Transcript Chapter 5. Isotope Cosmochemistry

Chapter 5
Meteorites
 Primitive


Chondrites (stones)
 Ordinary Chondrites (H, L, LL)
 Enstatite Chondrites (EH, EL)
 Carbonaceous Chondrites (CI,
CV, CM, CO, etc)
 Misc. rarer classes
Further subdivided by ‘petrologic
grade’
 1-2: hydrous low-T alteration
 3: least altered
 4-6: high-T metamorphosed
 Differentiated Meteorites



Achondrites (stones)
 primitive
 HED (from Vesta)
 Lunar (very rare)
 SNC (from Mars)
 misc.
Irons
 Magmatic (cores of asteroids)
 Non-magmatic (but
nevertheless were molten)
Stony Irons
 Pallasites
 Mesosiderites
Chondrite Components
 Calcium-aluminum Inclusions
(CAI’s):
 very high-T condensates or
evaporative residues
 Chondrules
 once molten droplets
 Ameboidal Olivine
Aggregates (AOA’s): high-T
condensates
Pb-Pb Ages of meteorites &
components
4568.22±0.17 Ma
CAI’s are oldest objects
Little difference between achondrites & chondrites
K-Ar (40-39) Ages of Ordinary
Chondrites
 K-Ar ages are often younger
than other ages
 Results from shock-resetting
of K-Ar ages.
4-Vesta as seen by DAWN
Eucrites, many of which are brecciated, yield K-Ar ages with peaks in the
distribution occurring at 4.48 Ga, 4.0 to 3.7 Ga and 3.45-3.55 Ga, which
date heating from large impacts on the surface.
On to 1-Ceres!
Short-lived Radionuclides
and the evolution of the solar nebula and formation of planetary bodies
Short-lived Chronometers
Table 5.1. Important short-Lived Radionuclides in the Early Solar
System
Radio- Half-life Decay Daughter
Initial Abundance
nuclide
Ma
Ratio
10Be
10B
10Be/9Be ~ 7.5 × 10–4
1.5
β
26Al
26Mg
26Al/27Al = 5.2 × 10–5
0.73
β
36Cl
36Ar/36S
36Cl/35Cl ~ 17 × 10-6
0.301
β
41Ca
41K
41Ca/40Ca ~1.4 × 10–8
0.15
β
53Mn
53Cr
53Mn/55Mn = 6.3 × 10–5
3.7
β
60Fe
60Ni
60Fe/56Fe ~ 5.8 × 10–8
1.5
β
107Pd
107Ag
107Pd/108Pd ~ 5.9 × 10–4
9.4
β
129I
129Xe
129I/127I ~ 1.2 × 10–4
16
β
146Sm
142Nd
146Sm/144Sm ~ 0.0094
103
α
182Hf
182W
182Hf/180Hf ~ 9.7 × 10–5
9
β
244Pu
244Pu/238U ~ 0.001
82
α, SF Xe
247Cm
247Cm/235U ~ 6 × 10-5
15.6
α, SF 235U
Short-Lived Chronometers
 For these isotopes, the parent has entirely decayed
and what we want to know was the initial abundance of
the parent (N0).
 As was the case for the isochron equation, we start
with:
D* = N 0 - N
 But rather than substituting for N0, we substitute for N
using
- lt
N = N0e
 And obtain:
D* = N 0 (1- e- lt )
 The equation for D is then
D = D0 + N0 (1- e- lt )
53Mn–53Cr
Example
 Written for the 55Mn–55Cr decay, we have:
Cr æ 53 Cr ö æ 53 Mn ö
= ç 52 ÷ + ç 52 ÷ (1- e- lt )
52
Cr è Cr ø 0 è Cr ø 0
53
 We do not know the 53Mn/52Cr ratio, but we measure
the 55Mn/53Cr ratio. Assuming that initial isotopic
composition of Mn was homogeneous the initial
53Mn/52Cr ratio is:
æ 53 Mn ö æ
çè 52 Cr ÷ø = çè
0
Mn ö æ
52
Cr ÷ø 0 çè
55
53
55
Mn ö
Mn ÷ø 0
 Substituting and noting that e–λt = 0:
Cr æ 53 Cr ö æ
=
+
52
Cr çè 52 Cr ÷ø 0 çè
53
Mn ö æ
52
Cr ÷øx0 çè
55
53
55
Mn ö
Mn ÷ø 0
53Mn–53Cr
 Our equation:
Cr æ 53 Cr ö æ
=
+
52
Cr çè 52 Cr ÷ø 0 çè
53
Mn ö æ
52
Cr ÷ø çè
55
53
55
Mn ö
Mn ÷ø 0
 has the form y =a + bx where b =(
55Mn/53Mn) , so plotting 53Cr/52Cr vs
0
55Mn/52Cr, the slope is proportional to
the initial Mn isotopic composition,
and the intercept gives the initial Cr
isotopic composition.
 Both the ( 55Mn/53Mn)0 and the
(53Cr/52Cr)0 are functions of time and
allow us to assign relative ages to
objects. We can then calibrate these
relative ages against absolute
chronometers such as Pb-Pb.
129I–129Xe
 The first short-lived radionuclide
discovered was 129I, recognized
from variable 129Xe/130Xe ratios.

