Download Lecture 9b: Upper Mantle Structure and Composition

to discuss how and why upper layers of Earth are mobile,
need to examine internal structure of Earth and plate tectonics
internal structure of Earth
Earth is layered
knowledge of layering
is recent (late 1800s);
prior to that, only knew
interior must be hot
(volcanoes)
astronomers calculated mass from radius and gravitational constant
(known for 2,000 years)
mean density ~ 5500 kg/m3, but surface rocks ~2300 kg/m3
therefore, density gradient exists
Mantle Pie Section and Seismic Velocities
Mantle Subdivisions
assumed density increased smoothly with depth due to increase
of pressure with depth
estimate for center of Earth was 10,000-12,000 kg/m3 (not bad!)
differentiation of Earth
early in its history
breakthrough came with idea that seismic waves generated by
earthquakes could travel through the entire Earth and
be recorded elsewhere on the surface: seismology
earthquake
seismic station
seismic station
travel paths of seismic waves
generated by earthquakes;
directly through the earth or
reflected by discontinuities
different behavior of P and S waves
led to idea of liquid layer in interior
S waves cannot travel
through liquid
creates S shadow zone
from: http://www.seismo.unr.edu/ftp/pub/louie/class/plate
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
by World War II, image of Earth as layered with layers
separated by discontinuities existed…
layered Earth
from: http://www.seismo.unr.edu/ftp/pub/louie/class/plate
depths of layers and
likely compositions
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
now have a good idea of seismic velocity with depth
seismic discontinuities define:
crust; upper mantle; transition zone; lower mantle
outer core; inner core
indicate changes in physical properties
with depth
(mostly density and elastic modulii)
still only a model to fit existing
measurements….
debate continues on
exact position of discontinuities
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
why density variation in Earth?
• changes in chemical composition (compositional changes)
• changes in mineral structures (phase changes)
what changes occur where is a large area of research…
……cannot make direct observation!
draw from geochemistry, mineral physics, meteoritics,
igneous petrology, seismology
crust: felsic (shallow) to mafic
mantle: ultramafic (peridotite)
outer core: liquid iron alloy
inner core: solid iron alloy
crust/mantle: Mohorovicic discontinuity (Moho)--compositional
mantle/core: Gutenberg discontinuity--compositional
inner/outer core: phase (liquid to solid)
400 km discontinuity: phase (olivine to spinel structure)
670 km discontinuity: phase (spinel structure to perovskite)
crust and mantle
(remember that they are distinguished on the basis of
their physical properties)
how do we know what is at depth?
electrical conductivity: identifies partial melts
exposed deep crust: occurs in mountain belts; 50 km originally
geochemistry and elemental abundances: tell range of composition
gravity anomalies: identifies density differences
lithospheric flexure: constrains rheology
magnetic anomalies: shows distribution of subsurface rocks
mineral physics: measures seismic velocities in rock samples
ophiolites: represents oceanic lithosphere
xenoliths in volcanic rocks: represents upper mantle
seismic reflection: identifies changes in lithology
seismic refraction: defines velocities of seismic waves at depth
seismic tomography: permits 3D visualization
crust
obvious from space that Earth has two fundamentally different
physiographic features: oceans (71%) and continents (29%)
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
global topography
bimodal distribution of topography is best illustrated with
a hypsometric curve (cumulative frequency curve)
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
high mountains and deep trenches are only a small portion
two modes (left) or two plateaus (right) on curve with little transition
continental crust: 1000 m
oceanic crust: -4000 m
why bimodal distribution?
differences in composition and thickness of
oceanic and continental crust
oceanic crust:
mafic; denser
continental crust:
felsic; less dense
isostasy:
columns of mass must
be the same at a certain depth
(compensation depth)
~ 50 km
continents have roots and stick-up
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
what controls differences in composition of oceanic and
continental crust?
