1. Effects of Volatiles at High Pressure
Morse 1980
Szabó Ábel
2016. Február 11.
Bazalt és fázisdiagramjai PhD kurzus

2. Melting temperatures are lowered dramatically when H2O is added under pressure to a system of anhydrous silicate crystals. This is because a silicate melt can readily dissolve H2O, thereby reducing the volume of H2O from that occupied by a gas to that occupied by a liquid.
Water and carbon dioxide are the chief volatiles of interest for the earth's mantle.
The presence of the hydrosphere and atmosphere require degassing of the planet through time, so the effect of volatiles in magmas plays an important role in planetary evolution.

3. 5 kbar & 500˚C 99% Ab + 1% H2O = crystals + supercritical gas
water pressure equals the total pressure, because the gas always fills the container and therefore bears the load, hydrostatically, with the crystals → PH2O = PT (?)
purists regularly and insistently decry the term "water pressure" or "PH2O“;
another helpful device is to use Paq, meaning "the pressure of an aqueous fluid"
purists are tedious people, and everybody knows very well that the stuff is a complex solution, so let's just call it water pressure and assume that everybody will read this as a euphemism for whatever it really is.

4. Forsterite-H2O
The diagram shows the dry melting curve Fo = L on the right, and the H2O - saturated curve Fo + G = L + G running out to the left, to lower temperatures.

5. Forsterite-H2O
A liquid field in the shape of a gore shows the range of bulk compositions that occur as gas-undersaturated liquid. This field is bounded to the left by the liquidus, and to the right by a saturation curve denoting saturation of the liquid with an aqueous gas phase.
The solvus curve delimits the two-phase L + G field from the L field on the left and the G field on the right. The gas phase contains a nontrivial amount of silicate component.
The fields L and Fo + L in diagram are those for which PH20 is clearly less than the total pressure, 10 kbar, because a gas phase is absent from those fields. Elsewhere in the diagram, Paq = PT
Isobaric T -x section at 10 kbar showing the eutectic like melting of the system Fo - H2O. (It is not truly eutectic because the gas phase contains much more silica than magnesia, so neither the gas nor the liquid lie in the Fo - H2O join

6. Forsterite-H2O
P-T-X prism for the system Fo-H2O, showing isobaric sections at 10 and 20 kbar.

7. SiO2-H2O

8. SiO2-H2O
The dashed curve in this figure is the critical curve L = G running from KS-H, the critical end point for SiO2 - H2O, to KS, the critical point for SiO2. The wet melting curve generates invariant points at its intersections with the tridymite field, and it terminates without metastable extension at the triple point for SiO2
The absence of a metastable extension here is due to the fact that the hydrous melting reaction is undefined in the anhydrous system SiO2
The wet melting curve originates at the triple point for SiO2 can be deduced from the fact that gases of compositions H2O and SiO2 are completely miscible in all proportions, and the resulting fact that the G field in the limiting system SiO2 must be continuous with the G field in SiO2 - H2O.

9. SiO2-H2O

10. SiO2-H2O
Isobaric T-X section at 15 kbar for the system SiO2 H2O. After Nakamura (1974).

11. Enstatite-H2O
P-T projection of the melting relations of MgSiO2 - H2O.

12. Enstatite-H2O
Enstatite dissolves incongruently with H2O at 10 kbar to Fo + G. It also melts incongruently to Fo + L + G, as shown by the transition from the 1300 °C to the 1400 °C isothermal, isobaric section

13. Enstatite-H2O
The wet melting of enstatite at 20 kbar is compared with the dry melting behavior, at the same pressure. The 20 kbar diagram emphasizes the fact that addition of H2O at high pressure changes the melting behavior of enstatite from congruent to incongruent, thus destroying the thermal barrier present in the dry system. Ultramafic sills that show cotectic crystallization of olivine and bronzite must have crystallized from relatively dry liquids.

14. Enstatite-H2O
The wet melting of enstatite at 20 kbar is compared with the dry melting behavior, at the same pressure. The 20 kbar diagram emphasizes the fact that addition of H2O at high pressure changes the melting behavior of enstatite from congruent to incongruent, thus destroying the thermal barrier present in the dry system. Ultramafic sills that show cotectic crystallization of olivine and bronzite must have crystallized from relatively dry liquids.

15. Forsterite-Diopside-Silica-H2O
The incongruent melting of enstatite - H2O extends far into the system Fo-Di-SiO2-H2O. Mantle-like compositions in the triangle Fo-Di88-En88 will begin to melt, with H2O, to a liquid at the peritectic composition X, 1220°C. Such a liquid may be likened to a basaltic andesite; it is clearly silica-saturated in terms of its normative composition.
The shift of quartz- saturated equilibria toward the SiO2 corner is remarkable.

16. Forsterite-Diopside-Silica-H2O
The incongruent melting of enstatite - H2O extends far into the system Fo-Di-SiO2-H2O. Mantle-like compositions in the triangle Fo-Di88-En88 will begin to melt, with H2O, to a liquid at the peritectic composition X, 1220°C. Such a liquid may be likened to a basaltic andesite; it is clearly silica-saturated in terms of its normative composition.
The shift of quartz- saturated equilibria toward the SiO2 corner is remarkable.

17. Forsterite-Diopside-Silica-H2O
Yoder (1973) argued that the liquid composition Y was an adequate proxy for rhyolite, and used the 20 kbar diagram to explain the contemporaneous eruption of rhyolite and basalt from the same vent.

