Transcript Lithospheric_Mantle_McDonough

What is the Lithosphere: it is not the asthenosphere
Lithosphere: mechanical boundary layer, dry-mostly,
stable for 108-109 a, possessing a steady-state conductive
geotherm with base in cratons at 4-7 GPa (170–250 km),
shallower (ca 100-150km) in off-cratons, and shallower
still in oceans (<100 km)
Asthenosphere: weak layer underneath the lithosphere,
area with pervasive plastic deformation deforming over
104-105 a. It is a region with small scale partial melt and
is electrically conductive (c.f., lithosphere).
LAB: Lithosphere-asthenosphre boundary, a transition
region of shear stress and anisotropic fabric, perhaps a
transition between diffusion vs dislocation creep. The
transition may or may not be sharp (up to tens of km).
lithosphere-asthenosphere boundary (LAB) properties
crust
mantle
w/ melt
Fischer et al (2010, Ann Rev)
Eaton et al (2009, Lithos)
Mantle
Crust
Composition of the lithospheric mantle
Approaches
geophysics: seismology, gravity, heat flow, tectonics
(rheology, deformation, uplift, erosion)
geochemistry: petrography, elemental, isotopic
Sampling the lithospheric mantle
Approaches
geophysics: 103 – 106 meters
geochemistry: 10-3 – 10-6 meters
- 6 to 12 orders of magnitude difference
Why study composition of the CLM?
- Place constraints on the timing and tectonic setting
for the formation of continents & their roots
- Examine consequences of the Earth’s secular
evolution
- Test models of basaltic source regions
- Characterize the inventory of elements in an Earth
reservoir
The different Lithospheres
one example
LID
Chemical
Mechanical
Thermal
Seismological
Tectosphere
Bottom: asthenosphere
(LAB)
Top: MOHO (seismic)
petrologic break
Oceanic
Continental:
craton vs off-craton
Where are the cratons and off-cratons
Pearson and Witting (2008, GSL)
Where are the cratons and off-cratons
Lee et al (2011, Ann Rev)
Growth of Lithospheric Mantle (LM)
- Mostly linked to crust production
- Different in oceanic vs continental setting
- Oceanic: crustal growth in divergent margin
settings, with LM growth via lateral accretion of
refractory peridotite, followed by conductive
cooling of deeper lithosphere
- Continental: mostly convergent margin tectonic
growth, with some intraplate contributions, LM
grows by accretion of refractory diapirs
Oceanic &
Continental
Crusts
60% of Earth’s surface consists
of oceanic crust
Oceanic lithosphere cools, thickens and
increases in density away from the ridge
Increasing density of lithosphere with age leads to
progressive subsidence (age-depth relationship)
Seafloor subsidence & heatflow reflect progressive
thickening of lithosphere with age
Depth
D(m) = 2500 +350t1/2
q = 480/t1/2
Heatflow
Wei and Sandwell 2006 Tectonophysics
Continental Lithospheric Mantle
CLM growth models
Lee et al (2011, Ann Rev)
Heat production in the Lithosphere
- Heat Producing Elements (HPE): K, Th, U
- Continental Surface heat flow (Q)
Craton 40 mW m-2
Off craton 55 mW m-2
- Near surface heat production
- Heat production versus depth
- Concentration of HPE in Lithospheric Mantle?
Earth’s Total Surface Heat Flow
40,000 data points
Conductive heat flow
measured from bore-hole
temperature gradient and
conductivity
Surface heat flow
463 TW (1)
472 TW (2)
mW m-2
(1) Jaupart et al (2008) Treatise of Geophys.
(2) Davies and Davies (2010) Solid Earth
Earth’s surface heat flow 46 ± 3 (47 ± 2)
Mantle cooling
(18±10 TW)
Crust R*
(7±3 TW)
Mantle R*
(13±4 TW)
*R radiogenic heat
after Jaupart et al 2008 Treatise of Geophysics
Core
(9±6 TW)
(0.4 TW) Tidal dissipation
Chemical differentiation
± are 1s.d. estimates
- linear relation between heat flow and radioactive heat production
- characteristic values for tectono-physiographic provinces.
