3. meteorites-bse comp

7/27/2019 3. Meteorites-BSE Comp

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Meteorites

Falls and finds

Find: typically, someone finds a strange rocky/metallicobject- may have been affected significantly byterrestrial weathering and alteration

Fall: the fireball of the falling meteorite is observed, and thefreshly fallen pieces are collected on the ground- noterrestrial alteration

Most meteorites derived from the Asteroid belt betweenMars and Jupiter

Some from Moon and Mars


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Meteorite classification

R- Rumuruti chondrite


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Meteorite types

Undifferentiated meteorites Chondrites

~85% of falls

- Closest to the solar nebula composition for non-volatile elements-condensed samples of undifferentiated cosmic material

- Thought to be specimens of planetary material in very nearly the same

state when the planets first formed

- Never experienced planetary melting or igneous differentiation

Differentiated meteorites

(1) Achondrites- (~8% of falls) e.g., Martian meteorites, lunar meteorites

- (2) Stony irons- (~1% of falls)

(3) Irons- (~6% of falls)

- formed by igneous processes on major or minor planets- E.g., fragmentation of core-mantle differentiated asteroids


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Meteorites

Irons(cores of differentiated planetesimals)

Stony-irons(mechanical mixes of Fe and rock)

Basaltic Achondrites(Crusts and mantles of

differentiated planetesimals)

Ordinary Chondrites(Aggregates of chondrules, CAIs, metal, matrix)

Carbonaceous Chondrites(Primitve, organic rich, contain CAIs)


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Iron meteorites Predominantly composed of

Fe-Ni alloy

Two basic minerals:kamacite(very little Ni) andtaenite(20-50% Ni)

The cubic crystal structurediffers between the twointergrown plates in acharacteristic pattern

The kamacite/taenite ratiodiffers between differentgroups: hexahedrites andoctahedrites

Widmansttten pattern


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Chondrites are stony meteorites that have not been modified due to melting ordifferentiation of the parent body

The most abundant constituents of chondrites are chondrules, which are igneous

particles that crystallized rapidly in minutes to hours

Chondrules composed largely of olivine and pyroxene, commonly contain metallicFe-Ni and are 0.0110 mm in size

Chondrites are the oldest known rockstheir components formed during the birth

of the solar system ca. 4,567 Ma

Best clues to the origin of the solar system - Building blocks of planets

Abundances of nonvolatile elements are close to those in the solar photosphere

Broadly ultramafic in composition, consisting largely of Fe, Mg, Si, O

Chondrites contain diverse proportions of three other components: refractoryinclusions (0.0110 vol.%), metallic Fe-Ni (,0.170%) and matrix material (180%)

Embedded in the matrix are presolar grains, which predate the formation of our

solar system and originated elsewhere in the galaxy

Chondrites


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Structure of chondrites

Matrix: dark, fine-grained background

CAI: whitish, irregularly shaped, calcium-aluminum-richinclusions-Hibonite,Pervoskite, Melilite, spinel, diop, An, Fo,

corundum-earliest condensed/crystallized refractory solids in the solarsystem

Chondrules show textural features, whichindicate they are the products of rapid

cooling of numerous ferromagnesiansilicate melt droplets

Matrix that binds the chondrules consistsof a disequilibrium mixture of mineralsranging from those formed at very hightemperatures (>1400 K) to very lowtemperatures (~273 K)

Preservation of disequilibriumassemblage of high temperature and low

temperature minerals attests to theprimitive nature of these meteorites

Chondrules: nearly spherical droplets,typically of mm-size-Ol, Py, Pl, Fe-Nimetal, FeS


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Chondrite formation Aggregate of high-and low-

temperature materials

CAIs may result fromextreme heating in the

innermost parts of theearly, active nebula

Chondrules were made byrapid, less extreme heatingand melting of large grains


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Chemical classes of chondritesCI (Ivuna)CM (Murchison)

CO (Ornans)CV (Vigarano)

Carbonaceous ~4% of falls

H (high iron)

L (low iron)

LL (low Fe, low metal)

Ordinary ~79% of falls

EH (high iron)

