ASTR 330: The Solar System
Announcements
• HW assignment #2 due back today, Tuesday, 9/26.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 8:
Meteorites (II)
Picture: The world’s largest
meteorite, the Hoba Iron, is
located near the town of
Grootfontein in Namibia. It
was first described in 1920
by J Brits, and weighs 60
tons. Dimensions: 2.95 by
2.84 meters, approximately
cubic. Age: 200 to 400
million years, and fell about
80,000 years ago.
Question: how high are
these people sitting off
the ground?
Picture credit: The Earth’s Memory
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Previous Class
• In the last class [Meteorites (I)] we discussed the different
categories of meteorites.
• Then we went on to look at primitive meteorites in detail, including
carbonaceous chondrites, and IPD particles.
• In this class we will further discuss meteorite types, including
differentiated meteorites.
• We will also consider different meteorite parent bodies.
• Then we will finish with a discussion of how to determine the ages
of rocks, including meteorites.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Differentiated Meteorites: Irons
•
Differentiated or igneous meteorites have been re-processed
(heated) after their initial condensation from the solar nebula.
• The primitive chondrules are absent: hence they may also be called
achondrites. In this category are the iron and stony-iron meteorites.
• Iron meteorites are the most obvious differentiated type.
• ‘Iron’ meteorites consist of nearly pure nickel-iron, along with trace
sulfur, carbon, and platinides. Nickel is about 10% by weight.
• The iron meteorites are dense and large by meteorite standards
(harder to break up in the atmosphere). They comprise 89% of the
weight of all recovered meteorites, yet only 6% or less of witnessed
falls.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Exterior of Iron Meteorites
• Iron meteorites often have a pattern of ‘thumbprints’ caused … how?
• During the fiery entry in the Earth’s atmosphere, the hot molten iron is
pushed back by the air and the pattern solidifies when it cools.
Picture: Carleton Moore, ASU.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Interior of Iron Meteorites
• Iron meteorites, when cut and polished, then etched with dilute acid,
show a beautiful crisscross pattern.
• This Widmanstatten
pattern shows the
crystalline structure,
created by slow cooling
of the nickel-iron melt
over millions of years.
Picture: NEMS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Iron Meteorites: Origins
• Where did the iron meteorites originate?
• We believe that these objects
are fragments from the metal
cores of differentiated parent
bodies, large asteroids for the
most part.
• Detailed chemical analysis
shows that there must have
been at least several dozen
different parent bodies for the
known iron meteorites.
Picture: SaharaMet, R&R Pellison
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Stony-Irons
• Stony-iron meteorites are part silicate mineral, as well as iron.
• A pallasite for example
consists of large crystals of
green olivine surrounded
by meteoric iron.
• This specimen, from the
Springwater pallasite
meteorite, fell on Canada in
1931, originally in three
pieces of 85, 41 and 23
pounds.
Picture: Mineralogical Research Co.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Stony-Irons: Origins
• Stony-iron meteorites are thought to come from the boundary layer
between the iron core and rocky mantle of their differentiated
parents.
• They are correspondingly rare: fewer than 1% of meteorites are
stony-irons.
• Their ages can be determined from the silicate component, using
radioactive isotopes (more on this later). Do you think iron
meteorites are similar ages to the stony-irons?
• The ages are around 4.4 to 4.5 billion years old, not much younger
than the primitive meteorites, which indicates that differentiation
took place early on.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Basaltic Meteorites
• Basaltic material is a low-density lava which rose to the
surface of the parent body during differentiation.
• Basaltic meteorites are therefore crustal material. Most
are also breccias, showing evidence of long exposure to
impact cratering.
• Basalts are the most common lava on the Earth, and
also the material which makes up the dark lunar maria
(‘seas’).
• We have found a few lunar meteorites on Earth, which
can be bought!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Eucrites and Vesta
•
Eucrites are a homogenous (self-similar) group of about 30
meteorites having distinctive oxygen isotopic ratios: they are believed
to come from the asteroid Vesta (e.g. Dar al Gani 609, below).
Picture: SaharaMet, R&R Pellison
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Martian Meteorites
• We have also identified about 30 basaltic meteorites, out of 22,000 or
so known meteorites, which show the distinctive isotopic composition
of Mars!
• These are the youngest meteorites, with ages of about 1 billion years.
• This youthfulness indicates
recent geologic activity:
volcanism.
•
Picture: Ron Baalke, JPL.
