solar system formation - meteorites classification and geologic context of meteorites. how are...

Solar System formation - Meteorites

• Classification and geologic context of meteorites.

• How are meteorites classified?– Hierarchy of classification, first based on process.

– Next level is based on texture, geochemistry, and isotopic composition.

– Names (How does a meteorite get an official name?).

MeteoritesPrimitiveAchondrites

BRAUREAUBACALODANGWINFigure 1ChondritesCarbonaceousC

OrdinaryO

EnstatiteE

COCVCKCI CRCHCBCMHLLLEHELRKUndifferentiatedAchondritesDifferentiatedVesta?MoonEUCDIOHOWSHENAKCHAOPXMGPALES

PALPP

PALMarsMESCVACVBCVredCBaCBbIABIIICDCM-COCV-CKCR clanH-L-LLCIEH-ELClassClanGroupSub-GroupClanGroupGroupClanACA-LODWIN-IAB-IICDICIIABIICIIDIIEIIIABIIIEIIIFIVAIVB


• Iron meteorites– Cores of small

planetesimals.

– FeNi - rich metal, which is only found essentially in the core of Earth (natural, one major location).

c

Isotope: equal protons but different # neutrons, hence different at mass

O-isotopes: 16, 17, and 18 with 16 being least abundant.

1234567CICMCRCHCBCVCOCKHLLLEHELRK*PristineIncrease in degree of thermal metamorphismIncrease in degree of aqueous alterationchd./typechd. - chondrite group, *groupletFigure 7

Figure 8

Why are chondrites important?

1. Age = ~ 4.566+2Ma/-1Ma billion years (Allégre et al, 1995) or 4.5647±0.0006 billion years (Amelin et al., 2002).

2. They are accretionary rocks formed within the protoplanetary disk.

3. They are relatively unprocessed planetary materials that have a solar-like composition.

4. They contain components that would not be predicted to exist if they did not exist.

Anatomy of a Chondrite

• Chondrules• CAI’s or Refractory

inclusions• Fragments of the

above• Matrix• Opaques: Fe-Ni, FeS

• Pre-solar grains


3. They are relatively unprocessed planetary materials that have a solar-like composition.

Rocks = Sun’s photosphere

0.5

1

Al Sc Ca La Sm Eu Yb Lu V Mg Cr Mn Na K

CR

CM

CO

CV

CK

H

L

LL

EH

EL

Mg-normalized abundance/CI

1.5 a

0.1

1

Al Sc Ca La Sm Eu Yb Lu V Mg Cr Mn Na K

CR

CH (ALH 85085)

CH (Acfer 182)

LEW 85332

CB (Bencubbin)


b

0.1

1

Os Ir Ni Co Fe Au As Ga Sb Br Se Zn

CRCMCOCVCKHLLLEHEL

c


Os Ir Ni Co Fe Au As Ga Sb Br Se Zn

CR

CH (ALH 85085)

CH (Acfer 182)

Ungrouped Lew 853320.1

1

Mg-normalized abundance/CICB (Bencubbin)

d


4. They contain components that would not be predicted if they did not exist.

• Chondrules (igneous rocks) and igneous CAIs

Chondrules and CAIs

Chondrule mineral components?

FeMg-rich chondrules:• Olivine (fosterite Mg2SiO4 in solid solution with

fayalite, Fe2SiO4)• Othropyroxene (enstatite, Mg2Si2O6 Ca-poor in solid

solution with ferrosilite, Fe2Si2O6)• Clinopyroxene (diopside, CaMgSi2O6)• Glass (trash can, varying amounts of Ca, Al, Na, K,

etc.)• ± minor abundances of chromite (Fe, Mg)Cr2O4, FeNi

metal, FeS, spinel MgAl2O4

CAI mineral components?

Refractory inclusions known as CAIs:• Spinel (MgAl2O4)• Melilite (gehlenite, Ca2Al2SiO7 in solid solution with

akermanite, Ca2MgSi2O7)• Fassaite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6: This is not an

official mineral name. It is a complex augite.• Anorthite (Ca2Al2Si2O8)• ± all kinds of other minerals, primary and secondary,

in varying abundances.

Fe, Mg-rich Chondrules

FeO-poor or type I

Fe, Mg-rich Chondrules

FeO-rich or type II

Textural Types

• Nonporphyritic textures = nearly complete to complete melting.

• Barred, radial, cryptocrystalline and glassy.

Textural Types

• Porphyritic textures = partial melting

• Porphyritic to micro-porphyitic.– Olivine-rich, olivine +

pyroxene, pyroxene-rich ± glassy mesostasis.

Textural Types

• Compound chondrules

• Information on local chondrule abundances during heating.

Why are igneous CAIs and chondrules important?

• They are considered free-floating wanders that are self-contained igneous rocks that reacted with ambient nebular gases.

• A high-temperature, transient heating event melted minerals and rocks within the protoplanetary disk.– The mechanism that melted these objects is not intuitive

to astrophysics, what was it?

• What was the mechanism that melted them?

Why are igneous CAIs and chondrules important?

• Why did these object not cool as a black body (or why did they cool slowly)?

• What can they tell us about the environment within the disk where they formed?

• What is their stable isotope composition telling us about the evolution of planetary materials.

• What is the relationship of these objects to terrestrial planet formation?

1. What was the mechanism(s) that melted these objects?

Constraints:

1. Detailed characterization

2. Experimental petrology


First, any model that reproduces these rocks MUST do so quantitatively and make testable predictions that match the rock record.

– Petrography/Petrology - This is a must!

– Geochemistry - Follows with above.

