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Lithospheric Plate Structure Lithosphere (or plate) = crust + uppermost, rigid part of the mantle Asthenosphere Low-velocity zone in the upper mantle at a depth of ~100-200 km due to 2-4% partial melt. Magma Types Based on Chemistry 50% SiO2 gabbro/basalt 60% SiO2 diorite/andesite 70% SiO2 granite/rhyolite 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) Partial Melting of Different Starting Materials Peridotite (mantle rock) Basaltic Magma Basaltic (oceanic crust) Andesitic Magma Andesitic (continental crust) Rhyolitic Magma ALL basaltic magmas are partial melts of the MANTLE Mantle Petrology - composed mostly of ultramafic rocks (peridotites) Olivine Dunite 90 Peridotites Lherzolite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 10 Orthopyroxene Websterite Clinopyroxenite Clinopyroxene 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 Fertile mantle = Four-phase lherzolite: Garnet Lherzolite (Ol + Opx + Cpx + Al-rich phase) garnet Fertile mantle = Four-phase lherzolite: (Ol + Opx + Cpx + Al-rich phase) Spinel Lherzolite Spinel (Mg,Fe)(Al,Cr)2O4 Fertile mantle = Four-phase lherzolite: (Ol + Opx + Cpx + Al-rich phase) Al-phase is P sensitive Plagioclase shallow Spinel 30-70 (<30 km) km Garnet >70 km Spinel 30-70 km Garnet >70 km Partial melting in the mantle occurs within spinel and/or garnet stability field Partial Melting of Different Starting Materials Peridotite (mantle rock) Basaltic Magma Basaltic (oceanic crust) Andesitic Magma Andesitic (continental crust) Rhyolitic Magma 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 How does the mantle melt?? Partial melting in the mantle occurs within spinel and/or garnet stability field How does the mantle melt?? 1) Increase the temperature How does the mantle melt?? Partial melting in the mantle occurs within spinel and/or garnet stability field Decompression Melting (just at melting temp) (200°C below melting temp) 2) Lower the pressure Adiabatic rise of mantle with no conductive heat loss Decompression melting could melt at least 30% Divergent Boundaries and Mantle Hotspots Decompression melting of mantle (peridotite) to produce basaltic magmas Can we partially melt the mantle by another mechanism?? Addition of Fluids Lowers melting °T Causes partial melting Add volatiles (especially H2O) Solid Solid Wet Melting Dry Melting Subduction Zone Convergent Boundaries “Flux melting” of mantle above subducting plate produces basaltic magmas Mantle Melting and the Origin of Basaltic Magma Basaltic melts can be created under realistic circumstances: Decompression melting Divergent boundaries plates separate and mantle rises at mid-ocean ridges Intraplate hot spots rise of localized mantle plumes Fluid fluxing important in subduction zones