METEORITES, ISOTOPIC DATATION. Aurélien CRIDA

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1 METEORITES, ISOTOPIC DATATION Aurélien CRIDA

2 MÉTÉORITES Falls : Stony-Irons (mésosidérites) Iron (sidérites) Achondrites Carbonaceous Chondrites Ordinary Chondrites

3 MÉTÉORITES : CLASSIFICATION

$Mg: red ; Ca: green ; Al: blue Refractory inclusions = CAI (Calcium-Aluminum rich Inclusions) Contain Ca- and Al-rich minerals. Varying sizes ( µm- few cm).$

4 COMPOSITION of CHONDRITES Chondrules (fr: chondres) Silicate/metal beads. Varying sizes ( µm- few mm). Mg: red ; Ca: green ; Al: blue Refractory inclusions = CAI (Calcium-Aluminum rich Inclusions) Contain Ca- and Al-rich minerals. Varying sizes ( µm- few cm). Matrix (fr: matrice) Silicates, oxides, amorphous. Fine-grained ( µm). Picture Sasha Krot

5 CHONDRULES : Proportion and Size CM: 2%,.3 mm L: 8%,.7 mm CV: 45%,. mm CI: %

6 Ex: PORPHYRITIC CHONDRULE Olivine [(Mg,Fe)2SiO4] and pyroxene [(Mg,Fe)SiO3] phenocrysts set in a glassy matrix

$Silicate Amorphous Silicate Carbonaceous matter Phyllosilicates Carbonaceous stuff Olivine Carbonates Metals and FeS Refractory material Présolar grains$

7 MATRIX of the CHONDRITES Chondrule fragments Magnesian critalline Silicates (forsterite et enstatite) FeO rich cristaline Silicates (olivine) Amorphous Silicate Amorphous Silicate Carbonaceous matter Phyllosilicates Carbonaceous stuff Olivine Carbonates Metals and FeS Refractory material Présolar grains Enstatite

8 CONNECTIONS ASTÉROÏDES-MÉTÉORITES

) ] N o r m a liz e d R e f le c ta n c e. N o r m a liz e d R e f le c ta n c e.3.8 C M M u r c h is o n.2.

9 CONNECTIONS ASTÉROÏDES-MÉTÉORITES C e re s..9 ]]]]]]]]]]] ] ] ] ] ] ] ] ] ]]]]]] ]] ]]] ]]]]]]]]]]]]]]]]]] ] ]]]]]]]] ]]]]]]]]] ]] ]]] ]]] ]]] ] ]]] ]] u n u s u a l C M Y ( 6 3 m ) ] N o r m a liz e d R e f le c ta n c e. N o r m a liz e d R e f le c ta n c e.3.8 C M M u r c h is o n W a v e l e n g t h ( m ) 3 Murchison (Australia, 969) c o l o r, 3 m 2 9 F o r t u n a C M L E W 9 5 ( m ).5..5 W a v e l e n g t h ( m ) E C A S, 5 2 -c o lo r 2. Burbine (2) Ph.D. Thesis M.I.T. Good agreement between asteroids of spectral type C and Carbonaceous Chondrites.

CONNECTIONS ASTÉROÏDES-MÉTÉORITES G ro u n d b a s e d S p e c tru m o f A s t e ro id 2 5 3 M a t h ild e.

10 CONNECTIONS ASTÉROÏDES-MÉTÉORITES G ro u n d b a s e d S p e c tru m o f A s t e ro id M a t h ild e. 5 R e la t iv e R e f le c t a n c e ( B in z e l e t a l ) C I M e t e o rit e..9 5 N E A R F ilt e r B a n d s ^.9 4 ^ 5 ^ 6 7 W a v e le n g t h ( Å ) ^ ^ 8 9 ^ ^

11 CONNECTIONS ASTÉROÏDES-MÉTÉORITES

CONNECTIONS ASTÉROÏDES-MÉTÉORITES N o r m a liz e d R e f le c ta n c e m e s o s id e r ite V e r a m in ( w h o le r o c k ).5.

