1. Class 8. Mantle Melting and Phase Diagrams William Wilcock
OCEAN/ESS 410
Class 8. Mantle Melting and Phase Diagrams William Wilcock

2. Lecture/Lab Learning Goals
Understand the basic concept of decompressional melting and know how its distribution in the mantle is related to plate tectonics.
Be able to define the terms liquidus, solidus, mantle adiabat and use them to explain mantle melting beneath mid-ocean ridges with a plot of depth (or pressure) against temperature.
Be familiar with the simple two phase diagrams and the lever rule
Know what a eutectic is and why it explains the relatively uniform composition of basalts in different tectonic settings (and on different planets).
Be able explain oceanic crustal thickness in terms of the degree of partial melting in the crust and be able to identify melting/solidification pathways on binary phase diagrams (LAB)

3. Tectonic Setting of Partial Melting of the Mantle

4. Mid-Ocean Ridges and Hotspot Plumes
Ridges are linear features underlain by flow rising from relatively shallow mantle depths
Hotspots are the result of cylindrical plumes of upflow rising from deep within the mantle (core-mantle boundary)

5. Temperature-Depth Plot for Mantle Beneath Oceanic Plates
Liquidus
Wet Solidus
Dry Solidus
Geotherm
for Old
Ocean Plate
Geotherm
beneath
mid-ocean
ridges

6. Terminology
Geotherm – Vertical temperature profile in the earth Adiabat – Temperature that a packet of the mantle that moves up/down without gaining or loosing heat Solidus – Temperature at which a rock will first start to melt Liquidus – Temperature at which a rock will be fully molten.

7. Percentage of melting
The pressure (or depth) versus temperature (P-T) path of upwelling mantle beneath a mid-ocean ridge leads to a maximum of ~25% melt

8. Melting beneath mid-ocean ridges
The melt rises out of the mantle because it is buoyant (less dense than the mantle)

9. NaCl Naturally occurring Inorganic
Fixed chemical formula (or range of formulas)
Unique, orderly internal arrangement of atoms (crystalline)
NaCl

10. Definitions
 A system can be defined as that part of the universe that is arbitrarily or naturally isolated for the purposes of consideration or experimentation. For our purposes it will generally be a sample of a rock
A phase is defined as a chemically and physically homogeneous part of a system. Each mineral in a rock is a phase as is a melt.
Components are the minimum number of chemical constituents necessary to describe the system

11. A simple 1-component system - Water
Pure substances (single component) melt at a single temperature (at a given pressure)

12. Olivine 70-80% of mantle (Mg,Fe)2SiO4 - solid solution
Mg2SiO4 - Forsterite
Fe2SiO4 - Fayalite
Mantle olivine is 90% forsterite
SiO42- tetrahedra with Mg2+ & Fe2+ cations in between
Peridotite – mantle rock

13. Olivine Binary Phase Diagram
Substances with >1 component melt over a range of temperatures

14. Equilibrium Melting
Equilibrium melting occurs when the solid and liquid phases are kept together as melting progresses.

15. Lever Rule S – solid composition L – liquid composition
A – system composition
We can write fraction x of solid as
xS + (1-x)L = A
which can also be written as
x (A –S) = (1-x)(L-A)
This is analogous to an old fashioned balance scale as shown in the figure
We can solve the above equations to get the proportion of solid
x = (A – L) / (S – L)
1-x x

16. Fractional Melting
Fractional melting occurs if the liquid is immediately removed from the solid as the solid melts.

17. Equilibrium Solidification
Equilibrium solidification occurs when the solid and liquid phases are kept together as solidifications progresses.

18. Fractional Solidification
Fractional solidification occurs if the solid is immediately removed from the liquid as it crystallizes.

19. Diopside (Clinopyroxene) – Anorthite (Plagioclase)
Diopside (CaMgSi2O6)
Dark mineral
Anorthite (CaAl2Si2O8)
Light mineral
Diopside is a chain silicate;
Plagioclase Framework (Al in tetrahedra)
Gabbro (coarse grained
equivalent of basalt)
– Oceanic Crust

20. Dipside-Anorthite Phase Diagram
Diopside
Anorthite
Eutectic – composition/temperature of 1st melt

21. Enstatite (Bronzite, Orthopyroxene)
MgSiO3
The second most common mineral in the mantle (20%)
Also known as orthopyroxene

22. MgO – SiO2 Phase diagram
Forsterite and enstatite undergo incongruent melting
En  Fo + Liquid
Mantle “composition”

23. Effects of pressure on melting of Forsterite – Enstatite mixtures
Surface
15 km
>15 km
Key Point – At depth of mantle melting, melt composition is somewhat pressure dependent but is not dependent on the relative proportions of the different minerals. Mantle melts in different environments tend to give similar rocks (basalts/gabbros)