Transcript Meteorites & Earth

The nebular
hypothesis
The solar nebula
(gas) contracted,
cooled and
condensed into
dust sized particles
that accreted (stuck
together as the
result of collisions)
into protoplanets
(asteroid sized
bodies) and then
larger planets
http://meteorites.asu.edu/
Evidence that chondrites are representative of
early solar system materials:
• 1:1 ratio of non-volatile elements to those in the sun
• “sedimentary”, non-equilibrated texture - formed by
accretion of condensed particles with little subsequent
heating or alteration
• presence of minerals predicted to be formed by
condensation of solar nebula, such as corundum, Fe
metal, troilite, etc.
Also inferred: achondrites, iron and stony-meteorites
represent fragments of accreted planetesimals that had
heated internally, melted and differentiated
The nebular
hypothesis
The solar nebula
(gas) contracted,
cooled and
condensed into
dust sized particles
that accreted (stuck
together as the
result of collisions)
into protoplanets
(asteroid sized
bodies) and then
larger planets
http://meteorites.asu.edu/
Earth (also, Mercury, Venus, Mars) is layered (core,
mantle, crust). How did it form, and what was its
original bulk composition?
Hypotheses for planet formation:
1) Heterogeneous accretion (layers form in condensation sequence from
refractory to volatile. Since iron condenses early – did the core form this
way?)
2) Homogeneous accretion - whole planet accreted from the same mixture
of condensing materials, and then differentiated by internal melting
3) “Chondrite mixing” - accretion of the constituents of chondrites in
varying proportions depending on time and distance from the sun,
followed by interior melting
Sample chondrite mixing model for Earth and Moon:
Chondrite mixing model: Use each element that can be estimated to
determine the proportion of a particular chondrite consitituent:
1)
2)
3)
4)
5)
Refractory elements in Calcium Aluminum Inclusions - U
Silicates - Si
Metal - Fe
Troilite (sulfide) - S
Volatile elements – K, Tl
Then add together all the other elements in those constituents:
1) Refractory elements: U, also Ca, Al, Ba, Sr, Sc, Ti, Pt, Re, Os……..
2) a) Elements in refractory silicates: Si, Mg…..
b) Elements in late condensing silicates: Na, K, Rb, Cs……..
3) Elements in metal phase: Fe, also Ni, Cr, Co, Au, P, As……..
4) Elements in troilite: Fe and S, also Zn, Cu, Ga, Se, Te……….
5) Highly volatile elements: Tl, Bi, Pb, Hg, Cl, I…………
What do we know about the bulk composition of the Earth
(and other planets and moons). What can we use to
constrain the proportions of chondrite constituents?
1) Amount of Fe can be estimated from size of core (if detectible), mass
and density of planet/moon
2) Amount of U can be estimated from surface heat flow measurements
and alpha particle emission (for planets and moons), and, on Earth,
by assuming most U is in the crust (which can be analyzed directly)
3) On Earth, amount of K can be estimated from amount of 40Ar in the
atmosphere, for Earth and differentiated planets and moons, also that
most K is in the crust (which can be analyzed directly).
http://earthref.org/GERM/
Estimate the composition of the Earth’s Core:
Fe-Ni based on Earth’s density and observations of iron meteorites
One or more light elements needed to account for lower than expected
density inferred from seismic wave velocities – S, O most likely
Trace elements with “siderophile” behavior (partition into molten metal)
added to core composition in accord with their “preference” for metallic
versus silicate melts (and subtracted from silicate earth)
(worked problem 12.1 in McSween et al.)
Result is: “silicate earth” and “primitive mantle” – the composition of the
silicate Earth following internal melting and core formation
For Tuesday:
The silicate earth (crust and mantle together) contains about 45%
SiO2 by weight (gram SiO2/gram rock X 100). Calculate the
concentration of K2O in weight percent and U, Sr and Tl in parts per
million (micrograms element/gram rock) in the silicate earth
assuming abundances relative to Si are the same as the solar (or
“cosmic”) abundances (use Table 2.1 in Faure).