Presentation on theme: "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."— Presentation transcript:
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
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 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 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 K 40 -> 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-Ar Advantage: No Ar initially, K relatively abundant (but small percentage of 40 K) But Ar diffuses in and out easily. Problem of atmospheric contamination of samples. Correction for atmospheric contamination based on Ar 36 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr ~ 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