LECTURE 5 Pressure, force, diamonds

1. LECTURE 5 Pressure, force, diamonds

2. 1 carat = 6,000$ • 1 carat = 6 mm diameter stone

3. Graphite C arranged in sheets Sheets are only weakly bonded to each other Graphite is soft

4. But if you increase pressure, You compress the graphite and force the C atoms to get closer together… This results in a structural change in the crystal We call this a phase change

5. High pressure form of C is DIAMOND … C atoms are bonded each to 4 other C atoms … structure is highly polymerized and densely packed This makes diamond the hardest mineral in the world and very dense

6. The densely packed nature of diamond gives it a very high refractive index This allows diamond to have strong internal reflections … giving it that brilliant luster (when cut appropriately)

7. Diamond occurrences = mostly in old continental interiors Why?

8. Old continental interiors often have the thickest lithosphere

9. At what pressure can diamonds form? 1 GPa = 10 kb

11. Graphite diamond Graphite diamond Graphite diamond Graphite diamond http://www.ei.lehigh.edu/learners/tectonics/heatflow/heatflow2.html

12. At what depth can diamonds form? 1 GPa = 10 kb

13. diamonds are stable at depth >150 km Sampled by deep-seated magmas called kimberlites

14. What is pressure?

15. PRESSURE = FORCE per UNIT AREA P = Force/Area F has units of Newtons Area has units of m2 Units of pressure = N/m2= Pa Pascal

16. PRESSURE = FORCE per UNIT AREA P = Force/Area Force = mass x acceleration F = m x a kg x m/s2 Your weight is a force weight = F = m x g g is gravity 9.8 m/s2

17. Force An influence that causes an object to accelerate Accelerate means the time rate of change in velocity Objects fall because they are attracted by the Earth’s gravitational field. Objects fall at an acceleration equal to gravity 9.8 m/s2

18. What is the pressure beneath a column of rock, water, etc? d P = mass x g / area = mg/d2 M = density x volume = ρ V Volume = h x d x d P = (ρ h d2) g / d2 P = r g h d Weight of column of rock h Density of water = 1000 kg/m3 Density of granite = 2700 kg/m3 Density of mantle = 3300 kg/m3

19. At what depth can diamonds form? 1 GPa = 10 kb

20. What depths do diamonds form? Consider the average temperature of the Earth to be 1400 C Then average pressure is 55 kbars 1 kbar = 0.1 GPa So this is equivalent to 5.5 Gpa or 5.5 x 109 Pa From P = ρgh, we find H = P/(ρg) = 5.5 x 109 Pa / (3300 kg/m3 x 9.8 m/s2) ~170 km

21. How does pressure drive flow in the mantle? Consider two columns, one hot, one cold With a temperature difference of T What is the pressure at the base of each column? Which way is the pressure gradient? T1 > T2 P1 ? P2

22. How does pressure drive flow in the mantle? Consider two columns, one hot, one cold With a temperature difference of T What is the pressure at the base of each column? Which way is the pressure gradient? T1 > T2 MANTLE FLOW P1 < P2

23. Earth loses its heat by convection

24. Convection occurs in the atmosphere as well Warm areas are of low density and low pressure, so they rise Cold areas are of high density and high pressure, so they sink

25. Hurricane Isaac August 27, 2012

27. Continental Crust Ocean Mid- ocean Ridge Base of lithosphere Oceanic Crust Plume Where is the carbon from in the deep parts of continents?

28. Or in mantle eclogites

29. 1 carat = 6,000$ • 1 carat = 6 mm diameter stone

30. Grade of kimberlite • 1 carat per ton • 1 carat = 0.2 g • 0.2 g per ton • 0.2 ppm C

31. How are diamonds mined today? Open pit mining

34. How do you find diamond-bearing kimberlites? 1 – look in the old interiors of continents 2 – look for indicator minerals

35. Typically found in mantle peridotites

36. [SiO4]4- Independent tetrahedraNesosilicates Olivine Mg2SiO4 (aka peridot) Abundant mantle mineral Garnet (Mg,Ca,Fe)3Al2Si3O12 Common crustal mineral

37. Search anthills?

38. Pan the sediments in rivers And follow the garnets to their source

39. Yet another way is to pan glacial sediments… When glaciers retreat, they leave behind sediments

40. This led to the discovery of diamonds in Canada

41. The Big Hole at Kimberley, 1874. Each digger had his own pulley.

43. To maximize profits, Cecil Rhodes bought up all the claims But a large amount of steady, cheap and reliable human labor This led to the formation of labor camps, identity cards, etc., ultimately paving the way to apartheid in South Africa Cecil Rhodes

45. Diamond flow chartDe Beer’s, up until 10 years ago, controlled every part of the diamond flow They invented the phrase “A diamond is forever”

46. PRESSURE = FORCE per UNIT AREA P = Force/Area P has units of Newtons Area has units of m2 Units of pressure = N/m2= Pa Pascal