Chapter 2 Meteorites
Underlying every topic in planetary science are two basic properties of the solar system that are determined from the analysis of meteorites. First, meteorites give us our best estimate of the age of the solar system. The time at which the solar system formed provides a time frame for judging the signi cance of many of the physical processes that a ect the planets. Second, meteorites give us our best estimate of the initial composition of the solar system. Incredibly, the elemental abundances found in the oldest meteorites, the chondrites, match point for point with the elemental abundances found spectroscopically in the Sun’s photosphere. The study of meteorites, which is quite interdisciplinary, is central to the study of planetary science as a whole.
2.1 Falls and Finds A meteoroid is a small rock that is orbiting the Sun. A meteoroid that happens to fall into Earth’s atmosphere heats up and becomes an incandescent meteor. If a piece of a meteoroid reaches the ground it is called a meteorite. A meteorite is described as a fall or a nd depending on whether witnesses saw it enter the atmosphere or not. One of the oldest preserved falls occurred in 1492, the same year that Columbus discovered the New World, when a 127 kg stony meteorite landed in a wheat eld near the Alsatian (now French) town of Ensisheim.
2–1 2.1.1 The Allende Fall An extremely important fall occurred in Chihuahua, Mexico, in 1969, when a large meteor was observed to come into the atmosphere in several pieces. The rst piece was found near a house in the small village of Pueblito de Allende. Following standard practice, all of the meteorite fragments that were recovered from that fall are collectively named Allende. The Allende fall occurred just as the Apollo program was swinging into full gear, and it gave scientists who were preparing for the arrival of moon rocks an opportunity to practice on an extraterrestrial sample. Because it is such an old meteorite, and because there is plenty of it to go around, analysis of the Allende meteorite has taught us much about the early solar system. Allende is a member of an important class of stony meteorites called chondrites. They are so named because they contain chondrules (from the Greek word condros, meaning “grain”), which are primitive, spherical objects that condensed out of the proto- planetary nebula before being incorporated into the larger rock. Chondrules are puzzling features because their existence implies signi cant heating event in the early solar nebula ( 0 to 10 Ma). The glassy texture of these igneous features implies heating to tempera- tures in excess of 1500 C to 1900 C, followed by rapid cooling, on a time scale of on the order of an hour. The mm-size of the chondrules indicates that they were distributed in the solar nebula, but the rate of cooling required to explain them rules out formation very close to the sun, where the solar nebula would have been much too warm. Many mechanisms for this ash heating event have been discussed, often relating the thermal or electromagnetic emissions from the variable, nascent sun. In the fall of 1999 scientists from the University of Dublin proposed that chondrules were produced by a gamma ray burst. Gamma ray bursts are rare and extremely poorly understood phenomena that may be related to ex- plosive end of life of supermassive stars. Another idea is that shock waves related to the formation of Jupiter provided the impetus for chondrule formation. It remains to be seen whether chondrule formation was a consequence of one of these low probability events.
2.1.2 The Tagish Lake Fall Another important fall occurrred in the Tagish Lake region in western Canada on January 18, 2000. The meteorite is a carbonaceous chondrite and it represents some of the most primitive soalr system materials ever recovered. Over 10,000 fragments were recovered, with more than 2000 fragments larger than a gram. Perhaps most importantly, approximately 0.85 kg was recovered in the days after the reball by an individual (Jim Brook) who had the good sense to bag the samples without touching them to minimize contamination. These samples were also kept frozen which will allow perhaps the best-yet characterization of volatiles and organics. Analysis of this meteorite is now underway.
2.1.3 Antarctic Finds Finds are more common than falls. Many meteorites are discovered serendipitously by hikers and by farmers plowing their elds. In recent years, the best source of meteorites has been the ice elds of Antarctica. In 1969, a group of Japanese geologists studying glaciers discovered nine meteorites laying on the bare ice near the Yamato Mountains in Queen
2–2 Maud Land. Meteorites are dark and easy to spot on ice. It turned out that these nine specimens were members of four di erent classes of meteorites. Since that time, thousands of meteorites have been recovered from Antarctica. The meteorites collect naturally at locations where the ice sheets, which ow several meters per year under their own weight, stagnate when they encounter mountain ranges. Wind erosion then ablates the top layers of the ice and, over time, a concentration of meteorites works its way to the top. Such ice is bluish in color and is easy to spot from the air, which allows scientists to plan ahead for the best places to look. A robot designed by Japanese scientists to search autonomously for meteorites made its rst nd in January, 2000.
