Terrestrial age

Meteorites are not in chemical equilibrium with the Earth s surface, and thus are quickly degraded and destroyed after they fall. The most striking example of this lack of equilibrium is the presence of Fe-Ni metal in many meteorites, which quickly rusts on Earth. The rate at which meteorites weather and disintegrate can be measured. The time since a meteorite fell is known as its terrestrial age. 

When a meteorite falls onto the Earth s surface, it becomes for the most part shielded from the effects of cosmic rays. The cosmic-ray-produced nuclides cease to form so the exposure clock stops. However, because many of the cosmic-ray produced nuclides are radioactive, several new clocks start that can be used to estimate the time that a meteorite has been on the Earth s surface. 

A large number of meteorites have been collected from hot desert areas. In these areas, rainfall is low, which helps to preserve the meteorites. In addition, the dry conditions make meteorites easier to find because vegetation is scarce and the wind blows away the fine dust, leaving rocks and meteorites as a lag deposit. 

Numerous meteorites have been collected in Western Australia and terrestrial ages have been determined for 50 of them. Ages range from very young to around 40 000 years. There is a rough exponential decline in the number of meteorites as a function of age. The distribution of ages gives a mean residence time of 10 000 years at this location. 

Thousands of meteorites have also been recovered from the deserts of North Africa. Terrestrial ages of these meteorites extend out to 50 000 years and again the number of meteorites shows a rough exponential decrease with age. The mean residence time inferred for these meteorites is 12 000 years. 

The production of radionuclides in meteoroids that are orbiting the Sun takes place by nuclear spallation and neutron-capture reactions with the atoms of the major elements (Bogard et al. 1995 Leya et al. 2000). The concentration of a particular radionuclide in a stony meteoroid exposed to cosmic rays in Fig. 18.14 initially increases with time until it reaches a state of equilibrium or saturation when its rate of decay is equal to its rate of production. When such a meteoroid enters the atmosphere of the Earth and explodes, the resulting meteorite specimens are assumed to be saturated with respect to the cosmogenic radionuclides they contain. After a meteorite has landed on the surface of the Earth, the production of radionuclides stops because the Earth is protected from cosmic rays by its magnetic field and by the atmosphere. Therefore, the rate of decay of cosmogenic radionuclides decreases with time as each nuclide continues to decay with its characteristic halflife. The terrestrial age of a meteorite specimen collected in Antarctica or anywhere else on the Earth is calculated from the rates of decay of the radionuclides (e.g., C1 or A1) that remain at the time of analysis (Jull 2001). 

The rate of decay of cosmogenic tadionucUdes in meteorites is so low that this method of dating was not practical until after tandem-accelerator mass-spectrometers became available in the 1980s (Elmore and Phillips 1987). As a result of this development. 

Chlorine-36 is favored by investigators because the grains of metallic iron are easily concentrated from crushed samples of stony meteorites and because the concentration of 1 atoms per gram of metal can be determined routinely by accelerator mass-spectrometry. The corresponding rate of decay of Cl, also called the activity, is calculated from the measured concentration by means of the law of radioactivity. The calculation of the terrestrial 1 age of the LL6 chondrite ALH 78153 is demonstrated in Appendix 18.12.2. 

The histogram of the terrestrial ages of meteorite specimens collected on the Allan-Hills ice fields in Fig. 18.16 demonstrates that more than 35% of the specimens that have been dated fell less than 100,000 years ago and that about 10% have been on the Earth for more than 5(X),(XX) years, including ALH88019 

The exponential decrease of the abundances of meteorite specimens with increasing terrestrial ages in Fig. 18.16 is noteworthy because it implies either that meteorites were progressively destroyed the longer they resided in the ice sheet or that they collect at the base of the ice sheet and are prevented from being 

COSMOGENIC NOBLE GASES PRODUCED BY SOLAR COSMIC RAYS (SCR) 

SCR protons and alpha-particles penetrate only a very few cm into solid matter. Therefore, SCR-produced cosmogenic nuclides are normally not expected to be observed in meteorites, because their outermost few cm usually were ablated in the Earth s atmosphere. SCR nuclide concentration profiles therefore mostly have been calculated for the Moon, i.e., for samples with lunar chemical composition irradiated at 1 AU from the Sun. Most of the recent work has been done by R.C. Reedy and coworkers (e.g., Rao et al. 1994 Reedy 1998a,b) and R. Michel and coworkers (Michel et al. 1996 Neumann 

The lunar data also allow one to study temporal variations of SCR fluxes (Reedy and Marti 1991 Reedy 1998b). 

