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
DRAMATICALLY LOWERS THE LIQUIDUS
T OF THE MANTLE
“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 A HYDRATED MANTLE
Ocean
Geothermal
Gradient
MELTING A HYDRATED MANTLE
Problem: Water
content of the mantle
typically <0.2% (far
from saturated) and it
is structurally locked
into hydrous mineral
phases like amphibole
and phlogopite (biotite)
Exceeding the melting
points of amphibole (b)
and phlogopite (c) will
release H2O and allow
small degrees (<1%) of
melting if saturation is
achieved.
CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS
IN NON-SUBDUCTION SETTINGS
ALKALINE AND SUBALKALINE (THOLEIITIC)
Incompatible Elements
in the Mantle
Qtz-under
saturated
Qtzsaturated
1713
Thermal
Divide
Liquid
Tr + L
1070
Ne + L
Ab + L Ab + L
Ne + Ab
Ne
Ab + Tr
Ab
Q
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
CHANGING PRESSURE
(96 km)
(65 km)
(34 km)
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
CHANGING VOLATILE CONTENT
Ne
P = 2 GPa
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
CHANGING DEGREE OF INCONGRUENT PARTIAL MELTING
Winter (2001) Figure 10.9 , After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160.
INCONGRUENT PARTIAL MELTING OF THE
MANTLE
Mantle Gt
Lherzolite
Fo65Py20Di15
(Fo57En17Py14Di12 )
Batch Melting
Experiments of
Natural
Lherzolites
Melt – Fo5Py45Di50 +En
From Yoder (1976)
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
CHANGING DEGREE OF PARTIAL MELTING AND
DEPTH OF MELTING
Winter (2010), Figure 10.11 After Kushiro (2001).
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
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
WHAT IS A PRIMARY MAGMA?
Magma that last equilibrated with the mantle and was
not subsequently modified by fractional crystallization
or assimilation
 Modified magmas are termed derivative or evolved.
 Criteria - Most Primitive Composition
 Highest mg# (MgO/(MgO+FeO) = 66-75
 Highest compatible element concentrations (Cr >

1000 ppm, Ni > 400-500 ppm)
Lowest incompatible element abundances
 Lowest radiogenic isotope ratios
 Multiply saturated

DETERMINING THE DEPTH OF PRIMARY MAGMA
FORMATION BY THE INVERSE METHOD OF FINDING
MULTIPLE SATURATION
Involves melting a rock suspected
of corresponding to a primary
magma and finding the P&T of
multiple saturation.
This corresponds to “eutectic” point
of the multi-component magma.
Winter (2010) Figure 10.13), Anhydrous P-T phase
relationships for a mid-ocean ridge basalt suspected
of being a primary magma. After Fujii and Kushiro
(1977). Carnegie Inst. Wash. Yearb., 76, 461-465.
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
COMPOSITIONALLY HETEROGENEOUS MANTLE
Two Layer
Mantle
Convection
Model
Upper deplete mantle
= MORB source
Lower undepleted mantle
= enriched OIB source
From Courtillot et al. (2003).
Whole
Mantle
Convection
Model
Winter (2010) Figure 10-17b After
Basaltic Volcanism Study Project
(1981). Lunar and Planetary Institute.
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
COMPOSITIONALLY HETEROGENEOUS MANTLE
Ocean Island Basalt
(plume-influenced)
Mid-ocean Ridge Basalt
(normal upper mantle)
increasing incompatibility
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
COMPOSITIONALLY HETEROGENEOUS MANTLE
increasing incompatibility
Winter (2010) Figure 10.14b. Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB).
From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
COMPOSITIONALLY HETEROGENEOUS MANTLE
Melting “Fertile” Mantle
Melting “Infertile”
(previously melted)
Mantle – MORB?
LREE depleted
or unfractionated
LREE enriched
LREE depleted
or unfractionated
From Rollinson (1996)
LREE enriched
Winter (2010) Figure 10.15 Chondrite-normalized REE
diagrams for spinel (a) and garnet (b) lherzolites. After
Basaltic Volcanism Study Project (1981).
CREATING COMPOSITIONAL TYPES OF MAFIC
MAGMAS IN NON-SUBDUCTION SETTINGS
COMPOSITIONALLY HETEROGENEOUS MANTLE
= EMORB/OIB
= MORB
% partial
melting
% normative
olivine
Winter (2010) Figure 10-18a. Results of partial melting experiments on depleted and enriched lherzolites. Dashed lines are contours representing
percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the
degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310
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 favored over tholeiites by
deeper melting and by lower % partial melting
o Crystal fractionation at moderate to great depths in
the mantle can also create alkaline basalts from
tholeiites
o At mod to high P, there is a thermal divide that
separates the two series
o Mantle varies in bulk composition and fertility due
to prior melting events (upper – depleted; lower
undepleted)
PRESENT-DAY MID-OCEAN RIDGES
BIRTHPLACES OF MORB
OCEANIC CRUST AND UPPER MANTLE STRUCTURE
EVIDENCE

