Faculty of Science and Technology

PO Box 6050 Langnes, NO-9037 Tromsø / +47 77 64 40 00 / postmottak@uit.no / uit.no

EXAMINATION

QUESTION PAPER

Exam in: GEO 2008 (Geochemistry)

Date: 3 June 2016

Time: 09.00 – 13.00

Place: Teorifagbygget,

hus 1, plan 3

Approved

aids:

Calculator

Ruler

Type of

sheets

(sqares/lines):

squares

Number of

pages incl.

cover page:

5 pages + 1 attachment (the attachment has only 1 page)

Contact

person during

the exam:

Sabina Strmic Palinkas

Phone:

417 97 833

NB! It is not allowed to submit rough paper along with the answer sheets

UiT / PO Box 6050 Langnes, NO-9037 Tromsø / 77 64 40 00 / postmottak@uit.no / uit.no 2

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)

MgCO3 + 2H+ ↔ Mg2+ + H2O + CO2 (g) rH<0 a) Decrease of pH

b) Increase of the Mg2+ concentration

c) Decrease of pressure

d) Increase of temperature

UiT / PO Box 6050 Langnes, NO-9037 Tromsø / 77 64 40 00 / postmottak@uit.no / uit.no 3

Problems (30 points) 1. Complete the following table: (4 points)

Isotopes # protons #neutrons #electrons

𝐾1939

𝐶𝑎2042

𝐹𝑒2656

𝑈92232

2. 146Sm 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)

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-ICP-

MS), they analyzed the concentration difference between sulfide droplets and their host

fresh mid-ocean-ridge basalt (MORB) glasses.

Samples Ni Cu

Sample 1 sulfide droplets (wt%) 17.4 12.5

silicate glass (ppm) 243 96.6

Sample 2 sulfide droplets (wt%) 10.1 8.68

silicate glass (ppm) 131 72.5

Mineral melt partition coefficients for mafic magma (based on values of www.earthref.org/KDD)

Minerals Cu Ni Sr Rb

Olivine 0.1 12 0.01 0.006

Clinopyroxene 0.1 3 0.14 0.04

Plagioclase 0.05 0.3 2 0.1

Sulfur ? ? 0.01 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 / postmottak@uit.no / uit.no 4

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) (Kyanite) (Quartz)

Phase/

Compound Formula

Hf0

(kJ/mol)

S0

(J/K-mol)

Gf0

(kJ/mol)

Pyrophyllite Al2Si4O10(OH)2 -5628.78 239.32 -5255.10

Kyanite Al2SiO5 -2581.10 83.68 -2426.91

Quartz SiO2 -910.65 41.34 -856.24

Water H2O -285.84 69.92 -237.18

6. 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) (Wollastonite)

Phase/

Compound Formula

Hf0

(kJ/mol)

S0

(J/K-mol)

Gf0

(kJ/mol)

Quartz SiO2 -910.65 41.34 -856.24

Calcite CaCO3 -1207.30 92.68 -1130.10

Wollastonite CaSiO3 -1630.04 82.00 -1544.84

CO2(g) CO2 -393.51 213.64 -394.39

7. 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 / postmottak@uit.no / uit.no 5

-0,8

-0,3

0,2

0,7

1,2

02

46

810

12

14

Eh (mV)

pH