2.2 Chemical Composition of a Rock Here we present the most common mineral types. These minerals occur in meteorites but also compose the crusts and mantles of the terrestrial planets, moons, and asteroids. Olivine is a silicate mineral rich in iron and magnesium, principally (Mg,Fe)2SiO4. Here magnesium and iron are in solid solution, which means that either element can occupy a given location in the cyrstal matrix. Olivine is the primary constituent of the Earth’s mantle and is found in igneous and metamorphic rocks. Pyroxene is any of a group of crystalline mineral silicates common in igneous and metamorphic rocks and containing two metallic oxides, as of magnesium, iron, calcium, or +2 sodium. A general formula is (Mg,Fe)SiO3, though Ca may also substitute. Pyroxene is a common mineral in the Earth’s oceanic crust. Plagioclase constitutes any of a common rock-forming series of triclinic feldspars, consisting of mixtures of sodium and calcium aluminum silicates that form a solid solution. The chemical formula is (K, Na, Ca)Al2Si2O8. Plagioclase feldspar is common in the oceanic and continental crust of Earth and in addition is the primary constituent of the lunar highlands.
2.3 Age of a Rock Rocks provide us with a means of measuring time in billions of years. The detailed study of the fossil record allows geologists and paleontologists to accurately determine the relative chronological order of the stratigraphic layers in Earth’s crust. Absolute ages are less accurately determined than relative ages, however. But since all rocks contain trace amounts of radioactive material absolute ages can be found by radiometric dating.
2.3.1 Poisson Probability Distribution and the Rate Equation Radioactive decay occurs because some of the mass of an atom is held in binding energy. (Recall that Einstein taught us that energy (E) is equivalent to mass (m) with the simple and elegant expression E = mc2, where c is the velocity of light.) If there is ”too much” binding energy (as determined by quantum mechanics), then the nucleus will decay spontaneously – by radioactive decay – or by an induced nuclear reaction (neutron bombardment) to a lower energy state. Decay can occur by three classes of mechanisms: - Alpha decay - escape of a He4 nucleus; the strong forces that bind nuclei dictates that it is di cult to escape from the potential well. particles are not very energetic
2–3 and cannot jump the potential well but Heisenbergs Uncertainty Principle tells us that a particle can sometimes exist outside the threshold due to uncertainty in position. - Beta decay - escape of an electron or positron by: (1) Electron emission ( ): for nuclides over-rich in neutrons (n p + e + ) (2)→Positron emission ( +): for nuclides depleted in neutrons (p n + e+ + ) (3) Electron capture (ec): an S electron in K shell on an atom has→ nite probability of being inside nucleus. - Gamma decay - emission of gamma radiation that occurs when nuclei are in excited energy states. Newly-produced nuclei are not in the ground state and produce -rays. Nuclear binding energies are very large and nuclei are so small (order 1 angstrom = 8 10 cm) that radioactive decay rates are not signi cantly a ected by physical conditions on Earth such as pressure and temperature. However, half-lives can be changed slightly by changes in bonding energy. For example, solar wind studies have shown that radioactive beryllium decays at slightly di erent rates on the sun and Earth. Radioactive decay is governed by the Poisson probability distribution. Other Poisson processes include the number of impact craters that form on a planet’s surface over a given time interval, the waiting time for a subway train, the number of typos made by an experienced typist, or any other process that involves random events that occur infrequently. It is useful to review the derivation of the Poisson probability distribution because it shows where the concept of a “half-life” in radioactive decay originates. Also, we will be using the same method for calculating expected values from random processes introduced here when we study blackbody radiation in Chapter 4. We start rst with the binomial probability distribution. Consider n independent tries of an experiment whose outcome is either “yes” or “no,” but not “maybe.” If the probability of “yes” is p, and the probability of “no” is 1 p, then the probability Pn(j) of getting “yes” j times out of n tries is given by the binomial probability distribution:
