Lithosphere : mechanical boundary layer, dry-mostly, stable for 10 8 -10 9 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 10 4 -10 5 a. It is a region with small scale partial melt and is electrically conductive (c.f., lithosphere). LAB : Lithosphere-asthenosphre boundary, a is the Lithosphere: it is not the asthenosp
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What is the Lithosphere: it is not the asthenosphere
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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).
What is the Lithosphere: it is not the asthenosphere
Lee et al (2011, AnnRev) Pearson and Witting (2008, GSL)
Isotope systems
NO: U-Pb, Sm-Nd, Rb-Sr, Lu-Hf (incompatible element systems)
YES: Re-Os (compatible element systems)
“Alumina-chron”
Data filter: - No peridotites with less than 0.5 ng/g Os plotted- No samples analyzed by sparging.
Al2O3 (wt. %)
187Os/ 188Os
PUM
J.G. Liu et al., 2009; 2011
TRD (Ga)0.5
2.5
1.0
1.5
2.0
Yangyuan Peridotites, North China Craton
Hannuoba Peridotites,Central Zone:1.9 Ga lithosphere
PUM
0.116
0.120
0.124
0.128
0.132
0 0.1 0.2 0.3 0.4
2 sigma error< spot size
Age = 1.94 ± 0.18GaInitial = 0.1155 ± 0.0008
Initial gOs = 0MSWD = 23
187Re/188Os
187Os/ 188Os
Gao et al., 2002, EPSL
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
ClinopyroxeneOrthopyx
mafic
ultramafic
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)
Lee et al (2011, AnnRev)
Olivine is important
MassifOff-craton
On-craton dunite
Prim. Mantle
meltingtrend
Secular decrease in the ambient mantle temperature – resulted in lower degrees of depletion in the CLM
Lee et al (2011, AnnRev)
Mafic Lithologies
pyroxenites eclogites
Median composition of the CLM
OPX-enrichment is secondary: melt addition or cumulate control
* 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
Composition of the CLM: trace elementsTreatment of data:
non-gaussian distributionaverage (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 samplingsampling by geologistsinfiltration 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
K, in Peridotites:Lithospheric Mantle
Heat Producing Elements
McDonough (1990, EPSL)
REE composition of CLM (median values only)
LREE-enrichmentnot strong
MREE ~ Primitive Mantle
Cratons are strongly HREE-depleted
Most depleted is most enriched – not explained feature
Primitive mantle normalized
McDonough (2000, EPSL)
Incompatible elements in CLM (median values only)
K-depletion - low % partial melt metasom.
~ Primitive Mantle
We can build a complete picture of elements in CLM!
Attributes of Continental Crust and Lithospheric Mantle
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!