GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 10
Mantle Melting and
the Generation of Basaltic Magma
February 16, 2009
What is this made of?
MELTING THE MANTLE MAKES MAFIC MAGMA
ALWAYS!
COMPOSITION OF THE MANTLE
LHERZOLITE
Evidence:
•Ophiolites
Slabs of oceanic crust and upper mantle
Obducted onto edge of continent at convergent
zones
•Dredge samples from oceanic fracture zones
•Nodules and xenoliths in some basalts
•Kimberlite xenoliths
Pipe-like intrusions quickly intruded from the
deep mantle carrying numerous xenoliths
+ Al-bearing Phase
• Plagioclase <30km
• Spinel 30-80 km
• Garnet >80km
Olivine
Tholeiitic basalt
15
Dunite
Melt
90
Peridotites
10
Lherzolite
40
5
Lherzolite
Harzburgite
Dunite
0
0.0
0.2
Mantle
Pyroxenites
Orthopyroxenite
Olivine Websterite
Residuum
Websterite
10
0.4
Wt.% TiO2
10
0.6
0.8 Orthopyroxene
Clinopyroxenite
Clinopyroxene
PHASE DIAGRAM OF NORMAL MANTLE
Mantle should
not melt
under”normal”
geothermal
conditions
How to get it to
melt?
Winter (2001) Figure 10-2
Phase diagram of aluminous
lherzolite with melting interval
(gray), sub-solidus reactions,
and geothermal gradient. After
Wyllie, P. J. (1981). Geol.
Rundsch. 70, 128-153.
MELTING THE MANTLE
INCREASING TEMPERATURE – MANTLE PLUMES
Zone of Melting
Normal
Geotherm
Plumeinfluenced
Geotherm
MELTING THE MANTLE
ADIABATIC DECOMPRESSION
(RISE OF THE MANTLE WITH NO CONDUCTIVE HEAT LOSS)
Adiabatic
Geotherm
MELTING THE MANTLE
ROLE OF VOLATILES
“Dry” curve has a positive slope because increased P favors lower V phase (solid),
increased T favors S phase (liquid)
H2O-saturated curve has negative slope because V of liq+vapor (Liqaq) is less than V of
solid+vapor (or fluid); change is most extreme at low overall pressures.
Melting point is lowered with increasing P by more volatiles being dissolved in the melt
MELTING A HYDRATED MANTLE
Ocean
Geothermal
Gradient
MELTING A HYDRATED MANTLE
Problem: Water
content of the mantle
Dehydration
melting curves
typically <0.2% (far from
saturated) and it is
structurally locked into
hydrous mineral phases
like amphibole and
phlogopite (biotite).
However, dehydration
melting of these phases
will only yield <1% H2O,
hardly enough to
saturate the mantle
CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS
IN NON-SUBDUCTION SETTINGS
FIVE WAYS
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
1) CHANGING PRESSURE
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
2) CHANGING VOLATILE CONTENT
Ne
P = 2 Gpa
(80 Km)
CO2
dry
Highly undesaturated
(nepheline-bearing)
alkali olivine
basalts
H2O
Ab
Oversaturated
(quartz-bearing)
tholeiitic basalts
Not really applicable to
non-subduction settings
Fo
En
SiO2
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
3) CHANGING DEGREE OF PARTIAL MELTING
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
4) FRACTIONAL CRYSTALLIZATION DURING ASCENT
Winter (2001) Figure 10-10
Schematic representation of the
fractional crystallization scheme of
Green and Ringwood (1967) and
Green (1969). After Wyllie (1971).
The Dynamic Earth: Textbook in
Geosciences. John Wiley & Sons.
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
5) COMPOSITIONALLY HETEROGENEOUS MANTLE
Melting “Fertile” Mantle
Melting “Infertile”
(previously melted)
Mantle
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
5) COMPOSITIONALLY HETEROGENEOUS
MANTLE
Upper deplete mantle=
MORB source
Lower undepleted mantle=
enriched OIB source
Partially
Melting
the Heterogeneous
Mantle
M
ELTING THE
MANTLE
MAKES MAFIC M
AGMA
makes Various Types of Mafic Magma
o A chemically homogenous mantle can yield a variety
of basalt types
o Alkaline basalts are favoured over tholeiites by
deeper melting and by low % partial melting
o Crystal fractionation at moderate to great depths in
the mantle can also create alkaline basalts from
tholeiites
o At low P, there is a thermal divide that seperates
the two series
o Mantle varies in bulk composition and fertility due
to prior melting events (upper – depleted; lower
undepleted)