ES 4971-4973 Petrology
Homework 9 Ch 10
Name _____________________
Part 1 Bemco Mining District
1. Define the following:
a. hydrothermal
b. caldera
c. sulfide
d. sulfate
e. soluble
f. ferrous
g. ferric
h. oxidizing
i. reducing
Part 2 Chapter 10, origin of Basalts
2. Partial melting of the mantle initially yields a magma that is
a. felsic (granitic) in composition
b. mafic (basaltic) in composition
3. Basalts in different Plate Tectonic settings are
a. chemically different
b. chemically identical
4. Which type of Basalt has common Olivine and no quartz?
a. Alkaline
b. Sub-alkaline (Tholeiites, etc)
5. Subalkaline basalts include Tholeiites and
a. Calc-alkaline basalts
b. Alkaline basalts
6. Fact: As Sub-alkaline magmas fractionate and their melts
change composition, different rock types solidify from the
melt, their lavas changing from Basalt to, eventually,
Rhyolite.
Question: In an AFM diagram, which is correct?
a. The Tholeiitic magmas curve towards ferro-basalts
before evolving toward Alkali compositions.
b. The Calc-alkaline magmas curve towards ferro-basalts
before evolving toward Alkali compositions.
7. Explain how diamonds reach the surface
8. Harzburgite and Dunite are solid residuum after partial
melting of Lherzolite. Harzburgite and Dunite are referred to
as being “deleted mantle”, depleted in Aluminum and
Titanium
a. True
b. False
9. A Dunite has
a. Olivine > 90%
b. Olivine < 40%
10.
Harzburgite has
a. less than 10% Clinopyroxene
b. less than 10% Orthopyroxene
11.
Peridotites (e.g. Lherzolite, Harzburgite and Dunite)
have
a. less than 40% Olivine
b. more than 40% Olivine
12.
Define adiabatic
13.
(Slide 24) When mantle rock begins rising
adiabatically, it is
a. initially solid
b. initially melted
14.
Name a plate tectonic setting when decompression
melting of the mantle would occur.
15.
If you add a phase to a system, say by adding water to a
the mantle near a subduction zone, the melting point of the
mantle
a. increases
b. decreases
16.
(Slide 29) In which setting does the temperature cross
the solidus at the greatest depth. (read the graphs)
a. MOR
b. rising plume
c. subduction zone
WRITE DOWN THE CROSSING To FOR EACH
17.
Fact: Increased pressure moves the ternary eutectic
minimum from the oversaturated tholeiite field to the undersaturated alkaline basalt field. The eutectic is the temperature
at which the melting starts.
Question: Therefore
a. Alkaline basalts are favored by greater depth of melting
b. Tholeiitic basalts are favored by greater depth of
melting
18.
Less partial melting, say 20 % melting at 60 km favors
the formation of an alkaline basalt, because incompatibles
(alkalis such as K+) go into the initial melts
a. True
b. False
19.
(Slides 43-45) Explain why the discontinuity between
~650-700 kilometers is considered to be a boundary to
convective flow, leading to two convection cell models of the
mantle.