Presentation on theme: "History of “primordial” Pb Meteorite samples Chondrite – a primitive, undifferentiated meteorite CI refers to a particular class of carbonaceous chondrite."— Presentation transcript:
History of “primordial” Pb
Meteorite samples Chondrite – a primitive, undifferentiated meteorite CI refers to a particular class of carbonaceous chondrite which has solar abundances for most of the elements except for the extremely volatile elements. chondrite achondrite Iron
Pb isochron from chondritic meteorites: 4.55 Ga (Patterson, 1955)
α 0 β 0 give us “primordial” Pb Only one isochron for all chondrites (within error limits) Chondritic initial lead composition can be calculated from isochron Primordial Earth close to chondrite Constraint on age of core formation (must be early)
Meteorites: summary All meteorites have about the same age ~ 4.55 Ga Some meteorites that have younger ages come from the moon. They were ejected after impact of large meteorite on Moon. A few are much younger (1.1 Ga). They are assumed to have been ejected by Mars after a large impact! Martian meteorites(?)
Moon Ages of Moon samples range from 3.1 to 4.51 Ga for highlands Moon is even more depleted in volatiles than Earth Moon has lower density than Earth Moon has very small core
Energy (heat) released during accretion could explain why Earth lost volatiles (what about Moon). Note that more energy will be released when the Fe sinks to form the core.
Moon formation hypotheses Moon was formed separately from and was captured by Earth Moon was extracted from Earth (G.H. Darwin’s tidal resonance) Moon formed in orbit at the same time as Earth
Giant impact is considered most likely (i.e. Mars size body impacted the Earth at end of accretion) Explains the geochemical trends (loss of volatiles) Might explain relative density and core size Several mechanical problems (angle of collision, etc.) have now been resolved by astronomers.
Time series of a Moon-forming impact simulation. Results are shown looking down onto the plane of the impact at times t = 0.3, 0.7, 1.4, 1.9, 3, 3.9, 5, 7.1, 11.6, 17 and 23 hours (from left to right); the last frame is t = 23 hours viewed on-edge. Colour scales with internal energy (shown on the colour bar in units of 6.67 times 108 erg g-1), so that blue and dark green represents condensed matter, and red particles signify either the expanded phase or a hot, high-pressure condensed phase; pressures at intermediate energies are computed by an interpolation between the Tillotson15 condensed and expanded phases. We form initial impactors and targets in hydrostatic equilibrium by pre-colliding smaller bodies together at zero incidence, resulting in realistically evolved internal energies, stratified densities (basalt mantle + iron core) and consistent pressures. Each particle's internal energy is evolved due to the effects of expansion/compression and shock dissipation, with the latter represented by artificial viscosity terms that are linear and quadratic in the velocity divergence of converging particles; effects of mechanical strength and radiative transfer are ignored. The momentum of each particle is evolved due to pressure, viscous dissipation and gravity. Gravity is computed using a binary tree algorithm, reducing the N2 calculation of particle– particle attractions into an NlogN calculation25. We use a beta spine kernel to define the spatial distribution of material represented by each SPH particle. The scale of each particle, h, is automatically adjusted to cause overlap with a minimum of 40 other particles, ensuring a 'smoothed' distribution of material even in low-density regions. The code is explicit, requiring a Courant-limited timestep Deltat < (c/h) where c is the sound speed. For a full description of the technique, see ref. 26, from whose efforts our present algorithm derives. Giant impact movie on youtube: http://www.youtube.com/watch?v=OY_5h5iPA8k
Isotopes with short half-life (i.e. τ<<4.5Gyr) are now extinct ParentDaughterHalf-life 26 Al 26 Mg0.72 Myr 60 Fe 60 Ni0.3 Myr 129 I 129 Xe16 Myr 146 Sm 142 Nd100 Myr Anomalies in isotopic composition (for daughter of short lived isotopes) indicate that parent was still present when mineral formed Meteorites ( 129 Xe) Early crust ( 142 Nd)
