Composition and Evolution of the Lithosphere

Composition and Evolution of
the Lithosphere
Matthias G. Barth
Universität Mainz
MYRES II: Dynamics of the Lithosphere
Contents
Earth’s Layers
The Earth is divided into
three chemical layers:
the core,
the mantle and
the crust
Chemical & mechanical
differences
Like boiled egg
A Geologist’s View of the Earth
The outermost sublayer is the most active geologically.
Large scale geological processes occur, including
earthquakes, volcanoes, mountain building and the creation
of ocean basins.
The Lithosphere
Greek (lithos = stone)
outermost layer made of
crust and uppermost
mantle
rigid
broken up into the moving
plates that contain the
world's continents and
oceans
asthenosphere = “fluidlike”
Isostasy
Continental lithosphere is less dense than oceanic lithosphere
and “floats” higher than oceanic lithosphere.
Oceans vs Continents
Oceanic Lithosphere
young
simple ☺
thin crust
mantle density related to
cooling
Continental Lithosphere
long history
complicated (uh-oh…)
thick crust
no simple density
relationships
Mineralogy and composition of the
Lithosphere
Mineralogy
plagioclase
K-feldspar
quartz
amphiboles, pyroxenes
(Fe, Mg minerals)
Continental Crust
hydrous minerals
intermediate
(micas, amphiboles)
Plagioclase
Oceanic Crust clinopyroxene
orthopyroxene
(olivine)
olivine
Upper Mantle orthopyroxene
(< 400 km)
(clinopyroxene)
(plagioclase, spinel, garnet)
Bulk Composition
mafic
ultramafic
Oceanic vs. Continental Crust
Oceanic Lithosphere
vP = velocity of the longitudinal wave
Detailed Structure of Ocean Crust
Oceanic Crust and
Upper Mantle
Structure
Lithology and thickness of a typical
ophiolite sequence, based on the Samial
Ophiolite in Oman.
An ophiolite is a sequence of rock that is
interpreted as representing oceanic
lithosphere.
After Boudier and Nicolas (1985)
Marine Sedimentary Rocks
Pillow Lavas
contact between lava and seawater
Sheeted Dyke Complex
feeder dykes of the basalt
Isotropic Gabbro
“magma chamber”
Layered Gabbro
“magma chamber”
Ultramafic Cumulates
transition zone between crust and mantle
MTZ – Moho Transition Zone
Residual Peridotite
“depleted mantle”
Thermal Structure of Oceanic
Lithosphere
z ⎞
⎟
⎝ 2 Kt ⎠
⎛
cooling of a half-space: T (z, t ) = T0 + (T∞ − T0 )erf ⎜
good approximation for lithosphere <80 Ma
after Boudier et al. (1988)
Igneous Processes at Mid-Ocean
Ridges
Melt Transport
after Niu (2004)
Peridotite Melting
polybaric melting at MOR
>2.5 – 0.8 GPa
10-25% partial melting
mostly in the spinel stability field
depleted source
magma separation
1.2 – 0.8 GPa (25-35 km)
~40% fractional crystallization of
olivine ± plagioclase
Abyssal Peridotites
slow-spreading ridges:
lherzolites
cpx-bearing harzburgites
fast-spreading ridges:
cpx-bearing harzburgites
cpx-poor harzburgites
after Niu and Hékinian (199
Chemical Composition of Abyssal
Peridotites
increasing degree of
melting
Al2O3
MORB
source
s
ou
dr
g
hy
an eltin
m
MgO increases
CaO, Al2O3, TiO2,
Na2O decrease
decreasing density
5
4
5%
3
near-fractional melting
incompatible trace
elements extremely
depleted
low H2O in residue
ab
y
10%
2
ss
al
p
er
id
o
15%
tit
e
20%
1
s
25%
SSZ
perid
otite
s
0
36
38
40
42
44
MgO [wt%]
46
48
50
The Axial Magma Chamber
original model:
semi-permanent
Periodic reinjection of fresh, primitive
MORB from below
Dikes upward through the extending and
faulting roof
Fractional crystallization
derivative
MORB magmas
Crystallization near top and along the
sides
successive layers of gabbro
(layer 3)
Layering in lower gabbros (layer 3B) from
density currents flowing down the sloping
walls and floor
Dense olivine and pyroxene crystals
ultramafic cumulates (layerafter
4)Byran und Moore (1977)
A modern concept of the axial
magma chamber beneath a
fast-spreading ridge
after Perfit et al. (1994) Geology, 22,
375-379.
