Download Global earth structure lecture 2 - UC Berkeley Earth and Planetary

Global earth structure
lecture 2
EPS 122 –Spring 2017
03/14/2017
Instructor: Barbara Romanowicz
Sharp density changes:
1) CMB: 5.57 103-à 9.90 103 kg/m3
2) At 410 and 660 km discontinuities
(about 5%)
3) At the inner core boundary:
about 4%
Density profile -> gravity -> pressure
inside the earth:
r
P(r) = − ∫ g(r)ρ (r)dr
0
1 GPa = 10 kbar
or 1 atm =105 Pa
410 km: P= 13.3 GPa
660 km: P= 23.8 GPa
CMB
P = 136 GPa
ICB
P = 329 GPa
Center P = 364 GPa
Silicate mantle
Mostly Mg
Iron rich
core
Composition constrained from
seismology+density+lab. experiments
on behavior of earth materials at high
P and T
Composition
•  Constraints:
extraterrestrial
–  Nucleosynthesis
–  Meteorites
•  Constraints: near surface
–  Xenoliths
–  Magma source
•  Constraints: Interior
–  Physical properties
•  Fractionation important
–  Earth-hydrosphere-space
–  Crust-mantle-core
èPyrolite/Lherzolite/Peridotite/…
A conceptual model of a high pressure device for experiments
under various conditions of pressure and temperature.
A diamond anvil cell
Samples ~30µm in diameter
Pressures > 500 GPa
T -> 6000 K
Multi-anvil presses and diamond cells generate pressures
that cover full range of conditions in the mantle and even
the core.
Multi-anvil apparatus “DIA”
3 pairs of octohedral pistons; internal heating -> 20 Gpa
Larger samples ~1 mm in diameter
Heated internally – can be used to study deformation
Elastic properties and changes in crystal structure can be recorded
“in situ” when combined with monochromatic X-ray beam
The silicate mantle
•  Main constituent: olivine
–  End members of olivine
•  Forsterite (Fo) Mg2SiO4
•  Fayalite (Fa) Fe2SiO4
–  “normal mantle”: very forsteritic :
Fo91-Fo94
•  Here 91= 91% of Mg in (Mg,Fe)2SiO4
•  Other major mantle minerals:
•  Orthopyroxene (Opx) (Mg,Fe)SiO3
•  Clinopyroxene (Cpx) Ca(Mg,Fe)Si2O6
•  Garnet (Gt) (Mg,Fe)3Al2Si3O12
Pyrolite model: composition of upper mantle
Pyrolite is a synthetic rock invented by Ringwood as a model
for experimenting with constitution of the upper an lower
mantle. This composition is generally accepted for the
uppermost part of the mantle. As we go deeper, differences
among various authors increase. In particular, it remains a
matter of a hot debate whether composition is constant
throughout the mantle.
Phase changes in olivine
•  Olivine undergoes phase changes to
denser structure at pressures
equivalent to depths ~400km,
~520 km and ~670 km.
–  400-670km: mantle
“transition zone”
•  At 400 km: olivine phase change:
–  Olivine (α) to spinel (β)
–  Exothermic (releases heat)
Clapeyron slope 2-3MPa/K
–  Density increases by about 5%
At 400 km: olivine phase change:
Olivine (α) to spinel (β)
Exothermic
Increase in density and seismic velocities ~5%
At 520 km:
(β)-> (γ) (wadsleite to ringwoodite)
Less effect on seismic velocities
Exothermic
At 670 km:
(γ)-> perovskite (Mg,Fe)SiO3+magnesiowüstite (Mg,Fe)O
Endothermic (negative Clapeyron slope (-2-6 MPpa/K)
Increase in density by ~10%
Melting temperatures such that mantle is solid
Phase diagrams
Two different models for upper mantle composition
Two different data types to investigate
upper mantle discontinuities …
* reflected waves
* both continents and oceans
* converted waves
* only beneath stations
Global Transverse Component Stacks
Shearer, 1991
Receiver function concept
Mantle Phases
•  Upper mantle
–  Olivine, orthopyroxene, clinopyroxene,
plag→spinel→garnet
•  Transition Zone
–  Olivine→Wadsleyite→
Ringwoodite
–  Pyroxenes dissolve into garnet
•  Lower mantle
–  Two perovksites + oxide
•  What else?
–  Most of interior still relatively little explored
–  Discovery of post-perovskite phase (2004)
Mineral sequence II
Lower Mantle
+
+
(Mg(1-x-z),Fex, Alz)(Si(1-y),Aly)O3
Mg/Fe perovskite
(Mgx,Fe(1-x))O
oxide
CaSiO3
Ca- perovskite
Minerals in Earth’s mantle
(Mg,Fe,Al)-MgSiO3 perovskite
post-perovskite?
CaSiO3 - perovskite
(Mg,Fe)O - ferropericlase
(Mg,Fe,Al,Ca)silicates
Courtesy of S. Sinogeikin
Now called
bridgmanite
Heterogeneity in the D” region of the mantle
Precursors to ScS
S
ScS
The lowermost mantle
Most of the lower mantle
appears to have a constant
composition and changes
in the seismic velocities and
density appear to correspond to
those predicted accounting
for the increasing pressure.
There are, however, complications
in the deepest 200-300 km of
the mantle. The example on left
is a collection of S-velocity models
in the lowermost mantle. The
origin of the discontinuity has
recently been attributed to a
newly discovered phase
transition:
perovskite-> post-perovskite
Discovery of post-perovskite at T and P conditions of the
base of the mantle (120 GPa, 2500oC)
MgSiO3 perovskite
MgSiO3 post-perovskite
N=
New
phase
M Murakami et al. Science 2004;304:855-858
Periclase: cubic
structure (cf NaCl)
Bridgmanite and post-perovskite:
Orthorombic structure
MgSiO3 phase diagram in the lower mantle based on LHDAC
Experiments of Murakami et al. (2004) and ab inito computations of
Tsuchiya et al. (2004)
Irifune and Tsuchiya (2015)