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VERCHOVSKY, SEPHTON: NOBLE GASES IN METEORITES SOME POSSIBLE SOURCE ENVIRONMENTS/PROCESSES FOR NOBLE GASES IN METEORITES 2 (a): Carbon stars (low-mass AGB stars in which atmosphere ratio of C/O is >1) such as IRAS 05316+1757. (Atlas image/2MASS) (b): Supernovae (generated by an explosion of a massive star) such as that which produced the remnant SNR0103-72.6. (NASA) (c): Novae (produced when a star ejects some of its material to form a luminous cloud) such as is seen for Nova Cygni 1992. (NASA) A noble record ABSTRACT Meteorites contain unusual mixtures of noble gases that reflect contributions from several extraterrestrial environments. Processes that predate the formation of the solar system – such as condensation in stellar atmospheres and violent supernovae explosions – have introduced noble gases into interstellar grains that include diamonds, silicon carbide and graphite. The solar wind has implanted solar mixtures of gas into the surfaces of asteroids from which meteorites originate. An enigmatic carrier of planetary noble gases remains unidentified. Cosmogenic and radiogenic noble gases reveal the timing of events during solar system formation and the eventual break-up of the asteroidal meteorite parent body. T he noble gases – helium, neon, argon, krypton and xenon – have chemical and physical properties that make them distinct from all other elements. Their most notable property is a lack of chemical activity in comparison to other elements; they tend not to combine easily with other elements, for example. Yet these properties ensure that noble gases are an important source of information for extraterrestrial investigations. That very lack of chemical activity means that they can preserve features of the early solar system that are obliterated by reactions among other elements. The relative abundances of noble gases are different for many objects in the cosmos (figure 1) and measurements in meteorites can therefore give an insight into their origins, as well as into extraterrestrial processes they may have undergone since. The rate at which noble gases physically melt, boil, diffuse, dissolve and adsorb varies systematically 2.12 10 9 10 7 10 5 103 101 10 –1 10 –3 10 –5 10 –7 10 –9 M/10 6 Si Alexander B Verchovsky and Mark A Sephton review the origins and significance of noble gases in meteorites, focusing on what this unique record reveals about the early solar system. with their mass and can be used to model both their original abundances and the processes that changed those abundances over time. Noble gas distribution is usually expressed in terms of the relative abundances of the different elements. Equally or even more important is their isotope abundance, because this reflects a wide range of processes, including nuclear reactions in the hearts of stars and the self-destructive process of radioactive decay. The noble gas content – elemental and isotopic – of ancient meteorites reveals a cornucopia of contributing environments. In this account, we will follow the noble gas evidence in meteorites from the early solar system and show how it illuminates different aspect of its history. Noble gas origins The first noble gas produced in the cosmos was helium, created seconds after the Big Bang as Sun Venus meteorites (Q) Earth Mars 4He 20Ne 36 Ar 84Kr 132 Xe 1: Abundances of noble gases (per 106 atoms of Si) in the Sun, atmospheres of terrestrial planets and meteorites (Q component). soon as temperatures dropped low enough to allow any neutrons and protons to combine. At that point helium made up around 25% of matter with hydrogen accounting for almost all of the rest. Around a billion years later matter aggregated into clumps to form galaxies where the first-generation stars formed, using primordial hydrogen and helium as fuel. The original Big Bang helium was joined by helium generated in stars from the process of hydrogen burning – the fusion of four hydrogen nuclei (single protons) into a single helium nucleus (two protons and two neutrons). Hydrogen burning is the most common nuclear process in stars, and the ultimate source of their luminosity. Over time, hydrogen burning is making the cosmos poorer in hydrogen and richer in helium. Many stars end their lives by expelling their matter into space and this material is then involved in the formation of a new generation of stars. ConseA&G • April 2005 • Vol. 46 VERCHOVSKY, SEPHTON: NOBLE GASES IN METEORITES (d): The solar wind (a flow of gas and energetic charged particles) originating from our Sun. (NASA) (e): Products from all of (a) to (d), and those from other environments/processes, are represented in primitive asteroids. (NASA) quently, younger generations of stars are increasingly rich in helium and the heavier elements. The relative amount of elements heavier than helium in a star is described as its “metallicity” and is an expression of both star age and the number of stellar cycles the constituent matter has experienced. Further processing in stars produced increasing amounts of heavier noble gases. The noble gas mixtures in meteorites are a combination of products from the Big Bang and subsequent star chemistry. When stars end their lives in supernova explosions, they eject small micrometre sized (or less) particles that join the interstellar medium and can become part of a dense molecular cloud. Such grains in one particular interstellar cloud were caught up in the developing solar nebula. Some of the grains survived formation of the Sun and avoided destruction by geological activity on the planets by being accreted into relatively small asteroid-size bodies. Fragments of asteroids that fall to Earth as meteorites provide us with tangible samples