n j n j P (j)= p (1 p) , (2.1) n j n where j is pronounced “n choose j,” and is de ned to be: n n! . (2.2) j j!(n j)! For example, the probability that exactly 2 students in a class of 20 were born on August 8 is: 20! 1 2 364 18 P (2) = =0.0014 . (2.3) 20 2!18! 365 365 The binomial distribution is the discrete analog of the bell-shaped normal distribution found in experiments with continuous sampling. Notice that (2.1) gives the j-th term of the binomial expansion of [p +(1 p)]n, which is the source of the distribution’s name. 2–4 Since p +(1 p) = 1, summing over all cases yields unity as required for a probability distribution. Now consider a group of N radioactive particles. We would like to know the expected number of particles that will decay in a particular time interval, say in 1 second. This is a quantum mechanical process, and is therefore random. The probability of decay is small for a given particle, and so we can always divide up any 1-second time interval into enough n smaller subintervals such that at most only one particle will decay during each subinterval. This is where the “events that occur infrequently in time or space” requirement is used. The total number of decays per second, j, will then have a binomial distribution. We don’t know p, but we do know that p will decrease as we make the subintervals shorter, and that p will increase as N increases. After enough subdivisions have been made such that each bin contains either 0 or 1 event, further doubling of the number of subdivisions will only reduce by half the probability that one subdivision contains an event. This means that we can write: N p = , (2.4) n where is a constant with units of inverse time. Using (2.4) in place of p in (2.1) and taking the limit n yields the Poisson probability distribution P (j): →∞ j n j n(n 1) (n j+1) N N P (j) = lim Pn(j) = lim 1 , n n j! n n →∞ →∞ j n ( N)j n n 1 n j+1 N N = lim 1 1 , j! n n n n n n →∞ j n j ( N) N ( N) N = lim 1 = e . (2.5) j! n n j! →∞ The last limit follows from: x n a = lim 1+ , n n →∞ x x log a = lim n log 1+ = lim n = x, n n →∞ n →∞ n a=ex . (2.6) ⇒ N N Since (2.5) gives e times the j-th term of the Taylor expansion of e , the sum of all the terms adds up to unity, as required. The physical signi cance of N is found by calculating the average, or expected, value of j, which we denote by j:
j ∞ ∞ ( N) N j jP (j)= j e j! j=0 j=1 X X j 1 N ∞ ( N) = N e = N . (2.7) (j 1)! j 1=0 X 2–5 Thus N is just the average number of particles that decay per second, and is based on the probability of a particular decay mechanism operating in an atom of a given element. The change in the total number N of radioactive particles over the time interval dt is therefore:
dN = N. (2.8) dt Rewriting, we write dN = dt N Integrating both sides we nd lnN = t + c where the constant c is found in the limit where t 0tobelnNo. Taking the exponential of both sides we may write → t N(t)=Noe , (2.9)
where No is the initial number of radioactive particles. Equation (2.9) is the rate law of radioactive decay. The half-life, T1/2, which represents the time it takes for half of the number of particles to decay, is found by setting N/No =1/2 such that
N 1 ln 2 0.69315 T1/2 = = e T1/2 = = . (2.10) No 2 ⇒
Note that the half-life represents an alternative way of expressing the decay constant . In principle, the experimentally-demonstrated accuracy of the simple expression (2.10) allows for the determination of the absolute ages of billion-year-old rocks. However, in practice the initial concentration of the radioactive parent element No is very often not known. We can more easily measure the concentration of the daughter product (D ), which is simply D = N N. (2.11) o We may substitute (2.9) for N to nd
t t D = N N e = N 1 e . (2.12) o o o We want to eliminate No so we divide by (2.9) which gives
t t D No 1 e 1 e = t = t . N N o e e or D = e t 1. (2.12) N Equation (2.12) can be used directly in the determination of ages if there is no initial non-radiogenic daughter component, or if that initial component can be estimated. If not,
2–6 the it is necessary to make an allowance for the non-radiogenic initial daughter product Do by writing t D = D + N(e 1). (2.13) o
2.3.2 Radiometric Dating To illustrate the radiometric dating technique, consider the decay of the unstable isotope of rubidium, 87Rb, into the stable isotope of strontium, 87Sr. This system is particularly simple because the parent element only decays into one type of daughter element, unlike 40K, say, which decays into both 40Ar and 40Ca. The Rb-Sr system is useful for dating old rocks because the decay constant and half-life T1/2 for the Rb-Sr system are well suited for the purpose:
11 1 9 =1.42 10 yr ,T =48.8 10 yr . 1/2 Only a fraction of the rubidium present in the solar nebula has so far decayed. If t is the time since some melting event reset the isotope ratios to their high-temperature values, 87 87 then by (2.9), the current amount of Rb is reduced from its initial amount Rb0 by:
87 87 87t Rb = Rb0 e .
