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 differenttechniques

Palme and O’Neill 2003