Download Fertile mantle = Four

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