OCEAN/ESS 410
Class 7. Mantle Melting and
Phase Diagrams
William Wilcock
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)
Tectonic Setting of Partial Melting of
the Mantle
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 (coremantle boundary)
TemperatureDepth Plot for
Mantle Beneath
Oceanic Plates
1300°C
Geotherm
beneath
mid-ocean
ridges
Geotherm
for Old
Ocean Plate
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.
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
Melting beneath mid-ocean ridges
The melt rises out of the mantle because it is buoyant (less
dense than the mantle)
NaCl
• Naturally occurring
• Inorganic
• Fixed chemical formula (or
range of formulas)
• Unique, orderly internal
arrangement of atoms
(crystalline)
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
A simple 1-component system Water
Pure substances (single component) melt at a single temperature
(at a given pressure)
Olivine 70-80% of mantle
(Mg,Fe)2SiO4 - solid solution
Mg2SiO4 - Forsterite
Fe2SiO4 - Fayalite
Mantle olivine is 90% forsterite
Peridotite – mantle rock
SiO42- tetrahedra with Mg2+
& Fe2+ cations in between
Olivine Binary Phase Diagram
Substances with >1 component melt over a range of temperatures
Equilibrium
Melting
Equilibrium melting
occurs when the
solid and liquid
phases are kept
together as melting
progresses.
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)
Lever Rule
1-x
x
Fractional
Melting
Fractional melting
occurs if the liquid is
immediately
removed from the
solid as the solid
melts.
Equilibrium
Solidification
Equilibrium
solidification occurs
when the solid and
liquid phases are
kept together as
solidifications
progresses.
Fractional
Solidification
Fractional
solidification occurs
if the solid is
immediately
removed from the
liquid as it
crystallizes.
Diopside (Clinopyroxene) – Anorthite
(Plagioclase)
Diopside (CaMgSi2O6)
Dark mineral
Gabbro (coarse grained
equivalent of basalt)
– Oceanic Crust
Anorthite
(CaAl2Si2O8)
Light mineral
Dipside-Anorthite
Phase Diagram
Diopside
Anorthite
Eutectic – composition/temperature of 1st melt
Enstatite (Bronzite, Orthopyroxene)
MgSiO3
The second most
common mineral in
the mantle (20%)
MgO – SiO2
Phase
diagram
Forsterite and
enstatite undergo
incongruent melting
En  Fo + Liquid
Mantle “composition”
Effects of pressure on melting of Forsterite –
Enstatite mixtures
Mantle
First melt
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)