[meteorite-list] Earth is Made of More Than Enstatite Chondrites

[Phil Whitmer]

http://www.huffingtonpost.com/2012/03/02/meteorites-that-formed-earth_n_1315903.html

Meteorites That Made Earth Are Of Many Types, Geochemists Report

By: Clara Moskowitz
Published: 03/01/2012 02:31 PM EST on SPACE.com
Earth's building blocks were more eclectic than once thought, according to a 
new study suggesting our planet formed from collisions of many different 
types of meteorites.
Our planet is thought to have formed around 4.5 billion years ago from a 
disk of dust grains left over from the cloud of material that built our sun. 
These grains slowly clumped together, drawn by gravity into pebbles, then 
boulders, then planetary embryos. Eventually, enough mass coalesced to form 
the planet Earth.
Scientists had thought that most of the bodies that merged to make Earth 
formed from a narrow zone in space and were similar to each other, belonging 
to a subclass of meteorites called enstatite chondrites. This idea was based 
on measurements of numerous striking similarities between different types of 
atoms (called isotopes) of elements such as oxygen, nickel and chromium, 
between the Earth and enstatite chondrites.
But a new study of the silicon isotope signature of Earth rock samples and 
meteorites suggests that Earth is made of a more diverse mix of meteorites. 
[The Solar System To Scale (Infographic)]
Geochemists Caroline Fitoussi and Bernard Bourdon of the Ecole Normale 
Supérieure de Lyon in France analyzed the silicon isotopes in terrestrial 
rock samples collected from diverse types of mantle rocks. They also 
analyzed lunar rock samples collected by NASA astronauts on moon missions. 
They compared these to meteorite samples, particularly enstatite chondrites 
and another type called enstatite achondrites. The measurements were done at 
the Swiss Federal Institute of Technology of Zurich in Switzerland.
Using computer models of Earth's formation, the researchers calculated that 
a mix of three types of meteorite ingredients could have produced the right 
blend of oxygen, nickel, and chromium isotopes previously measured in Earth 
samples, as well as their new findings about silicon isotopes in terrestrial 
and meteorite samples. The results suggest that a mixture of chondrites, 
rather than enstatite chondrites alone, probably combined to create the 
Earth.

"This is the first time that a different composition in isotopes is observed 
for a major element between the enstatite chondrites and the Earth," 
Fitoussi told SPACE.com. "So that's quite different from what has been 
observed before."
The fact that the silicon isotope compositions measured were similar in both 
the Earth and moon rock samples suggests that the material that formed the 
moon must have mixed with the Earth's mantle before the moon formed so that 
both bodies hold the same signature.
"That should tell us something about how the moon really formed and what are 
the constraints," Caroline Fitoussi said.
The moon is thought to have resulted when a giant asteroid slammed into the 
Earth not long after our own planet's birth. But the precise details of the 
process are still not well understood.
The new findings were published in the March 2 issue of the journal Science.
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Phil Whitmer