87 87 The current amount of strontium, Sr, is therefore increased from its initial amount, Sr0, by:
87Sr = 87Sr + 87Rb 87Rb 0 0 = 87Sr + 87Rb(e 87t 1) . (2.14) 0 To proceed with the dating, one uses a mass spectrometer to measure the amounts of 87Sr and 87Rb present in each sample. Since di erent parts of a rock will contain 87 di erent concentrations of the unknown quantity Sr0, we must normalize against another stable isotope with similar chemistry that occurs in proportional concentrations, like 86Sr. Dividing (2.14) by 86Sr yields:
87Sr 87Sr 87Rb = + (e 87t 1) . (2.15) 86Sr 86Sr 86Sr 0 The presence of initial daughter abundances also requires more than one measurement of the parent/daughter ratio to obtain an age. Samples that have di erent 87Rb/86Sr ratios can be plotted versus 87Sr/86Sr using (2.15). The 87Rb/86Sr ratio varies naturally from one mineral to another. For example it is typically higher in plagioclase than in pyroxene, so a spread in the samples is obtained by mineral separation. When plotted, the two ratios fall on a straight line called an isochron (meaning ”equal time”), which by (2.15) has a t 87 86 slope of (e 1) t and a y-intercept of Sr/ Sr 0. If the the decay constant of the radioactive parent is known, the isochron yields the age, t, of the rock. The most useful decay systems for radiometric dating are Rubidium-Strontium (Rb- Sr), Samarium-Neodymium (Sm-Nd), Potassium-Argon (K-Ar), Thorium-Lead (Th-Pb),
2–7 and the two Uranium-Lead (U-Pb) systems. In order for a parent-daughter system to be useful, a non-radiogenic reference isotope of the daughter must be present for comparison. In addition, the decay constant of the parent must be accurately known. The accuracy of radiometric dating also depends on to what extent the rock under study has been a chemically closed system with respect to the parent and daughter elements. If it has not been a closed system then the daughter/parent ratio will not be solely due to radioactive decay, and the time information will be corrupted. The U-Pb system is especially useful because only measurements of Pb are required, and Pb tends to be reliable because it is not too mobile in rock. Also, because of decades of nuclear research the decay constants for uranium are very accurately known. Zircon crystals are resistant to uranium di usion and are commonly used for this dating scheme. There are four isotopes of Pb: 204Pb, 206Pb, 207Pb, 208Pb. Only 204Pb does not have a radioactive progenitor, and the decay schemes for the other three isotopes are:
238 206 10 1 U Pb , =1.55 10 yr ,T =4.5By → 238 1/2 235 207 10 1 U Pb , =9.85 10 yr ,T =0.7By → 235 1/2 232 208 11 1 Th Pb , =4.95 10 yr ,T =14By. → 232 1/2 Using (2.13) and referencing to 204Pb:
206Pb 206Pb 238U = + e 238t 1 , 204Pb 204Pb 204Pb 0 207Pb 207Pb 235U = + e 235t 1 . 204Pb 204Pb 204Pb 0 Now take the ratio of 207Pb/204Pb to 206Pb/204Pb:
207Pb 207Pb 204Pb 204Pb 235U e 235t 1 0 = , 206Pb 206Pb 238U e 238t 1 204Pb 204Pb 0 and rewrite into an isochron equation:
207Pb 206Pb = M + B, (2.16) 204Pb 204Pb where the slope and y-intercept are:
235U e 235t 1 207Pb 206Pb M = =0.613 ,B= M =4.46 . (2.17) 238U e 238t 1 204Pb 204Pb 0 0 The age information is contained in the slope, M, using only isotopes of Pb. The value of 235U/238Uis1/137.88, and this ratio is very nearly constant in all natural materials.
2–8 To determine the initial lead ratios the standard practice is to look to meteorites. Iron meteorites have virtually no uranium. The least radiogenic lead found anywhere is in the Canyon Diablo meteorite. This is de ned as primordial lead, the best estimate of the initial lead ratio in the solar nebula.
2.3.3 Age of Earth’s Rocks The radiometric dating of rocks from Earth’s surface indicates that most of the surface is less than 100 million years old. The oldest rocks that have so far been found on Earth are slightly older than 4.0 billion years. We know that the Earth’s surface is young compared to the age of the solar system, for a number of reasons. For instance, while the absence of small craters on Earth can be explained by the protection of the surface by the atmosphere, the lack of many large craters on Earth, as compared to the Moon, tells us that Earth’s surface is constantly being renewed. Erosion is e cient on Earth because of the abundance of liquid water and because of the presence of a biosphere. Earth also renews its surface continuously through the action of plate tectonics, where new crustal material comes to the surface at mid-ocean ridges and old crustal material plunges below the surface at subduction zones. Given the great activity occurring on Earth’s surface, it is not surprising that terrestrial rocks are not the oldest rocks in the solar system. However, it is interesting to note that the Earth and the meteorites fall on the same lead-lead isochron, which is evidence that the lead and uranium were isotopically homogeneous in the solar nebula before accretion.
2.3.4 Age of Lunar Rocks The Moon has no owing water, air, or biosphere to cause erosion, and no plate tectonics to continuously fold the crustal rocks back into the interior. We can therefore expect that lunar rocks will be older than terrestrial rocks. As discussed in Chapter 5, there is a marked di erence in the number of craters on the Moon’s highlands and maria, the highlands being much more heavily cratered. This implies that a signi cant period of time separated the end of the heavy bombardment of the highlands and the later emplacement of the maria by volcanic processes. It is possible to sort out these relative ages in detail from telescopic and spacecraft images, but it is not possible to determine absolute ages from counting craters on a surface. The absolute ages of lunar rocks have been determined from the “ground truth” obtained by dating returned lunar samples. The total lunar rock and soil collection from the 6 manned Apollo missions and 3 unmanned Soviet missions is 382 kg. Basalts from lunar mare plains are dated at 3.1 to 3.8 billion years old, although some small fragments are 4.3 billion years old. The lunar landscape was found to be so pulverized by impact events that no traces of the original outer crust were observed. What was found instead were breccias, which are rocks composed of cemented fragments of previous rocks, and impact melts, which are rocks that show clear evidence of complete melting during an impact event. The lunar highlands’ breccias and impact melts are dated at 3.8 to 4.0 billion years old. This relatively short span of time came as a surprise to researchers. It could mean that there was an increase in cratering just before the end of the heavy bombardment, or simply that during the heavy bombardment rocks only lasted about 0.2
2–9 billion years before they were pulverized. Lunar rocks are indeed older than terrestrial rocks, but they are still not the oldest rocks in the solar system.