Garrison et al. (1995) and Weigel et al. (1999) conclude that SCR-Ne is also present or likely to be present in some acapulcoites and lodranites. For some reason, members of these meteorite classes often had a small preatmospheric size. It should be noted, however, that these conclusions are based on high Ne/ Ne ratios of samples without a documented relative position to each other (this is also true for most of the shergottites). A verification by means of depth profiles of Ne, Ne/ Ne as well as Al would be highly desirable. 

SCR noble gases have also been found in one iron meteorite (Lavielle et al. 1999b). Arlington has a highly unusual shape that led to a partial preservation of its preatmospheric surface. 

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Aluminum-26 is an important nuclide for investigating the cosmic-ray exposure history of meteorites on their way to Earth from the asteroid belt. It can also be used to estimate the terrestrial age of a meteorite. In both of these applications, the 26 A1 is alive in the samples, having been produced by cosmic-ray interactions with elements heavier than aluminum, primarily silicon. Cosmic-ray-exposure dating will be discussed in Chapter 9. 

Ejection ages (sum of cosmic-ray exposure age + terrestrial age) for Martian meteorites. The ages cluster by meteorite type, suggesting that each cluster represents a distinct impact (ejection) event. The only outliers are the EETA 79001 and Dhofar 019 shergottites and ALHA84001. Modified from McSween (2008). 

We also have over 120 lunar meteorites in our collections. Because the Moon has no atmosphere, the irradiation history of these meteorites can include an extended period in the lunar regolith. The transit times from the Moon to the Earth range from a few x 104 years to nearly 10 Myr. Detailed analysis of exposure ages and terrestrial ages indicate that at least three impact events in the lunar highlands and five events in the lunar mare ejected the meteorites that have been recovered to date. 

A third desert site is Eastern New Mexico, where nearly 200 meteorites have been found. Thirty-two of these meteorites have had their terrestrial ages determined. They fall in the range of 10 000 to 50 000 years. They do not show an exponential decrease in abundance... 

Cosmic-ray exposure ages are determined from spallation-produced radioactive nuclides. Cosmic-ray irradiation normally occurs while a meteoroid is in space, but surface rocks unshielded by an atmosphere may also have cosmogenic nuclides. These measurements provide information on orbital lifetimes of meteorites and constrain orbital calculations. Terrestrial ages can be estimated from the relative abundances of radioactive cosmogenic nuclides with different half-lives as they decay from the equilibrium values established in space. These ages provide information on meteorite survival relative to weathering. 

Jull, A. J. T. (2006) Terrestrial ages of meteorites. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H.Y., Jr. Tucson University of Arizona Press, pp. 889-905. This paper reviews what is known about terrestrial ages of meteorites. 

Before leaving the aminoacids problem, it is interesting to note that aminoacids have been detected in carbonaceous chondrites found in Antarctica. The risk of contamination is much less important in Antarctica than in Australia and this is one of the reasons why these studies were undertaken. They fully confirm the results obtained on Murchison 54,55), even if in one CM carbonaceous chondrite the amino acid content was only 10% of what was observed in Murchison 56,57). The contamination is in fact lower than in Murchison the aminoacid content was very similar for samples taken near the surface of the Antarctica chondrites or from their bulk. On the other hand, all the significant analyses on Murchison were performed on samples from the interior of the meteoritic fragments due to the high degree of surface contamination. In the case of the Allende meteorite, which has the same terrestrial age as Murchison, contamination was found to extend to a depth of more than 5 mm below the surface 52). 

The terrestrial age of meteorites is based on the measurement of long-lived radio nuclides such as 14C, 26Al, 36CI, 53Mn and more recently of 41Ca, the half-lives of which rank from 5730 years up to 3.7 million years. It is not possible with such measurements to distinguish which fraction of the terrestrial age the... 

Table 1. Comparison of terrestrial age and surface exposure for three Antarctic chondrites [110]...