128Xe
(a minor isotope of Xe) can
be produced by neutron capture
of 127I by irradiation in a reactor.
 The 128Xe/130Xe is then
proportional to 127I/130Xe.
 Very much analogous to 40Ar–39Ar
dating, an ‘isochron’ can be
produced by step-heating of the
irradiated sample.
Natural
ratio
Heavy Isotopes of Xe:
244
Evidence of Pu fission
I-Xe Calibrated Chronology
107Pd–107Ag
 Pd and Ag (t1/2 = 9.4 Ma) are
siderophile elements
concentrated in iron
meteorites, so this system is
useful in dating iron
meteorites.
 Most magmatic irons have
Pd-Ag ages less than 10 Ma
after CAI formation.
 Planetesimals formed and
differentiated into mantles
and cores quite early!
26Al–26Mg
 This is arguably the most
significant decay system
because:
 Half-live is short: 0.73 Ma,
providing for detailed
chronology
 Both elements are relatively
abundant, making analysis
relatively easy.
 Amount of 26Al in early solar
system was sufficient to be a
significant source of heat in
early-formed bodies and may
partly explain why they melted
and differentiated.
Initial 26Al/27Al in various
objects
Early Solar System Overview
182Hf–182W
 The 182Hf–182W chronometer (t1/2 = 9 Ma) is particularly
interesting because Hf is lithophile and concentrates in the
silicate parts of planetary bodies, W is siderophile and
concentrates in the cores of planetary bodies.
 Notation: 182W/184W ratio reported in εW notation - deviations
in parts per 10,000. However, unlike 143Nd/144Nd and
176Hf/177Hf, the deviations are from a terrestrial standard. All
modern terrestrial rocks reported thus far have εW, so this
value is also thought to be that of the bulk silicate Earth.
 Variation in 182W/184W are small, so a second notation, μw,
variations in ppm from a terrestrial standard, is also used.
Dating Core Formation



The fractionation of Hf from W results from segregation of metal from silicate and hence
dates the timing of core formation. The Earth’s formation was thought to be essentially
completed by the Moon forming impact, which would have been the last opportunity for
W isotopic equilibration between the mantle and core. This last equilibration is thought to
date the ‘age of the Earth’.
182Hf–182W measurements of magmatic irons indicates planetesimal (asteroidal) cores
formed within a few million years of CAI formation.
When did the Earth’s core form?
 Initial measurements suggested the Earth’s 182W/184W was the same as chondrites
(undifferentiated), suggesting core formation happened after 182Hf was extinct.


Subsequent measurements showed chondrites have εW ≈ –2, indicating the Earth’s core
formed before 182Hf was extinct.

The moon also apparently has εW ≈ 0, suggesting core formation was largely complete
before the Moon-forming impact.
Two scenarios:
 The Earth’s core formed in a single event (perhaps triggered by the Moon-forming impact).
In this case, dating core formation with Hf-W is straightforward.

The planetesimals that accreted to form the Earth has already differentiated into silicate
mantles and iron cores. How much W isotopic equilibration happened as these bodies
accreted and their cores merged with the Earth’s core? Dating in this case is complex and
model-dependent.
εW in Solar System Materials
Modeling Earth-Moon
Formation
f is the Hf/W difference between the silicate
Earth and the silicate Moon.
Post-Script
 Subsequent studies have identified small variations in
εW in Archean rocks.
 Initial interpretation was that this reflected addition of a
late accretionary veneer of chondritic material that had
not yet been mixed into the Earth.
146Sm–142Nd

146Sm
alpha decays to 142Nd with a half-life of 103 Ma. 142Nd
excesses first identified in the eucrite Juvinas several
decades ago. Long half-life suggests in should have
survived in the early Earth. First evidence found in 3.8 Ga
rocks from Isua, Greenland in 1990’s, but not confirmed for
a decade later.
 Values reported as ε142Nd or as μ142Nd deviations from a
terrestrial standard.
 Since both Sm and Nd are refractory lithophile elements, we
expect the Sm/Nd ratio of the Earth to be chondritic and
therefore the 142Nd/144Nd ratio of the bulk Earth to be
identical to chondrites.
 Guess what?
ε142Nd Variations in Terrestrial and Solar
System Materials

All modern terrestrial rocks have
uniform ε142Nd =0, but 20 ppm
greater than ordinary chondrites.
 Carbonaceous chondrites
averageε142Nd = –0.4, enstatite
chondrites average ε142Nd= –0.1.

