Faculty of Science and Technology
EXAMINATION
QUESTION PAPER
Exam in:
GEO 2008 (Geochemistry)
Date:
3 June 2016
Time:
09.00 – 13.00
Place:
Teorifagbygget,
hus 1, plan 3
Calculator
Ruler
Approved
aids:
Type of
squares
sheets
(sqares/lines):
Number of
5 pages + 1 attachment (the attachment has only 1 page)
pages incl.
cover page:
Contact
Sabina Strmic Palinkas
person during
the exam:
Phone:
417 97 833
NB! It is not allowed to submit rough paper along with the answer sheets
PO Box 6050 Langnes, NO-9037 Tromsø / +47 77 64 40 00 / [email protected] / uit.no
Review questions (25 points)
1. Define half life, parent element, daughter element (3 points)
2. Define the Goldschmidt’s rules (4 points)
3. What standards are used for comparing isotopic separations of O, H, C, and S? (1 point)
4. What are isotopes? (1 point)
5. Chondrites can be considered mixtures of three components: describe them and their
origin and relative ages. (3 points)
6. What are the main mineral phases in: (i) the upper mantle? (ii) the transition zone?
(iii) the lower mantle? (3 points)
7. Show the relation between the Gibbs free energy change for a reaction and the
equilibrium constant (1 point)
8. Explain how changes in Eh and pH affect mobility of iron in natural waters (3 points)
9. Explain the process of hydrolysis on an example of: (2 points)
a. strong acid + weak base
b. strong base + weak acid
10. Explain how pH affects dissociation of H2S (2 points)
11. Write the equilibrium constant expression for the following reaction and predict how
the equilibrium will be affected by the listed changes of selected variables: (2 points)
a)
b)
c)
d)
MgCO3 + 2H+ ↔ Mg2+ + H2O + CO2 (g)
Decrease of pH
Increase of the Mg2+ concentration
Decrease of pressure
Increase of temperature
rH<0
UiT / PO Box 6050 Langnes, NO-9037 Tromsø / 77 64 40 00 / [email protected] / uit.no
2
Problems (30 points)
1.
Complete the following table: (4 points)
Isotopes
# protons
#neutrons
#electrons
39
19𝐾
42
20𝐶𝑎
56
26𝐹𝑒
232
92𝑈
2.
Sm is radioactive and decays by alpha decay with a decay constant  = 6.73 x 10-9
yr–1. What is the half-life of 146Sm? (2 points)
146
3. A sample of Antarctic snow has an 18O/16O ratio of 0.001950 and a D/H ratio of
0.00010850. What are its δ18O and δD values? (2 points)
4. Patten et al. (2013) published a paper in which they determined the partition
coefficients between sulfide melt and silicate melt for several elements. Using the
technique of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS), they analyzed the concentration difference between sulfide droplets and their host
fresh mid-ocean-ridge basalt (MORB) glasses.
Samples
Sample 1
Sample 2
sulfide droplets (wt%)
silicate glass (ppm)
sulfide droplets (wt%)
silicate glass (ppm)
Ni
17.4
243
10.1
131
Cu
12.5
96.6
8.68
72.5
Mineral melt partition coefficients for mafic magma (based on values of
www.earthref.org/KDD)
Minerals
Olivine
Clinopyroxene
Plagioclase
Sulfur
Cu
0.1
0.1
0.05
?
Ni
12
3
0.3
?
Sr
0.01
0.14
2
0.01
Rb
0.006
0.04
0.1
0.01
Questions:
a. Calculate the partition coefficients (D) between sulfide droplets and silicate
melt for Ni and Cu. (2 points)
b. Which elements (Ni, Cu, Rb, Sr) are compatible and which are incompatible
in the sulfur droplets? Why? (2 points)
c. Using the partition coefficients for mafic magma listed above, calculate bulk
partition coefficient for each element: (4 points each)
(i) For a sulfur-poor magma with a composition of 20% olivine, 30%
clinopyroxene, 49,95% plagioclase and 0,05% sulfur
(ii) For a sulfur-rich magma with a composition of 20% olivine, 30%
clinopyroxene, 49,50% plagioclase and 0,5% sulfur
UiT / PO Box 6050 Langnes, NO-9037 Tromsø / 77 64 40 00 / [email protected] / uit.no
3
5.
Using the listed thermodynamic data calculate is the following reaction endothermic
or exothermic at the standard conditions. (2 points)
Al2Si4O10(OH)2 <=> Al2SiO5 + 3 SiO2 + H2O
(Pyrophyllite)
Phase/
Compound
Pyrophyllite
Kyanite
Quartz
Water
6.
Formula
Al2Si4O10(OH)2
Al2SiO5
SiO2
H2O
(Kyanite) (Quartz)
Hf0
S0
Gf0
(kJ/mol)
(J/K-mol)
(kJ/mol)
-5628.78
-2581.10
239.32
83.68
-910.65
41.34
-5255.10
-2426.91
-856.24
-285.84
69.92
-237.18
Using the listed thermodynamic data calculate is the following reaction spontaneous
or not at the standard conditions. (2 points)
SiO2 + CaCO3 <=> CaSiO3 + CO2 (g)
(Quartz) (Calcite)
Phase/
Compound
Quartz
Calcite
Wollastonite
CO2(g)
7.
Formula
SiO2
CaCO3
CaSiO3
CO2
(Wollastonite)
Hf0
S0
Gf0
(kJ/mol)
(J/K-mol)
(kJ/mol)
-910.65
41.34
-856.24
-1207.30
92.68
-1130.10
-1630.04
82.00
-1544.84
-393.51
213.64
-394.39
Within the attached Eh-pH diagram draw the boundary between the stability fields of
Fe2+ and Fe(OH)3 if the Fe2+ concentration is 10-6 mol/L. (6 points)
Species
Gf0 (kJ mol-1)
Fe2+
-90.0
Fe(OH)3
-696.5
H2O
-237.1
+
H
0
F=96.489 C/mol
*Hand in the attachment with the Eh-pH diagram together with your answer
UiT / PO Box 6050 Langnes, NO-9037 Tromsø / 77 64 40 00 / [email protected] / uit.no
4
UiT / PO Box 6050 Langnes, NO-9037 Tromsø / 77 64 40 00 / [email protected] / uit.no
5
Eh (mV)
1,2
0,7
0,2
-0,3
-0,8
0
2
4
6
pH
8
10
12
14