Transcript Document

Other clues to the formation of the Solar System
 Inner 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 meteorite
 CI chondrite: chondrite with composition close to Sun
 Volatile: condensation for T<800K
 Moderately volatile: condensation 800K < T <1200K
 Refractory: condensation T >1400K
Short and useful definitions
 Siderophile: elements that prefer to partition into the Fe-Ni
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core
Lithophile: elements that prefer to partition into silicates
Atmophile: elements that prefer to partition into the
atmosphere
BSE: hypothesized composition of the crust and mantle
Depleted mantle: mantle that is the source for MORB,
depleted in incompatible trace elements
Enriched mantle: enriched in incompatible trace elements
Pyrolite: 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 (methods)
Age of nuclear synthesis synthesis
Meteorites
Age of the Earth accretion
The moon
Formation of the core
Formation of crust
Plate tectonics starts
Dating the synthesis of elements
 Direct estimate from nuclear synthesis models and
present isotopic ratios
 Indirect dating
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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.
Geochronometry
 Radiogenic 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 λ
 Mineral 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 SmNd). Methods yield initial isotopic ratio of Sr87/Sr86 (important for tracing)
K-Ar
 Advantage: 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)
 Constraints
 Oldest rocks (Acasta
gneisses, 4.03 Ga,
Nuvvuagittuq
amphibolites, 4.18 Ga)
 Oldest minerals: detrital
zircons in Jack Hills,
Australia, 4.4Ga
History of Pb