Left: meteorite Los Angeles 001, a
shergottite. These 30 rocks have
been extremely well studied as
you might imagine.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Parent bodies of Primitives
• Aside from the rare lunar and martian meteorites, most meteorites
must come from asteroids and comets.
• Primitive meteorites come from un-differentiated bodies, which must
be small enough to form and cool from the original nebula.
• Calculations put an upper limit of several hundred km on the sizes of
these parent bodies, but they could be smaller.
• Moreover, from the different chemical and isotopic compositions of
meteorites, we know that many different parent bodies must have
been involved.
• Some must have contained volatiles such as water, to create the
carbonaceous chondrites.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Differentiated Parent Bodies
• Similarly, meteorites such as the irons must have come from bodies
which were large enough to differentiate.
• At the same time, we can tell from the crystalline patterns that they
cooled in about a million years: they did not remain hot like the
Earth’s core for eons.
• An upper limit to the differentiated bodies is of order 100 km again, as
small if not smaller than the ones which did not differentiate!
• Thus, a mystery remains, as to why some of the parent bodies
differentiated and some did not: what possible reasons might you
offer?
• Again, we note the multitude of different compositions indicating a
variety of parents.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
One parent or many?
• We are now in a very good position to refute the old hypothesis that
an exploded planet was the precursor of both the asteroids and the
meteorites. Let us summarize the evidence against:
• We see a variety of compositions, both differentiated and
primitive, and also a variety of ages, implying and requiring
different parent bodies no larger than 200 km.
• We also see breccias, evidence for impact-stirred surfaces of
airless bodies.
• Finally note that comets are a probable source for some meteorites
and dust grains.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ages of Rocks
• The technique used today to measure the ages of rocks is
radioactive or radioisotope dating.
• The advent of radioactive dating of rocks once and for all
set geological timescales on an absolute, as opposed to a
relative basis.
• Previously, you could look at sedimentary layers and say
that the upper layers were younger than the lower layers.
• But with radioactive dating we could date each layer
exactly, or date a rock which was out of place from its
original layer. Very useful!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Natural Radioactivity
• What is radioactivity? Radioactivity is the natural process whereby
isotopes of certain elements spontaneously change into other
isotopes; of the same element, or a different element entirely!
• Radioactivity was discovered by Henri Becquerel in 1896, who
noticed that photographic plates left in the dark beside uranium
became fogged (exposed).
• The name radioactivity was given by Marie and
Pierre Curie in 1898: from the Latin word for ‘ray’.
• The Curies also isolated two new elements which
were even more radioactive than uranium, from
uranium ore. These two new elements were
polonium (element 84) and radium (element 88).
Picture: lucid interactive
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
What is Radioactivity?
• What’s going on here? What do we mean by changing one isotope
into another?
• Radioactivity occurs when an unstable isotope decays by
spontaneously emitting a particle and transforming its nuclear
structure to a more stable form.
• There is a lot of information in the previous sentence! What sorts of
particle are emitted here? There are two types:
Alpha Decay: The emitted particle is a helium nucleus: 2 protons and
2 neutrons.
Beta Decay: The emitted particle is an electron (e-) or positron (e+).
Note that this electron is emitted from the nucleus, not the
electron cloud! (There is also a variation where an orbiting
electron is captured by the nucleus).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Alpha Decay
• Alpha decay occurs where the ratio of protons to neutrons is too high
for stability. A helium nucleus is emitted and the remaining nucleus is
more stable.
• E.g. here:
Picture: www.impcas.ac.cn
241
95
Am Np  He
237
93
4
2
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Beta Decay
• Normal Beta decay occurs where the ratio of neutrons to protons is
too high for stability. A neutron turns into a proton and an electron
(conserving charge) and the electron is emitted.
• E.g. here:
Picture: www.impcas.ac.cn
3
1
H  He  e
3
2

Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Gamma Radiation
• Gamma rays were the third type of radioactivity discovered, but
turned out not to be mass-carrying particles at all, and no isotope
transformation occurs.
• Gamma rays are just a type of EM radiation which we met in an
earlier lecture, although a very energetic type.
• Gamma rays are emitted when the nucleus is in an excited energy
state, often following alpha or beta decay.
• The nucleus emits
some of its energy
as a photon and
reaches a lower
energy state.
Picture: www.impcas.ac.cn
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
So What?
• You may well ask at this stage, what use is radioactivity to us? Isn’t it
just dangerous and harmful?
• Radioactivity is all around us, indeed it occurs inside our own bodies,
where unstable C-14 beta decays to N-14.