EL (low iron)

enstatite ~2% of falls

R Rumuruti ~0.1% of falls


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Classification of chondrite

Chondrites divided into three clans based on their major-element compositions

and the ratio of Fe bounded to metal and sulfides to Fe bounded as oxides insilicates

A function of the oxygen fugacity of the meteorite forming environment

In high oxygen fugacity environments, the Fe is bound up in silicates as Fe2+,

whereas in low oxygen environments, the Fe is bound in metals as Fe0


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Petrologic types of chondrites

Reflect the state of

alteration - eitheraqueous alteration

(carbonaceous) orthermal metamorphism

(other classes)

Most ordinary chondrites are

thermally metamorphosed

Most carbonaceous chondrites

show aqueous alteration


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Oxygen isotopes in chondrites


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Why are chondrites important ?

Goldschmidt, Suess, and Urey showed that chondrites provide best

estimates for mean abundances of condensable elements in solar system

These estimates were essential for developing theories for the formation

of elements in evolved stars Presolar grains provide additional clues to nucleosynthesis and the

subsequent growth of circumstellar grains

Chondrules, metal grains, refractory inclusions, and matrix materialsformed under very diverse conditions in the solar system and appear to

offer insights into processes that occurred during the formation of the Sun

and planets from a collapsing cloud of interstellar dust and gas

The rocks themselves provide clues to the geological processes including

impact processes that affected asteroids over 4.5 Ga

Studies of chondrites help us to match chondrite groups with asteroid

classes, to understand the origin and evolution of the asteroid belt


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General decrease in abundance with atomic number (H mostabundant, U least abundant)

Relative to this trend:

Big negative anomaly at Be, B, Li Moderate positive anomaly around Fe

Saw tooth pattern from odd-even effect

This data is obtained from spectroscopic observation of atomic

absorption lines in the solar spectrum, from light passing throughsolar atmosphere

99% of solar system mass is in the sun, so solar composition isgood approximation of bulk solar system composition

Some elements, for which spectroscopy is difficult, are filled in usingmeteorite data

Successful model of nuclear origins needs to explain all thesefeatures in the abundance pattern!

Solar abundance of the elements


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Solar abundance of the elements


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Solar composition vs. different groups of

chondrites Only one group ofmeteorites, the CIchondrites, closely matchsolar abundances forelements representing thevarious cosmochemicalgroups and excluding theextremely volatile elementssuch as the rare gases,

hydrogen, carbon, oxygen,and nitrogen

All other chondrite groupsdeviate from solar

abundances and thedeviations can beunderstood, at least inprinciple, by gassolidfractionation processes in

the early solar system


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Solar system composition- CI chondrite

Except for the most volatileelements (i.e., more volatile

than nitrogen), CI

carbonaceous chondrites are

excellent models of bulk

solar system compositionAre they also close to bulk

earth composition ?

While the sun is basically

H+He, the Earth is

dominated by O, Si, Mg, Fe.

Much Fe is in core, leaving

rocky earth dominated by O,Si, Mg, Fe

Volatile depletion indicates

incomplete condensation ofvolatiles

Zr


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Volatility trend

Among the several classes of carbonaceous chondrites, relative abundance of all

elements are controlled by volatility; this plot shows the CV chondrites versus CI

CV

vs.CI


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Bulk composition of meteorites

More recent work shows pervasive volatility control even among moderately

refractory elements; the Earth is on the Carbonaceous chondrite line, but ordinarychondrites are different except for the very most refractory elements


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Bulk composition of meteorites

Carbonaceous chondrites plot on simple volatility controlledlines in consistent order, OC do not

Variation due to incomplete condensation/retention of

volatiles

Earth is on CC line but does not match with any group


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Incomplete condensation (retention) of volatiles

Laboratory quantification of volatility by condensation temperature shows that relative

abundance in carbonaceous chondrites is controlled by pure vapor-solid equilibrium down to~900 K, then adsorption must become significant for retaining many highly volatile elements.


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Bulk composition of Earth and Volatility

Major element composition of Earth doesnot match CI chondrites

For volatile and semi volatile

elements Earth and CI chondritesare different


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Estimating BSE composition

Using mass balance approachProblems:

(1) We do not know composition of BSE

(2) We do not know the concentration of

siderophile elements in the Fe core.

