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

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

ALL basaltic magmas are partial melts of the MANTLE

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

Decompression Melting

(200°C below melting temp)

(just at melting temp)

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??

Addition of Fluids

Lowers melting °T

Causes partial melting

Add volatiles (especially HAdd volatiles (especially H22O)O)

Dry Melting

Solid

Solid

Wet Melting

Subduction Zone Convergent Boundaries

“Flux melting” of mantle above subducting plate produces basaltic magmas

Mantle Melting and the Origin of Basaltic MagmaMantle Melting and the Origin of Basaltic Magma

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