Earth History GEOL 2110 Lecture 11 Origin 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

Compositional Layers Structural Layers MANTLE SiO 2 – 45% MgO – 37% FeO – 8% Al 2 O 3 – 4% CaO – 3% others – 3% CORE Fe – 86% S – 10% Ni – 4% OCEANIC CONTINENTAL OCEANIC CONTINENTAL CRUST CRUST SiO 2 47% 56% Al 2 O 3 16% 18% FeO 13% 9% MgO 10% 3% CaO 10% 4% Na 2 O 2% 5.5% K 2 O 0.7% 2.5% TiO 2 1.1% 1.3% P 2 O 5 0.2% 0.7% Layers of the Earth = chondritic meteorites ---Mohorovicic Discontinuity

Igneous Mineralogy of the Earth’s Crust Ocean CrustMantleContinental Crust

Oceanic Crust Distillation of the Mantle by Partial Melting Magmatism at Mid-Ocean Ridges Peridotite

Oceanic Crust Distillation 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 Crust Distillation of the Mantle by Partial Melting Evidence in Solid Solution Minerals OCEANIC OCEANIC CRUSTMANTLE SiO 2 47% 45% Al 2 O 3 16% 4% FeO 13% 8% MgO 10% 37% CaO 8% 3% Na 2 O 2.5% 0.2% K 2 O 0.7% 0.1% TiO 2 1.1% 0.3 % P 2 O 5 0.2% 0.05% Ni200ppm 5000ppm Cr100ppm 7000ppm 94% 6% 76% 24% Na Fe Ca Mg Olivine/PyroxeneBasalt/ Gabbro

Continental Crust Distillation of Ocean Crust by Partial Melting Evidence in Solid Solution Minerals OCEANIC CONTINENTAL OCEANIC CONTINENTAL CRUST CRUST SiO 2 47% 56% Al 2 O 3 16% 18% FeO 13% 9% MgO 10% 3% CaO 10% 4% Na 2 O 2% 5.5% K 2 O 0.7% 2.5% TiO 2 1.1% 1.3% P 2 O 5 0.2% 0.7%

Present-day Formation of Crust A Double Distillation Process Melting the Mantle Makes Mafic Magma – ALWAYS Melting 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 Crust 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 Canada 4.03 Ga metamorphosed sedimentary rocks

Origin of Seawater and the Atmosphere Volcanic Outgassing

Photochemical Evolution of the Atmosphere Prior to development of the Ozone layer, ultaviolet radiation caused dissociation of water vapor to create oxygen: 2H 2 O + UV rad  2H 2 (lost) + O 2 Oxygen then transformed methane and ammonia to carbon dioxide and nitrogen gas CH 4 + 2O 2  CO 2 + 2H 2 O

Biogentic Build-up of Oxygen in the Atmosphere Banded Iron-formation deposition Fixing biogenic oxygen in chemical sediments

The Global Chemostat and Thermostat The Earth’s Capacity to Buffer

Summary of Early Earth Events

Next Lecture Theory of Plate Tectonics Part 1: Toward the Development of the Grand Paradigm By SARAH Quiz : Chapters 6 & 7