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Chapter 8 : Meteorites

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Meteorites

• Chapter 8 - All

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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

M t Th li ht h t d d• 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.

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MeteorMeteor

CometsAsteroids

MeteoroidMeteoroid

Meteor

Amount of meteoric material accreting to the EarthMass influx on the Earth

Big impactorsmm – cm sized meteoroids

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 called chondrules)• 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 ( ) ( ) gcores 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

6(brackets indicate approx. numbers known)

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Where are meteorites found…

• Hot and cold deserts:– Antarctica (10,000+ meteorites)– Sahara desert /Middle East – (10,000+)– Nullarbor plain (Australia) – thousandsp ( )– Desert South-West (USA) – thousands

• Approximately half a dozen meteorites are observed to fall (and then recovered) over the entire globe each year.

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Material accumulates at larger distances from the protosun and/or within smaller bodies and experiences LESS heating (top) – does NOT differentiate

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Larger bodies and/or bodies closer to the sun heated more significantly – minerals “homogenized” and body differentiates (if larger than D>70 km)

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13NWA 4818, a CV3 carbonaceous chondrite

Why should you care about chondrites?

1. They are 4.556 billion years old.2. They are the oldest rocks in our collection.3. Tell the story of the earliest time of our solar

t ’ f ti

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system’s formation 4. Are what the Earth-like planets were made

from.5. Contain mineral grains from other stars.

What are the components of chondrites?

• Chondrules• Calcium-rich,

aluminum-rich inclusions

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inclusions• Presolar grains• Matrix

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.

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Chondrules

• Chondrules in the 0.1 – 2 mm size range must have cooled on the order of 10 minutes to a few hours to explain ptheir crystalline properties

• Correlations between size and composition are difficult to explain, but chondrules must have formed before becoming part of larger bodies.

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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 chondritesMost common in CV chondrites

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Origin of Chondrules and CAIs

• Both require high temperatures followed by rapid cooling.Possible scenarios:– Drag during passage through an accretion

shockshock.– X-wind acceleration followed by cooling in a

shaded region.– Lightning– Nebular shock waves– Molten impact splash

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The matrix

• The matrix consists of smaller grains, a lot of olivine and pyroxene (also seen in IR spectra of extra-solar debris disks pand comets)

• Can also have grains from other stars (presolar grains) mixed in (including diamonds).

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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 carbideThey are made of diamond, graphite, silicon carbide,

spinel and other refractory materials

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Presolar graphite grain (left) and silicon carbide grain (right) from the Murchison carbonaceous chondrite

CAIs

chondrules

24NWA 4818, a CV3 carbonaceous chondrite

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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 processedcarbonaceous 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 plot, elemental abundances in the Sun (spectroscopic) and CI chondrites (lab

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chondrites (lab measurements). Ratios to 106 Si atoms. Inset shows an arithmetic plot, showing the differences between different measurements.

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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

Oxygen IsotopesSystem to classify meteorites using oxygen isotopes

is largely the work of Robert Clayton et al.

• Three stable isotopes (Earth abundance):– 16O (99.76%)– 17O (0.039%)

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( )– 18O (0.202%)

• Definition: δ is variation (‰) from SMOW (Standard Mean Ocean Water):

SMOW16

18

0 OO

⎥⎦

⎤⎢⎣

⎡=R

sample16

18

OO

⎥⎦

⎤⎢⎣

⎡=R [ ]‰1000Oδ

0

018 ⋅−

=R

RR

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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.

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 andSeven 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

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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. g

• 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).

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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, Al2O3 1647 (1393) Perovskite, CaTiO31625 (1450) Melilite, Ca2Al2SIO, Ca2MgSi2O71513 (1362) Spinel, MgAl2O1471 Fe, Ni metal1450 Diopside, CaMgSi2O6

O

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1444 Forsterite, Mg2SiO1362 Anorthite, CaAl2Si2O81349 Enstatite, MgSiO3<1000 Alkali-bearing feldspar,

(Na,K)AlSi3O8 – CaAl2Si2O8<1000 Ferrous olivines, pyroxenes

(Mg,Fe)2SiO4 , (Mg,Fe)SiO3700 Troilite, FeS405 Magnetite, Fe3O4

Data from Wood, J. A. (1998) Annu. Rev. Earth Planet. Sci 16, 57.

atur

e (K

)

1400

1700

Condensation sequencePerovskite Melilite

DiopsideFosterite

Anorthite

Plagioclase

Corundum

Spinel

Enstatite

Metallic Iron

Olivine/Pyroxene with

Refractory inclusions

Chondrules

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Tem

pera

270

420

670 Olivine/Pyroxene with higher iron content

Trolilite

SulfatesCarbonates

Phyllosilicates

Ices

Organic compounds

Magnetite

Matrix

Metals

Silicatesratu

re (K

)

Earth Jupiter Saturn Uranus

2000

Condensation sequence

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Silicates, rocky material

Water ice

Ammonia ice

Tem

per

Distance from Sun (AU)

1000

0 5 10 15 20

The differentiated stones I : Achondrites

• Mantle, crust and surfaces of a differentiated early (large) asteroid later smashed to pieces through collisions

• Howardites – Planetary soil, highly brecciated mixture of

Eucrite/Diogenites

Eucrite

g• 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)

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Vesta• The surface maps of the asteroid

Vesta are derived from the Dawn spacecraft. The southern crater is thought to be the source of HED meteorites

Model of Vesta surface

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Stages in the evolution of Vesta (HED parent body)

Early core formation and magma

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Early core formation and magma ocean 1500-1530 °C

Turbulent convection and equilibrium crystallization 1530 °C - 1220 °C

Convective lock-up and crystal settling

Intrusion and extrusion of residual liquids into and on to thin crust

Achondrites – II – Planetary Meteorites

• Shergottites-Nakhlites-Chassignites (SNCs) – linked to Mars since:1. Trapped gases match isotopic ratios measured by Viking of the Martian

atmosphere2. Isotopic dating of the melt ages show many values less than 2 Ga –p g g y

formed on a recently active (very large) planetary body3. Isotopes in SNCs match general chemistry known for Mars4. 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

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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 hi hl d ) Ri h i Al Chighlands). 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 elementAbundances different from Earth andordinary chondrite meteorites

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

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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.

