Week 2:

1
Week 2 When the Earth was young (Archean eon)

• EPSC 233-001 Fall 2002

2
Oceanic and continental crusts have different
proportions of the main elements.
continental (felsic) crust more Al, Si, less Mg,
Fe average density 2.7 g/cm3 oceanic (mafic)
crust less Al, Si, more Mg, Fe average density
3.0 g/cm3
3
Greenstone belts (shown in black on map)

• Podlike bodies consisting of
• metamorphosed mafic and ultramafic volcanic
  rocks (very Mg-rich, Si-poor, a mantle-like
  chemistry)
• felsic volcanics
• deep-water sediments chert (no limestone)

4
How do you form greenstone belts? 1) Mantle
plumes are columns of ultramafic magma rising
from deeper parts of the mantle... They could
explain the ultramafic lavas of greenstone
belts. They might have been more numerous in the
Archean than ever since because of a hotter
mantle.
5

• In Hawaii, the plate moves over the hot spot at a
  rate of several cm per year. This has created a
  string of basaltic (mafic) volcanic islands.

6

• Iceland, on the other hand, sits atop a
  mid-ocean ridge. The plate motion is slow (about
  2 cm/year) and the plume below melts and remelts
  the new oceanic crust.

7
Rocks are mixtures of minerals that do not all
melt at the same temperature. If you melt only
part of the mantle and send that melt upwards
through fractures in the rock, you are extracting
a more felsic magma (the aluminum, silica-rich
minerals melt earlier than the Mg-rich
silica-poor minerals.) This magma is less dense
than the mantle and tends to rise through faults
(large cracks in the rocks)... This magma may
crystallize near the Earths surface and be
remelted again to produce a more felsic
composition, more like continental crust.
8
Iceland is unusual because it is a baby continent
growing near a mid-ocean ridge (shown on left).
Most continental crust is probably created near
subduction zones (as shown below), where partial
melting of oceanic crust takes place as it is
subducted.
Greenstone belts were probably folded during
subduction of small Archean plates .
9
Earliest crust would have been basaltic (close to
average mantle composition), Mg-rich and
Si-poor. Continental crust grew more slowly, and
formed by partial melting of the early oceanic
crust.
10

• Within our solar system, our Earth is uniquely
  suited to life
• size and distance from Sun are right
• large enough to retain an atmosphere
• this atmosphere is thin enough to let sunlight
  reach its surface, thick enough that surface
  water does not evaporate
• surface water can exist in liquid state

11
Evidence of earliest life?
Isua Gneiss (3.8 Ga) from Greenland
(metamorphosed sandstones). It was suggested in
1996 that the proportions of carbon
isotopes 13C/12C in their graphite suggests that
the carbon was originally fixed
by photosynthesis.
The banded gneiss containing graphite flakes, and
the dated igneous rock that crosscuts it.
12
Carbon Isotopic Evidence for Early Life
A paper in Nature, this year, suggests that other
inorganic processes can produce a life-like
isotopic signature.
13
Modern and Ancient Stromatolites
Oldest ancient bacteria, 3.5 Ga, are preserved in
chert (SiO2) found with stromatolites.
14

• A life-like 13C/12C signature is a chemical
  fossil.
• Earliest fossils of cells, preserved in chert
  (soft gelatinous dissolved silica, common in
  areas of submarine volcanic activity) are body
  fossils.
• Stromatolites are bacterial condominiums
  layered sediments that accumulate as columns or
  domes where grains are trapped on sticky mats
  formed by bacterial colonies.

(It has been argued, however, that every one of
these features could form without the influence
of life... So these observations support an
hypothesis for the age of the earliest life.)
15
(Australia)
(Greenland)
(Nunavut, Canada)
16
Many chemicals are expelled at mid-ocean ridges,
some of which decompose in seawater and release
energy. Earliest bacteria could have evolved
there, by using various chemicals as energy
sources (chemosynthesis).
17
Banded iron formations are also found in
Greenland.
Layers rich in iron oxide alternating with
silica-rich layers. Submarine volcanic
eruptions were the source of the dissolved silica
that was precipitated as chert. What was the
source of the iron? Today, river waters carry
hardly any dissolved iron to the ocean because it
rusts on the continents.
18

• This world was fit for life, but alien to us . . .

• - no significant free oxygen
• - a CO2-rich atmosphere
• shorter days (a minimum of 15 hours . . . )
• stronger tides (Moon was closer)
• - iron deposition along the continental platform
• BUT . . .
• - ocean composition probably similar to today
• - temperature range of liquid water much like
  today
• - simple life forms may already be changing the
  composition of the atmosphere

19
The fragments of older rocks incorporated in
younger strata were produced during periods of
low sea level, when the older rocks were exposed
to erosion.
Is this list of the different rock types red,
pink, orange, yellow, aqua in the correct time
sequence ? No. It should be pink, red, orange,
yellow and aqua. The red igneous rocks must be
younger than the metamorphic rocks.