ES 4971-4973 Petrology

Homework 7

Name ______

  1. Define Cotectic
  2. In the Di-An-Fo ternary phase diagram shown below, if the system is at 2000C, what is the state?
  3. liquid
  4. solid
  5. liquids plus solids
  6. If the system of composition a

cools to 1700C, what phases

are present?

  1. Forsterite
  2. melt
  3. Forsterite plus melt
  1. Example: At the 1500C melt temperature shown at

composition x, what is the ratio of

melt to Fo crystals? The bulk is a.

Hint: compare the lengths a to Fo and x to a

a. I get a-Fo 42mm and x-a 25 mm

in an enlarged diagram.

Notice liquid x is closer to the fulcrum bulk composition a, so it is the predominant phase (it is the heavier end of the see-saw)

42/25 = liquid/solid = 1.68 is the ratio of liquid to solid

b. To get the weight %

The total of 42 + 25 is 67

42 liquid / total 67 = 0.628 or 62.8% liquid by weight. `

Problem: redo the example above for liquid composition c, shown above between x and b at 1400C.

  1. At the ternary Eutectic, liquid is in equilibrium with three solids. There
  2. the liquid temperature stays constant until all the liquid freezes
  3. the liquid temperature continues to fall before all the liquid freezes
  4. Increasing pressure
  5. increases volume
  6. decreases volume
  7. Increasing pressure will
  8. raise the melting point..
  9. lower the melting point.
  10. Adding water to a system
  11. lowers the melting point..
  12. raises the melting point
  13. Granite is a rock of the continental crust. Continental crust has an average density of 2.75 x 103 kg/m3 and is at most 80 x 103 m thick (half that is a more typical max). As such, granite occurs at pressures of (use 80km)
  14. more than 200 GPa
  15. no more than about 2.15 GPa
  16. Two Basalt samples from the big island of Hawaii have common Olivine, interstitial alkali feldspars or Feldspathoids, no quartz, and Plagioclase grows later (at cooler temps) and is uncommon. This is a
  17. Tholeiitic Basalt
  18. Alkaline Basalt
  19. Increased pressure moves the ternary eutectic (first melt) from silica-saturated Tholeiitic Basalts to highly undersaturated alkaline basalts.
  20. True
  21. False
  22. Water in a melt moves the 2 GPa eutectic toward higher silica Basalts, while CO2

moves it to more alkaline types.

  1. True
  2. False