12 CONNECTIONS ASTÉROÏDES-MÉTÉORITES N o r m a liz e d R e f le c ta n c e m e s o s id e r ite V e r a m in ( w h o le r o c k ) ] 22 ] ] ] ] ]] ] ] ]]]]]]]]]]]]]]] ] ]]]]] ]]]]]] 22 ] ] ] ] ] ]] ]] ] ]] ] ] ]]]]] ] ] ]] ]]]]]]]]]]]]]]]]] ]]]]]]]]]]]]]]]]] ]]]]]]]]]] 222 ] 2 2 ] ]]]] ] ] ]]]] ]]] ]]]]]]]]] ] 2 ]]]]]]]]]]]]]]] ]]]]]]]]] ] ] ]]]]]]]]]] ]]]]]]] ]2]]]] ]] ]]]22 ]]]]]]]]]]]]] 6 % I m ila c o liv in e, 4 % o c ta h e d r ite B u tle r ] ] ]] ]]]]]]]]]] 2 6 O t e r o ] ] ]]]]]]] ]] ]] ] ]]]]]]] ] ]] ]]]]]]]]]]]] ]]]] ]]]]]] ]] ]]]]] ]]]] ] ] ] ] W itt.6.8. W a v e l e n g t h ( m ) Burbine (2) Ph.D. Thesis M.I.T. Possible agreement between astéroides of type A and stony-iron meteorites.

13 CONNECTIONS ASTÉROÏDES-MÉTÉORITES

14 CONNECTIONS ASTÉROÏDES-MÉTÉORITES Iron has a smooth spectrum, inclined towards the red. Quite similar to asteroids of type M.

15 CONNECTIONS ASTÉROÏDES-MÉTÉORITES 2 Ostro et al. (2) km Radar measurements of M-type asteroids provide a strong link to iron meteorite compositions.

16 CONNECTIONS ASTÉROÏDES-MÉTÉORITES

17 CONNECTIONS ASTÉROÏDES-MÉTÉORITES.4 C C D ( K it t P e a k ) N S F C A M A s t e r o i d G r i s m ( IR T F ] _ ( S N R ~ 2 in t o t a l i n t e g r a t io n t i m e o f 3 m i n u t e s ) 997 BR R e la t iv e R e f le c t a n c e V = 6.5 J = L L 4 O rd in a r y C h o n d r it e M e t e o rit e W a v e le n g t h ( m ).2.4.6

18 CONNECTIONS ASTÉROÏDES-MÉTÉORITES

19 CONNECTIONS ASTÉROÏDES-MÉTÉORITES Spectral match between Vesta and achondrite meteorites (HEDs) first found by McCord et al. (97).

20 VESTA Impact basin of 46 km diametre Images HST Reconstruction topographique Thomas, Binzel et al. (997)

21 sin (i) 2: Resonance 5:2 Resonance 3: Resonance ASTÉROIDES Vesta semi-major axis [UA]

22 ASTÉROIDES Repartition of taxonomic types : From Gradie and Tedesco (982)

23 MARTIAN METEORITES Shergottite, Nakhlite, Chassignite (SNC) Meteorites:

24 Abundances

25 ABUNDANCES The abundance of an element El in the photosphere of the Sun is measured via emission lines. In the solar spectroscopic community: A(El) = log[n(el)/n(h)] + 2 In this scale, A(H) = 2. Meteorites abundances are measured in the laboratory [more precise] Meteorites abundances are rescaled so that A(Si)meteorites = A(Si)photosphere

26 ABNUDANCES A(El) average CI chondrites 2 O 8 H C N 6 4 Li 2 Data from Lodders 23 In A(El) - photosphere 2 4

27 CONDENSATION SEQUENCE Fe-Ni metal.4 Anorthite CaAl2Si 2O8.4 Fe-Ni metal Ca pyroxene ~CaMgSi 2O6 Fraction CI chondritic composition condensed.5 kermanite Ca2MgSi 2O7.3 Enstatite ~MgSiO 3.3 Gehlenite Ca2Al2SiO Hibonite CaAl2O9 Corundum Al2O3 Spinel ~MgAl 2O4. Forsterite ~Mg2SiO 4 Ca pyroxene ~CaMgSi 2O Albite NaAlSi 3O8 Perovskite CaTiO3 7 Anorthite CaAl2Si 2O8. 9 Spinel ~MgCr 2O Temperature (K) Davis and Richter, TOG 23

28 Age of Chondrites

29 RADIOACTIVE DECLINE Radioactivity discovered by Becquerel in 896 Radioactivity α: A Radioactivity β : A Radioactivity β+: A Z P Z P A Z P A or: A-4 Z-2 F + e- + νe (n -> p + e- + νe) F + e+ + νe (n -> p + e- + νe) Z+ Z- F + 4He N (t) = N() x exp(-λt) N (t) = N() x (/2)- t/t Radioactive decay characterized by: The decay constant (λ) The half-life (T/2)

