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Petrology Lecture 5

Petrology Lecture 5. Reaction Series and Melting Behavior GLY 4310 - Spring, 2014. Norman Levi Bowen. Canadian geologist who was one of the most important pioneers in the field of experimental petrology

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Petrology Lecture 5

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  1. Petrology Lecture 5 Reaction Series and Melting Behavior GLY 4310 - Spring, 2014

  2. Norman Levi Bowen • Canadian 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 rocks • Reaction principle. He recognized two types of reaction, continuous and discontinuous. (1922) • 1887 - 1956

  3. Continuous Reaction

  4. Discontinuous Reaction • The second reaction was seen before in the phase diagrams shown in mineralogy • What was that type of reaction called?

  5. Name of reaction? • This was the reaction

  6. Bowen’s Reaction Series

  7. Gibbs Free Energy Definition • We 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. Solid-Liquid Reaction • Considering a reaction between a solid and a liquid (S ↔ L) we can rewrite the previous equation as • Δ represents a change as the result of a reaction - here, going from solid to liquid or vice versa

  9. ΔV • Since most solids are denser than their liquids at the melting point, ΔV is positive on going from solid to liquid • Water is a notable exception

  10. Melting Reaction • Schematic P-T diagram of a melting reaction • This figure shows the behavior of an arbitrary phase • In 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. Isobaric System • Because Sliquid > Ssolid, the slope of G vs. T is greater for the liquid than the solid • At low temperatures the solid phase is more stable, but as temperature increases, the liquid phase becomes stable

  12. Equilibrium Temperature • Relationship between Gibbs Free Energy and temperature for the solid and liquid forms of a substance at constant pressure. • Teq is the equlibrium temperature

  13. Isothermal System • Because Vliquid > Vsolid, the slope of G vs. P is greater for a liquid than a solid • The liquid phase has lower G, and is thus more stable, at low pressure, but the solid phase is more stable at higher pressure • This is why the inner core is solid

  14. Equilibrium Presssure • V is positive, and therefore the slope of (δG/δP) is positive.

  15. Equilibrium Curve • Any two points on the equilibrium curve for a solid-liquid interface must have ΔG =0, and therefore dΔG = 0 • Substituting gives

  16. Clapeyron Equation • Rearranging the previous equation gives:

  17. Diopside – Anorthite System Figure 6-11.Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), American Journal of Science, 40, 161-185.

  18. Fluid Saturation • A fluid-saturated melt contains the maximum amount of dissolved volatile species possible at a given set of P-T-X conditions • Any increase in volatile content will produce one or more additional phases

  19. Fluid Pressure • The fluid pressure (Pf) is used to define the state of volatiles in a melt • If Pf = Ptotal, the melt is saturated with volatiles • If Pf = 0, the system does not contain volatiles, and is often called “dry”

  20. Le Châtlier’s Principle • Any change imposed on a system at equilibrium will drive the system in the direction that reduces the imposed change

  21. Melting of Hydrous Minerals • Adding water to the system should cause melting, according to Le Châtlier’s Principle • Adding water drives the reaction from left to right • Removing water, such as by loss of volatiles near the surface, should cause crystallization

  22. H2O Solubility • Solubility of H2O at 1100°C in three natural rock samples and albite • After Burnham (1979)

  23. Albite – H2O • Effect of H2O saturation on the melting of albite • After Burnham and Davis, 1974 • Dry melting curve from Boyd and England, 1963

  24. Melting of Albite • This reaction has a large negative ΔV on going from left to right, thus stabilizing the liquid phase and lowering the melting point • At higher pressures, ΔV is less negative, and the slope of the line is less

  25. Application of Clapeyron Equation • For the dry case, ΔV is positive, and the slope of the melting curve is positive • For the wet case, ΔV is negative, and the slope of the melting curve is negative (melting point is depressed with increasing pressure)

  26. Melting of Gabbro • Effect of H2O saturation on the melting of albite (Burnham and David, 1974) • Dry melting curve from Boyd and England (1963)

  27. Melting Curves • H2O saturated curves are solid • H2O free curves are dashed • Mafic rocks have higher melting points than felsic rocks

  28. Albite – H2O System • Pressure-temperature projection of the melting relationships in the system albite – H2O • After 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. Albite Melting Percentage • Percentage of melting for albite with 10 mol % H2O at 0.6 GPa as a function of temperature along traverse e-i

  30. Albite – H2O System • Pressure-temperature projection of the melting relationships in the system albite – H2O • After Burnham and Davis, 1974

  31. Melting Relationships • Pressure-temperature projection of the melting relationships in the system albite – H2O with curves representing constant activity of H2O • After Burnham and Davis, 1974

  32. Diopside-Anorthite Liquidus • The affect of H2O on the diopside-anorthite liquidus

  33. Albite Melting with Fluids • Experimentally determined melting of albite • Dry • H2O saturated • In presence of fluid containing 50% each of H2O and CO2

  34. CO2 Solubility

  35. Ne P = 2 GPa CO2 H2O dry Ab Highly undesaturated (nepheline-bearing) alkali olivine basalts Oversaturated (quartz-bearing) Undersaturated tholeiitic basalts tholeiitic basalts Fo En SiO2 Ternary Eutectic • Effect of volatiles on ternary eutectic in the system Forsterite – Nepheline – Silica at 2 Gpa • Water moves the (2 GPa) eutectic toward higher silica, while CO2 moves it to more alkaline types

  36. Ne Volatile-free 3GPa 2GPa 1GPa Ab Highly undesaturated (nepheline-bearing) 1atm alkali olivine basalts Oversaturated (quartz-bearing) Undersaturated tholeiitic basalts tholeiitic basalts Fo En SiO2 Ternary Eutectic • Effect of Pressure on the position of the eutectic in the basalt system • Increased pressure moves the ternary eutectic (first melt) from silica-saturated to highly undersat.alkaline basalts

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