• Normally, radioactive nuclei are not concentrated enough to have a
harmful effect on us, unless of course we artificially enhance the
concentration. Can you think of examples?
• As the amount of the parent nucleus decreases, the amount of the
daughter nucleus correspondingly increases. If the substance (rock,
wood, whatever) is no longer been modified externally, we can use
the measured concentrations to deduce how long the radioactivity
has been occurring.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rate of radioactive decay
• We must know three things to calculate an age:
1. The current parent/daughter isotope ratio.
2. The original parent/daughter isotope ratio.
3. The rate of decay.
• The rate of decay, fortunately, is an intrinsic property of the isotope,
independent of: temperature, pressure (unlike chemical reactions)
and chemistry.
• Although we can never predict exactly when a particular atom will
decay, we know on average what percentage will decay in a given
time. This is the decay rate.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Half-Life
• Usually the rate of decay is expressed in terms of half-life: the time for
one half of the radioactive atoms to decay.
• Example: a particular isotope has a half life of 1 year. If we initially
had 64 grams of the substance, then after:
•
•
•
•
•
•
1 year we have 32 g of the parent, 32 g of the daughter;
2 years we have 16 g of the parent, 48 g of the daughter;
3 years we have 8 g of the parent, 56 g of the daughter;
4 years we have 4 g of the parent, 60 g of the daughter;
5 years we have 2 g of the parent, 62 g of the daughter;
6 years we have 1 g of the parent, 63 g of the daughter;
•
How much of each after 8 years?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Time change of isotope populations
• As the amount of the
parent nuclei decreases
exponentially…
Picture: Univ. of South Carolina/CSE
… the amount of the
daughter nuclei
increases accordingly.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Aging Rocks
• We have taken a substantial detour into the realm of radioactivity, but
the reason is now clear:
• If we can find isotopes with sufficiently long half-lifes: 100s of millions
(108) to 10s of billions (1010) of years; then we can find how long since
the rock has been formed, or last processed (mixed) externally.
• Useful rock-dating isotopes:
Parent Isotope
Daughter Isotope
Samarium (Sm 147)
Rubidium (Rb 87)
Thorium (Th 232)
Uranium (U 238)
Potassium (K 40)
Neodymium (Nd-143)
Strontium (Sr 87)
Lead (Pb 208)
Lead (Pb 206)
Argon (Ar 40)
Half-Life
(billions of years)
106.00
48.80
14.00
4.47
1.31
• What types of decay are these?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Meaning Of Age
• Say we use radioisotope dating to measure the age of a rock, what
does that age mean?
• Normally, it will mean the solidification age, the time since the rock
was last molten. Since then, the parent isotope has not been able to
replenish itself by external mixing, and will decay undisturbed to
daughter nuclei.
• However, we may measure the age using two different radioactive
paths, and get two different ages! What’s going on?
• If, for example, the daughter isotope is a gas (e.g. the K/Ar route),
then we have measured the gas retention age: e.g. the time since an
event (e.g. a shock or impact) last disturbed trapped gases.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Original Isotope Fractions
• We mentioned that we needed three pieces of information to measure
rock age.
• The decay rate can be measured in a lab, and the present-day
isotope fractions we can also measure: what about the original
isotope fractions, how do we get that?
• The method is to use sister isotopes: isotopes which are the same
chemical element as a decay parent or daughter, but not involved in
radioactivity. Hence, their abundance will remain the same to the
present day.
• E.g. In Rb-Sr dating, we use the Sr-86 sister to Sr-87, to infer the
original amount of Sr-87 when the rock solidified.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Example of Meteorite Dating
• We will now work through the Rb-Sr example in the
quantitative supplement: section 4.7 of the textbook.
• Note that the formula is wrong and the text explanation is
garbled!
• The method we describe here as called the isochron
method, which accounts for the unknowns in the initial
isotope fractions.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rubidium and Strontium
• The rubidium-87 isotope is comprised of 37 protons and
50 neutrons. Strontium-87 isotope has 38 protons and 49
neutrons.
• Therefore, Rb-87 has excess neutrons and beta-decays
to Sr-87 by converting a neutron into a proton and an
electron, with a half life =48.8 billion years (48.8 Gyr).
• Let R be the amount of Rb-87 and S be the amount of Sr87, at time t.
• Let’s also define R0 as the initial amount of Rb-87 and S0
as the initial amount of Sr-87, at time t=0.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Formula for R and S
• We will use formula for R and S at time t as follows:
1.
2.
 t /
R  R0  2
 t /
S  S 0  R0 (1  2 )
• We can check that these are correct, by considering their behavior at
time t=0, t= and as time t∞.
