Chondrite Components Calcium-aluminum Inclusions (CAI’s): very high-T condensates or evaporative residues Chondrules once molten droplets Ameboidal Olivine Aggregates (AOA’s): high-T condensates
Pb-Pb Ages of meteorites & components CAI’s are oldest objects Little difference between achondrites & chondrites 4568.22±0.17 Ma
K-Ar (40-39) Ages of Ordinary Chondrites K-Ar ages are often younger than other ages Results from shock-resetting of K-Ar ages.
4-Vesta as seen by DAWN Eucrites, many of which are brecciated, yield K-Ar ages with peaks in the distribution occurring at 4.48 Ga, 4.0 to 3.7 Ga and 3.45-3.55 Ga, which date heating from large impacts on the surface.
Short-lived Radionuclides and the evolution of the solar nebula and formation of planetary bodies
Table 5.1. Important short-Lived Radionuclides in the Early Solar System Radio-Half-lifeDecayDaughterInitial Abundance nuclideMaRatio 10 Be1.5β 10 B 10 Be/ 9 Be ~ 7.5 × 10 –4 26 Al0.73β 26 Mg 26 Al/ 27 Al = 5.2 × 10 –5 36 Cl0.301β 36 Ar/ 36 S 36 Cl/ 35 Cl ~ 17 × 10 -6 41 Ca0.15β 41 K 41 Ca/ 40 Ca ~1.4 × 10 –8 53 Mn3.7β 53 Cr 53 Mn/ 55 Mn = 6.3 × 10 –5 60 Fe1.5β 60 Ni 60 Fe/ 56 Fe ~ 5.8 × 10 –8 107 Pd9.4β 107 Ag 107 Pd/ 108 Pd ~ 5.9 × 10 –4 129 I16β 129 Xe 129 I/ 127 I ~ 1.2 × 10 –4 146 Sm103α 142 Nd 146 Sm/ 144 Sm ~ 0.0094 182 Hf9β 182 W 182 Hf/ 180 Hf ~ 9.7 × 10 –5 244 Pu82α, SFXe 244 Pu/ 238 U ~ 0.001 247 Cm15.6α, SF 235 U 247 Cm/ 235 U ~ 6 × 10 -5 Short-lived Chronometers
Short-Lived Chronometers For these isotopes, the parent has entirely decayed and what we want to know was the initial abundance of the parent (N 0 ). As was the case for the isochron equation, we start with: But rather than substituting for N 0, we substitute for N using And obtain: The equation for D is then
53 Mn– 53 Cr Example Written for the 55 Mn– 55 Cr decay, we have: We do not know the 53 Mn/ 52 Cr ratio, but we measure the 55 Mn/ 53 Cr ratio. Assuming that initial isotopic composition of Mn was homogeneous the initial 53 Mn/ 52 Cr ratio is: Substituting and noting that e –λt = 0: x
53 Mn– 53 Cr Our equation: has the form y =a + bx where b =( 55 Mn/ 53 Mn) 0, so plotting 53 Cr/ 52 Cr vs 55 Mn/ 52 Cr, the slope is proportional to the initial Mn isotopic composition, and the intercept gives the initial Cr isotopic composition. Both the ( 55 Mn/ 53 Mn) 0 and the ( 53 Cr/ 52 Cr) 0 are functions of time and allow us to assign relative ages to objects. We can then calibrate these relative ages against absolute chronometers such as Pb-Pb.
129 I– 129 Xe The first short-lived radionuclide discovered was 129 I, recognized from variable 129 Xe/ 130 Xe ratios. 128 Xe (a minor isotope of Xe) can be produced by neutron capture of 127 I by irradiation in a reactor. The 128 Xe/ 130 Xe is then proportional to 127 I/ 130 Xe. Very much analogous to 40 Ar– 39 Ar dating, an ‘isochron’ can be produced by step-heating of the irradiated sample. Natural ratio
Heavy Isotopes of Xe: Evidence of 244 Pu fission
107 Pd– 107 Ag Pd and Ag (t 1/2 = 9.4 Ma) are siderophile elements concentrated in iron meteorites, so this system is useful in dating iron meteorites. Most magmatic irons have Pd-Ag ages less than 10 Ma after CAI formation. Planetesimals formed and differentiated into mantles and cores quite early!
26 Al– 26 Mg This is arguably the most significant decay system because: Half-live is short: 0.73 Ma, providing for detailed chronology Both elements are relatively abundant, making analysis relatively easy. Amount of 26 Al in early solar system was sufficient to be a significant source of heat in early-formed bodies and may partly explain why they melted and differentiated.
182 Hf– 182 W The 182 Hf– 182 W chronometer (t 1/2 = 9 Ma) is particularly interesting because Hf is lithophile and concentrates in the silicate parts of planetary bodies, W is siderophile and concentrates in the cores of planetary bodies. Notation: 182 W/ 184 W ratio reported in ε W notation - deviations in parts per 10,000. However, unlike 143 Nd/ 144 Nd and 176 Hf/ 177 Hf, the deviations are from a terrestrial standard. All modern terrestrial rocks reported thus far have ε W, so this value is also thought to be that of the bulk silicate Earth. Variation in 182 W/ 184 W are small, so a second notation, μ w, variations in ppm from a terrestrial standard, is also used.