Trace of 26 Al from 26 Mg variations in meteorites. Line is not an isochron: slope = ( 26 Al/ 27 Al)
Similar observations with Xe 129 Xe 129 daughter of short half life isotope I 129 Meteorites formed shortly after nucleosynthesis. Xe 129 in earth atmosphere (I 129 in primitive earth) comes from degassing of mantle Earth and meteorites have ~ same age
130 Xe and 128 I are not radiogenic Slope of line is radiogenic 129 Xe, i.e. 129 I/ 128 I in sample when it closed 129 I/ 128 I decreases rapidly with time
Meteorites must have condensed shortly after nucleosynthesis and within a 30 Myr time span
Dating core formation Hafnium Hf and Tungsten W Hf 182 -> W 182 (half life ~9 Myears) Hf 180 reference Hf stays in mantle W goes in core Initial ratio Hf 182 /Hf 180 in solar system different from that of mantle
εw versus 180Hf/184W for different fractions of the H chondrites Ste Marguerite (a) and Forest Vale (b). NM-1, NM-2 and NM-3 refer to different nonmagnetic fractions, M is the magnetic fraction. We interpret the positive correlation of εw with 180Hf/184W as an internal Hf–W isochron whose slope corresponds to the initial 182Hf/180Hf ratio at the time of closure of the Hf–W system.
εw values of carbonaceous chondrites compared with those of the Toluca iron meteorite and terrestrial samples analysed in this study. The values for Toluca, Allende, G1-RF and IGDL-GD are the weighted averages of four or more independent analyses. Also included are data from ref. 16 (indicated by a), ref. 30 (b), and ref. 2 (c). For the definition of εw see Table 1. The vertical shaded bar refers to the uncertainty in the W isotope composition of chondrites. Terrestrial samples include IGDL-GD (greywacke), G1-RF (granite) and BB and BE-N (basalts).
Timing of core formation. The Earth formed through accretion, absorbing planetesimals (lumps of rock and ice) through collisions. Did the Earth accrete undifferentiated material that then separated into shell and core — in which case, did the planet reach its present mass before differentiating, or was it a more gradual process? Alternatively, core formation might have happened rapidly inside growing planetesimals, so that the Earth's core is a combination of these previously formed cores. Isotopic evidence supports the latter model.
Core formation (energy aspects) Gravitational potential energy decreases when core forms Moment of inertia decreases Angular velocity of rotation increases Rotational energy increases Increase in energy of rotation << Decrease in gravitational potential energy Total energy must be conserved Difference goes into heat Estimates: Core formation -> 1000-2000K temperature increase
How did volume of continental crust change with time?
Tracing with isotopes Crust Rb/Sr high Sm/Nd low Sr 87 /Sr 86 increases faster in crust than mantle Nd 143 /Nd 144 decreases compared with mantle Mantle Rb/Sr low Sm/Nd high
142 Nd anomalies in oldest crustal rocks (Isua gneisses, Greenland) 142Nd daughter of 146Sm that has “short” (100Myr) half life. Anomalies in old rocks imply very early crustal differentiation. Similar conclusion from Lu/Hf on inclusions in Jack Hill zircons.
It happened fast! Chondrites condensate very early after nuclear synthesis Condensation lasts several 10 Myrs Early Earth accretion Core formation well in progress at time Giant impact Moon formed Core completely separated at 4.45Ga Oldest minerals on Earth (4.4Ga) Crust present very early
He It is assumed that volatiles were lost during accretion Very little He in atmosphere (too light, lost to space) He in mantle He 3 is primitive, He 4 primitive + decay of radioelements He 4 /He 3 ratio (initial ratio same as that of universe) He 4 /He 3 ratio grows with time Some degasing Shows mantle is not well mixed
Time of core formation in Myr after CAI condensation for Vesta, Mars, Earth and Moon versus planet radius as deduced from Hf–W systematics. For the Moon, the two data points refer to the endmember model ages. The Moon plots distinctly to the left of the correlation line defined by Vesta, Mars and Earth, suggesting a different formation process.