Class 7. Mantle Melting and Phase Diagrams William Wilcock

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Class 7. Mantle Melting and Phase
DiagramsWilliam Wilcock
OCEAN/ESS 410
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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/solidificati
  on pathways on binary phase diagrams (LAB)

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Tectonic Setting of Partial Melting of the Mantle
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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)
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Temperature-Depth Plot for Mantle Beneath Oceanic
Plates
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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.

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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
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Melting beneath mid-ocean ridges
The melt rises out of the mantle because it is
buoyant (less dense than the mantle)
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• Naturally occurring
• Inorganic
• Fixed chemical formula (or range of formulas)
• Unique, orderly internal arrangement of atoms
  (crystalline)

NaCl
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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

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A simple 1-component system - Water
Pure substances (single component) melt at a
single temperature (at a given pressure)
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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
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Olivine Binary Phase Diagram
Substances with gt1 component melt over a range of
temperatures
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Equilibrium Melting
Equilibrium melting occurs when the solid and
liquid phases are kept together as melting
progresses.
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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
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Fractional Melting
Fractional melting occurs if the liquid is
immediately removed from the solid as the solid
melts.
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Equilibrium Solidification
Equilibrium solidification occurs when the solid
and liquid phases are kept together as
solidifications progresses.
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Fractional Solidification
Fractional solidification occurs if the solid is
immediately removed from the liquid as it
crystallizes.
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Diopside (Clinopyroxene) Anorthite (Plagioclase)
Diopside (CaMgSi2O6) Dark mineral
Anorthite (CaAl2Si2O8) Light mineral
Gabbro (coarse grained equivalent of basalt)
Oceanic Crust
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Dipside-Anorthite Phase Diagram
Eutectic composition/temperature of 1st melt
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Enstatite (Bronzite, Orthopyroxene)
MgSiO3 The second most common mineral in the
mantle (20)
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MgO SiO2 Phase diagram
Forsterite and enstatite undergo incongruent
melting En ? Fo Liquid
Mantle composition
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Effects of pressure on melting of Forsterite
Enstatite mixtures
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)