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?
a. liquid
b. solid
c. liquids plus solids
3. If the system of composition a
cools to 1700C, what phases
are present?
a. Forsterite
b. melt
c. Forsterite plus melt
4. 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.
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Problem: redo the example above for liquid composition c, shown above
between x and b at 1400C.
5. At the ternary Eutectic, liquid is in equilibrium with three solids. There
a. the liquid temperature stays constant until all the liquid freezes
b. the liquid temperature continues to fall before all the liquid freezes
6. Increasing pressure
a. increases volume
b. decreases volume
7. Increasing pressure will
a. raise the melting point..
b. lower the melting point.
8. Adding water to a system
a. lowers the melting point..
b. raises the melting point
9. 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)
a. more than 200 GPa
b. no more than about 2.15 GPa
10. 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
a. Tholeiitic Basalt
b. Alkaline Basalt
11. Increased pressure moves the ternary eutectic (first melt) from silica-saturated
Tholeiitic Basalts to highly undersaturated alkaline basalts.
a. True
b. False
12. Water in a melt moves the 2 GPa eutectic toward higher silica Basalts, while CO2
moves it to more alkaline types.
a. True
b. False