Chapter 8 : Meteorites. Chapter 8 - All. Meteorites

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1 Chapter 8 : Meteorites 1 Chapter 8 - All Meteorites 2 1

2 I. Basic Definitions Meteoroid : A solid particle in orbit about the sun; composed of silicates and hydrocarbons and generally lower in density than common terrestrial rock Meteor : The light, heat and sound phenomena produced when a meteoroid enters the atmosphere and is heated by friction through collisions with air molecules. Meteorite : A meteoroid which survives atmospheric entry and lands on the Earth. 3 Asteroids Comets Meteor Meteor Meteoroid 2

Amount of meteoric material accreting to the Earth Mass influx on the Earth mm cm sized meteoroids Big impactors Dust Meteorites and their origins From collected meteorites several major types:

3 Amount of meteoric material accreting to the Earth Mass influx on the Earth mm cm sized meteoroids Big impactors Dust Meteorites and their origins From collected meteorites several major types: Stoney (mostly silicate minerals) (25306) Chondrites (relatively unprocessed rock formed in smaller bodies containing small spherules ues called chondrules) ues) Achondrites (processed, igneous-type meteorites, which formed as part of a larger body. No chondrules) Irons (rich in free iron and nickel) (965): originate as the cores of larger differentiated bodies Stoney-Irons (mixtures of silicates with metal veins) (122): formed in large bodies between iron core and mantle crusts Lunar (52) and Martian (33) meteorites (brackets indicate approx. numbers known) 6 3

Where are meteorites found Hot and cold deserts: Antarctica (10,000+ 000+

thousands Desert South-West (USA) thousands Approximately half a dozen

4 Where are meteorites found Hot and cold deserts: Antarctica (10, meteorites) Sahara desert /Middle East (10,000+) Nullarbor plain (Australia) thousands Desert South-West (USA) thousands Approximately half a dozen meteorites are observed to fall (and then recovered) over the entire globe each year

5 9 Material accumulates at larger distances from the protosun and/or within smaller bodies and experiences LESS heating (top) does NOT differentiate Larger bodies and/or bodies closer to the sun heated more significantly minerals homogenized and body differentiates (if larger than D>70 km) 10 5

6 11 6

7 NWA 4818, a CV3 carbonaceous chondrite 13 Why should you care about chondrites? 1. They are billion years old. 2. They are the oldest rocks in our collection. 3. Tell the story of the earliest time of our solar system s formation 4. Are what the Earth-like planets were made from. 5. Contain mineral grains from other stars. 14 7

Matrix 15 Chondrules Chondrules are spherical glassy igneous

Because they are glassy, they probably cooled very fast (minutes

8 What are the components of chondrites? Chondrules Calcium-rich, aluminum-rich inclusions Presolar grains Matrix 15 Chondrules Chondrules are spherical glassy igneous inclusions. Because they are glassy, they probably cooled very fast (minutes hours). There is a correlation between chondrule size and the types of crystals in them, suggesting that the cooling rate is somehow set by the size of the globule. 16 8

and composition are difficult to explain, but

9 17 Chondrules Chondrules in the mm size range must have cooled on the order of 10 minutes to a few hours to explain their crystalline properties Correlations between size and composition are difficult to explain, but chondrules must have formed before becoming part of larger bodies. 18 9

$refractory components of meteorites and the oldest Most common in CV chondrites 19 Origin of Chondrules and CAIs Both$ require high temperatures followed by rapid cooling.

require high temperatures followed by rapid cooling.

10 Calcium Aluminum Inclusions CAIs are white/light grey inclusions 1 10 mm size and usually not round Among the most refractory components of meteorites and the oldest Most common in CV chondrites 19 Origin of Chondrules and CAIs Both require high temperatures followed by rapid cooling.possible ibl scenarios: Drag during passage through an accretion shock. X-wind acceleration followed by cooling in a shaded region. Lightning Nebular shock waves Molten impact splash 20 10

11 21 The matrix The matrix consists of smaller grains, a lot of olivine and pyroxene (also seen in IR spectra of extra-solar debris disks and comets) Can also have grains from other stars (presolar grains) mixed in (including diamonds)

$They are made of diamond, graphite, silicon carbide, spinel and other refractory materials Presolar graphite$

