Other clues to the formation of the Solar SystemInner planets are small and dense Outer planets are large and have low density Satellites of the outer planets are made mostly of ices Cratered surfaces are everywhere in the Solar System Saturn has such a low density that it can't be solid anywhere Formation of the Earth by accretion: Initial solar nebula

consists of mixtures of grains (rock) and ices. The initial ratio is about 90% ices and 10% grains

The sun is on so there is a temperature gradient in this mixture:

Refractory elements (condense at T>1400K)

Moderately volatile (condense at 800<T<1200K)

Volatile (condense at T<800K)

Short and useful definitions Chondrite: a primitive, undifferentiated

meteoriteCI chondrite: chondrite with composition

close to SunVolatile: condensation for T<800KModerately volatile: condensation 800K < T

<1200KRefractory: condensation T >1400K

Short and useful definitions Siderophile: elements that prefer to partition into the Fe-Ni

coreLithophile: elements that prefer to partition into silicatesAtmophile: elements that prefer to partition into the

atmosphereBSE: hypothesized composition of the crust and

mantleDepleted mantle: mantle that is the source for

MORB, depleted in incompatible trace elementsEnriched mantle: enriched in incompatible trace

elementsPyrolite: a hypothetical mixture of ("depleted")

mantle peridotite and basalt

Earth and Planets formed by accretion from meteorites

There are small differences in composition between Earth and chondritic meteorites because of the accretion processes

Accretion by collisions gives a lot of heat => some “volatile elements” are lost.

Geochronometry-Isotope tracingAge and early evolution of the Earth

Geochronometry (methods)Age of nuclear synthesis synthesis

MeteoritesAge of the Earth accretion

The moon Formation of the core

Formation of crustPlate tectonics starts

Dating the synthesis of elementsDirect estimate from nuclear synthesis models and

present isotopic ratiosIndirect dating

Age of Earth Determining how long after nucleo-synthesis did Earth form

Geochronometry is based on development of mass spectrometry Mass spectrometer allow to determine

the ratio of different isotopes of an element.

Sample is ionized and ions are accelerated into a magnetic field

Deflection of ion by field (i.e. acceleration) inversely proportional to mass.

Recent technical improvements allow precise measurements on samples with extremely low concentration of analyzed elements.

GeochronometryRadiogenic isotopes Decay mechanisms (α decay, β decay, electron capture) Main isotopic systems for dating Rb-Sr K-Ar U-Pb Th-Pb Other isotopes used mainly for “tracing” (Sm-Nd, Re-Os, …)

Another implication of the radio-isotopes is that their decay yields energy.

What does radiometric age mean?Time when the system closed. Determined by temperature. Time when

mineral crossed an isotherm. Temperature depends on mineral and isotopic

system we are considering About 800C for U-Pb on zircons, but much

less for most other minerals. Cooling (or metamorphic) history could be

inferred by using different minerals.

Geochronometry (hypotheses)Parent -> daughter decay probability λMineral closes at temperature (depends on type: zircons 800 deg, feldspars

350, …)No daughter present at closure (or it can be accounted for)No loss or gain of parent or daughter after mineral closes No physical fractionation when mineral form (only chemical)

Counting P/D gives the time that elapsed since the system closed

Geochronometry (particulars)K->Ar is a branching decay K40 -> Ar 40 or Ca 40

U -> Pb two different isotopes of same element give two independent age estimates (must be concordant)

Rb/Sr requires different minerals with variable Rb/Sr ratios (same for Sm-Nd). Methods yield initial isotopic ratio of Sr87/Sr86 (important for tracing)

K-ArAdvantage: No Ar initially, K relatively abundant (but small percentage of

40K)But Ar diffuses in and out easily. Problem of atmospheric contamination of samples. Correction for

atmospheric contamination based on Ar36

Also Ar is easily lost Retrace loss by step heating of samples and Ar-Ar ages

The isochron: Rb/Sr system. Similar method and equations are used for other isotopic systems (U-Pb, Sm-Nd)

Note that the 87Sr/86Sr increases with the concentration in Rb. This provides a useful tracer.

In the Earth, Rb is preferentially concentrated in the crust relative to the mantle.

Depleted mantle is poorer in Rb and enriched mantle has higher Rb relative to “primitive mantle”

Present samples from mantle have 87Sr/86Sr ~0.705. Higher ratios would indicate that the source has been enriched in Rb relative to mantle, most likely that the source is crustal.

Interpretation of discordant ages: Evolution of the Pb/U as a function of time

Age of the Earth? What does that

mean? (Accretion took some time)

ConstraintsOldest rocks (Acasta

gneisses, 4.03 Ga, Nuvvuagittuq amphibolites, 4.18 Ga)

Oldest minerals: detrital zircons in Jack Hills, Australia, 4.4Ga

History of Pb