lithospheric plate structure lithosphere (or plate) = crust + uppermost, rigid part of the mantle
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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
gabbro/basalt diorite/andesite granite/rhyolite
50% SiO2 60% SiO2 70% SiO2
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
OlivineOlivine
ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene
LherzoliteLherzoliteH
arzb
urgi
te
Wehrlite
Websterite
OrthopyroxeniteOrthopyroxenite
ClinopyroxeniteClinopyroxenite
Olivine Websterite
PeridotitesPeridotites
PyroxenitesPyroxenites
90
40
10
10
DuniteDunite
Mantle Petrology - Mantle Petrology - composed mostly of composed mostly of
ultramafic rocks (peridotites)ultramafic rocks (peridotites)
LherzoliteLherzolite is considered “fertile” mantle is considered “fertile” mantle capable of producing basalt capable of producing basaltDuniteDunite and and harzburgiteharzburgite are refractory residuum after basalt has been extracted are refractory residuum after basalt has been extracted
by partial melting by partial melting “infertile”, incapable of producing basalt “infertile”, incapable of producing basalt
Fertile mantle = Four-phase lherzolite:Fertile mantle = Four-phase lherzolite:(Ol + Opx + Cpx + Al-rich phase)(Ol + Opx + Cpx + Al-rich phase)Garnet Lherzolite
garnet
Spinel Lherzolite
Fertile mantle = Four-phase lherzolite:Fertile mantle = Four-phase lherzolite:(Ol + Opx + Cpx + Al-rich phase)(Ol + Opx + Cpx + Al-rich phase)
(Mg,Fe)(Al,Cr)2O4
Spinel
Fertile mantle = Four-phase lherzolite:Fertile mantle = Four-phase lherzolite:
PlagioclasePlagioclase shallow (<30 km)shallow (<30 km)
SpinelSpinel 30-70 km30-70 km
GarnetGarnet >70 km>70 km
Al-phase is P sensitiveAl-phase is P sensitive
(Ol + Opx + Cpx + Al-rich phase)(Ol + Opx + Cpx + Al-rich phase)
Partial melting in Partial melting in the mantle occurs the mantle occurs within spinel and/or within spinel and/or garnet stability fieldgarnet stability field
SpinelSpinel 30-70 km30-70 km
GarnetGarnet >70 km>70 km
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?? What does partial melting actually mean??
Why does it produce magmas of different Why does it produce magmas of different composition from the starting material??composition from the starting material??
Conduct “partial melting” experiment with snow, sugar, butter, and chocolate chips
How does How does the the mantle mantle melt??melt??
Partial melting in Partial melting in the mantle occurs the mantle occurs within spinel and/or within spinel and/or garnet stability fieldgarnet stability field
How does the mantle melt??How does the mantle melt??1) 1) Increase the temperatureIncrease the temperature
How does How does the the mantle mantle melt??melt??
Partial melting in Partial melting in the mantle occurs the mantle occurs within spinel and/or within spinel and/or garnet stability fieldgarnet stability field
2)2) Lower the pressureLower the pressure AdiabaticAdiabatic rise of mantle with no conductive heat loss rise of mantle with no conductive heat loss Decompression meltingDecompression melting could melt at least 30% 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 Can we partially melt the mantle by another mechanism??by another mechanism??
Subduction Zone Convergent Boundaries
“Flux melting” of mantle above subducting plate produces basaltic magmas
Basaltic melts can be created under Basaltic melts can be created under realistic circumstances:realistic circumstances:
Decompression meltingDecompression melting Divergent boundaries Divergent boundaries plates separate and plates separate and
mantle rises at mid-ocean ridgesmantle rises at mid-ocean ridges Intraplate hot spots Intraplate hot spots rise of localized rise of localized
mantle plumesmantle plumes Fluid fluxingFluid fluxing
important in important in subduction zonessubduction zones