12 Presolar grains Solid grains that condensed around other stars (giant stars or in supernovae) or in the interstellar medium and survived the formation of the solar system without melting. Rare. They are made of diamond, graphite, silicon carbide, spinel and other refractory materials Presolar graphite grain (left) and silicon carbide grain (right) from the Murchison carbonaceous chondrite 23 chondrules CAIs NWA 4818, a CV3 carbonaceous chondrite 24 12

13 Meteorites I The Chondrites Least altered of all meteorite-types Elemental abundances of most unaltered members very close to sun and original solar nebula (except volatile elements) Elemental patterns differ for each class of chondrite Most primitive members are highly oxidized (i.e. carbonaceous chondrites), while more processed members have more free metal (ordinary chondrites, Enstatite chondrites) E-chondrites have very low oxygen content may have formed interior to Mercury s orbit Size and types of chondrules, plus degree of heating determine petrological-type (1 7) Carbonaceous chondrites rich in volatiles, exposed to water and less heat than ordinary chondrites Log-log gp plot, elemental abundances in the Sun (spectroscopic) and CI chondrites (lab measurements). Ratios to 10 6 Si atoms. Inset shows an arithmetic ti plot, showing the differences between different measurements

14 Chondrite taxonomy based on variations in minor element abundances Degree of oxidation (lower right fully oxidized) compared to reduction (fully reduced to upper left) in OCs 27 Oxygen Isotopes System to classify meteorites using oxygen isotopes is largely the work of Robert Clayton et al. Three stable isotopes (Earth abundance): 16 O (99.76%) 17 O (0.039%) 18 O (0.202%) Definition: δ is variation ( ) from SMOW (Standard Mean Ocean Water): R 0 = O O SMOW = O O 18 0 R δ O = 1000 [ ] sample R R R

15 Oxygen 3-isotope plot. TFL: terrestrial mass-dependent fractionation line. Carbonaceous chondrites tend to plot below the line (except CI). The isotopes also reflect the role of water in mass-dependent fractionation, isotopic mixing, and oxidation. 29 Carbonaceous Chondrites Less heated than other chondrites - show evidence of exposure to water Rich in carbon, water (up to 30% water by weight) and volatile compounds Seven classes based on chemistry and isotopic composition Rarest type = CI are extremely fragile and least heated of all meteorites Best analog to original solar nebula material 15

16 Cosmochemical Classification Elements are classified on the basis of their stability in a gas of solar composition. The distribution of an element between gas and solid is controlled by pressure and temperature of gas. The temperature at which various elements condense or evaporate in a gas of solar composition has been estimated by using principles of chemical thermodynamics (under equilibrium condensation). 31 Equilibrium condensation sequence: major minerals that would condense in a gas with solar system abundances at 10-3 bar. Temperatures in parentheses are those at which the phase has been converted to other minerals. Temperature (K) Mineral, Formula 1758 (1513) Corundum, Al 2 O (1393) Perovskite, CaTiO (1450) Melilite, Ca 2 Al 2 SIO, Ca 2 MgSi 2 O (1362) Spinel, MgAl 2 O 1471 Fe, Ni metal 1450 Diopside, CaMgSi 2 O Forsterite, Mg 2 SiO 1362 Anorthite, CaAl 2 Si 2 O Enstatite, MgSiO 3 <1000 Alkali-bearing feldspar, (Na,K)AlSi 3 O 8 CaAl 2 Si 2 O 8 <1000 Ferrous olivines, pyroxenes (Mg,Fe) 2 SiO 4, (Mg,Fe)SiO Troilite, FeS 405 Magnetite, Fe 3 O 4 Data from Wood, J. A. (1998) Annu. Rev. Earth Planet. Sci 16,

17 Temperature (K) Condensation sequence Perovskite Diopside Anorthite Plagioclase 420 Sulfates Carbonates 270 Melilite Fosterite Spinel Corundum Enstatite Olivine/Pyroxene with higher iron content Phyllosilicates Ices Organic compounds Metallic Iron Trolilite Magnetite Refractory inclusions Chondrules Matrix 33 Condensation sequence 2000 Metals ) Temperature (K 1000 Earth Jupiter Saturn Uranus Silicates, rocky material Water ice Ammonia ice Distance from Sun (AU) 34 17

