ASTR 330: The Solar System Announcements Dr Conor Nixon Fall 2006 HW assignment #2 due back today, Tuesday, 9/26

ASTR 330: The Solar System

Announcements

Dr Conor Nixon Fall 2006

• HW assignment #2 due back today, Tuesday, 9/26.


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.

Picture credit: The Earth’s Memory

Question: how high are these people sitting off the ground?


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.


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.


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.


Interior of Iron Meteorites


• Iron meteorites, when cut and polished, then etched with dilute acid, show a beautiful crisscross pattern.

Picture: NEMS

• This Widmanstatten pattern shows the crystalline structure, created by slow cooling of the nickel-iron melt over millions of years.


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


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.


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 isotopesradioactive 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.


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!


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


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.

• Left: meteorite Los Angeles 001, a shergottite. These 30 rocks have been extremely well studied as you might imagine.

Picture: Ron Baalke, JPL.


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.


Differentiated Parent Bodies


• Similarly, meteorites such as the ironsirons 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.


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.


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!


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).

Picture: lucid interactive

• 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).


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).


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

HeNpAm 42

23793

24195 +→


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:


−+→ eHeH 32

31


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.


So What?


• You may well ask at this stage, what use is radioactivityradioactivity 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.


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.


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?


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.


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 Half-Life(billions of years)

Samarium (Sm 147) Neodymium (Nd-143) 106.00Rubidium (Rb 87) Strontium (Sr 87) 48.80Thorium (Th 232) Lead (Pb 208) 14.00Uranium (U 238) Lead (Pb 206) 4.47Potassium (K 40) Argon (Ar 40) 1.31

• What types of decay are these?


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.


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.


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.


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 Sr-87, 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.


Formula for R and S


• We will use formula for R and S at time t as follows:

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 S=S0 • t=: R=R0/2 and S=S0 + R0/2 • t∞ R=0 and S=S0 + R0

• Also, add (1) plus (2) and find that: R + S = R0 + S0

/0 2

tRR −⋅=)21( /

00tRSS −−+=


Formula from the book


• Morrison and Owen (p 92) give the following formula:

3.

• 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?

)1(00 −+= kteRSS

)1(00kteRSS −+=


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/SrM2 has the second lowestM3 has second highestM4 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.


Initial Ratios

Dr Conor Nixon Fall 2006Figure credit: Charles Cowley U mich

• 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’).


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.


Final Ratios

Dr Conor Nixon Fall 2006Figure credit: Charles Cowley U mich

• The final data point positions are the empty black circles.

• For each mineral, M1, M2 etc, you can see that the amount of Rb-87 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.

• 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:


Textbook Solution


)21(0.170.0764.0 /t−−+=/21064.0 t−−=

936.02 / =− t

0954.0)2(log/)936.0(log)936.0(log/ 2 −===− eet

0954.0/ =t

• Take logs of both sides (need to convert to base e or base 10):

• t= 0.0954 x 48.8 Gyr = 4.65 Gyr.


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?


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?


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?

Documents

ASTR 330: The Solar System Announcements Dr Conor Nixon Fall 2006 HW assignment #2 due back today, Tuesday, 9/26