of this star dust. There have been several presolar grains discovered so far (Zinner 1998, Nittler 2003, Anders and Zinner 1993), and some contain noble gases whose isotopic compositions are significantly different from what is normally observed in the solar system. These anomalous isotopic compositions were the key evidence for their extrasolar origin and the main feature that allowed identification of the grains in painstaking laboratory isolation procedures. Nanometre-sized diamonds (average grain size ~3 nm) were the first presolar gains identified (Lewis et al. 1987) and they are the most abundant presolar material in meteorites, at up to 1500 ppm. But their origin is the least well understood among the known types of presolar grains. Xenon isotopes suggest that they originate from mixing of different layers of an explosive supernova (Ott 1993). Yet correlation between isotopic compositions of noble gases, carbon and nitrogen with grain size of the diamonds indicates that they consist of several populations formed in different stellar environments (VerA&G • April 2005 • Vol. 46 chovsky et al. 1998). Diamonds, it seems, may be ubiquitous components of interstellar space. Krypton and xenon in presolar silicon carbide grains (typical grain size from submicrometre to a few micrometres) point directly to a provenance in the atmospheres of carbon stars (lowmass Asymptotic Giant Branch stars in which atmosphere ratio of C/O is >1) in which carbonaceous grains condense. The carbon star origin of silicon carbide grains has been supported by their detection in these environments by spectroscopic observations. In contrast to the relatively straightforward provenance of presolar silicon carbide, noble gases in presolar graphite reveals a more complicated history (Zinner 1998). As well as a carbon star component similar to that for silicon carbide, presolar graphite contains neon that has decayed from a short-lived isotope of sodium with a half life of 2.6 years. Its origin requires a quick explosive supernovae. Isotopic compositions of other elements – such as krypton – in the graphite grains also point to diverse stellar sources, including carbon stars, novae and supernovae (Zinner 1998). Like diamonds, graphite appears to be widespread in cosmic environments. Solar noble gases The most abundant noble gases in the solar system are those in the Sun. In meteorites the distinctive patterns of solar noble gases are rare, depleted by many orders of magnitude compared to their abundance in the Sun. The small amounts that are present were implanted into the surfaces of asteroids by the solar wind. The process that separated solids – the meteorites – from the noble gases of solar composition must have occurred very early in the history of the solar system and appears to be associated with the violent solar winds common during the tempestuous T-Tauri stage of the early Sun (Kallenbach et al. 2003). Yet the element abundance of noble gases in meteorites is usually different from that observed in the solar wind because lighter noble gases are preferentially lost relative (f): Meteorites such as Murchison are fragments of primitive asteroids that have fallen naturally to Earth. to heavier ones during metamorphism (mineral changes resulting from heat and pressure) on the parent asteroid. Planetary noble gases The term “planetary” was first used to describe mixtures of noble gases in an attempt to distinguish between the solar noble gases and those observed in the Earth’s atmosphere. The latter has a significant (several orders of magnitude) depletion of light noble gases compared to the solar component. What was formerly called the planetary component in meteorites initially bore a superficial resemblance to the mixtures seen in the Earth’s atmosphere, but later turned out to be a mixture of several components including those from presolar grains. It was during the separation of presolar grains in the laboratory that a phase was identified that appeared to host a very high concentration of planetary noble gases. This was named phase Q, for quintessence (Lewis et al. 1975), and was found to be a separate carbonaceous phase comprising less than 0.03% of the total meteoritic mass. Q accounts for all of the noble gases in meteorites that do not belong to presolar grains, implanted solar wind, or matter of cosmogenic or radiogenic origin (the latter two discussed below). To this day, the identification of Q remains elusive and only its eventual isolation in pure form and its subsequent chemical characterization will provide an answer to the questions about both its provenance and associated planetary noble gases (Verchovsky et al. 2002). Cosmogenic noble gases Cosmogenic isotopes are formed in meteorites as a result of the interaction between galactic cosmic rays – high-energy protons – and meteoritic matter (Wieler 2002). The protons break atomic nuclei in spallation reactions to create new lighter ones, as well as inducing nuclear reactions with secondary neutrons that also form new nuclides. Among the many isotopes formed in this way only noble gases show sig2.13 VERCHOVSKY, SEPHTON: NOBLE GASES IN METEORITES Table 1: Radiogenic noble gas isotopes and their parents Parent isotope Half-life (years) Daughter isotope(s) 40 1.277×109 4.468×109 7.038×108 1.405×1010 1.57×107 8.08×107 40 K U 235 U 232 Th 129 I 244 Pu 238 Ar He 4 He 4 He 129 Xe 4 131,132,134,136 Xe (a) (b) 3: An example of a presolar grain and its stellar source. (a) SiC grain from Murchison meteorite. (b) Planetary nebula NGC6751. (From L R Nittler 2003 Earth and Planet. Sci. Lett. 209 259) nificant shifts in the ratios of different isotopes, because their background concentrations in meteorites are extremely low. Cosmic rays