2. Lithospheric Plate Structure Lithosphere (or plate) = crust + uppermost, rigid part of the mantle

3.  Asthenosphere Low-velocity zone in the upper mantle at a depth of ~100-200 km due to 2-4% partial melt.

4. Magma Types Based on Chemistry gabbro/basaltdiorite/andesitegranite/rhyolite 50% SiO 2 60% SiO 2 70% SiO 2

5. Processes That Change Magma Compositions Primary Process  Partial Melting of Different Materials (e.g. mantle, oceanic crust, continental crust, etc.) Secondary Processes - “Magmatic Differentiation” Fractional Crystallization Assimilation/Magma Mixing Combined Process (AFC)

6. Partial Melting of Different Starting Materials Peridotite (mantle rock)  Basaltic Magma Basaltic (oceanic crust)  Andesitic Magma Andesitic (continental crust)  Rhyolitic Magma

7. ALL basaltic magmas are partial melts of the MANTLE

8. OlivineClinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites 90 40 10 Dunite Mantle Petrology - composed mostly of ultramafic rocks (peridotites) Lherzolite is considered “fertile” mantle  capable of producing basalt Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting  “infertile”, incapable of producing basalt

9. Fertile mantle = Four-phase lherzolite: (Ol + Opx + Cpx + Al-rich phase) Garnet Lherzolite garnet

10. Spinel Lherzolite Fertile mantle = Four-phase lherzolite: (Ol + Opx + Cpx + Al-rich phase) (Mg,Fe)(Al,Cr) 2 O 4 Spinel

11. Fertile mantle = Four-phase lherzolite: l Plagioclase F shallow (<30 km) l Spinel F 30-70 km l Garnet F >70 km Al-phase is P sensitive (Ol + Opx + Cpx + Al-rich phase)

12. Partial melting in the mantle occurs within spinel and/or garnet stability field l Spinel F 30-70 km l Garnet F >70 km

13. Partial Melting of Different Starting Materials Peridotite (mantle rock)  Basaltic Magma Basaltic (oceanic crust)  Andesitic Magma Andesitic (continental crust)  Rhyolitic Magma

14. What does partial melting actually mean?? Why does it produce magmas of different composition from the starting material?? Conduct “partial melting” experiment with snow, sugar, butter, and chocolate chips

15. How does the mantle melt?? Partial melting in the mantle occurs within spinel and/or garnet stability field

16. How does the mantle melt?? 1) Increase the temperature

17. How does the mantle melt?? Partial melting in the mantle occurs within spinel and/or garnet stability field

18. Decompression Melting (200°C below melting temp) (just at melting temp)

19. 2) Lower the pressure F Adiabatic rise of mantle with no conductive heat loss F Decompression melting could melt at least 30%

20. Divergent Boundaries and Mantle Hotspots Decompression melting of mantle (peridotite) to produce basaltic magmas

21. Can we partially melt the mantle by another mechanism??

22. Addition of Fluids Lowers melting °T Causes partial melting

23. Add volatiles (especially H 2 O) Dry Melting Solid Wet Melting

24. Subduction Zone Convergent Boundaries “Flux melting” of mantle above subducting plate produces basaltic magmas

25. Mantle Melting and the Origin of Basaltic Magma

26. Basaltic melts can be created under realistic circumstances: l Decompression melting  Divergent boundaries  plates separate and mantle rises at mid-ocean ridges  Intraplate hot spots  rise of localized mantle plumes l Fluid fluxing F important in subduction zones