30 AGES and ISOTOPIC COSMOCHEMISTRY One of the most important successes of isotopic cosmochemistry is the ability to date rocks, events Radioactive chronometers are used to date events and processes chronometers Long-lived chronometers are still alive (T/2 > 2 Ma) Short-lived chronometers are extinct (Al-Mg; Hf-W; ) Main long-lived chronometers are U decaying into 26Pb (T/2 = 4.5 x 9 yr) fission U decaying into 27Pb (T/2 = 7.3 x 8 yr) fission Rb decaying into 87Sr (T/2 = 4.8 x yr) β

31 PRINCIPLES of DATATION Quantité de Parent ou de fils Abundance Father Isotope 9 S = F (- e-λt) = F (eλt - ) 8 7 Son Isotope F = F e-λt de temps, (Ma) millions d'années TT/2Intervalle Time If F is known, and S= at t=, one just needs to measure F or S to deduce t. But in general, neither F nor S(t=) are known...

32 ABSOLUTE DATATION: long period isotopes The system Pb-Pb :

33 ABSOLUTE DATATION: long period isotopes The system Pb-Pb : Pb Pb Pb 24 Pb 27 Pb 24 Pb 26 Pb 24 Pb ( Hyp : 238U/235U = Cte=37,88 ) = e 37,88 e 235 t 238 t or: (y-y)/(x-x) = a(t) Equation defining a straight line of slope depending on time : the slope gives the age! This straight line is an isochrone. For an isochrone, at least 2 points are needed, from 2 minerals of a same object, having different initial U/Pb.

34 ABSOLUTE DATATION: long period isotopes The system Pb-Pb : Consider the formation of an asteroid. N kinds of minerals are created, with various µ=u/pb (chemistry). Then, the system doesn't evolve chemically anymore, but U Pb inside the minerals. In the Pb-Pb diagramm, the N points are aligned on a line of slope a ( age of the asteroid ). (x,y)

35 ABSOLUTE DATATION: CAI & CHONDRULES Age difference The oldest dated CAI has ±.7 Ma Compatible with 4566 ± 2 Ma (Manhès et al. 988) Does that dates the origin of Solar System? Amelin et al., Science 22

36 RELATIVE DATATION: short period isotopes If extinct radioactivity, F=, S=S+F. Ex: The system Hf-W (t/2~ 9 Myr) : Hafnium 82 decomposes into Tungsten W = 82 W init 82 Hf init ( ) ( ) ( ) ( ) 82 W = 84 W y 82 W + 84 W init b 82 8 Hf Hf 84 8 Hf init W a x The higher the rate 82Hf/8Hf was, the more the proportion of 82 W inside the tungsten increases with the general Hf/W ratio.

37 RELATIVE DATATION: short period isotopes The system Al-Mg (t/2~.7 Ma) : extinct radioactivity Mg Mg 26 = Mg Mg t=t f 24 Al Mg ( tf = formation time ) t=t f Al Mg = 27 t =t f 27 Al Al 24 t =t f 26 Al 27 Mg ( ) ( ) ( ) 26 Mg = 24 Mg y 26 Mg + 24 Mg t=t f b Al Al = t= t f 27 Al Al t f t e t =t ( ) 27 Al Al λ(t t ) e Al Mg t =t f a x If one measures x and y of several minerals of a same object, the points align on a straight line of slope a : an isochrone. Same slope a => same age t f (under the assumption that (26Al/27Al)t is the same in all the Solar System). EXO

38 RELATIVE DATATION: short period isotopes Sp Px An Mel CAI MRS6 (Leoville, CV3) Crystallized Ga (BSE image) Melilite Anorthite Spinel Pyroxene

39 Al inside CHONDRULES 26 Maximum for a =(26Al/27Al)t e-λ(t-t) inside chondrules : ~-5. Inside CAI : ~ 4,5 x -5. Huss et al., MAPS 2

40 Al inside CHONDRULES 26 Maximum for a =(26Al/27Al)t e-λ(t-t) inside chondrules : ~-5. Inside CAI : ~ 4,5 x -5. Thus exp(-λ(tcai-t)) / exp(-λ(tchondres-t)) = 4,5 so exp(-λtcai) = 4,5*exp(-λtchondres) so tcai = tchondres - ln(4,5)/λ Conclusion: chondrules formed 2 Myrs after the CAIs. We set tcai = t =, birth of the Solar System.

41 PRINCIPLES of DATATION Fundamental assumptions : The isotopic distribution of the chemical elements was the same at t= everywhere in the Solar System. Chemical fractionation doesn't induce any isotopic fractionation. Attention : The ages obtained are closure ages (i.e. the time for which the system hasn't exchanged elements with the outer world). In fact, datation is made from isochrones (so from the analysis of several chemical components of the sample), thus one date the age of chemical fractionation of the system.