18. Forsterite-Diopside-Silica-H2O
The analysis requires a mantle source composed of two pyroxenes plus quartz. Such a composition would melt, with water, to a rhyolitic liquid Y at 960˚C. If the melt were fractionally removed, or removed in a batch when the TSC reached the joint En-Di, no further melting would occur until the temperature rose to 1220°C, supposing a new supply of H2O to be added to replace that removed in the rhyolitic liquid. The basaltic liquid would be generated at 1220°C until the TSC reached the tie line Enss - Fo, at which point melting would cease unless the temperature rose above 1220°C.

19. Forsterite-Diopside-Silica-H2O
BC in the field Enss + Diss + Q + H2O
TSC: BC → C
TLC: Y
TSC: C → D
TLC: X → Y (FL = 0.38.) Di
TSC: D → Fo
TLC: XY (FL = 0.38.) → E En
TSC: Fo → 0
TLC: E → BC

20. Albite-H2O
P-T projection for the melting of Ab-H2O is shown in Fig. The figure also shows three liquidus curves for the cases where insufficient H2O is present to saturate the liquid at all pressures; these are curves for Paq < PT
The curve Ab + G + L is, in principle, the solidus curve for all compositions; it is also the liquidus curve for vapor - saturated conditions.

21. Albite - H2O liquidus H2O-saturated solidus melt+vapor
BUT the only water available is 1-2%
contained in amphibole or mica
Albite example above assumed
10 wt% water
melt+vapor
BUT the only water available is 1-2% contained in amphibole or mica
Albite example above assumed 10 wt% water
(H2O solubility in albite-melt system)
Pressure-temperature projection of the melting relationships in the system albite-H2O.
From Burnham and Davis (1974)
Am. J. Sci., 274,

22. Albite-Orthoclase-H2O

23. Albite-Orthoclase-H2O
The entry of H2O into the liquid at high pressures so reduces the solidus temperature that it cuts into the Ab-Or solvus at 5 kbar. The reaction Ab + Sa + G = L occurs at 701°C at 5 kbar. The solvus crest rises with pressure, at a rate we shall take to be 18°/kbar. The intersection of the solidus (trace of the minimum temperature for Ab-Or-H2O) and the solvus critical line (trace of Tc with pressure) occurs at about 4.2 kbar, 715°C (Morse, 1970). However, this is an indifferent crossing because nothing happens there.

24. Albite-Orthoclase-H2O

25. Diopside-H2O and Diopside-H2O-CO2
The P-T projection for Di-H2O is shown in Fig. The wet melting curve is unusual in showing a minimum near 20 kbar, implying that at higher pressures, the volume change on melting is positive. At low pressures carbon dioxide enters silicate liquids only in small quantity; it thus lowers the melting temperatures only moderately, as shown by the Di-CO2 curve in Fig. When CO2 and H2O are mixed in a molar ratio CO2/(CO2 + H2O) = 0.8 and added to diopside, the solidus lies as shown by the dashed line in Fig. The effect of H2O, even in small amounts, clearly dominates the slope of the melting curve. The solubilities of H2O and CO2 in silicate melts are reviewed in detail by Mysen (1977).

26. Phlogopite-H2O
When a hydrous phase is heated under pressure, it will tend to decompose by dehydration to an assemblage of anhydrous crystals plus gas. The gas so released will tend to cause melting, leading to a water-poor melt which absorbs all the released H2O and is therefore far from being saturated with a gas phase. The case of the magnesian mica phlogopite furnished the first demonstration of this important principle (Yoder and Kushiro, 1969). An isobaric 10 kbar T-X projection of the relevant system is shown in Fig. In the absence of gas, the first liquid is generated at 1220°C from an assemblage of phlogopite with forsterite, leucite, and kalsilite. This liquid contains only about 2.5% H2O by weight.
Moreover, the diagram shows that upon crystallization from melts poorer in H2O than the phlogopite composition, the phlogopite extracts H2O from the melt, causing the latter to become even poorer in H2O. The initiation of melting in a phlogopite bearing mantle would presumably be like that on the H2O - poor side of phlogopite in the diagram. It is interesting to note that the H2O – poor melting temperature, 1220°C, is not very much higher than that of the gassaturated melting reaction at 1185°C. These two temperatures, in fact, shift past one another as the pressure varies

27. General P-T-X Relations for the Model System A-H2O

28. General P-T-X Relations for the Model System A-H2O

29. General P-T-X Relations for the Model System A-H2O

30. G-X Diagrams for Ab-Or-H2OT4
three segments: the outer two of which are concave up and the middle of which is concave down
a mechanical mixture of the two nodal compositions is minimized in G relative to homogeneous solid solution. T3
much the same, but the nodes have drawn closer together T2
the critical temperature, the nodes have united at the critical composition Xc, and the solid solution is now described in G-X space by a single concave up curve. T1
the system is at the minimum melting point

31. G-X Diagrams for Ab-Or-H2OT3
liquid has a high G relative to any solids and is therefore metastable: the system is solid
T2 = Te
the node of the liquid curve just touches the binodal tangent connecting the stable feldspar compositions, and this is the eutectic condition. At any temperature slightly above this, the binodal tangent is interrupted by two crystal-liquid tangents, which are minimized in G relative to feldspars on the solvus. The solvus is therefore metastable above T2.
It continues to be metastable up to the critical temperature, above which it does not exist.

32. Summary
The component H2O drastically reduces the beginning-of-melting (solidus) temperature.
The amount of liquid generated can be predicted from the lever rule: for example if the system contains 1% H2O and the first liquid contains 10% H2O, then 10% liquid will be generated at the isobaric invariant point involving S-L-G. This is generally accounted sufficient to cause magma separation by intergranular flow.
If 0.1% H2O is present in such a system, 1% melt will be generated. This is generally accounted insufficient for magma separation, but sufficient for seismic attenuation to yield the Low Velocity Zone (LVZ, asthenosphere) of the upper mantle (roughly km deep). This interpretation of the LVZ must be viewed with caution.