GLY 4310 Spring 2020 2 Norman Levi Bowen Canadian geologist who was one of the most important pioneers in the field of experimental petrology Widely recognized for his phaseequilibrium studies of silicate systems as they relate to the origin of igneous rocks ID: 1021455
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1. 1Petrology Lecture 5Reaction Series and Melting BehaviorGLY 4310 - Spring, 2020
2. 2Norman Levi BowenCanadian geologist who was one of the most important pioneers in the field of experimental petrology Widely recognized for his phase-equilibrium studies of silicate systems as they relate to the origin of igneous rocksReaction principle. He recognized two types of reaction, continuous and discontinuous. (1922)1887 - 1956
3. 3Continuous Reaction
4. 4Discontinuous Reaction The second reaction was seen before in the phase diagrams shown in mineralogy What was that type of reaction called?
5. 5Name of reaction? This was the reaction
6. Bowen’s Reaction Series6
7. 7Gibbs Free Energy DefinitionWe can formulate a differential equation to represent changing geologic conditions: In igneous petrology, we are most often interested in the conditions involved at the liquid-solid phase boundary
8. 8Solid-Liquid ReactionConsidering a reaction between a solid and a liquid (S ↔ L) we can rewrite the previous equation asΔ represents a change as the result of a reaction - here, going from solid to liquid or vice versa
9. 9ΔVSince most solids are denser than their liquids at the melting point, ΔV is positive on going from solid to liquidWater is a notable exception
10. 10Melting ReactionSchematic P-T diagram of a melting reactionThis figure shows the behavior of an arbitrary phaseIn the region labeled “Solid” the solid phase is stable, because GS < GL In the region labeled “Liquid” the liquid phase is stable, because GS > GL
11. 11Isobaric SystemBecause Sliquid > Ssolid, the slope of G vs. T is greater for the liquid than the solidAt low temperatures the solid phase is more stable, but as temperature increases, the liquid phase becomes stable
12. 12Equilibrium TemperatureRelationship between Gibbs Free Energy and temperature for the solid and liquid forms of a substance at constant pressure.Teq is the equlibrium temperature
13. 13Isothermal SystemBecause Vliquid > Vsolid, the slope of G vs. P is greater for a liquid than a solid
14. 14Equilibrium PresssureV is positive, and therefore the slope of (δG/δP) is positiveThe liquid phase has lower G, and is thus more stable, at low pressure, but the solid phase is more stable at higher pressureThis is why the inner core is solid!
15. 15Equilibrium CurveAny two points on the equilibrium curve for a solid-liquid interface must have ΔG = 0, and therefore dΔG = 0Substituting gives
16. 16Clapeyron EquationRearranging the previous equation gives:
17. Diopside – Anorthite System17
18. 18Fluid SaturationA fluid-saturated melt contains the maximum amount of dissolved volatile species possible at a given set of P-T-X conditionsAny increase in volatile content will produce one or more additional phases
19. 19Fluid PressureThe fluid pressure (Pf) is used to define the state of volatiles in a meltIf Pf = Ptotal, the melt is saturated with volatilesIf Pf = 0, the system does not contain volatiles, and is often called “dry”
20. 20Le Châtlier’s PrincipleAny change imposed on a system at equilibrium will drive the system in the direction that reduces the imposed change
21. 21Melting of Hydrous MineralsAdding water to the system should cause melting, according to Le Châtlier’s PrincipleAdding water drives the reaction from left to rightRemoving water, such as by loss of volatiles near the surface, should cause crystallization
22. 22H2O SolubilitySolubility of H2O at 1100°C in three natural rock samples and albiteAfter Burnham (1979)
23. 23Albite – H2OEffect of H2O saturation on the melting of albiteAfter Burnham and Davis, 1974Dry melting curve from Boyd and England, 1963
24. 24Melting of AlbiteThis reaction has a large negative ΔV on going from left to right, thus stabilizing the liquid phase and lowering the melting pointAt higher pressures, ΔV is less negative, and the slope of the line is less
25. 25Application of Clapeyron EquationFor the dry case, ΔV is positive, and the slope of the melting curve is positiveFor the wet case, ΔV is negative, and the slope of the melting curve is negative (melting point is depressed with increasing pressure)
26. 26Melting of GabbroEffect of H2O saturation on the melting of gabbro (Burnham and David, 1974)Dry melting curve from Boyd and England (1963)
27. 27Melting CurvesH2O saturated curves are solidH2O free curves are dashedMafic rocks have higher melting points than felsic rocks
28. 28Albite – H2O SystemPressure-temperature projection of the melting relationships in the system albite – H2OAfter Burnham and Davis, 1974 Red curves = melting for a fixed mol % water in the melt (Xw) Blue curves tell the water content of a water-saturated melt
29. 29Albite Melting PercentagePercentage of melting for albite with 10 mol % H2O at 0.6 GPa as a function of temperature along traverse e-i
30. 30Albite – H2O SystemPressure-temperature projection of the melting relationships in the system albite – H2OAfter Burnham and Davis, 1974
31. 31Melting RelationshipsPressure-temperature projection of the melting relationships in the system albite – H2O with curves representing constant activity of H2OAfter Burnham and Davis, 1974
32. 32Diopside-Anorthite LiquidusThe affect of H2O on the diopside-anorthite liquidus
33. 33Albite Melting with FluidsExperimentally determined melting of albiteDryH2O saturatedIn presence of fluid containing 50% each of H2O and CO2
34. 34CO2 SolubilitySystemPressureCO2 SolubilityAlbite-H2O-CO22 GPa5-6%Enstatite-H2O-CO22 GPa18%Diopside-H2O-CO22 GPa35%
35. 35Ternary EutecticEffect of volatiles on ternary eutectic in the system Forsterite – Nepheline – Silica at 2 GpaWater moves the (2 GPa) eutectic toward higher silica, while CO2 moves it to more alkaline typesNeFoEnAbSiO2Oversaturated(quartz-bearing)tholeiitic basaltsHighly undesaturated(nepheline-bearing)alkali olivinebasaltsUndersaturatedtholeiitic basaltsCO2H2OdryP = 2 GPa
36. Ternary EutecticEffect of Pressure on the position of the eutectic in the basalt systemIncreased pressure moves the ternary eutectic (first melt) from silica-saturated to highly undersat.alkaline basalts36NeFoEnAbSiO2Oversaturated(quartz-bearing)tholeiitic basaltsHighly undesaturated(nepheline-bearing)alkali olivinebasaltsUndersaturatedtholeiitic basalts3GPa2GPa1GPa1atmVolatile-free