2.4 Chondrites
2.4.1 Meteorite Classi cations Chondrites are meteorites that are chemically similar to the Sun. A comparison of elemental abundance in a chondrite versus elemental abundance in the Sun’s photosphere, as determined by spectroscopy, yields an astonishing one-to-one correspondence. The only elements that don’t match well are the most volatile elements, which tend to escape incorporation into a meteorite as it cools, and lithium, which is depleted in the Sun due to destructive nuclear reactions. The name chondrite has come to refer more broadly to any meteorite with a chemical composition that is similar to that of the Sun. Chondrites fall into several sub-classes: - ordinary chondrites - the most abundant meteorite class - enstatite chondrites - chondrites that formed under reducing environment - carbonaceous chondrites - chondrites rich in volatiles, including organic material. A stony meteorite that deviates from solar composition is called an achondrite. The chemical and mineralogical information contained in the various types of mete- orites yields some of our most important clues about the nature of the early solar system. In order to make inferences about the early solar system from analysis of meteorites, sev- eral assumptions must be made, and it is important to be aware of these assumptions. The rst question to come to mind is one of origin. Most meteoriticists agree that mete- orites are fragments of asteroids that have collided relatively recently in the asteroid belt. Two complications that have for some time prevented acceptance of this idea are: i) it is not obvious at rst what dynamical mechanisms can explain the observed mass ux and orbits of meteorites, and ii) spectrophotometric data indicate that the most common type of asteroid is not mineralogically similar to the most common type of meteorite, the ordinary chondrites. Work by Jack Wisdom of MIT, has shown that resonant dynamical processes can explain the rst problem of mass ux and orbits. And according to Richard Binzel of MIT, the second problem of mineralogical mismatch may be explained by space weathering. Another origin issue is the nature of the parent body or bodies that are the source of meteorites. How many di erent kinds of parent bodies, and how many of each kind, are required to account for the various classes of meteorites? The diversity seen in meteorites is partially explained by the idea that the parent bodies were large enough to have di erentiated. Then, the less dense meteorites can be traced back to the crust of the parent, and the more dense meteorites can be traced back to the core of the parent. Meteorites are classi ed in many ways, according to composition, textures, etc. A useful classi cation metric uses oxygen isotopes, thanks to pioneering geochemical work by Robert Clayton. Oxygen has three isotopes, the most common being 16O and the others being 17O and 18O. The approach is to plot 17O/16O vs. 18O/16O and to look for varia- tions, using terrestrial seawater as a standard (standard mean ocean water, aka SMOW). Plotted in this manner, most chondrite classes appear as distinct clumps, thought to re ect various meteorite parent bodies that accreted in di erent parts of the asteroid belt. Rocks
2–10 from Earth, the Moon and Mars plot along straight lines with the same slope (=-1/2). This is expected, and re ects one and two extra neutrons, respectively for 17O and 18O compared to 16O. The straight lines re ect the partitioning of oxygen isotopes into silicate rocks during melting and crystallization. Terrestrial and lunar rocks plot along the same straight line indicating that they formed in the same part of the solar system. The Martian meteorites form a line that parallels the Earth-Moon line. Achondrites also form a line that parallels the Earth-Moon line, consistent with their supposed history of igneous processes. The carbonaceous chondrites form a pseudo-line, but it does not have the same slope as for the igneous planetary and asteroidal bodies. The slope is steeper, which suggests that some process injected 16O into the solar nebula, lowering the 17O/16O and 18O/16O ratios at the same rate. The process that caused this 16O infusion isn’t known.
