Stories of the Lithosphere
Gold, Pollution, and Farmland--Part 2, Core formation, Ore formation, and Pollutant Migration
Earth Science Essentials
by Russ Colson
Formation of Earth's Core and Crust
Our goal is to understanding conceptually how elements partition to differentiate the Earth. However, understanding the conceptual idea often depends on applying that idea to specific problems. We're going to go through some puzzles in differentiation and partitioning. Be sure to look over your lecture notes before starting.
Given the model for formation of the Earth's core and crust that we talked about in lecture, and the following partition coefficients, predict the relative concentrations of Rubidium (Rb) in the Earth's core, mantle, and crust
D(Rb) = concentration Rb in metal/Concentration Rb in silicate <<1
D(Rb) = concentration of Rb in solid/Concentration of Rb in liquid <<1
Given the model for formation of the Earth's core and crust that we talked about in lecture, and the following partition coefficients, predict the relative concentrations of Rhodium (Rh) in the Earth's core, mantle, and crust
D(Rh) = concentration Rh in metal/Concentration Rh in silicate >>1
D(Rh) = concentration of Rh in solid/Concentration of Rh in liquid >1
In the lecture, we talked about the difference between a chemical component and a phase. Phases are made up of chemical components, but properties like density and the conditions under which the phase forms depend on the properties of the phase and trace elements that partition into those phases may 'go along for the ride'. Based on the ideas presented in lecture, we can say that Uranium is
Consider the table below showing estimated concentrations of elements in Earth's core, mantle, and crust (these are estimates because it is not possible to collect an "average" sample of Earth's crust, and we can't collect a sample of Earth's core at all. We make our estimates based on the mass of the core, our observation of the composition of our solar system, especially as shown by meteorites, experimentally measured partition coefficients, and the composition of iron meteorites thought to have formed in the core of small planetesimals. Values are in weight percent. Values for the core and mantle come from W. F McDonough, "The Composition of the Earth", in the book Earthquake thermodynamics and phase transformations in the Earth's interior.
|
Element |
Core |
Mantle |
Crust |
|
Si |
6.0 |
21.0 |
27.7 |
|
Al |
0.0 |
2.4 |
8.1 |
|
Fe |
85.0 |
6.25 |
5.0 |
|
Ca |
0.0 |
2.55 |
3.6 |
|
Na |
0.0 |
0.27 |
2.8 |
|
K |
0.0 |
0.02 |
2.6 |
|
Mg |
0.0 |
22.8 |
1.5 |
|
U |
0.0 |
0.02 |
1.8 |
|
Cr |
0.9 |
0.26 |
0.01 |
|
Ni |
5.2 |
0.20 |
0.01 |
|
Au |
0.00005 |
0.0000001 |
0.0000003 |
Ore formation
Mercury (Hg) dissolves in hot water, but precipitates (crystallizes) at lower temperatures. Which one of the following correctly models how a Mercury (Hg) ore might form in an underground hot-water (hydrothermal) system? Assume arrows going left to right in all cases.
In lecture, we learned about the widely applicable concept of mass transport and precipitation. Conditions in one region might cause a particular element to dissolve and be transported to a new location.
Different conditions (temperature, pressure, pH, oxygen, or something else) in the new location can result in precipitation. This can result in an element dissolving from a large area and getting concentrated into a small area.
Let's apply this broad concept to a specific situation.
Copper often dissolves in oxidizing conditions (high O2) and precipitates in reducing conditions (low oxygen) conditions. Iron, in contrast, precipitates where there is lots of oxygen and dissolves where there is little oxygen. In general, as water seeps from the surface downward, it has the most oxygen closer to the surface and the least oxygen at and below the water table.
For the following question, consider the diagram below showing a schematic representation of a supergene enriched copper ore deposit.
Precipitation of blue copper minerals on rocks in a pool at a copper mine in Arizona.
Toxicant Attenuation:
A toxicant (pollutant) introduced into the natural environment at a point source will generally decrease in concentration with time and distance from the point source. Example processes that cause the concentration to decrease include the following:
1) dilution—for example a pollutant in a river is diluted where the river is joined by a tributary. A pollutant introduced into the atmosphere becomes diluted when it is stirred into the higher atmosphere by currents in the air.
2) Evaporation—for example, a pollutant introduced into water evaporates into the atmosphere (or it partitions into the atmosphere).
Generally this process is faster when the water is warmer and when the surface area of the water is large compared to the volume.
3) Photolysis—exposure to sunlight breaks the toxicant down into a non-toxic form. This will be more effective in shallow water or air, and ineffective in landfills or deep water.
4) Hydrolysis-- exposure to water breaks the toxicant down into a non-toxic form—generally more effective at higher temperature and of course in the presence of water.
5) Biodegradation—bacteria or other living things break the toxicant down into a non-toxic form—this was an important process that cleaned up the big Deepwater Horizon oil spill in the Gulf of Mexico in 2010.
6) Sedimentation--Partitioning into sediment and settling—this is most effective where the partition coefficient for the toxicant between sediment/water is high and where the water velocity slows to the point where the sediment can settle out.
Consider the situation below, showing how two different pollutants (Case 1 and Case 2) change downstream from a toxicant point source.
Case 1:
The toxicant is not significantly volatile (notice how its attenuation is not affected by a change in water temperature).
The toxicant partitions into sediment which settles in the lake (notice how the biggest drop in concentration occurs where the muddy stream flows into the lake). Additional attenuation occurs due to dilution from the tributary stream (notice the point drop in concentration and the confluence of the streams).
Case 2: The toxicant does not partition into the sediment (D<<1) (notice that there is not much change in concentration when the stream flows into the lake and the sediment settles out). Attenuation occurs more rapidly as water temperature increases, suggesting either volatile loss (higher T increase volatility) or T-related degradation of the pollutant. Additional attenuation occurs due to dilution by the tributary stream.
You try to make up some puzzles. Consider that pollutant X is dumped in a stream at point A.
Do a series of thought puzzles related to the figure above in which you choose how the pollutant behaves and model the resultant behavior. Construct concentration versus distance graphs for each scenario. Consider the variables:
Pollutant is affected by hydrolysis that is T-dependent
Pollutant is affected by photolysis, dependent on water depth
Pollutant is affected by volatility, which is T and surface area dependent
Pollutant is affected by sediment/water partitioning, which depends on the partition coefficient, the amount of sediment, and the energy of the water.
A toxicant is spilled into a river. As it goes downstream, its concentration gradually decreases with time, as shown in the figure below. Although more than a single process appears to be acting on the toxicant, one important process is
A point source of a contaminate is at "P" in the figure below. The variation in concentration downstream is shown in the figure, with properties of the stream shown in the associated map. It is likely the primary means by which the contaminate concentration is being decreased (naturally) in the water are
Consider the variation in concentration of a toxicant shown in the diagram. What are the partition coefficients for the toxicant between sediment and water (sed/wat) and between air and water (air/wat)?
Last updated Nov 21, 2016. All text and pictures are the property of Russ Colson.
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