GEOL 0008 Geochemistry 2022
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GEOL 0008 Geochemistry 2022
Exercise 5: How did the Earth-Moon system form? (graded)
Background: Since the Apollo landings , scientists have been able to compare directly samples of the Moon’s surface with rocks on Earth . In this exercise, you will find out how we know the chemical composition of the solid interior of the Earth and Moon, and make a reasoned hypothesis to explain how (and when) the Earth-Moon system formed .
THE CRUST
In order to estimate the composition of Earth’s crust, geochemists had to make assumptions about the average age of the crust, the relative proportion of basaltic versus granitic “precursor rocks” , and analyze thousands of sediments to integrate an average composition . The best estimate we have of crustal composition is: Si = 26.8%, Al = 8.4%, Fe = 7.06%, Ca = 5.3%, Mg = 3.2%, Na = 2.3%, K = 0.9% and Ti = 0.5%.
Because these elements are most commonly bound to oxygen in the form of silicates, we generally present crustal abundances as oxides, by assuming that all Si exists in as SiO2 , Al as Al2O3 , Fe is equally divided between FeO and Fe2O3 , Ca as CaO, Mg as MgO, Na as Na2O, K as K2O and Ti as TiO2 .
Question 1 (20 marks) Complete the table . Submit only the oxide weight % online
Symbol Si |
Oxide SiO2 |
Molecular weight
60.085 |
Weight (wt) % oxide in crust
57.33% |
Al |
Al2O3 |
|
|
Fe |
FeO |
|
|
Fe |
Fe2O3 |
|
|
Ca |
CaO |
|
|
Mg |
MgO |
|
|
Na |
Na2O |
|
|
K |
K2O |
|
|
Ti |
TiO2 |
|
|
Compare these numbers to some measured values in the table below . Are they the same? What assumptions did you make in your calculation that might explain the difference?
Compound |
SiO2 |
Al2O3 |
FeO |
Fe2O3 |
CaO |
MgO |
Na2O |
K2O |
TiO2 |
Crust, % |
59.19 |
15.82 |
3.58 |
3.41 |
3.07 |
3.30 |
2.05 |
3.93 |
0.79 |
THE MANTLE
Estimating the composition of the Earth’s mantle is a little trickier . For this we have to rely on analyses of “ultramafic” inclusions in igneous rocks and on theoretical constraints based on density, seismic velocity and radioactive heat production . A reasonable estimate of undepleted mantle based on lherzolite is given here: SiO2 = 45 .0%, MgO = 39 .0%, FeO = 8 .0%, Al2O3 = 3 .5%, CaO = 3 .25%, Na2O = 0 .28%, K2O = 0 .04%, TiO2 = 0 .09% . Undepleted mantle refers to the mantle before crustal differentiation and its composition is sometimes referred to as the SILICATE EARTH.
Question 2 (20 marks) The Earth’s crust was formed by partial melting (depletion) of the upper mantle and is enriched in incompatible elements , which segregated preferentially into the residual melt. Arrange these eight elements in order of
incompatibility (or tendency to be more concentrated in the crust) . For estimating the
THE CORE
By contrast to the mantle and crust, estimates of the composition of the Earth’s core rely solely on informed assumptions based on density, seismic velocity and magnetic properties . About 31 .5% of the Earth’s mass resides in the core, which is composed of 87 .5% iron, 5 .4% nickel and 5 .0% sulphur, but this last estimate is uncertain , and there could be other lighter elements in the Earth’s core, too .
THE MOON
We can estimate the Moon’s composition from its surface rocks and constraints from its density, seismic velocity, albedo and magnetic properties . For every 1000 moles of Si in the SILICATE MOON, there are 1000 moles of Mg, 220 moles of Al, 200 moles of Fe, 150 moles of Ca, 5.3 moles of Na, 5.1 moles of Ti and 0.35 moles of K .
Question 3 (20 marks)
Using the above information , make a table with three columns for the concentrations of the eight major elements (Si, Al, Mg, Ca, K, Na, Fe and Ti) in the 1) silicate Moon , 2) silicate Earth and 3) bulk Earth (or whole Earth , including the core) . The Moon’s core is so small we can assume that bulk Moon = silicate Moon . Note the major differences .
Question 4 (40 marks)
In no more than 400 words, outline how the volatile elements (Na, K) are distributed in these two planets compared with the more refractory elements (Fe, Ca, Mg, Ti)? Use your own observations above to put forward a reasoned, chemically based hypothesis of how the Moon formed . In your hypothesis address at least the following possible scenarios: 1) that they formed as a twin planetary system; 2) that the Moon formed elsewhere and was captured; 3) that the Moon formed after violent collision with Earth (if so, when?) . Base your answers primarily on the findings above but use internet and textbook resources in addition . Don’t forget to quantify chemical differences . Look up isotopic evidence and the latest findings , but do not use these exclusively .
2022-11-11