• t=0: R=R0
 and
• t=: R=R0/2  and
• t∞ R=0
 and
S=S0
S=S0 + R0/2
S=S0 + R0



• Also, add (1) plus (2) and find that: R + S = R0 + S0 
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Formula from the book
• Morrison and Owen (p 92) give the following formula:
3.
S  S 0  R0 (e  1)
kt
• Again, we can check the correctness, by considering the behaviour at
time t=0 and as time t∞. We’ll assume for the minute that the time
constant k is negative (as it should be):
• t=0:
S=S0
• t∞ S=S0 - R0


• How could we fix their formula?
S  S 0  R0 (1  e )
kt
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rb/Sr Ratios In Mineral Grains
• Within a particular rock, e.g. granite; rubidium and strontium may
crystallize in different ways and at different times, as the original
magma cooled. This is called fractional crystallization.
• Strontium tends to preferentially leave the magma first, so for
example, if mineral M1 forms first, followed by M2, M3, and M4, then:
M1 has the lowest Rb/Sr
M2 has the second lowest
M3 has second highest
M4 has highest Rb/Sr
• However, the Sr-87/Sr-86 in these minerals will be constant: there is
no preference for difference isotopes of the same chemical element.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Initial Ratios
• In the figure (left) the
filled-in black dots
represent the initial
ratios when the rock
formed (t=0).
• Note how the different
minerals, M1, M2 etc
have the same
Sr-87/Sr-86 ratios, but
different Rb-87/Sr-86
ratios, which depend
on chemistry.
• The line connecting
the dots is called the
isochron (‘same
times’).
Figure credit: Charles Cowley U mich
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Change in Fractions Over Time
• What happens over geologic time? The radioactive Rb-87 decays to
Sr-87. For every lost Rb-87 atom there is now an Sr-87 atom.
• Hence, for each of the minerals, the Rb-87/Sr-86 ratio decreases, and
the Sr-87/Sr-86 ratio increases. The dots (or data positions)
representing each element migrate upwards and to the left.
• The lines of migration are 45°, because the overall amount of atoms
(R + S) is conserved.
• The dots stay in a line, because the more Rb-87 there was, the more
decays in a fixed time.
• The result is that the isochron rotates anticlockwise over time (this is
the opposite to what the book says!).
• The amount of rotation tells us the amount of time which has passed.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Final Ratios
• The final data point
positions are the
empty black circles.
• For each mineral, M1,
M2 etc, you can see
that the amount of Rb87 has decreased
while the amount of
Sr-87 has increased.
• We can measure the
age of a rock by
plotting the data
points and measuring
the angle of the final
isochron.
Figure credit: Charles Cowley U mich
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Textbook Solution
• In the textbook Figure 4.15, the final isochron for the Guarena
meteorite is shown. Let’s use that to put some numbers into our
equation (2);
• S0 = 0.70, S(t) = 0.764, R(t) = 1.0
• Therefore:
0.764  0.70  1.0(1  2t / )
0.064  1  2  t /
2 t /  0.936
• Take logs of both sides (need to convert to base e or base 10):
 t /   log 2 (0.936)  log e (0.936) / log e (2)  0.0954
t /   0.0954
• t= 0.0954 x 48.8 Gyr = 4.65 Gyr.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary (i)
1. Name some typical physical properties of an iron meteorite.
2. To what (formation) category of meteorites does it belong?
3. What are the typical exterior and interior features of an iron
meteorite?
4. Where and how did iron meteorites form?
5. How might you identify a stony-iron meteorite?
6. Basaltic meteorites are another type of differentiated meteorite. What
does the term mean?
7. From what part of the parent body do basalts come?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary (ii)
8. Someone suggests to you that all meteorites come from a single
original progenitor. What evidence might you use to refute their
claim?
9. What is a radioisotope?
10. What is the meaning of (a) parent nucleus (b) daughter nucleus (c)
half-life?
11. What are the three main types of radioactivity, and which one is the
odd-one out?
12. What is meant by (a) solidification age, (b) gas retention age of a
mineral?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary (iii)
13. A certain radioisotope, X has a half-life of 100 years.
How much of an original sample of X, of mass 400 g, is
left after 300 years?
If you find a sample bearing 10 g of X and 150 g of its
daughter isotope Y, and suspect that all the Y is
radiogenic, what is the solidification age of the sample?
Could you use an isotope such as X for dating real
meteorites? Why?
Dr Conor Nixon Fall 2006