Transcript Solar System formation - Meteorites

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?).
Meteorites
Und ifferentiated
Chondrites
Car bonace ous
C
Class
Or dinary
O
Clan
CI
CM-CO CV-CK
Group
CI
CM CO CV CK CR CH CB
Sub -Group
CVA CVB CVr ed
CR clan
H-L-LL
H
L
LL
Ens tatite
E
EH -EL
EH
EL
R
K
CBa CBb
Primitive
Achondrites
Clan
Group
ACA-LOD
URE
BRA
ACA
LOD
WIN-IAB-IICD
WI N IAB
II ICD
Differentiated
Achondrites
Clan
Group
Figur e 1
Vesta?
ANG
AUB
EUC DIO HOW
Moon
MES
MG
PAL
ES
PAL
PP
PAL
IC II AB II C II D II E II IAB II IE II IF IVA IVB
Mars
SHE NAK CHA OPX
Solar System formation - Meteorites
• Iron meteorites
– Cores of small
planetesimals.
– FeNi - rich metal,
which is only found
essentially in the core
of Earth (natural, one
major location).
Solar System formation - Meteorites
a
O-isotopes: 16, 17, and 18 with 16 being least abundant.
4
2
ANG BRA
IVA
SNC
IIE
AUB
HED
MG-PAL
IIICD
17O (‰ )
IIIAB
PP-PAL
ACA-LOD
0
URE
TF
-2
IRA
WIN
-4
CCAM
-6
c
ES-PAL
-2
IAB
0
2
4
6
8
18O (‰ )
Isotope: equal protons but different # neutrons, hence different at mass
ANG
MG-PAL
4
BRA
IIE
AUB
WIN
17O (‰ )
IIIAB
2
IVA
TF
IIICD
URE
HED
IAB
0
ACA-LOD
PP-PAL
-2
2
CCAM
4
d
6
18O (‰ )
8
Increase in degree of
aqueous alteration
chd./type
1
2
CI
CM
CR
CH
CB
CV
CO
CK
H
L
LL
EH
EL
R
K*
chd. - chondrite group, *grouplet
Figure 8
Figure 7
Increase in degree of thermal
metamorphism
Pristine
3
4
5
6
7
a
10
CI
17O (‰ )
EH-EL
R
5
CR
LL
H-L-LL
L
H
CM
K
0
TF
CH
CR
-5
CV
CB
CO
CK
CCAM
-10
-5
0
5
10
18O (‰ )
15
6
b
5
R
17O (‰ )
4
LL
L
3
EH-EL
H
CR
2
CH
1
0
CM
K
2
3
4
5
18O (‰ )
6
7
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
Why are chondrites important?
3. They are relatively
unprocessed
planetary materials
that have a solar-like
composition.
Rocks = Sun’s photosphere
1.5
a
1
0.5
CR
CK
EH
CM
H
EL
CO
L
CV
LL
Al Sc Ca La Sm Eu Yb Lu V Mg Cr Mn Na
K
b
1
CR
CH (ALH 85085)
CH (Acfer 182)
0.1
LEW 85332
CB (Bencubbin)
Al Sc Ca La Sm Eu Yb Lu V Mg Cr Mn Na
K
Mg-normalized abundance/CI
c
1
CR
CM
CO
CV
CK
H
L
LL
EH
EL
0.1
Os
Ir
Ni
Co
Fe
Au
As
Ga
Sb
Br
Se
Zn
d
1
CR
CH (ALH 85085)
CH (Acfer 182)
0.1
Ungrouped Lew 85332
CB (Bencubbin)
Os
Ir
Ni
Co
Fe
Au
As
Ga
Sb
Br
Se
Zn
Why are chondrites important?
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 microporphyitic.
– Olivine-rich, olivine +
pyroxene, pyroxenerich ± 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
1. What was the mechanism(s) that melted these objects?
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
1. What was the mechanism(s) that melted these objects?
Petrology/Petrography:
1.
Igneous textures
2.
Fractionated chemistry of crystals
3.
Bulk composition
4.
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.
1. What was the mechanism(s) that melted these objects?
• 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%
3. Constraints on Cooling Rate
• Porphyritic chondrules:
Texture and chemistry
• PO = 5-1000 K /hr with
5-100 K/hr best for
producing the majority.
• Abundance = 85%
3. Constraints on Cooling Rate
• 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.
3. Constraints on Cooling Rate
• Type B1 CAIs
• Porphyritic Chondrules
• 0.5 -50 K/hr
• 5-100 K/hr
• 0.5-10 K/hr best
• 10 K/hr preferred
– Stolper and Paque, 86
– 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!
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. Lightning
2. Chemical energy
3. Frictional heating -disk edge & infall
4. Planetesimal-bow shocks
5. Nebular shock waves
6. Magnetic current sheets
7. Gamma ray bursts
8. 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.
–
3.
It was present and energetic.
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.
Three Major Hypotheses - 2
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).
Three Major Hypotheses - 2
Three Major Hypotheses - 2
X-wind model:
Unfortunately this model does meet our firstorder constraint in detail. Thus, it makes no
detailed predictions on thermal histories.
–
Excellent idea with aspects of a quantitative
model.
Three Major Hypotheses - 3
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.