Dating Core Formation The fractionation of Hf from W results from segregation of metal from silicate and hence dates the timing of core formation. The Earth’s formation was thought to be essentially completed by the Moon forming impact, which would have been the last opportunity for W isotopic equilibration between the mantle and core. This last equilibration is thought to date the ‘age of the Earth’. 182 Hf– 182 W measurements of magmatic irons indicates planetesimal (asteroidal) cores formed within a few million years of CAI formation. When did the Earth’s core form? Initial measurements suggested the Earth’s 182 W/ 184 W was the same as chondrites (undifferentiated), suggesting core formation happened after 182 Hf was extinct. Subsequent measurements showed chondrites have ε W ≈ –2, indicating the Earth’s core formed before 182 Hf was extinct. The moon also apparently has ε W ≈ 0, suggesting core formation was largely complete before the Moon-forming impact. Two scenarios: The Earth’s core formed in a single event (perhaps triggered by the Moon-forming impact). In this case, dating core formation with Hf-W is straightforward. The planetesimals that accreted to form the Earth has already differentiated into silicate mantles and iron cores. How much W isotopic equilibration happened as these bodies accreted and their cores merged with the Earth’s core? Dating in this case is complex and model-dependent.
Modeling Earth-Moon Formation f is the Hf/W difference between the silicate Earth and the silicate Moon.
Post-Script Subsequent studies have identified small variations in ε W in Archean rocks. Initial interpretation was that this reflected addition of a late accretionary veneer of chondritic material that had not yet been mixed into the Earth.
146 Sm– 142 Nd 146 Sm alpha decays to 142 Nd with a half-life of 103 Ma. 142 Nd excesses first identified in the eucrite Juvinas several decades ago. Long half-life suggests in should have survived in the early Earth. First evidence found in 3.8 Ga rocks from Isua, Greenland in 1990’s, but not confirmed for a decade later. Values reported as ε 142 Nd or as μ 142 Nd deviations from a terrestrial standard. Since both Sm and Nd are refractory lithophile elements, we expect the Sm/Nd ratio of the Earth to be chondritic and therefore the 142 Nd/ 144 Nd ratio of the bulk Earth to be identical to chondrites. Guess what?
ε 142 Nd Variations in Terrestrial and Solar System Materials All modern terrestrial rocks have uniform ε 142 Nd =0, but 20 ppm greater than ordinary chondrites. Carbonaceous chondrites averageε 142 Nd = –0.4, enstatite chondrites average ε 142 Nd = –0.1. Variable ε 142 Nd have now been found in many Archean rocks, particularly Eoarchean rocks. ε 142 Nd is even more variable in meteorites from Vesta and Mars. The Moon appears to have average ε 142 Nd = 0 (same as observable Earth). How do we explain this?
1. Isotopically heterogeneous solar nebula 146 Sm is a p-only nuclide produced in small quantities in supernovae. Analysis of other isotopes (e.g., 148 Nd) shows anti- correlated variations in r- and s-process nuclides, suggesting a late addition of supernova debris that did not completely mix. (But is possible this results from incomplete dissolution of pre-solar grains present in meteorites). Earth still plots 10 ppm off correlation.
2. Formation of an Early Enriched Reservoir Boyet and Carlson proposed that an incompatible-element (and therefore low Sm/Nd) reservoir formed early in Earth’s history, sunk to the deep mantle and has remained there ever since. The crust and rest of the mantle and all subsequent volcanic rocks are derived from the Early Depleted Reservoir (with high Sm/Nd and therefore high 142 Nd/ 144 Nd).
Issues with the EER Moon and Earth has the same ε 142 Nd – so the hypothetical EER would have had to form before (and hence survive) the Moon forming impact. A very substantial fraction (50%) of the Earth’s U, Th, and K would also end up in the EER, meaning very high heat production in it. This would ultimately make it dynamically unstable (should rise). But, could the LLSVP’s be the EER?
Hypothesis 3: Collisional erosion produces a non-chondritic Earth As planets grow, collisions are energetic enough to partially melt them; producing a magma ocean that crystallizes to produce an incompatible element-enriched crust. Collisions also blast away a fraction of this crust, depleting the planet of elements enriched in the crust (incompatible elements). (Caro et al., 2008). There is indeed a growing body of evidence from astronomy and dynamic modeling that planetary accretion is a non-conservative process. Mercury’s core is way too big. Could collisional erosion be the explanation?
Short-lived Radionuclide Summary Short-lived, extinct radionuclides provide a chronology of the early-solar system, including nebular processing and early planetary differentiation. They also provide evidence of the presence of newly synthesized elements in the solar nebula. Some, such as 10 Be likely synthesized by spallation in high energy environments within the nebula. Others, such as 26 Al, likely synthesized in red giants (AGB stars) and seeded into surrounding galaxy by their very enhanced solar winds. Yet others, such as 60 Fe, likely produced in supernovae. Finally, some long-lived ones, such as 129 I and 182 Hf, have abundances similar to steady-state galactic background (maintained by occasional supernovae).
Short-lived Radionuclides & Star Formation Stars are born in stellar nurseries, such as the Great Nebula in Orion when (small) parts of these nebulae collapse under gravity. The stars formed may include a few very large ones, with very short lives that could feed newly synthesized elements into the nebulae while younger stars are still forming. Did a shockwave from nearby supernova trigger the collapse that resulted in formation of the Sun? Smaller, longer-lived stars eventually wander away from the nebular birthplaces.