1. Geol 2312 Igneous and Metamorphic Petrology
Lecture 10
Mantle Melting and
the Generation of Basaltic Magma
February 16, 2009

2. Melting the Mantle Makes Mafic Magma ALWAYS!
What is this made of?
Melting the Mantle Makes Mafic Magma ALWAYS!
How does this happen?
But aren’t there different
types of “mafic” magmas?

3. 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 km
Garnet >80km
Olivine
Clinopyroxene
Orthopyroxene
Lherzolite
Harzburgite
Wehrlite
Websterite
Orthopyroxenite
Clinopyroxenite
Olivine Websterite
Peridotites
Pyroxenites 90 40 10
Dunite 15 10 5
0.0
0.2
0.4
0.6
0.8
Wt.% Al2O3
Wt.% TiO2
Dunite
Harzburgite
Lherzolite
Tholeiitic basalt
20% Partial Melting
Residuum
Melt
Mantle

4. Phase Diagram of Normal Mantle
Mantle should not melt under”normal” geothermal conditions
How to get it to melt?
Winter (2001) Figure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

5. Melting the Mantle Increasing Temperature – Mantle Plumes
Zone of Melting
Normal
Geotherm
Plume-
influenced
Geotherm

6. Melting the Mantle Adiabatic Decompression (Rise of the Mantle with no Conductive Heat Loss)
Geotherm

7. 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

8. Melting a Hydrated Mantle
Ocean
Geothermal
Gradient

9. 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).
However, dehydration melting of these phases will only yield <1% H2O, hardly enough to saturate the mantle
Dehydration
melting curves

10. Creating Compositional Types of Mafic Magmas in Non-Subduction Settings Five Ways

11. Creating Compositional Types of Mafic Magmas in Non-Subduction Settings 1) Changing Pressure

12. Creating Compositional Types of Mafic Magmas in Non-Subduction Settings 2) Changing Volatile ContentNe Fo En Ab
SiO2
Oversaturated
(quartz-bearing)
tholeiitic basalts
Highly undesaturated
(nepheline-bearing)
alkali olivine
basalts
Undersaturated
CO2
H2O
dry
P = 2 Gpa
(80 Km)
Not really applicable to non-subduction settings

13. Creating Compositional Types of Mafic Magmas in Non-Subduction Settings 3) Changing Degree of Partial Melting

14. Creating Compositional Types of Mafic Magmas in Non-Subduction Settings 4) Fractional Crystallization during Ascent
Winter (2001) Figure 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.

15. Creating Compositional Types of Mafic Magmas in Non-Subduction Settings 5) Compositionally Heterogeneous Mantle
Melting “Fertile” Mantle
Melting “Infertile”
(previously melted)
Mantle

16. 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

17. Melting the Mantle makes Mafic Magma
Partially Melting the Heterogeneous Mantle makes Various Types of Mafic Magma
A chemically homogenous mantle can yield a variety of basalt types
Alkaline basalts are favoured over tholeiites by deeper melting and by low % partial melting
Crystal fractionation at moderate to great depths in the mantle can also create alkaline basalts from tholeiites
At low P, there is a thermal divide that seperates the two series
Mantle varies in bulk composition and fertility due to prior melting events (upper – depleted; lower undepleted)