Seismic Velocities

Deep Sea Drilling Program

Ophiolites

Dredging of Fracture Zone
Scarps
OCEANIC CRUST AND UPPER MANTLE FORMATION
Winter (2010) Figure 13.15. After Langmuir et al. (1992). AGU.
COMPOSITION OF MORB
Primitive
Magma
Table 13-2. Average Analyses and CIPW Norms of MORBs
(BVTP Table 1.2.5.2)
Oxide (wt%)
SiO2
TiO2
Al2O3
FeO*
MgO
CaO
Na2O
K2O
P2O5
Total
All
50.5
1.56
15.3
10.5
7.47
11.5
2.62
0.16
0.13
99.74
MAR
50.7
1.49
15.6
9.85
7.69
11.4
2.66
0.17
0.12
99.68
EPR
50.2
1.77
14.9
11.3
7.10
11.4
2.66
0.16
0.14
99.63
IOR
50.9
1.19
15.2
10.3
7.69
11.8
2.32
0.14
0.10
99.64
Norm
q
or
ab
an
di
hy
ol
mt
il
ap
0.94
0.95
22.17
29.44
21.62
17.19
0.0
4.44
2.96
0.30
0.76
1.0
22.51
30.13
20.84
17.32
0.0
4.34
2.83
0.28
0.93
0.95
22.51
28.14
22.5
16.53
0.0
4.74
3.36
0.32
1.60
0.83
19.64
30.53
22.38
18.62
0.0
3.90
2.26
0.23
All: Ave of glasses from Atlantic, Pacific and Indian Ocean ridges.
MAR: Ave. of MAR glasses. EPR: Ave. of EPR glasses.
IOR: Ave. of Indian Ocean ridge glasses.
Fractional
Crystallization
COMPOSITIONAL VARIABILITY IN MORB
Fractional crystallization appears to
play a minor role.
Suggests instead, incompatible
element-rich and incompatible
element-poor mantle source regions
for MORB magmas

N-MORB (normal MORB) taps
the depleted upper mantle
source


Mg# > 65: K2O < 0.10
TiO2 < 1.0
E-MORB (enriched MORB)
taps undepleted (deeper?)
mantle

Mg# > 65: K2O > 0.10
TiO2 > 1.0
ORIGIN OF NMORB AND EMORB
DISTRIBUTION OF
NMORB & EMORB
AT FAST SPREADING
RIDGES
X-Sectional View
PROMOTES DEVELOPMENT OF
LARGER MAGMA CHAMBERS
MORE FRACTIONATED BASALT
COMPOSITIONS
Winter (2001) Figure 13-15,
After Perfit et al. (1994)
Geology, 22, 375-379.
Longitudinal View
Winter (2001) Figure 13-16 ; After
Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
SLOW-SPREADING RIDGES
Smaller, crystal-rich, dike-like
magma bodies results is less
fractionation  less evolved MORB
Depth (km)
2
Rift Valley
4
6
Moho
Transition
zone
Gabbro
Mush
8
10
5
0
5
Distance (km)
Winter (2001) Figure 13-16 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
10
OCEAN ISLAND BASALTS (OIB)
PLUME-INFLUENCED VOLCANISM
Figure 14.1. Map of relatively well-established hotspots and selected hotspot trails (island chains or aseismic ridges). Hotspots and trails
from Crough (1983) with selected more recent hotspots from Anderson and Schramm (2005). Also shown are the geoid anomaly contours
of Crough and Jurdy (1980, in meters). Note the preponderance of hotspots in the two major geoid highs (superswells).
HAWAIIAN VOLCANISM
STAGING OF ALKALINE AND THOLEIITIC MAGMA SERIES
MAJOR ELEMENT CHEMISTRY OF OIB
Variable alkalinity likely reflects
variable depths and degrees of
partial melting in the plume and
variable degrees of mixing and reequilbration as magma rises
through the mantle plume to the
ocean crust.
Table 14-4. Alkali/silica ratios (regression) for selected
ocean island lava suites.
Island
Alk/Silica Na2O/SiO2
Tahiti
0.86
0.54
Principe
0.86
0.52
Trinidade
0.83
0.47
Fernando de Noronha
0.74
0.42
Gough
0.74
0.30
St. Helena
0.56
0.34
Tristan da Cunha
0.46
0.24
Azores
0.45
0.24
Ascension
0.42
0.18
Canary Is
0.41
0.22
Tenerife
0.41
0.20
Galapagos
0.25
0.12
Iceland
0.20
0.08
K2O/SiO2
0.32
0.34
0.35
0.33
0.44
0.22
0.22
0.21
0.24
0.19
0.21
0.13
0.12
Winter (2001) Figure 14-2.
After Wilson (1989) Igneous
Petrogenesis. Kluwer.
REE COMPOSITIONS OF OIB
Alkali basalts strongly
LREE-enriched  low
degrees of partial melting
Similar to E-MORB, but
stronger depletion of HREE
for both tholeiitic and alkali
magmas series  deep
garnet-bearing source for
both with re-equilibration at
shallower depths
Lack of Eu anomaly indicates
no significant fractionation of
plagioclase
TRACE ELEMENTS OIB / MORB
Winter (2001) Figure 14-3. An Introduction
to Igneous and Metamorphic Petrology.
Prentice Hall. Data from Sun and
McDonough (1989).
A MODEL FOR OCEAN MAGMATISM
Mantle
Reservoirs
(based on radiogenic isotopes)
Chondritic (undepleted) mantle
From recycled
Low Sr87/Sr86 mantle
Low Nd143/Nd144 mantle
ocean crust
Previously melted mantle
Winter (2001) Figure 14-10. Nomenclature
U-enriched mantle
from Zindler and Hart (1986). After Wilson
Chondritic (undepleted) mantle (1989) and Rollinson (1993).