Geology assignment about describing graph and show crystallization and Bowmen series
Examine the graph below and answer the following questions.
Points:
145
The graph shows the chemical composition (as weight %) of the liquid magma remaining after
10 stages of fractional crystallization.
The mineral crystallizing are ones that you know from Bowen’s Reaction Series. There are no
trick minerals.
15-
A103
10
Wt % Oxide
s
cao
Feot Fezo
Mgo
5
Na, o
:
.
.
KO
0
45
50
65
70
75
55 60
Wt % SiO2
1. Why is SiO2 used as the X-axis? Explain your answer.
(3)
GEOL286
Take-Home Test 2
NAME:
2a. What was the starting composition of the magma? Plot it carefully as a small “1” on the
accompanying TAS diagram and complete the name in the table.
(1)
2b. Plot each other stage (numbered 2-10) on the TAS diagram. Then complete the table. (9)
Na,0 + K/O wt%
15
phonolite
13
tephr
trachyte qk204)
2a. What was the starting composition of the magma? Plot it carefully as a small “1” on the
accompanying TAS diagram and complete the name in the table.
(1)
2b. Plot each other stage (numbered 2-10) on the TAS diagram. Then complete the table. (9)
Na,O + KO wt%
15
phonolite
tephr
phonolice
trachyte q20%)
phono-
tephrite
7
foidice
trachy
andesne
thyolite
1 ephine
7
basaltic
Tokio
basanite
andesite
trachy
(ob 10%) basalt
5
dacite
anciesite
basaltic
andelite
3
basak
piro
basalt
1
37
41
45
53
57
61
65
69
7)
Sio, wt%
Stage
SiO2 wt%
TAS Classification Name
1
47.5
2
3
4
5
6
7
8
9
10
GEOL286
Take-Home Test 2
NAME:
3a. At what SiO2 value does an Fe-rich mineral begin to form for the first time?
(1)
3b. What Fe-rich mineral starts to form? Explain your answer. (HINT: what other oxides are
crystallizing at the same time? How would these combine?)
(3)
4a. At what SiO2 value does the first feldspar form?
(1)
4b. Using the plagioclase phase diagram provided below, together with the original diagram,
answer: What is the composition of the first plagioclase crystal (An% value)? (HINT: – the
starting composition will be dictated by the ratio Ca:Na when plagioclase crystallization starts
(i.e. the answer to Q4a)
1600
1550
1500
Liquid
1400
rature C
1300
3a. At what SiO2 value does an Fe-rich mineral begin to form for the first time?
(1)
3b. What Fe-rich mineral starts to form? Explain your answer. (HINT: what other oxides are
crystallizing at the same time? How would these combine?)
(3)
4a. At what SiO2 value does the first feldspar form?
(1)
4b. Using the plagioclase phase diagram provided below, together with the original diagram,
answer: What is the composition of the first plagioclase crystal (An% value)? (HINT: – the
starting composition will be dictated by the ratio Ca:Na when plagioclase crystallization starts
(i.e. the answer to Q4a)
(2)
1600
1553
1500
Liquid
1400
1300
Temperature C
1200
Plagioclase
1118
1000
40
60
900
o
NaAlSiz08
Ab
Weight % An
100
CA 251208
An
4c. At what temperature does it form?
(1)
4d. At what temperature AND what SiO2 value does the liquid reach a value of An50? (2)
4e. At what temperature does crystallization produce An50 crystals and crystal rims? (1)
4f. In which rock type (i.e. TAS diagram zone) does the An50 plagioclase crystallize? (1)
5a. Returning to the first diagram, what minerals co-crystallize first AND how do you know?
Explain your answer. (HINT – It is not one mineral alone. Consider which oxides are or are not
being used-up.) (3)
5b. Use the AFM ternary diagram to plot each of the 10 stages. F= the wt% Fe oxide present
(i.e. the FeO+Fe2O3 line on the first graph). M = wt% MgO. A = wt% (Na2O + K20). (HINT –
you will have to normalize the values A+F+M to equal 100% before plotting. That means that
Al2O3 and CaO have to be excluded.)
(10)
100%
0%
90%
80%
70%
60%
20%
10%
30%
40%
50%
Tholeiitie
50%
40%
60%
30%
70%
Calc-alkaline
80%
20%
10%
90%
100%
0%
10%
20%
30%
40%
50%
%09
70%
%08
90%
YOOOT
5c. Is this magma evolution in the ‘tholeiitic’ or ‘calc-alkaline’ series?
(1)
5d. Based on your answer to Q5c, what is a likely tectonic regime for forming these magmas: a
rift (i.e. decompression melting) or an arc (i.e. flux-melting)? Explain your answer. (3)
6. Based on the data available for this magma, does alkali feldspar (orthoclase / potassium
feldspar) ever crystallize? Explain your answer.
(3)
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