(3) BSE has differentiated into manyreservoirs, may be difficult to find a relict

sample of it.

(4) Adding up these derivative silicatereservoirs is not a trivial task as anumber of uncertainties can arise.

(5) Composition of each reservoir may be uncertain due to either the lack of samplesor to heterogeneity within a reservoir (e.g., the continental crust is highly

heterogeneous vertically and laterally).

(6) It must be assumed that the presence of other reservoirs has not been overlooked.Size of depleted and undifferentiated mantle is not known


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Using mantle peridotite compositions

Whitearrows:

direction ofincreas

ingmeltextraction

Approach

isto


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Major element compositions of terrestrial rocks not matched by any chondrite groups

While it is valid to assume that the Earths refractory lithophile elements are inchondritic proportions, the fact that the major element composition of the bulk Earthis not matched by any meteorite group indicates:

That not only can we not obtain direct information on absolute elemental abundancesin the Earth from meteorites, but we also cannot obtain the relative abundances of

moderately volatile and volatile elements in the Earth by direct comparison tometeorites

Using primitive (chondritic) meteorite compositions

Assuming primitive Earth formed fromundifferentiated rocky planetesimals, bulkEarth should have chondritic or solarabundances of refractory elements

For those refractory elements that do notenter the core but instead reside in thesilicate mantle (refractory lithophileelements), BSE, or primitive mantle,possesses chondritic relative abundances


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BSE composition of the Earth

BSE Mg/Si and Al/Si composition is atintersection between the terrestrial mantlexenolith array and the chondrite array

BSE major element composition is not

equal to any class of meteorites, so if bulkearth is, e.g., CI chondrite in composition,then lower mantle must be compositionallydistinct (or Si is a major constituent of core)

Some of these primitive mantle samples have chondritic refractory element ratios

Those that have been depleted in meltable components have non-chondritic

refractory element ratios due to the different behaviors of these elements during

melting

One could thus determine absolute elemental abundances by extrapolating

compositional trends in melt-depleted peridotites back to where refractory element

ratios become chondritic

Once the absolute abundance of one element has been determined by such anextrapolation, the abundances of other elements follow from their ratios to each other


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X

X/Y refractory major element

Chondritic ratio

BSE composition Melting trend


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Bulk composition of the Earth

Mg/Fe vs. Si/Fe for CC,the Sun and Earth

CC have constant Mg/Si

ratios, but variable Fecontents

The Mg/Si-ratio of theEarths upper mantle is1.029 which issignificantly higher thanthe CI-ratio of 0.90


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Solar Nebula

CAI - Formation

Chondrule - Formation

Accretion

Differentiation

Earth evolution-From Dust to Planets

Allende Ca, Al-rich inclusion

(CAI) 4.567 Ga Pb-Pb age(Amelin et al., 2002)


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nternal structure of the Earth

How and when did the Earth develop this structure ?

Metallic core

Silicate mantle

Heterogeneouscrust


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Homogeneous vs. heterogeneous accretion

How and when was Earths radial

heterogeneity established?

Hypothesis 1- Heterogeneousaccretion: Radial heterogeneity

primordial. Earth layered from theoutset, i.e., solid particles making upEarth were assembled in order ofdecreasing density. Fe, being verydense, accreted first to form the

core, while silicates, being lessdense, accreted afterwards, formingthe mantle.

Hypothesis 2- Homogeneous accretion: Earth was homogeneously accreted. Itwas initially a homogeneous mixture of metallic Fe and silicates.Metallic core formation is the result of metal-silicate segregation after most of theEarth had been accreted.

It is now believed that theHomogeneous accretion

is most likely but that core-mantle segregation occurred very shortly after the Earth accreted


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Agee (2004)

Planetary differentiation

Magma ocean formation ?


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(Jochum, MPI)

ment fractionation during differentiation

Used as

tracers for

geochemical processes

geochronology


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Sm-Nd fractionation during silicate

differentiation

Sm

Nd

CORE

MANTLE(Depleted Reservoir)

CRUST(Enriched Reservoir)Low Sm/Nd142Nd, 143Nd

DSm > DNd

High Sm/Nd142Nd, 143Nd

3. meteorites-bse comp

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