The variation of Ir and Ni among several differing iron meteorite groups. Iridium shows a variation over five orders of magnitude (Wasson 1985)

% Ni

Stony-irons

Mesosiderite

Pallasite

• 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.

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Location of meteorite Parent Bodies in the Solar System

1. Direct measurements of ~10 meteorite orbits : all asteroidal-type orbits

2. Reflectance spectra of asteroids matched to some meteorite classessome meteorite classes

3. Models of orbital dynamics and space residence times of meteorites based on cosmic-ray exposure ages consistent with material delivered from Main-belt to Earth on timescales of millions – tens of millions of years

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Measured orbits for meteorite falls

Collisions can send fragments to nearby resonances and then to Earth

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then to Earth

The Asteroid-Meteorite Connection

D=Tagish Lake

53(Courtesy: T. Hiroi, NASA/JSC, JPL, H. Yano, NHK)

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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 agesexposure 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 ν6 resonance)– Gefion family (near 5:2 reonance

with Jupiter) 54Gefion family (red) and modelled fragments(black dots) (Nesvorny et al., 2008)

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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 y) y pfew 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.

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Atmospheric entry

56Ceplecha 1998

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 py g10 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.

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- Conservation of momentum and energy are used to calculate the velocity, mass and temperature of the particle

Kinetic energy ofBlackbody Radiation

Energy Momentum

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Kinetic energy of air atoms

Ablating mass Momentum transfer from air atoms

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛−−−=

dtdmLTTvc

cmA

dtdT

aah

m

443

3/23/1 42

σερρ 3/23/1

2

m

ad

mvAc

dtdv

ρρ

=

Love and Brownlee (1991)

Particles decelerate very high (90-100 km) – experience low aerodynamic stressesMore fragile material

i th i

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can survive than in meteorites, which decelerate lowerVery small particles (<10 microns) never ablate no matter what density

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• 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: 6x106 J/kg The heat transfer and drag coefficient may depend in

general on the size and velocityBelow ~3 km/s, ablation is no longer important.

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Velocity and drag• Very small/fast

meteoroids do not slow down appreciably before they ablatethey ablate completely.

• Meteoroids which survive to low altitudes experience significant deceleration.

61Homestead fall ellipse

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

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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 2. 63

The decay rate

• The abundance of a parent species as a function of time is:

⎟⎟⎠

⎞⎜⎜⎝

⎛ −−=

mpp

tttNtNτ

00 exp)()(

τwhere is a decay constant which depends on the particular nuclide in question.

The half life is the time at which

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mτ

)(21)( 0tNtN pp =

2ln2/1

2/1

/2/1

m

t

te m

τ

τ

== −

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Dating rocks containing radioactive isotopes

Consider the abundance of the parent and daughter species:

⎟⎟⎠

⎞⎜⎜⎝

⎛ −= pp

tttNtN 00 exp)()(

We have two unknowns: the age and the initial amount of daughter species.

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⎟⎠

⎜⎝ m

pp τ

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ −−+=

mpdd

tttNtNtNτ

000 exp1)()()(

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. yCompare 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. 68

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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 equalinitial ratio (D/S)o but different ratios (N/S). In this case equation 3.6 defines a line on an isochron plot:

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N/ S

Do/ So

slope = eλt -1D/S

t

DS

⎛ ⎝

⎞ ⎠ =

o

DS

⎛ ⎝

⎞ ⎠ +

t

NS

⎛ ⎝

⎞ ⎠ eλt −1( )

y = intercept + x * slope

Isochron diagram

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87Rb – 87Sr isochron for the unequilibrated H3 chondrite Tieschitz meteorite (Taylor 1992)

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.

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Gas retention: back before the beginning

• For example, Xenon is fairly rare, much rarer than Iodine. 129I beta decays to 129Xe with a half life of 17 million years.

• The total amount of Xenon in the meteorite• The total amount of Xenon in the meteorite is related to the initial amount of radioactive 129I.

• But I-129 was produced before the solar system formed in a supernova explosion

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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 distributionThe r process produces a particular nuclide distribution.• Unstable r-process elements, including 129I, decay after

formation. The amount of 129I 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. 74

Light elements with short half lives

• It turns out that there is a correlation in chondrites between Al abundance and 26Mg/24Mg ratio.

• This cannot be a result of mass dependent fractionation because 25Mg/24Mg is normal.

• So probably 26Al beta decays into 26Mg: the half life is• So probably 26Al beta decays into 26Mg: 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.

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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 ) pfirst solid materials to form, they are often taken to define the age of the Solar System

• Moon rocks are younger (3 – 4.45 Gy) so have melted since then. Terrestrial rocks are less than 4 Gy old.

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The EndThe End

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