The differentiated stones I : Achondrites Mantle, crust and

smashed to pieces through collisions Howardites Eucrite

Eucrites Extrusive basaltic lava; lava flows on or near surface

large crystals form portion of the mantle of original parent

18 The differentiated stones I : Achondrites Mantle, crust and surfaces of a differentiated early (large) asteroid later smashed to pieces through collisions Howardites Eucrite Planetary soil, highly brecciated mixture of Eucrite/Diogenites Eucrites Extrusive basaltic lava; lava flows on or near surface of parent body Diogenites Intrusive magma slow cooling with large crystals form portion of the mantle of original parent asteroid HEDs linked to pieces of the asteroid Vesta (through reflectance spectra) 18

20 Vesta The surface maps of the asteroid Vesta are derived from the Dawn Model of Vesta surface spacecraft. The southern crater is thought ht to be the source of HED meteorites 39 Stages in the evolution of Vesta (HED parent body) Early core formation and magma ocean C Turbulent convection and equilibrium crystallization 1530 C C Convective lock-up and crystal settling Intrusion and extrusion of residual liquids into and on to thin crust 40 20

Isotopic dating of the melt ages show many values less than 2 Ga formed on a recently active (very large) planetary body 3.

21 Achondrites II Planetary Meteorites Shergottites-Nakhlites-Chassignites (SNCs) linked to Mars since: 1. Trapped gases match isotopic ratios measured by Viking of the Martian atmosphere 2. Isotopic dating of the melt ages show many values less than 2 Ga formed on a recently active (very large) planetary body 3. Isotopes in SNCs match general chemistry known for Mars 4. Numerical simulations demonstrate impacts on Mars can release SNCs into orbit and computer determined transit times consistent with measured ages based on radioactive isotopes induced by cosmic-ray bombardment in space

Achondrites III Lunar Meteorites Like meteorites from Mars, lunaites are ejecta from recent impacts which collide with the

4 km/s) make it easier to leave than from Mars Most common lunaites are anorthositic breccias (containing plagioclase

Chemical compositions, isotope ratios, minerals, and textures of the lunar meteorites are all similar to those of samples

22 Achondrites III Lunar Meteorites Like meteorites from Mars, lunaites are ejecta from recent impacts which collide with the Earth. Low lunar escape velocity (2.4 km/s) make it easier to leave than from Mars Most common lunaites are anorthositic breccias (containing plagioclase feldspar crystals from the lunar highlands). Rich in Al-Ca. Chemical compositions, isotope ratios, minerals, and textures of the lunar meteorites are all similar to those of samples collected on the Moon during the Apollo missions. Anorthosite abundant in lunar highlands, but very rare on Asteroids and unknown on other planetary bodies Achondrites III Lunar Meteorites Lunar meteorites show element Abundances different from Earth and ordinary chondrite meteorites 22

23 The Iron meteorites Iron meteorites are classified according to structure and chemistry Structural classification is determined by nickel content in mineral phases and forms the basis for the broad group taxonomy Chemical sub-division into more refined groups based on amounts of the trace elements Ga, Ge, Ir. Irons come from larger bodies since: 1. Only differentiation could segregate heavy elements to produce nearly pure lumps of Fe-Ni 2. Criss-crossing pattern of Fe-Ni crystals (Widmanstatten pattern) in Irons can be used to measure rate at which metal cooled; low result of 1-10C per million years can only be achieved in large (~100 km) parent body 45 Kamacite Taenite Widmanstatten pattern of criss-crossing crystals of Kamacite and Taenite. Width of the kamacite bands is correlated with their Ni content and is used to classify irons into broad classes. The width of these bands is also determined by the meteorite s cooling history inside its parent body. 23

The variation of Ir and Ni among several

Iridium shows a variation over five orders

Stony-irons Mesosiderite Only about 10% of

24 The variation of Ir and Ni among several differing iron meteorite groups. Iridium shows a variation over five orders of magnitude (Wasson 1985) % Ni Pallasite Stony-irons Mesosiderite Only about 10% of meteorites, these are a combination of rock and metal. They may be difficult to distinguish from ordinary meteorites from their surfaces. 24

25 Location of meteorite Parent Bodies in the Solar System 1. Direct measurements of ~10 meteorite orbits : all asteroidal-type t orbits 2. Reflectance spectra of asteroids matched to some meteorite classes 3. Models of orbital dynamics and space residence times of meteorites based on cosmic-ray exposure ages consistent t with material delivered d from Main-belt to Earth on timescales of millions tens of millions of years 49 Measured orbits for meteorite falls 25

26 Collisions can send fragments to nearby resonances and then to Earth 52 26

The Asteroid-Meteorite Connection D=Tagish Lake P 53 (Courtesy: T.

