm)

MsChlGrtRt (-Ilm)

MsGrtRt (-Ilm) MsGrtKyRt

(-Ilm)

MsChlGrtSt

MsGrtStAnd

MsGrtStSil

MsGrtStRt (-Ilm)

Average Buchan metapelite MnNCKFMASHTO; Fe3+/FeTotal = 0.15HP98 ds 5.5; 2005 a-X models

MsGrtStKy

MsChlRt

MsChlRtEp

MsChlEp

CrdKfsOpxL (-Bt)

GrtCrdOpxKfsL (-Bt)

KfsLSil

MsGrtStMsChlGrt

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MnNCKFMASHTO Fe3+/Fetotal = 0.15

Pattison & Goldsmith (submitted, J Geol Soc)


MsChlGrt Ms GrtSt

MsGrtSil

MsSil

MsGrtLSil

MsGrtKyL

MsGrtKy

MsChl

KfsGrtLSil

MsLSil

MsCrdCrdKfs

CrdKfsL

GrtCrdKfsL

GrtCrdKfsL (-Bt)

KfsGrtLSil (-Bt)

KfsGrtRtLSil (-Ilm)

MsChlGrtRt (-Ilm)


(-Ilm)

MsChlGrtSt

MsGrtStAnd

MsGrtStSil

MsGrtStRt (-Ilm)


MsGrtStKy

MsChlRt

MsChlRtEp

MsChlEp

CrdKfsOpxL (-Bt)

GrtCrdOpxKfsL (-Bt)

KfsLSil

MsGrtStMsChlGrt

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Subsolidus Suprasolidus

Excess H2O Fixed H2O

Pl + Qtz + Ms + H2O ±… = L

Pl + Qtz + Kfs + H2O ±… = L



Rationale: low grade mineral assemblages have a lot of hydrous minerals like clays, chlorite, muscovite. These get consumed in prograde dehydration reactions to produce 1) less hydrous minerals like garnet, cordierite, etc., and 2) free H2O fluid that escapes the rock. Exceptions (i.e., prograde H2O-consuming reactions) occur but are relatively rare.

Excess H2O = enough H2O to ensure the lowest grade, most hydrous mineral assemblages on the phase diagram are stable (usually those in the low-T, high-P corner of the phase diagram). In practice, this means adding enough H2O to saturate the entire phase diagram (have free H2O present as a phase everywhere on the phase diagram). Adding more H2O than needed to saturate the entire phase diagram makes no difference to the calculated phase equilibria, but having too little may result in incorrect phase equilibria –even if they don’t look obviously incorrect! You must check the phase diagram to be sure that H2O is present as a phase in every predicted subsolidus mineral assemblage.

Upshot: you do not use the measured H2O content or LOI (loss on ignition) of a subsolidus rock (e.g., a garnet-kyanite schist) to calculate a subsolidus phase diagram that extends to P-T conditions below those of the rock: the lower grade phase equilibria will be wrong once H2O is no longer present as a free fluid phase.

Example input file in Theriak-Domino (using HP ds5.5) for the illustrated subsolidus phase diagram:

SI(091.5798)TI(1.3637)AL(42.8147)FE(09.1486)F3(1.6145)MG(7.0338)MN(0.1416)CA(1.8735)NA(5.7763)K(8.6406)H(80)O(?)

Subsolidus phase diagram – excess H2O

Excess H

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Bulk(1)= SI(091.5798)AL(42.8147)FE(09.1486)F3(1.6145)MG(7.0338)MN(0.1416)CA(1.8735)NA(5.7763)K(8.6406)TI(1.3637)H(80)O(?)

22/11/2020 14:23:59 CPU-Time (hhh:mm:sec): 001:58:45.796

domino version: 01.08.09 (08.07.14, src10e1rf)database: ds55hp1.txtH2O: HHP98PLC1: site mixing+margulesWM02V: site mixing+margulesGTT01: site mixing+margulesBI05: site mixing+margulesCHL: site mixing+margulesILMTERN: site mixingMT00: site mixing+margulesOPX: site mixing+margulesEP98: site mixing+margulesCRD: site mixingST: site mixing+margulesCTD: site mixing+margulesLIQtc2: external+margules

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ds 5.5 Avg Buchan Fe3/FeT=0.15 H=80(Mainly) subsolidus phase diagram calculated with excess H2O (H2O = 40, same as H = 80)

Pl + Qtz + Ms + H2O ±… = L


Pattison (unpublished)

and liquid (melt)

Increasing amt of H2O needed to saturate low grade hydrous mineral assemblages

Moles of H2O contained in solid or melt phases - excess H2O (H2O = 40, same as H = 80)


Abundant H2O-bearing melt(not realistic)

Moles of H2O contained in solid or melt phases - not enough H2O to saturate (H2O = 12.5, same as H = 25)

and liquid (melt)

Maxed out H2O in solids (12.5) – no free H2O phase

Strange low grade mineral assemblages involving Grt, St, Ky!