2.4.2 Age of the Solar System Any object with such a “chondritic” or “solar” composition is assumed to have changed very little since the solar system formed, and the age of chondrites gives us our best determination of the age of the solar system. In particular, the calcium, aluminum-rich inclusions, of the carbonaceous chondrites, like Allende, yield the most accurately determined age of the solar system:
9 age(solar system) = age(CAI0s)=4.559 0.004 10 yrs . (2.18)
2.4.3 Secondary processing If we want to use chondrites and other meteorites as a probe of the early solar system, we need to have a good idea of the types of processes that may have clouded or corrupted the information contained in these meteorites. Igneous meteorites, like the howardite- eucrite-diogenite suite (HED), have obviously undergone secondary processing. These meteorites provide unique information about the evolution of planetary objects, includ- ing basalt generation and core formation. Chondrites may be the least altered class of meteorites, but they too have undergone secondary processing. Most chondrites have ex- perienced thermal metamorphism. The result is changes in texture and mineralogy, and possibly chemical composition. The temperatures necessary to cause metamorphism are in the 400 to 1000 C range for the relatively low pressures encountered in small parent bodies. Possibly important heat sources are the decay of short-lived radionuclides, electro- magnetic induction, and accretion of material. The least metamorphosed type 3 chondrites probably carry the most information about the early solar system, but even these have been e ected somewhat by secondary thermal processing. Along with heat, the chemical reactivity of water has played an important role in the secondary processing of some of the most compositionally primitive meteorites. This process, called aqueous alteration, tends to replace the pre-accretionary lithology with new mineralizations, although the bulk chemistry is apparently preserved. Along with internal processing, meteorites can be changed by exogenic processes like collisions. Violent impacts produce shock metamorphism of individual mineral grains, and also produce rocks called breccias that contain mixtures of di erent previous rocks,
2–11 just like the breccias found on the lunar highlands. The study of breccias has provided information on the accretional growth and processing of parent bodies. The e ects of shock metamorphism have been seen in all major groups of meteorites. It appears that collision- induced high speed impacts took place before, during, and after the initial accretion and di erentiation of the parent bodies. Meteorites contain information related to their long exposures to galactic cosmic rays, solar radiation and the solar wind. It is possible to determine how long a meteorite existed free of its parent body before it impacted Earth by examining the cosmic ray damage. Noble gases are the most volatile elements in meteorites, but they are nonetheless present in measurable quantities in virtually all meteorites. Trapped noble gases are either “solar” or “planetary.” The solar noble gases are actually implanted solar-wind or solar- are material, and provide relatively direct information about the sun. The planetary noble gases have elemental abundances similar to those found in Earth’s atmosphere.
2.5 Achondrites Eucrites, Diogenites and Howardites. These are igneous meteorites that lack water-bearing (hydrous) or oxidized minerals. It is believed that these meteorites formed in a chondritic parent body that underwent partial melting. We will consider the possible heat source later in this chapter. Ureilites. These meteorites contain olivine and pigeonite (a calcium-rich pyroxene), and the matrix contains graphite or diamond. The carbon content suggests a that these meteorites are re-processed carbonaceous chondrites. Meteorites from the Moon. Some meteorites have breccias with white clasts in darker matrix, like lunar rocks. The lunar origin of some meteorites was established with- out contention because there exist many lunar samples which provided close geochemical and petrological matches to the meteorite samples. The identi cation of lunar meteorites opened the door to dynamical studies that subsequently established the possibility that meteorites can viably be deposited at Earth from Mars. Shergottites are named after a meteorite that fell in 1865 in Shergotty, which is located in the northeastern Indian state of Bihar, which borders Nepal. Unlike the eu- crites, their pyroxene and plagioclase mineralogy is strikingly similar to terrestrial basalts. They also have small amounts of the hydrous mineral amphibole kaersutite, whereas eu- crites show no evidence of water in their minerals, and they have some magnetite, which contains iron in oxided form (Fe+3), whereas eucrites contain only reduced iron. Their pyroxene crystals are elongated and arranged horizontally the way such crystals would ac- cumulate after settling to the bottom of a magma chamber. Such igneous rocks are called cumulates, and they can only occur in bodies with a large enough gravity eld to allow gravitational settling. Two other types of cumulate meteorites have hydrous and oxided minerals like the shergottites, these are the nakhlites, which contain the dark-green to black pyroxene mineral augite, and a unique meteorite that fell in Chassigny, France called chassignite, which contains mostly olivine. These three types of meteorites had for nearly two decades been referred to as the SNC (pronounced “snick”) meteorites. The SNC meteorites have been dated by radioactive methods and most, though not all, have igneous crystallization ages of 1 BY. Thus these rocks formed on a body in which 2–12 igneous volcanism occurred over 3 BY after the age of the solar system, which suggests a rather large (read: planet-sized) body. Since it is di cult to move material away from the sun, planets beyond Earth were the most likely candidates. By process of elimination Mars was originally hypothesized as the parent body for the SNC meteorites. Originally there was a great deal of skepticism about the possibility of transporting material from Mars to Earth, but it has now been shown to be dynamically possible. However, many workers have rightly asked why the source planet could not be Earth itself. In other words, rocks blasted o Earth into space by impacts that eventually re-impacted after some time in space. The ”smoking gun” in the Martian origin of SNCs came from analysis of gas inclusions in some of these rocks. Noble gas ratios are diagnostic of source regions because these gases do not readily combine and fractionate. Noble gas abundances of the SNCs were shown to be much di erent than the Earth’s atmosphere but identical to the atmosphere of Mars as determined by the Viking landers. Recently, many more examples of putative Martian meteorites have been discovered that have a range of characteristics that do not make them easily classi able under the SNC designation. The meteorites are more now more generally referred to as simply Martian meteorites. It is now understood that these rocks can be ejected from Mars during the impact process without being strongly shocked or heated. The martian meteorite ALH84001, discovered in Antarctica, was originally not des- ignated as originating from Mars because it has a crystallization age of 4 BY, much older than most SNCs. The Martian origin of this rock was estabilshed on the basis of its oxygen isotope signature, which matched that of other Martian meteorites and was much di erent that that of terrestrial rocks. ALH84001 gained a great deal of attention because it was proposed to contain evidence of ancient Martian life by David McKay and his colleagues. The evidence included the presence of nano-features reminiscent of bacteria, but many times smaller, the presence of organic material called polyachromatic hydrocarbons, and the presence of magnetite of a form that is often deposited by biogenic processes. There has been a great deal of controversy about whether these features do indeed represent evi- dence for past life on Mars. Research continues on this and other Martian meteorites that contain such features, with a careful eye towards understanding the potential for terrestrial contamination.
