Presentation on theme: "Mantle composition 1800s meteorites contain similar minerals to terrestrial rocks Hypothesis that meteorites come from asteroid belt and originate from."— Presentation transcript:
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 20 th 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.
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.
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