Other clues to the formation of the Solar System
y Inner planets are small and dense
y Outer planets are large and have low density
y Satellites of the outer planets are made mostly of ices
y Cratered surfaces are everywhere in the Solar System
y Saturn has such a low density that it can
can'tt be solid anywhere
y Formation of the Earth by accretion: Initial solar nebula
consists of mixtures of ggrains ((rock)) and ices. The initial ratio
is about 90% ices and 10% grains
y The sun is on so there is a temperature gradient in this mixture: y Refractory elements
(condense at
T>1400K)
y Moderately volatile
(condense at
800<T<1200K)
y Volatile (condense at
T<800K)
Short and useful definitions
y Chondrite: a primitive, undifferentiated meteorite
y CI chondrite: chondrite with composition close to Sun
y Volatile: condensation for T<800K
y Moderately volatile: condensation 800K < T <1200K
y Refractory: condensation T >1400K Short and useful definitions
y Siderophile: elements that prefer to partition into the Fe-Ni
y
y
y
y
y
y
core
Lith hil elements
Lithophile:
l
t that
th t prefer
f to
t partition
titi into
i t silicates
ili t
Atmophile: elements that prefer to partition into the
atmosphere
p
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 (
a hypothetical mixture of ("depleted") mantle depleted ) mantle peridotite and basalt
y Earth and Planets formed by accretion from meteorites
y There are small differences in composition between Earth and chondritic meteorites because of the accretion processes
y Accretion by collisions gives a lot of heat => some “ l l l
“volatile elements” are lost. ” l
Geochronometry (methods)
Age
g of nuclear synthesis
y
synthesis
y
Meteorites
Age of the Earth accretion
The moon
Formation of the core
Formation of crust
Plate tectonics starts
Dating the synthesis of elements
y Direct estimate from nuclear synthesis models and
present isotopic ratios
y Indirect dating
y
y
Age of Earth
Determining how long after nucleo-synthesis
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.
S
Sample
l is
i ionized
i i d andd ions
i
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
y Radiogenic isotopes
y Decay mechanisms (α decay, β decay, electron capture)
y Main isotopic
p systems
y
for dating
g
y Rb-Sr
y K-Ar
y U-Pb
U b
y Th-Pb
y Other isotopes used mainly for “tracing” (Sm-Nd, Re-Os, …)
y Another implication of the radio-isotopes is that their decay yields energy.
What does radiometric age mean?
y Time when the system closed. y Determined by temperature. Time when mineral crossed an isotherm. d i h
y Temperature depends on mineral and isotopic system we are considering y About 800C for U‐Pb on zircons, but much less for most other minerals. y Cooling (or metamorphic) history could be inferred by using different minerals. Geochronometry (hypotheses)
y Parent -> daughter decay probability λ
y Mineral closes at temperature (depends on type: zircons 800 deg, feldspars
350,, …))
y No daughter present at closure (or it can be accounted for)
y No loss or gain of parent or daughter after mineral closes
y No physical fractionation when mineral form (only chemical)
y Counting P/D gives the time that elapsed since the system closed
Geochronometry (particulars)
y K->Ar is a branching decay K40 -> Ar 40 or Ca 40
y U -> Pb two different isotopes of same element give two independent age
estimates ((must be concordant))
y 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
y Advantage: No Ar initially, K relatively abundant (but small percentage of
40K)
y But Ar diffuses in and out easily.
y
y Problem of atmospheric contamination of samples. Correction for
atmospheric contamination based on Ar36
y Also Ar is easily lost
y Retrace loss by step heating of samples and Ar-Ar ages
The isochron: Rb/Sr system.
y
Similar method and equations are used for other isotopic
systems (U-Pb, Sm-Nd)
y Note that the 87Sr/86Sr increases with the concentration in
Rb. This provides a useful tracer.
y In the Earth, Rb is preferentially concentrated in the crust
relative to the mantle.
y Depleted
D l t d mantle
tl is
i poorer in
i Rb and
d enriched
i h d mantle
tl has
h
higher Rb relative to “primitive mantle”
y Present samples from mantle have 87Sr/86Sr ~0
~0.705.
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?
y What does that mean? (Accretion took some time)
y Constraints
y Oldest rocks (Acasta
gneisses, 4.03 Ga, Nuvvuagittuq
amphibolites, 4.18 Ga) hib li 8 G ) y Oldest minerals: detrital
zircons in Jack Hills, Australia, 4.4Ga
Hi t
History
off Pb