2.6 Irons and Stony-Irons Iron and stony iron meteorites make up several percent of the meteorite population. They constitute 4% of the meteorites that fall to Earth but are the most common nd since they look so much di erent than the Earth’s crustal rocks. Stony iron meteorites consist of roughly equal parts of rock and iron, with the rock component consisting of olivine, the most common mantle material, and minor amounts of other silicate phases. Stony irons make up only about 1% of meteorites that fall to Earth. Pallasites, composed of metal and the silicate mineral olivine, are meteorites that are believed to have crystallized at the core-mantle boundaries of di erentiated parent bodies. Mesosiderites are composed of iron and achondrite material, and are believed to have formed when already-di erentiated parent bodies collided. The metallic component of these meteorites is predominantly iron with nickel in solid
2–13 solution averaging usually about 10% but sometimes up to 20%. There are also smaller amounts of sul de, graphite and occasionally silicate inclusions. Within the iron there are two metal phases: - the body-centered cubic ( ) form kamacite (5.5% nickel) - the face-centered cubic ( ) taenite (variable, but usually > 27% nickel) These phases occur because iron and nickel form a solid solution when mixed and are not completely miscible as they begin to cool. The iron and nickel are structurally similar but not identical. At high temperatures they exchange freely because the crystal lattice is expanded. But when cooling sets in their slight di erences produce lattices with slightly di erent structures. At a point the total energy of the system is minimized by segregating the elements into 2 separate lattices: one rich in iron and the other poor. To minimize the mismatch where the lattices connect, newly formed lattices form in preferred orientations called exsolution lamellae. Approximately 75% of iron meteorites exhibit a crystal pattern called Widmanstat- ten structure, which is the term used for these exsolution lamellae. The pattern is observed by taking a meteorite that is cut and polished and dipping it in acid. Because this pattern forms when an iron-nickel alloy crystallizes, it is an indication that some as- teroids were at least partially melted after they formed. In fact, the details of the pattern tell the cooling history of the meteorite parent body from which it was derived. From the iron-nickel phase diagram we can see the evolution of the relative amounts of iron and nickel that crystallized as the iron cooled. And from the variations in composition across Widmanstatten structure boundaries it is possible to constrain the rate of cooling.
2.6.1 Iron meteorites and planetary cores
In the iron-nickel system, equilibrium is maintained at temperatures above 650 C. Below 350 C crystal structures are frozen in. So the Widmanstatten structures yield the cooling rate in this temperature range. In the meteorites wide di usion boundaries cor- respond to slow cooling and narrow di usion boundaries correspond to rapid cooling. In the meteorites there are iron-nickel crystals that grown with lengths of up to several cen- timeters, which correspond to rates of 0.4 40 /MY. So the Widmanstatten structures yield cooling rates of many millions of years.→ For a cooling periods in this range radii of meteorite parent bodies in the range 100–200 km are implied. Identi cation of a signi cant metallic group within the meteorites made it the natural presumed component of dense planetary cores. It is known from the masses of the planets that the interiors are (in most cases), after correcting for the e ect of self-compression, denser than rock. An iron-nickel alloy with approximate meteoritic proportions is the leading candidate for the dense component. Shock wave experiments indicate that seis- mic wave velocities in the Earth’s deep interior are consistent with a predominantly iron composition and thus support the contention. The other piece of evidence for iron planetary cores comes from the process of nucle- osynthesis. Iron has the highest binding energy for nucleon and is thus highly stable and produced in abundance in stellar evolution. The equilibrium process (also known as e-process) in stellar thermonuclear reactions breaks apart silicon atoms and re-arranges
2–14 them to convert silicon to heavier and more stable nuclei. The most stable and thus most abundant element produced in the e-process is iron.