Domain of H2O present as phase

Maxed out H2O in solids and melt (12.5) – no free H2O phase, but still too much melt


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ds 5.5 Avg Buchan Fe3/FeT=0.15 H=80

These suprasolidus reactions assume excess H2O! They do not correspond to natural reactions (except inunusual situations).

Therefore, delete all reactions above the wet solidus.

Pl + Qtz + Ms + H2O ±… = L


(Mainly) subsolidus phase diagram calculated with excess H2O (H2O = 40, same as H = 80)


Subsolidus phase diagram calculated with excess H2O (cleaned up but unlabelled, with some less relevant reactions removed )

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Suprasolidusdomain

Same details as forcompleted diagram

Rationale: in suprasolidus rocks above the wet solidus, all hydrous fluid is consumed by H2O-consuming melting reactions.

Because the porosity of high grade rocks is likely to be near-zero, so is the volume of free H2O fluid contained in the pores, meaning that imperceptible volumes of melt are produced at the wet solidus. That is why “excess H2O” above the wet solidus is not realistic, unless externally-derived fluid infiltration occurs when the rocks are at P-T conditions above the wet solidus.

In dehydration reactions above the wet solidus (e.g., Ms- or Bt-consuming suprasolidus reactions), the H2O released is dissolved into the melt phase, hence the term dehydration-melting reactions. Thus, in contrast to subsolidus reactions in which the H2O released in dehydration reactions escapes the rock, the H2O content of suprasolidus rocks is retained in the rock (in the melt phase) – that is, unless some of the melt escapes (next talk!).

Upshot: use a fixed H2O content for suprasolidus phase equilibria.

Complication: melt has a variable H2O content (higher H2O content at higher P, lower H2O content at lower P).Therefore, more than one fixed-H2O suprasolidus phase diagrams may need to be calculated, as explained below.

Suprasolidus phase diagram – fixed H2O

Pl + Qtz + Ms-or-Kfs + H2O ±… = L


MsChlGrt Ms GrtSt

MsGrtSil

MsSil

MsGrtLSil

MsGrtKyL

MsGrtKy

MsChl

KfsGrtLSil

MsLSil

MsCrdCrdKfs

CrdKfsL

GrtCrdKfsL

GrtCrdKfsL (-Bt)

KfsGrtLSil (-Bt)

KfsGrtRtLSil (-Ilm)

MsChlGrtRt (-Ilm)


(-Ilm)

MsChlGrtSt

MsGrtStAnd

MsGrtStSil

MsGrtStRt (-Ilm)


MsGrtStKy

MsChlRt

MsChlRtEp

MsChlEp

CrdKfsOpxL (-Bt)

GrtCrdOpxKfsL (-Bt)

KfsLSil

MsGrtStMsChlGrt

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Dehydration-meltingreactions

Pl + Qtz + Ms + H2O ±… = L


Suprasolidus domain


Fixed H2O suprasolidus phase diagram - Step 1.

Obtain the H2O content of the rock (contained in hydrous phases like muscovite and biotite) just below the wet solidus, i.e., modes x H-content (easy in T-D and Perplex, less so in Thermocalc?). Use this as a fixed value for the H2O content (or, in T-D, the H-content) of the rock. Typically one chooses a point on the solidus ~ in the middle of the pressure range of the diagram (in the example shown here, 3.6 kbar).

Example input file in Theriak-Domino (using HP ds5.5) for the illustrated phase diagram:


Note that the bulk composition is identical to the bulk composition for the subsolidus phase diagram except for the reduced H2O (H) content.

Fixed (non-excess) H

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Determine H2O content of rock on wet solidus (contained in hydrous phases like Ms, Bt):

P = 3.6 kbarT = 653 ⁰CH2O = 8 (H = 16)

This choice is not unique; other points on the solidus could have been chosen (more discussion below)

(Mainly) subsolidus phase diagram calculated with excess H2O (H2O = 40, same as H = 80)


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ds 5.5 Avg Buchan Fe3/FeT=0.15 H=16Suprasolidus phase diagram calculated with fixed (non-excess) H2O (H2O = 8, same as H = 16)

Note:

+ L (suprasolidus)

+ H2O phase (subsolidus)

no H2O phase (subsolidus)


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ds 5.5 Avg Buchan Fe3/FeT=0.15 H=16Suprasolidus phase diagram calculated with fixed (non-excess) H2O (H2O = 8, same as H = 16)

For this reduced-H2O bulk composition, these subsolidusreactions do not involve a free H2O phase, and so they do not correspond to natural reactions (in a prograde sense at least).

Therefore, delete all reactions below the wet solidus.