Variable ε142Nd have now been found
in many Archean rocks, particularly
Eoarchean rocks.

ε142Nd is even more variable in
meteorites from Vesta and Mars.

The Moon appears to have average
ε142Nd = 0 (same as observable
Earth).

How do we explain this?
1. Isotopically heterogeneous
solar nebula

146Sm
is a p-only nuclide
produced in small quantities in
supernovae.
 Analysis of other isotopes
(e.g., 148Nd) shows anticorrelated variations in r- and
s-process nuclides,
suggesting a late addition of
supernova debris that did not
completely mix.
 (But is possible this results
from incomplete dissolution of
pre-solar grains present in
meteorites).
 Earth still plots 10 ppm off
correlation.
2. Formation of an Early
Enriched Reservoir
 Boyet and Carlson proposed
that an incompatible-element
(and therefore low Sm/Nd)
reservoir formed early in
Earth’s history, sunk to the
deep mantle and has
remained there ever since.
 The crust and rest of the
mantle and all subsequent
volcanic rocks are derived
from the Early Depleted
Reservoir (with high Sm/Nd
and therefore high
142Nd/144Nd).
Issues with the EER
 Moon and Earth has the same
ε142Nd – so the hypothetical
EER would have had to form
before (and hence survive) the
Moon forming impact.
 A very substantial fraction
(50%) of the Earth’s U, Th,
and K would also end up in
the EER, meaning very high
heat production in it. This
would ultimately make it
dynamically unstable (should
rise).
But, could the LLSVP’s be the
EER?
Hypothesis 3: Collisional erosion
produces a non-chondritic Earth

As planets grow, collisions are
energetic enough to partially melt
them; producing a magma ocean that
crystallizes to produce an incompatible
element-enriched crust.

Collisions also blast away a fraction of
this crust, depleting the planet of
elements enriched in the crust
(incompatible elements). (Caro et al.,
2008).

There is indeed a growing body of
evidence from astronomy and dynamic
modeling that planetary accretion is a
non-conservative process.
Mercury’s core is way too big. Could
collisional erosion be the explanation?
Short-lived Radionuclide
Summary
 Short-lived, extinct radionuclides provide a chronology of the
early-solar system, including nebular processing and early
planetary differentiation.
 They also provide evidence of the presence of newly
synthesized elements in the solar nebula.
 Some, such as 10Be likely synthesized by spallation in high
energy environments within the nebula.
 Others, such as 26Al, likely synthesized in red giants (AGB
stars) and seeded into surrounding galaxy by their very
enhanced solar winds.
 Yet others, such as 60Fe, likely produced in supernovae.
 Finally, some long-lived ones, such as 129I and 182Hf, have
abundances similar to steady-state galactic background
(maintained by occasional supernovae).
Short-lived Radionuclides &
Star Formation

Stars are born in stellar nurseries,
such as the Great Nebula in Orion
when (small) parts of these nebulae
collapse under gravity.

The stars formed may include a few
very large ones, with very short lives
that could feed newly synthesized
elements into the nebulae while
younger stars are still forming.

Did a shockwave from nearby
supernova trigger the collapse that
resulted in formation of the Sun?

Smaller, longer-lived stars eventually
wander away from the nebular
birthplaces.
High-Energy Jets from DG
Forming Star Tau B
Star Dust
and Isotopic Anomalies in Meteorites
Neon Alphabet Soup
Xe-S
 Isotopic anomalies in Xe
found in SiC grains in
Allende (CV-3).
 Associated with Neon E(L)
 Likely s-process enrichments
- similar to calculated
production in AGB stars
Xe-HL
Pre-Solar SiC Grain
C and N isotopes in SiC grains
from Murchison (CM2)
Mo Isotopes in SiC & graphite
Oxygen Isotopes in
Meteorites
O isotopes in Meteorites
Laboratory Induced MassIndependent Fractionation
CO Self-Shielding Hypothesis
Sun is 16O-rich
Mass-Dependent
Fractionation within Classes
Cosmic Ray Exposure Ages
of Ordinary Chondrites