Melting
Lars Stixrude
U. Roma Tre Short Course
27/3/13
Magma
Dynamics
.
.
Liquid-solid density
contrast
Origin of melt
.
.
MORB Generation
Water Partitioning
Inluence of water on
viscosity
Lithospheric thickness
Thermal History
Deep Heterogeneity
Compressibility
Liquid-solid density
inversion
Liquid-crystal density
inversion
Origin of Earth
—  Was Earth initially
molten?
—  How did Earth evolve
from this state?
—  What are the
consequences for
Earth’s present state?
—  What fossil evidence
can we find?
—  What was Earth’s first
atmosphere like?
Deep Melt
Deep Melting
First Principles Molecular Dynamics
Example: MgSiO3; Two-fold compression: V/VX=0.5; 6000 K
Initial condition: pyroxene structure, Maxwellian velocities
Liquid Structure
P=0 GPa
T=3000 K
Si-O polyhedra
Mg ions
P=140 GPa
T=3000 K
Melting
Volume and
entropy
Grüneisen Parameter
—  Increase on compression
appears to be a
universal feature of
silicate liquids
—  Prediction confirmed by
Hugoniot data
(Mosenfelder et al.,
2007 JGR)
—  Tendency for γ to
decrease with increasing
polymerization (NBO/T)
Original Thermal State of
Earth
—  Complete melting much
easier than previously
thought
—  TP~2450 K sufficient to melt
entire mantle
—  Steep liquid-state isentropes
(large γ)
—  Crystallization of magma
ocean begins at mid-mantle
depths
—  TP~2000 K (Archean?)
produces melt in lower
mantle
—  Melting at base of present
mantle
Molten Earth
Lower Magma Layer
Source of lower
mantle chemical
heterogeneity?
Upper
Magma
Layer
SIlicate/
Steam
Atmosphere
Source of chondritic
complement?
ULVZ a remnant?
Lower
Magma
Layer
Volatile reservoir?
Reaction with core?
Crystallizing
Layer
Mantle Heterogeneity