Hadean and Archean Eons

Key events in the Hadean Eon

1) Separation of core and mantle For about 800 million years after Earth accreted, it was heavily bombarded This would liquefy the Earth's crust and mantle repeatedly Evidence from the moon Dating craters reveal range 4.6-3.8 billion years Preserved on moon 2) Formation of Moon 3) First atmosphere and oceans 4) First basaltic and granitic crust

Evidence for Oxygenation: Changes in chemistry of sediments

1. Before 2.2 billion years the pyrite and uranium minerals that are unstable in presence of oxygen are present as clastic grains in sedimentary rocks. After 2.45 billion years ago the uranium minerals disappear. In Archean sediments - find minerals that form only in non-oxidizing environments Pyrite (FeS2) and Uraninite (UO2) in fluvial deposits and unoxidized paleosols indicate non-oxidizing atmosphere as late as 2.2 Ga These minerals easily dissolved out of rocks under present atmospheric conditions Suggests: Throughout Archean little to no free oxygen in atmosphere (<1% of PAL) Little produced by cyanobacteria probably consumed by weathering or respiration 2. Before 2.2 billion years (Banded Iron Formations abundantly formed in oceans). After 1.8 billion years ago we have very few BIF deposits The BIF deposits are important because they form most of the iron ores of the world.

How much geologic time does the precambrian make up?

90%

What is the standard explanation for banded iron formations or BIFs?

After cyanobacteria appear and start to photosynthesize in the shallow ocean...oxygen levels build up in the shallow ocean (i.e., in the well mixed layer). The abyssal layer remains reduced and anoxic (oxygen free). Iron in the oxidized shallow ocean would precipitate out (iron is not "mobile" in oxidized settings). In deep water, the iron would go back into solution as it fell through the water column down into the anoxic abyssal layer. However, in shallow water (continental shelves), where the whole water column was well mixed and oxygenated, it would accumulate on the seafloor. This would only deposit a thin layer; episodic upwelling of abyssal water onto continental shelves would deliver a steady supply of fresh, reduced iron to the shallow water shelves, which would quickly oxidize and precipitate out, generating thick banded iron formations in these areas.

Archean crust

Archean crust is also rare. It only accounts for about 7% of the exposed rock in Earth's Precambrian Shield areas. They are commonly overlain by younger sedimentary rocks and are commonly bordered by younger mountain chains.) Archean crust hosts abundant mineral resources (e.g., gold, nickel, copper, uranium, platinum, chromium and diamonds).

Archean Continental Crust

Archean shield consists of highly metamorphosed protocontinents and slightly metamorphosed belts. Records violent nature of Archean tectonic activities that are no longer active.

What do we know about the precambrian?

Comes from looking at cratons A precambrian shield is a large portion of a craton exposed on the surface The Canadian Shield is the largest example

Archean Plate Tectonics Model

Continental crust grows in size through differentiation, accretion, and possibly subduction Archean protocontinents were amalgamated (formed) through continental-continental collision

Origin of the Crust

Crust began to form when the final magma oceans cooled Originally the crust consisted of basaltic material (like modern ocean basins) Later felsic material began to segregate to form nuclei of continental crust

Origin of Basins

Deep sea basins exist: Presence of Banded Iron Formations Pillow basalts of Archean age Extensive chert beds Also: Continental surfaces form first terrestrial basins

Appearance of life, what do we expect to find?

Earliest evidence of life after 4 Ga 3.5Ga rock abundant in compounds with C12 What kind do we expect to find? Photosynthetic organisms Prokaryotes Stromatolites

How did atmosphere form?

Early degassing during molten phase gave rise to atmosphere 4.5 billion years ago Degassing produces CO, CO2, SO2, NH3, N2, CH4 and H2O Early atmosphere likely much higher in CO2, H and H2O than current atmosphere Remember evolution of the atmosphere!

Coevolution of atmosphere and life

Estimates from paleosols show early (pre-3.8 Ga) O2 levels to be 1.4x10-4 PAL (Present Atmospheric Levels) Before Life, small amounts of O2 produced by photodissociation of H2O in atmosphere (H2O almost certainly present from beginning)

Hadean Eon (formation of earth)

Formation of Earth: Flattening of solar nebula into disk Accretion of material in into planets Continual bombardment Melting: heavy material to core, light material to crust

Origin of Continental Crust

Hadean continental crust, if existed, was small Most has been destroyed Oldest continental crust still preserved about 4 billion years old, Archean Oldest crustal materials 4.1 to 4.2 billion years old For continental crust to form, felsic components must be extracted from mafic rocks Igneous processes associated with hot spots in oceanic crust can produce felsic rock

What are the three eons formally recognized?