penetrate to about one metre below the surface of an object and, therefore, cosmogenic noble gases are found in meteorites representing either the very surface of their parent bodies or small fragments that resulted from asteroid collisions. The main implication of the existence of cosmogenic isotopes is that it makes it possible to calculate exposure ages of meteorites, i.e. the time between separation of a small, metre-size, fragment from its parent body and its fall to the Earth’s surface as a meteorite (Wieler 2002). The exposure age is calculated from the production rate of noble gas isotopes by cosmic rays, based on the chemical composition of meteorites and the assumption of constant cosmic ray intensity throughout time. The production rate depends also on the geometry of the meteorite body before it entered Earth’s atmosphere and the position of the sample inside the meteorite. Exposure ages of meteorites of the same chemical group can reveal complex histories. For example, plotting the exposure ages on agedistribution diagrams can often reveal several peaks, indicating that many fragments were formed at these times, suggesting the ages of major collision events in the asteroid. For instance, one of the largest meteorite groups, the H-chondrites (meteorites with relatively high metallic iron content), show signs of three large collision events that happened 7, 24 and 33 million years ago. Radiogenic noble gases Radiogenic noble gases are formed as a result of the radioactive decay of unstable isotopes. The most common radiogenic noble gases and their parent isotopes are listed in table 1. Some of them, produced from long-lived parents, are generated in meteorites throughout the life of the solar system while others, originating from short-lived precursors, are produced only dur2.14 ing the very early stages of the formation of the solar system. An important factor is the abundance of the parent isotope in meteorites, which in turn determines how widespread is the corresponding radiogenic product. The radioactive isotope of potassium (40K) is by far the most abundant parent isotope in meteorites, and its decay product, the radiogenic noble gas argon (40Ar), is also relatively abundant. Because almost no 40Ar was incorporated into asteroids when they formed, nearly all of the 40Ar present in meteorites is assumed to be radiogenic. Because the decay rate of 40K is well known, comparing the ratio of 40K/ 40Ar in a meteorite can be used to determine when the object formed. The radiogenic noble gases, therefore, are valuable for establishing a solar system chronology (Swindle 2002). Events that can be dated by potassium–argon (K–Ar) methods include mineral formation or recrystallization as well as various metamorphic alteration processes on the meteorite parent bodies. One of the examples of a successful use of the method is related to dating of lunar rocks that established that a major catastrophic impact crater formation event occurred early in the solar system around 3.8–4 Ga, the so-called late heavy bombardment (Swindle 2002). Radiogenic products of short-lived and therefore now extinct isotopes have important implications for chronology of the very early solar system (Swindle 2002). For example, the presence of 129Xe implies the existence of its parent, radioactive iodine (129I), in the early solar system. 129I is produced by nucleosynthesis in stars, but it is so short-lived (half life of 15.7 Ma) that the time interval between inputs of nucleosynthetic products, including this, into the solar nebular and formation of the first solids must be short, estimated at ~100 Ma. As 129I decayed, its decrease relative to the background nonradiogenic 127I was locked up in solids as they formed and the ratio can be used for relative dating of the first 50 Ma of solar system history with a resolution of <1 Ma. The method has been used for establishing formation sequences of the major types of meteoritic parent bodies and characteristic inclusions found within them such as enigmatic spherical silicate blobs (chondrules) and refractory objects enriched with Ca and Al (calcium aluminium inclusions; CAIs) the origins of which are hotly debated. Conclusions Noble gases found in meteorites are valuable probes of extraterrestrial environments. Noble gas abundances and isotopic compositions reflect various processes including nucleosynthesis in stars that existed before our solar system was born, radioactive decay, and interaction with cosmic rays and solar winds. Their ability to act as fingerprints for specific sources and indicators of the means by which solid objects formed, make noble gases vital to constraining the environments that contributed raw materials to our solar system and deducing the sequence of events that followed. ● Alexander B Verchovsky, PSSRI, Open University, Milton Keynes. Mark A Sephton, Dept of Earth Sciences and Engineering, Imperial College London. References Anders E and Zinner E 1993 Meteoritics 28 490. Kallenbach R, Robert F, Geiss J, Herbst E, Lammer H, Marty B, Millar T J, Ott U, Pepin R O 2003 Space Science Reviews 106 58. Lewis R S, Srinivasan B and Anders E 1975 Science 190 1251. Lewis R S, Ming T, Wacker J F, Anders E, Steel E 1987 Nature 326 160. Nittler L R 2003 Earth and Planetary Science Letters 209 16. Ott U 1993 Nature 364 25. Swindle T D 2002 Review in Mineralogy and Geochemistry 47 101. Verchovsky A B, Fisenko A V, Semjonova L F, Wright I P, Lee M R, Pillinger C T 1998 Science 281 1165. Verchovsky A B, Sephton M A, Wright I P, Pillinger C T 2002 Earth and Planetary Science Letters 199 243. Wieler R 2002 Review in Mineralogy and Geochemistry 47 21. Zinner E 1998 Annual Review of Earth and Planetary Sciences 26 42. A&G • April 2005 • Vol. 46