2.7 Radioactivity as a heat source The second important role of radioactive nuclides besides age dating is as a source of heat in planetary interiors. The important heat-producing radionuclides form two classes: long-lived nuclides and short-lived nuclides. The long-lived radionuclides, of which the most important are 238U, 235U, 232Th, and 40K, are a primary source of heat over the span of planetary history. They provide heat which drives present-day mantle convection on the Earth and they likely fueled in large part the global resurfacing event on Venus during the last billion years. Long-lived radioactive elements have combinations of valence states and ionic radius that prevent them from being easily accommodated into the crystal lattices of the most common silicate rocks. They are examples of lithophile elements, which preferentially concentrate in the liquid phase; the majority of them are incorporated into the rst few percent of a melt. For this reason, a signi cant fraction of the Earth’s radioactivity is concentrated in the continental crust. The short-lived radionuclides may have been an important source of heat respon- sible for the early melting of meteorites. They may also have provided an early heat source for planets, depending on the time between nucleosynthesis and planetary accretion. The most abundant of the short-lived radionuclides is 26Al, which decays with a half-life of 720,000 years to 26Mg. The evidence for 26Al being an important source of heat in the early history of the solar system comes from excess amounts of 26Mg found in CAI’s in the Allende meteorite. The isotope 26Mg was enriched relative to the most common isotope 24Mg compared to solar abundance. The heat-producing ability of this isotope is such that solid objects a few km or greater would have been heated to melting if they formed with the ratio of 26Al/27Al implied to have been present in Allende. A major question is: how could 26Al be incorporated fast enough into early solar system objects to melt them? With such a short half-life, radioactive decay begins to produce heat after a cosmically short period of time. 26Mg is only produced by decay of 26Al, and 26Al is only produced in supernovae. This suggests that our solar system might have formed close to a supernova. Another piece of information in support of the supernova hypothesis is the fact that very small diamonds have been found in some meteorites. On Earth diamond forms at great depths due to very high pressures which contract carbon to the closely-packed state, characterized by all covalent bonds. In space the pressures required to form diamond can only be achieved in a supernova. One way around the supernova explanation is that experiments have recently shown that diamonds can form from a gas phase outside the stability eld. If the solar system did form near a supernova it would solve the problem of the mechanism that caused the proto-solar cloud to collapse, as the shock waves that emanate from supernovae would provide a natural mechanism for compression. However, supernovae are rare events and if it is necessary to invoke the participation of one it would imply that the formation of our solar system was a chance event. Given the number of other planetary systems that are being identi ed this is an unsatisfying explanation.
2–15 Problems
1. Most of the meteorites that have been found on Earth are irons but most meteoroids in Earth-crossing orbits are rocky. (a) How do we know this? (b) Why are rocky meteorites under-represented in the terrestrial record?
2. Show that for atoms of a radioactive species with decay constant , the mean life 1 is . Start o by writing an expression for the number of atoms that decay in the time period from t to t + t. Then integrate over all time to get the mean life.
3. Consider the cooling and crystallization of an iron-nickel meteorite with a bulk composition of 95% iron and 5% nickel. Use the phase diagram on the attached sheet to answer the following questions: (a) What phase crystallizes rst? What are the percentages of iron and nickel in the rst-formed crystals? (b) At what temperature does the second phase begin to exsolve? What is its composition? (c) What are the compositions of each of the phases at 570 C? What is the bulk composition of the system at this temperature? Show your work on the phase diagram.
4. Prepare a report on Martian meteorites that examines critically the evidence that they come from Mars [see McSween, 1994]. Can this evidence bear the weight of the suggestion by McKay etal. [1996] that ALH84001 contains evidence of biology?
References
Keizo, Y. & H. Kojima, Photographic catalog of the Antarctic meteorites, National Institute of Polar Research, Tokyo, 1987. Kerridge, J.F. & M.S. Matthews, Meteorites and the early solar system, University of Arizona Press, 1988. McKay, D.S, E.K. Gibson Jr., K.L. Thomas-Keprta, H. Vali, C.S. Romanek, S.J. Clemett, X.D.F. Chillier, C.R. Maechling, R.N. Zare, Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001, 1996. McSween, H.Y., Meteorites and their parent planets, Cambridge University Press, 1987. McSween, H.Y., What we have learned about Mars from SNC meteorites, Meteoritics 29, 757-79, 1994. Mason, B., Handbook of elemental abundances in meteorites, Gordon and Breach, New York, 1971. Mendenhall, W., R.L. Schea er & D.D. Wackerly, Mathematical statistics with applications, 2nd ed., Duxbury Press, Boston, 1981. Sears, D.W., The nature and origin of meteorites, Oxford University Press, New York, 1978. Wasson, J.T., Meteorites: their record of early solar system history, Freeman and Co., New York, 1985.
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