earth history geol 2110 lecture 11 origin and early evolution of the earth part 2: differentiation...
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Earth History GEOL 2110
Lecture 11Origin and Early Evolution of the Earth
Part 2: Differentiation of the Earth’s Spheres
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Major Concepts• Early Earth became strongly heated by gravitational
condensation, radioactive heating of short-lived isotopes, and impacting of asteroids; this resulted in the differentiation of the dense iron core and the light crust from the primordial mantle
• Partial melting of the earth’s ultramafic mantle created the chemically distinctive crust of mafic and felsic composition
• Outgassing of volatiles and photochemical dissociation during early crust formation created an early hydrosphere and atmosphere that quickly evolved to create the dominant chemistry we observe today
• The evolution of the biosphere has subsequently modified the atmosphere’s composition by enriching it in oxygen
• The chemical balance of the earth’s spheres have a great capacity to buffer pertubations to the system
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CompositionalLayers
StructuralLayers
MANTLE
SiO2 – 45%
MgO – 37%
FeO – 8%
Al2O3 – 4%
CaO – 3%
others – 3%
COREFe – 86%S – 10%Ni – 4%
OCEANIC
CONTINENTAL
CRUST CRUST
SiO2 47% 56%
Al2O3 16% 18%
FeO 13% 9%
MgO 10% 3%
CaO 10% 4%
Na2O 2% 5.5%
K2O 0.7% 2.5%
TiO2 1.1% 1.3%
P2O5 0.2% 0.7%
Layers of the Earth
= chondritic meteorites
---MohorovicicDiscontinuity
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Igneous Mineralogy of the Earth’s CrustOcean Crust MantleContinental Crust
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Oceanic CrustDistillation of the Mantle by Partial Melting
Magmatism at Mid-Ocean Ridges
Peridotite
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Oceanic CrustDistillation of the Mantle by Partial Melting
Incongruent Melting – mineral phases don’t melt in equal proportions
Normal Ocean Geotherm
Upwelling Geotherm
Pl
Sp
Gt
Komatiite
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Oceanic CrustDistillation of the Mantle by Partial Melting
Evidence in Solid Solution Minerals
OCEANIC
CRUST MANTLE
SiO2 47% 45%
Al2O3 16% 4%
FeO 13% 8%
MgO 10% 37%
CaO 8% 3%
Na2O 2.5% 0.2%
K2O 0.7% 0.1%
TiO2 1.1% 0.3 %
P2O5 0.2% 0.05%
Ni 200ppm
5000ppm
Cr 100ppm
7000ppm
94%6%
76%24%
NaFe
CaMg
Olivine/Pyroxene Basalt/Gabbro
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Continental CrustDistillation of Ocean Crust by Partial Melting Evidence in Solid Solution
Minerals
OCEANIC
CONTINENTAL
CRUST CRUST
SiO2 47% 56%
Al2O3 16% 18%
FeO 13% 9%
MgO 10% 3%
CaO 10% 4%
Na2O 2% 5.5%
K2O 0.7% 2.5%
TiO2 1.1% 1.3%
P2O5 0.2% 0.7%
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Present-day Formation of CrustA Double Distillation Process
Melting the Mantle Makes Mafic Magma – ALWAYSMelting Crust Makes Intermediate to Felsic Magma
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Crust Formation in Early Earth
Not likely that Plate Tectonics, as we know it today, existed in the early Earth (>3.0Ga). Ocean crust too thick and too hot (plastic) to behave rigidly. Probably overthickened in compression zones causing partial melting of basal zones
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Evidence of Early Continental Crust4.3-4.4 Ga zircon grains found in Sedimentary Rocks in Western Australia
Zircons are rare in mafic rocks but plentiful in granitic rocks
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Formation of the Hydrosphere and Atmosphere
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Evidence of Early Formation of the Hydrosphere
Acta Gneiss of the Slave Province in northwestern Canada4.03 Ga metamorphosed sedimentary rocks
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Origin of Seawater and the AtmosphereVolcanic Outgassing
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Photochemical Evolution of the Atmosphere
Prior to development of the Ozone layer, ultaviolet radiation caused dissociation of water vapor to create oxygen:2H2O + UV rad2H2 (lost) + O2
Oxygen then transformed methane and ammonia to carbon dioxide and nitrogen gasCH4 + 2O2 CO2 + 2H2O
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Biogentic Build-up of Oxygen in the Atmosphere
Banded Iron-formation depositionFixing biogenic oxygen in chemical sediments
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The Global Chemostat and ThermostatThe Earth’s Capacity to Buffer
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Summary of Early Earth Events
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Next Lecture
Theory of Plate TectonicsPart 1:
Toward the Development of the Grand ParadigmBy SARAH
Quiz : Chapters 6 & 7