180
(b)
160
Q = Q0 + Ab
140
mW m-2
120
100
80
EUS
SN
B&R
60
(Q0)
40
20
0
0
2
4
6
uW m-3
Birch et al., (1968)
8
(A)
10
12
Q = Q0 + Ab
1 Baltic Shield
2 Brazil Coastal
3 Central Australia
4 EUS Phanerozoic
5 EUS Proterozoic
6 Fennoscandia
7 Maritime
8 Piedmont
9 Ukraine
10 Wyoming
11 Yilgarn
Mahesh Thakur & David Blackwell (in press)
Archean lithosphere is thick & cold
0
Slave
50
2
100
4
150
6
Lesotho
Jericho
Kimberley
Lac de Gras
Letlhakane
8
Best Fit
200
Depth (km)
Pressure (GPa)
Kalihari
Kalihari
Torrie
250
Grizzly
300
10
0
200 400 600 800 1000 1200 1400 1600
Temperature (oC)
200 400 600 800 1000 1200 1400 1600
Temperature (oC)
From Rudnick & Nyblade, 1999
Lee et al (2011, Ann Rev)
Fischer et al (2010, Ann Rev)
Age of CLM
Isotope systems
NO: U-Pb, Sm-Nd, Rb-Sr, Lu-Hf
(incompatible element systems)
YES: Re-Os
(compatible element systems)
Lee et al (2011, AnnRev)
Pearson and Witting (2008, GSL)
“Alumina-chron”
Yangyuan Peridotites, North China Craton
PUM
TRD (Ga)
0.5
187Os/
1.0
188Os
1.5
2.0
2.5
Al2O3 (wt. %)
Data filter: - No peridotites with less than 0.5 ng/g Os plotted
- No samples analyzed by sparging.
J.G. Liu et al., 2009; 2011
Hannuoba Peridotites,Central Zone:
1.9 Ga lithosphere
0.132
PUM
2 sigma error
< spot size
0.128
187Os/
0.124
188Os
0.120
Age = 1.94 ± 0.18Ga
Initial = 0.1155 ± 0.0008
Initial g Os = 0
0.116
MSWD = 23
0
Gao et al., 2002, EPSL
0.1
0.2
187Re/188Os
0.3
0.4
Sm-Nd isotopes do not tell you about the age of the CLM
McDonough (1990, EPSL)
Lithospheric Mantle samples: Oc. vs Cont.
- On-Craton xenoliths
- Archean
- Off-Craton xenoliths*
- post-Archean
- Massif peridotites
- post-Archean
- Abyssal peridotites
- Phanerozic
- Oceanic Massifs
- Phanerozic
*no compositional distinction in Protoerzoic and Phanerozoc Off-Craton
Mineralogy of the Lithospheric Mantle
Olivine
*
ultramafic
mafic
Orthopyx
Clinopyroxene
Mafic assemblages in the CLM
Pyroxenites versus Eclogites
- Archean roots have distinctive assemblages
- Diversity of d18O values (evidence for
recycling)
- Probably ~5% by mass in CLM (…squishy #)
- Which ones are lower crustal vs those resident
in the CLM? …. what is the Moho?
Mafic lithologies are there, but what to do with them?
