Review Questions for Chapter 10:
Mantle Melting and the Generation of Basaltic Magma
1. Name as many sources of mantle samples as you can.
2. Use Figure 2.2c to describe the differences between dunite, harzburgite, and lherzolite.
15
Tholeiitic basalt
Wt.% Al2O3
3. What does Fig. 10.1 suggest about the relationship
between dunite, harzburgite, and lherzolite
samples?
10
a
rti
Pa
ing
elt
lM
5
um Lherzolite
idu
s
ReHarzburgite
0 Dunite
0.0
T C
o
0
1000
2000
3000
Plagioclase
Spinel
0.2
0.4
Wt.% TiO2
0.6
Figure 10.1. From Brown and Mussett (1993).
Liquid
Garnet
Lherzolite
idus
Liqu
m
ther
10
Depth (km)
200
Geo
P (GPa)
5
400
15
Solid
High-Pressure
Phases
us
20
600
Figure 10.2. Phase diagram of aluminous lherzolite with melting
interval (gray), sub-solidus reactions, and geothermal gradient.
4. What features in Fig. 10.2 suggest that mantle
melting is not a “normal” process in the “typical”
Earth?
0.8
5. You described the REE diagrams on the left
in the review questions in Chapter 9.
Suppose you demonstrated that the two
rocks represented primary melts. Using
Fig. 10.2, what could you say about the
depth of melting for each? Explain.
6. If melting is not a “normal” process, under what circumstances can it be accomplished? Note the
principles and limitations for each method.
7. As H2O-undersaturated rocks are heated by burial
along the shield geotherm in Fig. 10.6, what
should occur at point d? Will any melt be
generated? Why? If not, when will melt be
generated, and why?
6
4
160
c
R
gw
in g
ie
Sh
ld
he
ot
e
g
1
a
Oce
0
400
600
100
80
m
2
120
Depth (km)
d
ol e
140
o
eo o d
th O
er c e
m an
amp hi
b
3
H2O-s
aturate
P (GPa)
us
e
ite
p hlogop
5
d solid
180
r
a b
60
rm
he
eot
g
n
800
40
1000
1200
1400
T oC
Figure 10.6. Phase diagram (partly schematic) for a hydrous mantle
system, including the H2O-saturated lherzolite solidus, the
dehydration breakdown curves for amphibole and phlogopite, plus the
ocean and shield geotherms.
8. As H2O-undersaturated rocks are heated by burial along the ocean geotherm in Fig. 10.6, what
occurs at point a? Will any melt be generated? Why? If not, when will melt be generated, and why?
9. In our discussions of partial melting in conjunction with the ternary eutectic phase diagrams in
Chapter 7, melting begins at the eutectic point and continues as melt increments are removed until
what occurs? What happens as heating continues? How might such a process affect the termination
of melting in some source area of the mantle?
10. In what principal ways does a tholeiitic basalt differ from an alkaline basalt?
11. Use Figures 7.27, 9.3 and 10.8 to suggest three plausible ways that might tend to yield an alkaline
basalt rather than a tholeiite by partial melting of a chemically homogeneous mantle. (there is more
room on the next page, if required)
10
8
Rb
6
4
2
Sr
0
0
0.2
0.4
0.6
0.8
1
F
Figure 9.3. Change in the concentration of Rb and
Sr in the melt derived by progressive batch melting
of a basaltic rock.
Nepheline
E 3 GPa
Volatile-Free
Jadeite
E 2 GPa
E 1 GPa
Albite
Figure 7.27. Effect of volatiles on the ternary eutectic
point (minimum melt composition) in the system FoNe-SiO2 at 2 GPa.
Highly undersaturated
(nepheline-bearing)
alkaline basalts
E 1 atm
d
ate
tur salts
a
s
a
r
de c b
Un leiiti
o
h
t
Forsterite
Enstatite
Oversaturated
(quartz-bearing)
tholeiitic basalts
Silica
Figure 10.8. Change in the eutectic (first melt) composition with
increasing pressure from 1 to 3 GPa projected onto the base of
the basalt tetrahedron. All but the low-T ends of cotectic curves
have been omitted to avoid clutter.
12. Although it is possible to derive both tholeiites and alkaline basalts via partial melting of a
chemically homogeneous mantle, and “Ockham’s razor” (or the “law of parsimony”) suggests that
such a simple explanation is most likely to be correct, what about Fig. 10.13 below suggests that
the mantle is not a chemically uniform “fertile” mantle?
1000
a.
Rock/Chondrite
Rare Earth Elements
100
OIB
10
MORB
1
La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu
1000
b.
Spider Diagram
Rock/Chondrite
OIB
100
10
MORB
1
Rb Ba Th U Nb Ta K La Ce Sr NdSm Zr Ti Gd Y
Figure 10.13. REE diagram (a) and spider diagram (b) of a typical
alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge
basalt (MORB).
13. The “Mantle Array” in Figure 10.15a below shows the common range of isotopic analyses for
oceanic island basalts. These basalts represent relatively primitive mantle melts from throughout
the ocean basins, where the contaminating effects of thick ancient continental crust is not a factor.
What would be the simplest model for the sub-oceanic mantle that could explain the narrow linear
array? Explain how such a model could produce the array. You may want to refer to your answers
to review questions 9.46, 9.47, and 9.51 on Sr and Nd isotopes.
14. Discuss the relative merits of the whole-mantle vs. two-layer convection models.