Yano, NHK) L Chondrites origin About 2/3 of L chondrites are heavily shocked.

exposure ages, and more recent falls have longer exposure ages.

disruption of an asteroid near a resonance.

27 The Asteroid-Meteorite Connection D=Tagish Lake P 53 (Courtesy: T. Hiroi, NASA/JSC, JPL, H. Yano, NHK) L Chondrites origin About 2/3 of L chondrites are heavily shocked. Many have been found in ~470 Myr old sedimentary rocks with short cosmic ray exposure ages, and more recent falls have longer exposure ages. Some evidence that a meteorite shower occurred at this time as a result of the disruption of an asteroid near a resonance. There are two candidates: Flora family (near ν 6 resonance) Gefion family (near 5:2 reonance Gefion family (red) and modelled with Jupiter) fragments(black dots) (Nesvorny et 54 al., 2008) 27

28 Atmospheric Entry The outer part of the meteoroid gets hot enough to melt rock. However, it falls so fast (and conducts heat slowly) that the rock only melts to a depth of a few mm, and iron to about 1 cm. If the object survives ablation, when it reaches the ground it is cold. The crust is still altered (becoming magnetized for irons) but the interior may be undisturbed. 55 Atmospheric entry Ceplecha

Dust Photos courtesy of D. Brownlee, Univ of Washington. Because the particles are so small, their surfaces can only be studied in detail using electron microscopy.

The ice sublimes immediately when the particles are ejected from comets, leaving a framework of dust similar in composition to carbonaceous chondrites.

29 Dust Photos courtesy of D. Brownlee, Univ of Washington. Because the particles are so small, their surfaces can only be studied in detail using electron microscopy. This grain is about 10 microns wide, and may once have been filled with icy comet material. The ice sublimes immediately when the particles are ejected from comets, leaving a framework of dust similar in composition to carbonaceous chondrites Conservation of momentum and energy are used to calculate the velocity, mass and temperature of the particle Kinetic energy of air atoms Energy Blackbody Radiation Ablating mass Momentum Momentum transfer from air atoms dt A = /3 dt cm chρav dm ( ) T Ta L dt 3 2 4σε dv Acd ρav 1 2/3 ρ = m 1/ 3 2/ 3 dt m ρm 58 29

Love and Brownlee (1991) Particles decelerate very high (90-100 km) experience low aerodynamic stresses More fragile material can survive than in meteorites, which decelerate lower

It cannot radiate away the energy if it is above a certain size and velocity. It gets hotter than ~2000K, at which point it starts to melt. Liquid evaporates and sprays off the surface.

30 Love and Brownlee (1991) Particles decelerate very high ( km) experience low aerodynamic stresses More fragile material can survive than in meteorites, which decelerate lower Very small particles (<10 microns) never ablate no matter what density 59 Fall Phenomenon Larger objects The surface of the meteoroid is heated by an atmospheric shock front. It cannot radiate away the energy if it is above a certain size and velocity. It gets hotter than ~2000K, at which point it starts to melt. Liquid evaporates and sprays off the surface. Typical heat of ablation for stoney material: 6x10 6 J/kg The heat transfer and drag coefficient may depend in general on the size and velocity Below ~3 km/s, ablation is no longer important

31 Very small/fast meteoroids do not slow down appreciably before they ablate completely. Meteoroids which survive to low altitudes experience significant deceleration. Velocity and drag Homestead fall ellipse 61 Age Scales Three distinct ages for meteorites: 1. Formation age Time since meteorite was liquid (or gas) 2. Gas Retention Age Time since meteorite was last shocked/strongly heated (but not melted) 3. Cosmic-Ray Exposure Age Time since meteorite was exposed to cosmic rays, ie. since ejected from parent as meter-sized body 62 31