Slow-Spreading Ridge
Dike-like mush zone and a smaller transition zone beneath welldeveloped rift valley
Most of body well below the liquidus temperature, so convection and
mixing is far less likely than at fast ridges
Magmas at slow-spreading ridges are generally less differentiated than
fast ridges
2
Depth (km)
Rift Valley
4
6
Moho
Transition
zone
Gabbro
Mush
8
after Sinton and Detrick
(1992) J. Geophys.
Res., 97, 197-216.
10
5
0
Distance (km)
5
10
Continents “Float” on top of the Mantle
Density of continental crust = 2.7
Density of oceanic crust = 3.1
The Structure of the Continental Crust
The continental crust is the layer of granitic and
sedimentary rock which forms the continents and the
areas of shallow seabed close to their shores, known as
continental shelves.
NNE
Scandinavian
Caledonides,
Barents Sea
Archean
Crustal
Province
Svecofennian
Crustal
Province
SSW
0
Depth 20
[km]
40
60
0
after Wedepohl (1995)
200
400
600
km
800
1000
1200
Sediments, Granites, Gneisses
vp < 6 - 6.5 km/s
Mafic Granulites
vp = 6.9 - 7.5 km/s
Felsic Granulites
vp = 6.5 - 6.9 km/s
Lithospheric Mantle
vp = > 8.1 km/s
1400
Upper continental crust (UCC)
Most accessible; but also heterogeneous and
differentiated.
About 30% of the continental area is submerged
beneath the oceans.
Precambrian shields and platforms (cratons) structure well-known, with Z = 35 - 45 km; Vp = 5.8
- 6.4 km/s (UCC), 6.5 - 7.2 km/s (LCC)
Conrad discontinuity - present or absent
Orogenic belts - crustal structures very
complicated. In some areas, the Moho is
transitional, rather than discontinuous.
Methods for determining the composition of the
UCC
a) Using geological maps to obtain weighted averages
(Clarke, 1889; Clarke and Washington, 1924).
(b) Analysis of composite samples of large surface areas
(Shaw et al., 1967).
(c) Geochemical approach - analysis of fine-grained
sediments (shales or loess) and determine the
composition of insoluble elements. Estimation of other
elemental abundances from a variety of geochemical
principles (Goldschmidt, 1933; Taylor and McLennan,
1985; Rudnick and Fountain, 1995).
Upper Continental Crust
composition of UCC ≈ granodiorite
sedimentary and granitic rock
31.7% of CC (constrained by heat flow data)
thickness 10-13 km
major element compositions of different estimates
after Rudnick and Gao (2004)
Trace Elements in the Upper Crust
highly enriched in incompatible trace
elements
heat-producing elements (K, Th, U) are
concentrated
negative Eu anomaly
low Nb & Ta
high Pb
after Rudnick and Gao (2004)
The Deep Continental Crust
Vp Data of Crustal Sections
after Rudnick and Fountain (1995)
vP - density
correlation
linking geophysical data to
deep crustal lithologies
P-wave velocity
seismic anisotropy
density
heat flow
lower crustal xenoliths
high-grade metamorphic
terranes
lithology
after Rudnick and Fountain (1995)
Composition of the Deep Crust
less well constrained than upper
crust
middle crust
29.6% of bulk crust
amphibolite facies metamorphic rocks
similar to upper crust
lower crust
38.8% of bulk crust
mafic
granulite facies country rocks and
basic intrusives and/or cumulates
less enriched in incompatible trace
elements
not strictly residual (or complementary
t
t)
after Rudnick and Gao (2004)
Continental Crust – Summary
intermediate composition
relatively high Mg#
enriched in incompatible
elements
low Nb/La and low Nb/Ta
inconsistent with single-stage
melting of peridotitic mantle
ca. 30-40
Additional Processes
delamination
density foundering of mafic lower crust
silicic melts derived from subducted oceanic crust
more prevalent in the Archean?
