Earth History GEOL 2110

Lecture 11Origin and Early Evolution of the Earth

Part 2: Differentiation of the Earth’s Spheres

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

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

Igneous Mineralogy of the Earth’s CrustOcean Crust MantleContinental Crust

Oceanic CrustDistillation of the Mantle by Partial Melting

Magmatism at Mid-Ocean Ridges

Peridotite

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

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

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%

Present-day Formation of CrustA Double Distillation Process

Melting the Mantle Makes Mafic Magma – ALWAYSMelting Crust Makes Intermediate to Felsic Magma

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

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

Formation of the Hydrosphere and Atmosphere

Evidence of Early Formation of the Hydrosphere

Acta Gneiss of the Slave Province in northwestern Canada4.03 Ga metamorphosed sedimentary rocks

Origin of Seawater and the AtmosphereVolcanic Outgassing

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

Biogentic Build-up of Oxygen in the Atmosphere

Banded Iron-formation depositionFixing biogenic oxygen in chemical sediments

The Global Chemostat and ThermostatThe Earth’s Capacity to Buffer

Summary of Early Earth Events

Next Lecture

Theory of Plate TectonicsPart 1:

Toward the Development of the Grand ParadigmBy SARAH

Quiz : Chapters 6 & 7