Evolution of Igneous Rocks

1. Evolution of Igneous Rocks

2. Simple Eutectic • Two components that don’t mix in the solid state • One or the other begins to form as melt cools • When temperature minimum reached, other component starts to form • Both components crystallize • Temperature remains constant until melt completely solidifies

3. As Component A is Removed, the Melt Shifts Toward B

4. At Any Given Point We Can Determine How Much Melt and Solid Are Present

5. Once the Temperature Minimum (Eutectic) is Reached, B Forms as Well

6. The Overall Eutectic Diagram

7. A Familiar Eutectic

8. Evolution of a Eutectic Melt

9. Evolution of a Eutectic Melt

10. Intermediate Compounds

11. Solid Solution • Two components mix freely in solid state • A melts at higher temperature than B • As melt cools, the first crystals to form are richer in A than the melt • As A is taken out, the mineral and the remaining melt become richer in B • Last melt is much richer in B than the original melt • Final solid has same composition as original melt.

12. First Solid is Richer in Fo than the Melt

13. As Fo is Removed, Both the Remaining Melt and Resulting Solid Get Richer in Fa

14. A Simple Rule Allows Us to Tell How Much of Each Component We Have

15. The Final Melt is Much Richer in Fa Than the Original

16. Simple Solid Solution

17. How Our System Evolves on the Phase Diagram

18. A Solid Solution, Animated

19. How To Read Any Phase Diagram • Read the Field Labels • Note What Changes at Boundaries • Track All Phases • Use Proportions to Determine Quantities

20. Bowen's Reaction Series • The geologist N.L. Bowen found that minerals tend to form in specific sequences in igneous rocks • These sequences could be assembled into a composite sequence.

21. Bowen's Reaction Series • Why “Reaction?” • Solid Solutions may or may not remain in equilibrium with liquid • Some solids (enstatite) break down on melting and others may dissolve in their own magma • Why “Series?” • Solid solutions evolve as melt solidifies • Eutectic relationships determine solidification sequence.

22. Bowen's Reaction Series No igneous rock ever displays the whole sequence, just a slice across the sequence.

23. Bowen's Series and Igneous Rocks

24. Incongruent Melting • Some minerals break down as they melt • Example: Enstatite (MgSiO3) breaks down to Forsterite (Mg2SiO4) + Liquid • When cooling, the reverse happens: Fortsrite and Liquid react to make Enstatite • Sometimes Forsterite disappears completely

25. Incongruent Melting

26. Incongruent Melting

27. Incongruent Melting

28. Incongruent Melting

29. Incongruent Melting

30. Incongruent Melting

31. Incongruent Melting

32. Incongruent Melting

33. A Ternary System 3 3

34. A Ternary System 3 3

35. A Ternary System 3 3

36. A Ternary System 3 3

37. How Simple Ternary Systems Evolve • First phase crystallizes. Melt moves radially away from that corner of plot • Second phase starts to form. Melt moves away from both corners of plot toward eutectic • Once eutectic is reached, all three phases crystallize

38. A Ternary System 3 3

39. A Ternary System

40. Bowen's Series and Igneous Rocks Volcanic Rocks (Rare)  Basalt   Andesite                 Rhyolite Plutonic Rocks Dunite  Gabbro   Diorite                   Granite 1200 C            Melting Point              700 C Mg, Fe              Rich In...             Si, Na, K Rapid              Weathering                   Slow Usually Dark       Color              Often Light

41. Bowen's Series and Volcanoes Volcanic Rocks (Rare)  Basalt   Andesite                 Rhyolite Plutonic Rocks Dunite  Gabbro   Diorite                   Granite Fluid               Lava Is...                 Viscous Mild                Eruptions                 Violent Type of Volcano Shield Volcano     Stratovolcano    Plug Dome