– Isotopic Signatures - Critical? – Environment of formation vs. mechanism– Mass-dependant fractionation


Petrology/Petrography:1. Igneous textures2. Fractionated chemistry of crystals3. Bulk composition4. Redox conditions (e.g., FeO vs. Fe)5. They are controlled by kinetic reactions and are not systems in

equilibrium.Basically, they are igneous rocks and all characteristics of such rocks

must be determined and reproduced.


• The first-order constraint that must be quantitatively predicted and hence tested by any model is the reproduction of the rock’s thermal histories.

• Zero-order observation = Igneous rock• This means whether we are discussing Fe, Mg-rich

chondrules, Al-rich chondrules, type B or C, CAIs, etc.

• Critical to understanding the problem.

Thermal Histories of Chondrules

1. Pre-melting conditions

2. Peak melting

3. Cooling rates

4. Post-melting conditions

First order constraints on thermal histories.

1. Constraints on Pre-melting

• Limited to temperatures between 650 - 1000 K for seconds to many minutes (Lauretta et al., 2001).

• Abundances of primary S phases and moderately volatile elements such as Na.

2. Constraints on Peak Melting

• Tmax set from Tliq of barred olivine chondrule production - Texture

• Tmax = 1700-2100 K

• Time = Minutes

• Average (porphyritic) = ~1800 K

Bulk Composition!

2. Constraints on Peak Melting

• Constraints on CAI formation are essentially restricted to type B1.

• Tmax = 1750 K– Melilite appearance

• Time = Mins - “Hrs”

3. Constraints on Cooling Rate

• Non-porphyritic chondrules: Textures

• BO = 500-3000 K/hr

• Radial = 5-3000 K/hr

• Abundance = 10-14%


• Porphyritic chondrules: Texture and chemistry

• PO = 5-1000 K /hr with 5-100 K/hr best for producing the majority.

• Abundance = 85%


• Type B1 CAIs

• Cooling rate 0.5-50 K/hr but best results are from 0.5 - 10 K/hr.

• Based on melilite texture and composition.


• Type B1 CAIs

• 0.5 -50 K/hr

• 0.5-10 K/hr best– Stolper and Paque, 86

• Porphyritic Chondrules

• 5-100 K/hr

• 10 K/hr preferred– Jones and Lofgren, 93

– Type II bulk

4. Constraints on Post-melting

1. Recycling of chondrules and CAIs

• With some CAIs, this occurred after alteration in the nebula.

2. That’s about it!

Dusty relict olivine grains

What was the mechanism(s) that melted these objects?

Are there constraints on the overall duration of the mechanism(s) that produced chondrules and igneous CAIs?

– Well, the party line is probably yes. Approximately 2.5 million years of processing with an apparent gap of ~ 1 million years.

Party line!

Party line?

Not party line - Yet?

What was the mechanism(s) that melted these objects?

• Implications:1. Did the heating/mechanism last for over 2 million

years?2. Did the heating/mechanism turn off and on?3. Did more than one mechanism produce the rocks?4. If co-evolved, then they must have formed by the

same mechanism?

– Was material transported to the area?– Was it localized?

Proposed Heating Mechanisms

1. Lightning2. Chemical energy 3. Frictional heating -disk edge & infall 4. Planetesimal-bow shocks5. Nebular shock waves 6. Magnetic current sheets7. Gamma ray bursts8. And the list goes on, however…

Three major ‘paradigms’ (hypotheses) exist.

Three Major Hypotheses

1. Impacts or collisions between bodies in the earliest stages of planet formation.

– Known to have occurred.

2. Interactions between rock-forming materials and the early active Sun.

– It was present and energetic.

3. Those almost purely in the realm of hypothesis.– Of which, nebular shock wave is most quantitative.

Three Major Hypotheses - 1

Not a new idea (Tschermak, 1874; Merrill 1920).Key feature: We know it occurred and to some level it has been

observed!

Major implications: – Chondrules and igneous CAIs were not free-floating wanderers.– By-product of a mechanism.– Did not form BEFORE planetary bodies.

• No quantitative modeling has been performed --- Does not meet our first-order constraint!

– It’s an idea, and potentially a very good one.


First discussed by Sorby, 1877.

Key feature: We observe some activity of YSOs.

• Most stimulating model is the “X-wind” model (Shu et al., 1996; Shu et al., 1997; Shu et al., 2001).


X-wind model:

Unfortunately this model does meet our first-order constraint in detail. Thus, it makes no detailed predictions on thermal histories.

– Excellent idea with aspects of a quantitative model.


First discussed by John Wood, 1963.

Key feature: Most quantitative model to date: Hood and Horanyi, 1991; Hood and Horanyi, 1993; Hood and Kring, 1996; Ciesla and Hood, 2002; Desch and Connolly, 2002; Nakamoto and co-workers, 2002, 2004, 2005; Desch et al., 2005; Uesugi et al., 2005; Connolly et al., 2006, etc., still being modeled!

The first-order constraint on chondrule and CAI formation are given a quantitative treatment.

How Shocks Melt

• 1. Gas-drag friction• 2. Thermal

exchange with hot gas

• 3. Thermal radiation from dust and spheres.

vs = 7 km/s gas= 1x10-9 g/cm3

What would strengthen the case for shock waves (or how to do you kill the hypothesis)?

1. If they could be observed.

2. If a mechanisms that produces such waves could be convincingly modeled.

– Spiral Density Waves?

Additional Predictions of Shocks

• Predicts observed abundance of different chondrule textural types.

• Predicts observed compound chondrule frequency.

• An increase in pressure, almost 2 orders of magnitude.

• We can obtain oxygen fugacity needed for chondrule and CAI formation.

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