weathering
preferential recycling of Mg ± Ca into the mantle
ultramafic cumulates in the uppermost mantle
unlikely – uppermost mantle dominated by restitic peridotite
crustal recycling important throughout Earth history
Origin of Continental Crust
4.5
4.0
Archean
TTG
La / Nb
3.5
Arc
3.0
2.5
continental crust
2.0
1.5
1.0 Intraplate lava
0.5
0
20
40
60
80
Growth in convergent margins (%)
after Barth et al. (2000)
100
Two-Stage Process
1) mantle melting
lower crust
2) lower crustal melting
upper crust
after Hawkesworth and Kemp (2006)
Recycling of the Residue
residue of lower crustal melting is complementary to
upper crust
return of the residue to the mantle
shorter residence time of the lower crust than the upper
crust
after Hawkesworth and Kemp (2006)
Heat Flow
heat flow and surface heat production are correlated
average heat production of CC
bulk continental crust: 0.79 – 0.95 µW/m3
heat flow component: 32 – 38 mW/m2
Precambrian crust:
0.77 ± 0.08 µW/m3
23 – 30 mW/m2
Phanerozoic crust:
1.03 ± 0.08 µW/m3
37 – 43 mW/m2
after Jaupart and Mareschal (20
When did the continental crust begin to
form?
Oldest known rocks are
from the Great Slave
Province in Canada and
are approximately 4 Ga
old.
Oldest known mineral is
a zircon is from
Australian sediments
whose metamorphic
age is 3.5 Ga.
Inherited zircons are as
old as 4.4 Ga!
Age of the Continental Crust
When did the continents grow?
The Continental Lithospheric Mantle
thick (60 to >250 km)
age ± 200 Ma of the overlying crust
thickness & composition varies systematically with age
Phanerozoic:
60-130 km
Proterozoic:
150-180 km
Archean:
180-250 km
Archean CLM is strongly depleted and highly buoyant
± metasomatically overprinted
enriched in incompatible elements
“enriched” isotopic signature (low 143Nd/144Nd, high 87Sr/86Sr)
Age-Dependant Composition
older
after Griffin et al. (2003)
more depleted
lower intrinsic density
Archean CLM is unique
not simply more depleted
higher Si/Mg
lower Cr#, Ca/Al, Fe/Al at a given
Mg#
subcalcic garnets & diamonds
often overprinted
cryptic metasomatism
modal metasomatism
original composition difficult to
reconstruct
are only the depleted ones
preserved?
after Griffin et al. (2003)
How was the unique Archean CLM
produced?
high-degree melting at high
pressure
“komatiite extraction”
subduction-related
“lithospheric stacking”
after Griffin et al. (2003)
Two Major Archean Crustal Rock
Associations
Granulites (granite/gneiss comlexes): gneisses of
tonalites, granodiorites, granites, and layered intrusive
gabbros
Greenstones: basaltic, andesitic, and rhyolitic volcanic
rocks with metamorphosed sediments and basaltic pillow
lavas (sequential transition)
Tectonics
Granulites: island arc and continental margins
Greenstones: back arc basins
Granites: later intrusions
Greenstone
Belts
“Greenstone Belts” are basically metamorphosed basalts
and graywacke (discussed below) sandstones deposited
as pillow lavas and turbidity flows on the floors of ancient
seas.
When protocontinents collided and accreted, the ocean
floors filled with these basalts and graywackes
collapsed, forming greenstone belts that also accreted to
the growing protocontinent.
Thus some of the early seafloor survived destruction (by
subduction) and became part of the stable craton.
Evolution of greenstone belts
A. Basins between protocontinents fill with basalts,
B. when protocontinents collide, they “collapse” the oceans filled with
basalts and graywackes, forming greenstone belts.
Archaean domeand-keel
patterns
Vertical tectonics
(“sagduction”)
Zimbabwe (2.7 Ga)
Pilbara (3.5 Ga)
TTGs and adakites
Are TTGs and adakites similar?
Y
!
s
e
No
That’s the stuff active
scientific research is made of
…
!
MgO increases inTTG in course of time
SiO2 decreases inTTG in course of time
Adakites have exactly the same evolution pattern as (young)
TTG are systematically MgO poorer than
For the same SiO , experimental melts
2
TTG