Hadean, Archean and Proterozoic

Solidification in the Archean

Heat source = radioactivity and asteroid impact (high in early Earth history) Continental crust formed after oceanic crust

How did we get to today's level of oxygen?

Hypothesis: amount of O2 in the atmosphere has increased with time and Life Evidence from the Rock Record: Iron (Fe) extremely reactive with oxygen - look at the oxidation state of Fe in the rock record, can infer a great deal about atmospheric evolution

How was moon formed?

Mars sized protoplanet collides with Earth Earth was beginning to differentiate, but still partly molten - magma and silicate vapor Accretion of outer disk into moon; inner disk re-accreted to crust

When would crust have begun to form in Hadean? What is it's composition?

Rate of radioactive decay decreased, cooled First crust would have began to form at the end of the Hadean Basaltic and granitic composition Oceanic and continental crust

Late Heavy Bombardment

Period of increased impacts from left over solar system material and planetesimals with the same orbits

The Hadean Eon

Runs from 4.6 Ga (the estimated age of the Earth) to 4 Ga. Name derived from hellish conditions thought to exist at the time: Hades 4.6-4 billion years ago Begins with the origin of solar system and the formation of Earth No rocks on earth are this old, only meteorites Use meteorites and lunar rocks to learn Earth was formed at least 4.6 billion years ago

Greenstone belts

Slightly metamorphosed zones Formed along subduction zones Zones of variably metamorphosed volcanic rock Greenish hue Important sources of precious ores Gold Silver Copper Zinc Lead

Archean Protocontinents

Small continents, resembling Iceland, would have formed in the Archean Weathering and metamorphism would generate additional felsic rock Weathering: removes Fe and Mg from mafic rocks, leaves behind felsic clays Metamorphism: converted felsic shales to durable felsic metamorphic rock

Archean Atmosphere

Still low in oxygen Evidence: BIFs When photosynthesis started, the Earth's surface was likely reducing. Oxygen was absent. Because of the reducing environment, iron would have been mobile on the Earth's surface. It would have been leached from soils and sediments, then delivered in abundance by rivers to the ocean, where it would have built up over time in dissolved, reduced form. In addition, hydrothermal vents would have delivered iron to the ocean.

When was the earth's surface cool enough for oceans to form and stay?

The 4.4 Ga zircon has oxygen isotopes consistent with magma that reacted with liquid water Would need an atmosphere to be present, as surface temperatures still quite high High pressure CO2 atmosphere would allow liquid H20 to exist

When does the Archean Eon happen? What are its key events and concepts?

The Archean Eon runs from 4 to 2.5 Ga. Key events and concepts End of heavy bombardment First large cratons form (but most preserved Archean rocks are greenstone belts) Life emerges First photosynthesis, but oxygen levels stay low. First terrestrial basins develop

Why do BIFs form today?

The oxygen-rich (20%) atmosphere and the oxygen mixed throughout the oceans prevents banded iron formation from forming today To get iron into solution to precipitate from water it must be in the reduced state (Fe+2), but with oxygen available, iron is almost insoluble

The Earth must also have suffered similar heavy bombardment until the start of the Archean. However, the Earth bears little evidence of this; it is not pockmarked like the Moon. Why?

The reason for this is that Earth is not a "dead" planet. Surface renewal is ongoing. Surface renewal continues to this day on Earth thanks to plate tectonics, volcanism, sedimentation, and erosion. These processes "wipe the slate clean" over time, erasing the visible signs of many impact craters soon after they form.

Why do we know so less about the precambrian compared to the Phanerozoic?

This is mostly due to the nature of the geologic record: Precambrian rocks rare, less than 20% of all exposed rocks on the Earth's surface Tectonics and erosion has destroyed many The rocks that make up the Precambrian record are mostly metamorphosed Precambrian sedimentary rocks, for the most part, lack index fossils or fossils at all for that matter

How are oceans formed?

Water vapour from degassing and volcanic emissions form oceans

Sink

a reservoir that grows to take up a chemical as rapidly as it is produced

Meteorite Impacts

did not stop following the giant impact hypothesized to have formed the Moon. In fact, most geologists believe that heavy bombardment by asteroids continued into the Archean. As such, the great reduction in impact crater density from lunar highlands (Hadean) to lunar maria (Archean) areas suggests that heavy bombardment of the Moon had largely stopped by the end of the Hadean (~4 Ga).

Terrain

geologically distinctive region of Earth's crust

Highly metamorphosed zones

gneiss

Basins

large scale structural formation of rock strata formed by tectonic warping of previously flat-lying strata also known as Geological depressions

Stromatolites

some of the oldest, and most convincing, fossils on Earth. They have been found in rocks as old as 3.5 Ga (Warrawoona Fm, Australia).

Precambrian Shield

the large portions of cratons exposed at the surface of the Earth