Transcript The bulk composition of the Earth

Mantle composition
• 1800s meteorites contain similar minerals to
terrestrial rocks
• Hypothesis that meteorites come from
asteroid belt and originate from a single
planet with metallic core a silicate mantle
and crust
• Earth must be similar
Mantle composition
• Beginning 20th century, meteorites give
better estimate of bulk composition of the
Earth than rocks collected at its surface
• First estimate of earth composition from
data of all know meteorites
• Wiechert suggested iron core from
meteorite evidence before core was
established from seismic data
• Spectroscopy established similarity between
meteorite and solar composition
Meteorites are essential for mantle
geochemistry
• Offer the possibility to look back in time
and deep into planets
• Stone and iron bodies that arrive on Earth in
small numbers (mostly from asteroid belt)
• Most finds in Antarctica
• Classification
Meteorite Types & Percentage that Falls to the Earth
•Stony meteorites
•Chondrites (85.7%) (named after inclusion of chondrules)
•Carbonaceous
•Enstatite
•Achondrites (7.1%)
•HED group
•SNC group
•Aubrites
•Ureilites
•Stony iron meteorites (1.5%)
•Pallasites
•Mesosiderites
•Iron meteorites (5.7%)
Cosmic abundances
• High similarity between relative
abundances of atoms in solar atmosphere,
chondrites and the planets
• But
• Most abundant elements in Earth are
• O (highest in volume % )  isototopes
important for geochemical models
• Na
• Mg
• Al
• Si
• Ca
• Fe (highest in mass % )
Bulk composition based on cosmic
abundances
• Cosmic ratio Mg/Si=1.07
• More orthopyroxene than olivine (dominant in
uppermost mantle) in the average mantle
• Crust: SiO2, Al2O3 (Sial), CaO, Na20
• Cosmic model contains 5.8%, but total crust 0.5%
 missing crust in the mantle or core?
The bulk composition of the
Earth
Where does it all come from?
Drake and Righter, Nature, 2002
The overall composition of the Earth is difficult to
evaluate.
Intense processing and geochemical variety of terrestrial
samples accessible to observations together with the
existence of inaccessible domains in the deep Earth, do not
allow us to built a verifiable picture of the mean
composition of the Earth.
 What is the primitive composition from which all
known rocks must evolve.
Heterogeneous accretion hypotheses explains staircase diagram
Building blocks of planets
Chondrites are meteorites that have changed little since
they first formed 4.5 billion year ago. They have a primitive
(close to solar minus volatiles) composition.
The composition of the Earth's primitive upper mantle
(PUM, the Earth's mantle immediately after core formation)
is distinct from that of any kind of primitive meteorite.
Geochemical processes on differentiated planets tend to
raise the Mg/Si ratio and lower the Al/Si ratio in mantle
materials from which magma has been extracted, reflecting
the compatible nature of Mg and the incompatible nature of
Al. Thus, Mg/Si and Al/Si ratios in samples from both Earth
and Mars correlate with a negative slope. In contrast,
primitive materials show a loose positive correlation of
unknown meaning.
Why has PUM higher ratios?
• Si could have entered core during
formation, but this is difficult
• PUM is different from bulk mantle. This is
not very likely given geophysical evidence.
• Most likely, Earth accreted from material
different form any extant meteorite types.
Oxygen isotopes
Oxygen isotopes
Earth same oxygen reservoir than moon and enstatite
meteorites.
Different from Mars (which is close to Earth)
 Distinct oxygen reservoirs over small distances in solar
nebula
Osmium ratios
Osmium ratios
PUM overlaps with anhydrous ordinary chondrites
 Explains late veneer: addition of chondritic material after
core formation
D/H ratios
D/H ratios
Earth similar to carbonaceous chondrites
Different from Mars
 Local reservoirs in solar nebula
Where does the water come
from?
• Earth accreted dry, water comes later from
carbonaceous meteorite, but problem with
osmium ratios.
• Earth accreted wet, with some addition from
comets, but problem with other ratios
• Earth accreted from hydrous and anhydrous
materials, explains oxygen, osmium and
hydrogen ratios.
• Water probably important for the onset of
plate tectonics
Summary
• Earth accreted in parts from hydrous material not
present in our meteorite collections.
• Some elements of PUM are from extant meteorite
material, but no meteorite type shares all
properties.
• Composition of the Earth is unique and is a
consequence of distinct reservoirs within solar
nebula. There was thus no mixing of the bulk
material in the inner solar system during accretion.
Average PMs from different
techniques
Palme and O’Neill 2003