– they do not dominant CLM chemical budget
Significant findings:
- Cratonic roots are melt residues of circa ≤ 30% depletion
- Off-cratonic regions are dominantly post-Archean, with no
chemical distinction in suites over the last 2.5 Ga
- Melt depletion occurred at <3 GPa in all regions
- Re-Os system yield robust ages for the CLM that can be
correlated with the ages of local surface rocks
- No evidence for vertical compositional gradients in the CLM
- CLM growth during crustal genesis via residual diapiric
emplacement (conductive cooling additions – negligible)
Spinel- facies mineralogy
(<70 km)
Garnet- facies mineralogy
(>70 km)
Olivine is important
Lee et al (2011, AnnRev)
melting
trend
Massif
Off-craton
On-craton
dunite
Prim. Mantle
Secular decrease in the ambient mantle temperature
– resulted in lower degrees of depletion in the CLM
Mafic Lithologies
pyroxenites
eclogites
Lee et al (2011, AnnRev)
Median composition of the CLM
*
* In Kaapvaal, less so Siberian, much less elsewhere is the CLM OPX-enriched
- System is modeled w/ differ ratios of “basalt” + residue = PM
- Fe-depletion @ hi melt depletion
most bouyant residues
OPX-enrichment is secondary: melt addition or cumulate control
Composition of the CLM: trace elements
Treatment of data:
non-gaussian distribution
average (not a good measure)
median (better)
log-normal avg (better, will equal mode)
Sampling biases:
fraction of ultramafic to mafic
analytical (below detection (reported?), not measured)
geological sampling
sampling by geologists
infiltration by host magma, weathering of xenoliths
Is it an enriched mantle region?
- mantle metasomatism?
- source of basalts?
Characterization of elements in peridotites
Compatible to mildly incompatible elements
Di = Ci in residue/Ci in melt
Di > 1, compatible element
Di <1, incompatible element
Highly incompatible elements
Heat Producing
Elements
K, in Peridotites:
Lithospheric Mantle
McDonough (1990, EPSL)
REE composition of CLM (median values only)
Primitive mantle normalized
LREE-enrichment
not strong
MREE ~ Primitive Mantle
Cratons are strongly
HREE-depleted
Most depleted is most enriched – not explained feature
McDonough (2000, EPSL)
Incompatible elements in CLM (median values only)
K-depletion - low %
partial melt metasom.
~ Primitive Mantle
Primitive mantle normalized
We can build a complete picture of elements in CLM!
Incompatible element Budget in CLM
two-stage production of composition
Places limits on
heat production in CLM
compatibles, never
>factor 2 times PM
Primitive mantle normalized
degree of depletion
Constrained from Ca, Al & Ti
Th
Nb
La
Nd
Zr
Ti
Yb
Ca
Sc
Al
Ga
Re
Si
Fe
Mn
Mg
Ni
Ir
Integration of major, minor and trace elements
Attributes of Continental Crust and Lithospheric Mantle
±U
U (ng/g) (ng/g)
%
Thickness
(km)
Mass
(1022 kg)
Mass %
Continental crust
40
2.17
0.54%
1300
30%
35%
Cont. Lithospheric
Mantle
~160
8
2%
30
50%
3%
Mantle
(all else down there)
2695
395
98%
13
20%
62%
Silicate Earth
2895
404.3
100%
20
--
100%
Reservoir
U (%)
For cratonic & off-cratonic regions
- melt depletion is a continuum with no significant
differences in time or space (also cannot identify
regional distinctions*)
- OPX-enrichment is an overprinted feature found in
some cratons and is dominant in the Kaapvaal
cratonic and immediate off-cratonic area
- residual peridotites were produced at <3 GPa and
have been overprinted by low degree undersaturated
melts
- CLM is not a significant chemical reservoir, for the
Earth’s budget its compositional contribution = mass
contribution
(*Large scale perspective, regional features not highlighted)
For cratonic & off-cratonic regions
- elements show a non-normal log distribution
- median composition characterizes the abundances of the
moderately to highly incompatible trace elements in
the Lithospheric Mantle (Oceanic and Cont.)
- absence of chemical signature in CLM for growth in
convergent margin settings
- the absence of this signature does not mean the CLM
was not developed dominantly in such a tectonic
setting
- Stability of CLM…. this is another lecture, but let’s
discuss!
Thank you.