32 Radiometric Dating Most meteorites have formative ages from 4.53 to 4.57 Gy. These dates are estimated from long lived radioactive isotopes. Types of radioactive decay include β-decay (electron emitted) and α-decay (Helium nucleus emitted). For β-decay, a neutron is converted to a proton so that atomic number increases by 1. For α-decay, the atomic weight is reduced by 4 and atomic number decreases by The decay rate The abundance of a parent species as a function of time is: N p ( t ) = N τ m p ( t t t ) exp 0 τ m where is a decay constant which depends on the particular nuclide in question. The half life is the time at which 1/ 2 t 1/ 2 t = e = τ m 1/ 2 / τ ln 2 m 0 N 1 ( t) N p ( t0) 2 p = 64 32

34 Dating rocks containing radioactive isotopes Consider the abundance of the parent and daughter species: t t N ( = p t ) N ( p t ) exp 0 0 τ m t t 0 N = d ( t) Nd ( t0) + N p ( t0) 1 exp τ m We have two unknowns: the age and the initial amount of daughter species. 67 Ratios of parent/daughter nuclides To break down the uncertainty caused by the two unknowns, different samples of rock from the same meteorite are used. Each crystal has a different ratio of elements. Compare the ratio of parent and daughter elements to another nuclide which is stable and not changing. Use another isotope of the same element as the daughter nuclide. The initial isotope ratios should be the same in the different rock samples

35 69 Isochron method Most often the initial concentration of neither parent nor daughter is known, and more than one measurement is required to extract a meaningful date and also solve for the initial (D/S) ratio. Ideally we need multiple samples of equal age with equal initial iti ratio (D/S) o but different ratios (N/S). In this case equation 3.6 defines a line on an isochron plot: D/S slope = e λt -1 D S = D S + N S t o t ( e λt 1) y = intercept + x * slope D o / S o N/ S 70 35

36 Isochron diagram 87 Rb 87 Sr isochron for the unequilibrated H3 chondrite Tieschitz meteorite (Taylor 1992) 71 Gas retention ages Some elements decay into noble gases which are trapped in the rock. If the rock is heated enough (not necessarily melted) the gas can be liberated. If the radioactive parent remains, it continues to decay and rebuilds the gas supply. The clock is thus reset by heating events. The gas retention age can also tell you about the cooling history of the rock

37 Gas retention: back before the beginning For example, Xenon is fairly rare, much rarer than Iodine. 129 I beta decays to 129 Xe with a half life of 17 million years. The total amount of Xenon in the meteorite is related to the initial amount of radioactive 129 I. But I-129 was produced before the solar system formed in a supernova explosion 73 The interval between nucleosynthesis and condensation In a supernova, r-process elements are produced when there is a high flux of energetic neutrons. The unstable nuclei do not necessarily have time to β-decay before they gain another neutron. The r-process produces a particular nuclide distribution. Unstable r-process elements, including 129 I, decay after formation. The amount of 129 I inferred in the rock (by looking at the amount of Xenon present) gives a time between the supernova and the condensation of the iodine into the grains of the solar nebula. The protosolar nebula probably condensed ~80 million years after a supernova enriched the gas which was incorporated into the solar nebula

38 Light elements with short half lives It turns out that there is a correlation in chondrites between Al abundance and 26 Mg/ 24 Mg ratio. This cannot be a result of mass dependent fractionation because 25 Mg/ 24 Mg is normal. So probably 26 Al beta decays into 26 Mg: the half life is 720,000 year. Since Al is abundant, this could have provided a substantial amount of energy for melting planetesimals, an extremely important factor in considering why certain bodies melted and differentiated. The decay also produces a gamma ray which is detected from the Galaxy. 75 Clues to the formation of the solar system The Allende CV3 meteorite is 4.563±0.004 billion years old Because meteorites can be dated precisely (see error on date above) and are expected to be the first solid materials to form, they are often taken to define the age of the Solar System Moon rocks are younger ( Gy) so have melted since then. Terrestrial rocks are less than 4 Gy old

39 77 The End 78 39

Dating. AST111 Lecture 8a. Isotopic composition Radioactive dating

Dating. AST111 Lecture 8a. Isotopic composition Radioactive dating Dating Martian Lafayette Asteroid with patterns caused by the passaged through the atmosphere. Line on the fusion crust were caused by beads of molten rock. AST111 Lecture 8a Isotopic composition Radioactive