Sample location Signature Exposure duration on the ice in years Terrestrial age in years Class... 

Weathering of meteorites influences the oxidation state of the material besides an alteration of the composition of many elements. Since meteorite studies allow to gain information on the early solar system not accessible by any other means it is important to know which fraction of the solid meteorite under investigation has not undergone any secondary alteration and therefore has preserved its pristine composition. This may be of essential interest for additional detailed investigations of extraterrestrial material. For meteorites, weathering is proportional to the exposure duration on the Antarctic ice shield which is given by the F contamination as explained above and not by the terrestrial age of the observed meteorite. 

K.C. Welten, L. Lindner, C. Alderliesten, K. van der Borg, Terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area, East Antarctica, Meteor. Planet. Sci. 34 (1999) 558-570. 

Eugster O., Michel Th., and Niedermann S. (1991) " Pu-Xe formation and gas retention age, exposure history, and terrestrial age of angrites LEW86010 and LEW87051 comparison with Angra dos Reis. Geochim. Cosmochim. Acta 55, 2957-2964. 

Welten K. C., AlderUesten C., Van der Borg K., Lindner L., Loeken T., and Schultz L. (1997) Lewis Cliff 86360 an Antarctic L-chondrite with a terrestrial age of 2.35 million years. Meteorit. Planet. Sci. 32, 775-780. 

Range of applicability. Metal or magnetite under all shielding conditions all falls and those finds for which the terrestrial age is known or known to be short compared to the half-life of C1. 

Limitations, (i) As with cosmogenic Ar, the deconvolution of the cosmogenic component of Ar requires corrections for the presence of trapped Ar. (ii) Long terrestrial ages will lower appreciably the measured C1 contents. An independent measure of terrestrial age may be necessary to correct for this effect, (iii) The measurements are sensitive to the presence of calcium- and of rare potassiumbearing impurities in the metal phase. 

Range of applicability. Falls and finds for which terrestrial age is known. For exposures long... 

Limitations, (i) As with other radionuclide-based ages, the terrestrial age of the sample must be known, (ii) Concentrations of Kr are quite low in most meteorites, typically just 5 X 10 atomg in chondrites. For this reason, Kr measurements are still scarce and their uncertainties can be relatively large, often —20%. (iii) Production rates for krypton isotopes may vary with the abundances of rubidium, yttrium, and zirconium relative to strontium. It should be understood that the original basis for the calculation of Pgi/Fgs was a set of relative cross-section measurements for the production of krypton from silver (Marti, 1967). 

Pair refers paired meteorites from the same locality. Mass is the recovered mass. Z)2a-is the depth at which irradiation on the Moon took place. 72 is the duration of the lunar irradiation. / 4 isthe radius of the meteoroid while in transit to Earth. 74 is the duration of transit to Earth. Ti is the terrestrial age. (i) Greshake et al. (2001) note similarities to MAC 88104/5. (ii) Assume density 2.7 g cm. (iii) T2tt before compaction, (iv) Full model has three stages on Moon. References (a) Eugster and Lorenzetti (2001). (b) Nishiizumi and Caffee (2001a). (c) Warren (1994). (d) Nishiizumi and Caffee (2001b). (e) Shukolyukov et al. (2001). (f) Scherer et al. (1998). (g) Nishiizumi et al. (1998). 

As all known lunar meteorites are finds (and therefore have nonzero terrestrial ages), we need at least four measured quantities to determine the four parameters of a simple one-stage history. Similarly, for a simple two-stage history, we need at least six measured quantities. Typically the data set available comprises He, Ne, Ne, Ar, C1, A1, and e. Occasionally we may have other information— the concentrations of spallo-genic krypton isotopes, spallogenic xenon isotopes, " Ca, and Mn, the densities of nuclear tracks (tracks/unit area), and the concentrations of certain isotopes produced by thermal neutrons, e.g., Ar (from C1) and Gd. 

Miura Y., Nagao K., and Fujitani T. (1993) Kr terrestrial ages and grouping of Yamato eucrites based on noble gas and chemical compositions. Geochim. Cosmochim. Acta 57, 1857-1866. 

Schultz L. and Freundel M. (1984) Terrestrial ages of Antarctic meteorites. Meteoritics 19, 310. 

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