process of formation
oceanic crust: forms at mid-ocean ridges by seafloor spreading
partial melting of mantle peridotite (high Mg and Fe)
mafic magma (basaltic composition)
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
fast-spreading: magma enters a large magma chamber in crust;
broad bulge exists at ridge
slow-spreading: magma chamber freezes between pulses of
spreading; axial rift valley occurs
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
cross-section through a slow spreading ridge
(Mid-Atlantic Ridge)
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
oceanic crust forms both by intrusion and extrusion of magma…
layers (top to bottom) are similar everywhere:
pillow basalt; gabbro dikes; olivine cumulates
oceanic crust thickness: 6-10 km
as seafloor spreading continues, old seafloor moves away
from ridge axis and marine sediment is deposited on top
layered structure long recognized by seismologists
study of ophiolites (exposed oceanic crust) confirmed compositions:
layer 1: marine sediments
layer 2a: pillow basalt
layer 2b: sheeted-dike (gabbro) complex
layer 3: massive gabbro
below: cumulates (base of crust/top of upper mantle)
ophiolites and seismic layering of oceanic crust
Kearey and Vine, 1972
Oman ophiolite
harzburgite
upper mantle
both from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html
upper mantle
mylonitized dunite
layer 3
olivine gabbros
both from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html
layer 2b: sheeted dikes
layer 2a: pillows
both from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html
dolerite dike intruding pillow basalt
from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html
from: Seth Stein
magnetic anomalies allow dating of oceanic crust…
…for basalts intensity of remanent magnetism > induced
anomalies will vary with latitude and ridge orientation
if oceanic crust acquires its magnetism at high latitudes…
magnetization vector dips steeply…
in northern latitudes…
…dips steeply north for normal
…points steeply up and south for reversed
…closer to equator,
magnetization vector not as steep
…at equator,
magnetization vector horizontal
negative anomaly coincides
with normal blocks
as a consequence of seafloor spreading (and subduction),
oceanic crust is < 200 Ma old (with exception of ophiolites)
note pattern of increasing age away from ridges
continental crust
• 5-10 times thicker than oceanic crust (40-70 km thick)
• average chemical composition is similar to granodiorite
• heterogeneous vertically and laterally
• wide range of ages
most elements forming continental crust migrated from Earth’s
interior during Archean (3.8-2.5 Ga): differentiation
Earth was too hot to form permanent crust prior to 3.8 Ga;
surface likely convecting ultramafic material
at ~ 3.8 Ga, interior of Earth cooled enough to allow a crust to form;
only partial melting occurred (minerals melt at low temperature)
subsequent fractionation and crystallization led to variations in
composition from mafic to silicic
mantle
general composition of peridotite
from seismic velocities, xenolith compositions
seismic tomography (similar to CAT scan of Earth)
suggest inhomogeneous in 3 dimensions
--variations in composition? temperature? both?
composition:
extraction of basaltic magma to produce oceanic crust
“depleted” (without basalt component) mantle
“undepleted” (with basalt component) mantle
temperature:
convection and mantle plumes
models
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
convection in the mantle
observed heat flow
warm: near ridges
cold: over cratons
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
rheology of Earth
lithosphere and asthenosphere
distinguished by response to stress (their “strength”)
---not by seismic discontinuities
thermal boundary (more in a minute)
lithosphere first proposed to explain isostasy
--response of Earth’s surface to vertical loads
(growths of glaciers, formation of islands)
upper most rheologic layer of Earth (lithos-rock):
exhibits flexural rigidity on geological time scales
(resistance to bending)
steel: high flexural rigidity
rubber: low flexural rigidity
over long time periods, lithosphere does flow (more later)
lithosphere moves as a coherent entity: plate
• contains crust and uppermost mantle
• base is the 1280°C isotherm (thermal boundary)
at this temperature, peridotite weakens due
to easy deformation of olivine
• base is not fixed depth; depth of 1280°C isotherm varies
below ridges, temperatures high (lithosphere thin-few km)
below cratons, temperatures low (lithosphere thick-150 km)
asthenosphere behaves like a viscous fluid on geological time scales
• layer of mantle below lithosphere
• composed of predominantly solid, although, weak rock
• low flexural rigidity
• material flow (crystal plastic flow, diffusion): convection
• low velocity zone exists in asthenosphere below oceans
(partial melting? rheology of olivine?)
• base of asthenosphere problematic: 400 km; 670 km; core?
(layered convection? whole mantle convection?)
lithosphere “strong”
asthenosphere “weak”
The layered Earth and ophiolites
websites from which images are drawn:
http://www-personal.umich.edu/~vdpluijm/gs205.html
http://www.seismo.unr.edu/ftp/pub/louie/class/plate
http://www.geo.lsa.umich.edu/~crlb/COURSES/270/
http://www.bris.ac.uk/Depts/Geol/vft/oman.html
http://pubs.usgs.gov/publications/text
sources:
Kearey, P. and F. Vine, 1996, Global tectonics, second edition, Blackwell
Scientific, 333 p.