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Suprasolidus phase diagram (cleaned up but unlabelled)

Subsolidusdomain

Subsolidus phase diagram (cleaned up but unlabelled)

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Suprasolidusdomain

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Combined subsolidus-suprasolidus phase diagram (unlabelled)

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Note:

+ L (suprasolidus)

+ H2O (subsolidus)

no H2O (subsolidus)

Notice the blue line in the suprasolidus field – it is an H2O-consuming reactionabove the wet solidus(i.e., it is an artifact of the modelling, due to the progressively lower H2Ocontent of the melt)

This results in too much meltbeing generated at low P, which has a knock-on effect on the lower-P phase equilibria!

Suprasolidus phase diagram calculated with fixed (non-excess) H2O (H2O = 8, same as H = 16)

Complication

Fixed H2O suprasolidus phase diagram - Step 2.

Calculate another suprasolidus phase diagram, but with a fixed H2O content for a low pressure (e.g., 1.8 kbar).

Example input file in Theriak-Domino (using HP ds5.5) for the illustrated phase diagram:


Notice again that the bulk composition is identical to the bulk composition for the other phase diagrams except for the further-reduced H2O (H) content.

Fixed (non-excess) H

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Bulk(1)= H(6)O(?)FE(09.1486)F3(1.6145)SI(091.5798)AL(42.8147)MG(7.0338)MN(0.1416)CA(1.8735)NA(5.7763)K(8.6406)TI(1.3637)


Fe3/(FeTot) = 0.150domino vers ion: 01.08.09 (08.07.14, s rc10e1rdatabase: ds55hp1.txtH2O: HHP98PLC1: s i te mixing+margulesWM02V: s i te mixing+margulesGTT01: s i te mixing+margulesBI05: s i te mixing+margulesCHL: s i te mixing+margulesILMTERN: s i te mixingMT00: s i te mixing+margulesOPX: s i te mixing+margulesEP98: s i te mixing+margulesCRD: s i te mixingST: s i te mixing+margulesCTD: s i te mixing+margulesLIQtc2: external+margules

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+ L (suprasolidus)

+ H2O (subsolidus)

no H2O (subsolidus)

Notice the different phase equilibria involving Bt going to Opx between phase diagrams calculated for H2O=8 vs. H2O=3


Note:

Bt Opx


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Not

+ L (suprasolidus)

+ H2O (subsolidus)

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+L

+L

+L


Bt Opx

Notice the different phase equilibria involving Bt going to Opx between phase diagrams calculated for H2O=8 vs. H2O=3


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Bulk(1)= H(6)O(?)FE(09.1486)F3(1.6145)SI(091.5798)AL(42.8147)MG(7.0338)MN(0.1416)CA(1.8735)NA(5.7763)K(8.6406)TI(1.3637)


Fe3/(FeTot) = 0.150domino vers ion: 01.08.09 (08.07.14, s rc10e1rdatabase: ds55hp1.txtH2O: HHP98PLC1: s i te mixing+margulesWM02V: s i te mixing+margulesGTT01: s i te mixing+margulesBI05: s i te mixing+margulesCHL: s i te mixing+margulesILMTERN: s i te mixingMT00: s i te mixing+margulesOPX: s i te mixing+margulesEP98: s i te mixing+margulesCRD: s i te mixingST: s i te mixing+margulesCTD: s i te mixing+margulesLIQtc2: external+margules

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Position of solidus for three different assumed values of molar H2O (excess, 8 and 3)

Not


Yes, if focusing just (mainly) on the P-T conditions of that rock. But you need to be very careful if using the resultant phase diagram to make petrological inferences at higher and especially lower P-T conditions because of the H2O matter! Plus, good to compare thermodynamically-computed H-content of rock, based on the predicted hydrous phases, with the measured H2O/LOI content.

Can I use the measured H2O content of a suprasolidus rock as the value of the fixed H2O content for the phase diagram? (Or, use LOI - loss on ignition - if the rock isn’t too altered?)


MsChlGrt Ms GrtSt

MsGrtSil

MsSil

MsGrtLSil

MsGrtKyL

MsGrtKy

MsChl

KfsGrtLSil

MsLSil

MsCrdCrdKfs

CrdKfsL

GrtCrdKfsL

GrtCrdKfsL (-Bt)

KfsGrtLSil (-Bt)

KfsGrtRtLSil (-Ilm)

MsChlGrtRt (-Ilm)


(-Ilm)

MsChlGrtSt

MsGrtStAnd

MsGrtStSil

MsGrtStRt (-Ilm)


MsGrtStKy

MsChlRt

MsChlRtEp

MsChlEp

CrdKfsOpxL (-Bt)

GrtCrdOpxKfsL (-Bt)

KfsLSil

MsGrtStMsChlGrt

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The final result: a combined subsolidus-suprasolidus phase diagram



Documents

Building combined subsolidus-suprasolidus phase diagrams