42 THE Hf W SYSTEM DIFFERENTIATION The Hf/W :system The system Hf-W (t/2~ 9 Ma) : Hafnium 82 declines into Tungstène 82. Similarly as for the Al-Mg, system, one finds : ( ) ( ) ( ) 82 W = 84 W y 82 W + 84 W CAI b 82 ( ) 8 Hf Hf λ(t t ) e 8 84 Hf W CAI CAI a x But Hafnium is lithophile (goes into the mantle) while the Tungsten is siderophile (goes into the core) : chemical fractionation. One date the closure of the system mantle or core, that is the differenciation of a planet!

43 THE Hf W SYSTEM : DIFFERENTIATION The system Hf-W (t/2~ 9 Ma) : Late differenciation: all the 82Hf has declined, all the W in the core. Fast différentiation : 82Hf in the mantle, produces there some 82W.

44 THE Hf W SYSTEM : DIFFERENTIATION Age of the Moon, through 82Hf / 82W chronometer : 82 Hf 82W, half life τ = 9.6 years. Late core formation : no excess 82W in the mantle. 82 Core forms Undiff. planet Hf (rock-loving) 82 W (iron-loving) Early core formation : excess 82W in the mantle. Undiff. planet Core forms Differentiated mantle (sketch courtesy: F. Nimmo ) Excess 82W in lunar rocks Moon solidified within 6 Myrs (Kleine et al. 25, Lee et al. 997, 22)

45 THE Hf W SYSTEM : DIFFERENTIATION Definition : 82 W = [ 82 W 84 W 82 W 84 W objet ] reference 4 Analyse of 3 kinds of lunar rocks (Kleine et al, 27, Science): ε82w : Hf/84W: 8 KREEP Low W basalt High W basalt,6 +/-,2,8 +/-,2 2,4 +/-,57 +/- 26,5 +/- 6,5 > 45 EXERCICE: Deduce the age of solidification of the Moon. One gives (82W/84W)ref=, (82Hf/8Hf)CAI=-4.

46 THE Hf W SYSTEM : DIFFERENTIATION Unluckily, 82W is also coming from 8Ta capturing a neutron (cosmic rays). Touboul et al. (27, Nature) : The dominant 82W component in most lunar rocks reflects cosmogenic production New data from unpolluted samples : lunar and terrestrial mantles have identical 82 W / 84W. This [ ] constrains the age of the Moon and Earth to 62 Myrs (+9/-). Touboul et al. (28) : - Same result for plagioclase separates from two ferroan anorthosites (i.e. crust). - Measure of basalts cosmogenic pollution. ε 82 W

47 MOON FORMATION The Moon formed by a giant impact on the proto-earth of a proto-planet of the size of Mars. Do we date the differenciation of the proto-earth? That of the impactor? The collision?

48 MOON FORMATION Re-equilibration of the mantles : It depends... If reequilibration, the anomaly in 82W is reduced (or reset) by the impact. Total reequilibration -> one dates the impact. Core merging : No reequilibration : we get the average ε82w. Oservations of terrestrial planets and asteroids suggest that there has been many giant impacts after Ma, with requilibration (Nimmo & Agnor 26). (sketches courtesy: F. Nimmo )

49 MOON FORMATION Isotopic equilibration just after the impact, between a magma ocean Earth, and a molten disc, via an atmosphere of silicates (Pahlevan & Stevenson 27). Disc during to years, at 25 K, to Bar. Typical echange time between vapour and liquid : week. Convection in the ocean : week. In the disc or the gas : day.

50 MOON FORMATION Problem : radial mixing. It needs more than 3 years to diffuse material to 4 terrestrial radii.

51 MOON FORMATION Problem : radial mixing. It needs more than 3 years to diffuse material to 4 terrestrial radii. Salmon & Canup (22) : Moon accretion stalls for ~ years because the growing Moon repels the disc, and prevents its spreading.

52 MOON FORMATION Conclusion : The Moon formed after a giant impact on Earth. This impact caused an Earth-Moon isotopic equilibration. As the mantles of the Earth and the Moon still have the same 82W/84W, 82Hf was extinct at that time : the impact took place (at least) 6 Myrs after the CAI. New result: Jacobson, Morbidelli, et al, DPS meeting, oct. 23, Denver The amount of «late veneer» that the Moon accreted after it formed is correlated with the time of the last giant impact. For reasonable estimates of the late veneer, the impact must be late (~95+/-3 Myrs).

Composition and the Early History of the Earth

Composition and the Early History of the Earth Sujoy Mukhopadhyay CIDER 2006 What we will cover in this lecture Composition of Earth Short lived nuclides and differentiation of the Earth Atmosphere and