CRUSTAL EVOLUTION

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CRUSTAL EVOLUTION
Thermal history of Earth
Decay of U, Th, K isotopes: produce heat
Heat loss on ocean ridges = twice heat produced
Earth must have cooled since Archaean
Komatites indicate Archaean was hotter
Viscosity of mantle increased with time- slower convection
First crust: mafic or felsic?
low degrees of partial melting produce felsic magmas
higher degrees of melting produces mafic magma
Early mantle was hot- implies larger degrees of melting i.e. mafic
Lunar anorthosite model: lunar magma ocean
Plagioclase floated to form anorthosite; Ol, Pyx sank
But in hydrous magmas (Earth) plagioclase sinks.
Basalt model: Greenstone belts- basalt important
Magma ocean- ol, pyx and plag crystaliize= basalt favored
OLDEST ROCKS
Oldest rocks: 4.0 Ga Acasta gneiss (NW Canada)
Oldest mineal: U/Pb 4.3 Ga zircon in 3.2 Ga quartzites
Acasta gneisses range from felsic to mafic- maybe chemically
similar to greenstone belts.
Greenland Archaean
Itsaq gneisses: 3 terranes assembled by 2.7 Ga.
1) Akulleq terrane- mainly Amitsoq tonalite-granodiorite. Age3.9 – 3.8 Ga. Metamorphism at 3.6 Ga
2) Akia terrane- 3.2 to 3.0 Ga, Tonalite. Metamorphism at 3.6 Ga
3) Tasiusarsuaq terrane: 2.7 to 2.8 Ga. Late Archaean
metamorphism
Supracrustal rocks: basalts, komatities, BIF, volcanic turbidites
Overall greenstone type tectonic seting by 3.9 Ga.
No major continents in early Archaean- why not?
1) Fragments re-cycled into mantle (see Nd evidence)
2) Too few collisions
Early crust
Composition
Origin
Oceanic
4.5 Ga
basalt
mantle
partial melt
Continental
<4.3 Ga
tonalite
subduction
(garnet in source)
Why Earth has continents- none on Mars, or Moon (or
Venus?)
Earth is 1) wet and 2) had subduction
True granites only after 2.6 Ga – melting of pre-existing
tonalite
Fractionation events- produced granites with high Rb/Sr ratios
HOW CONTINENTS GROW
Problem: island arcs and ocean plateaux are basaltic,
but average continental crust is andesite.
Lower crust = basalt
Upper crust = granodiorite
Delamination of lower crust (back into mantle) after accretion.
Some seismic evidence favors cold slabs in upper mantle
Plate Tectonics with time
1) Collisional orogens: e.g. Alps, Himalayas (Phanerozoic);
Wopmay Canada- early Proterozoic.
2) Accretionary orogens (terrane accretion): Archaean to
Phanerozoic (Fig. 8.7)
Continental age patterns (Fig. 8.11)
Granatoid peaks:
2.7 – 2.5 Ga
2.0 – 1.7 Ga
1.3 – 1.0 Ga
Greenstone peaks also at 2.7, 1.9, 1.3 Ga
i.e. related to granitoids
Peaks correspond to subduction/accretion events.
Minima correspond to supercontinent existence- little
subduction.
Continental growth rates
Net change in volume of continental crust
Possibilities: positive, negative or zero.
Four models of continental growth (fig. 8.10)
1) Rapid early, slow late growth
2) Continuous growth
3) Slow early, rapid late growth ?
4) Episodic growth
Model 3: old-isotopic ages were re-set by metamorphism
Model 1 implies re-cycling back into mantle- because early
crust rare.
Re-cycling
1) Subduction of sediment
2) Subduction erosion (Fig. 3.21b, Condie)
3) Delamination of lower crust into mantle
Evidence for recycling
Neodymium isotopes
147
Sm decays to 143 Nd,
144
Nd stable
143
Nd/144Nd increases with time.
Fractionation behavior during partial melting:
Nd prefers felsic magmas,
Sm prefers mafic magmas
143
Nd/144Nd ratio will be LOWER in felsic (continental)
rocks, HIGHER in mafic (mantle) rocks of same age.
Epsilon parameter: Nd – compare to meteorites (CHUR)
Nd = [(143/144 sample)/(143/144 CHUR)] x 104
Nd is positive for primitive mantle sources;
negative for enriched continental sources
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