Meteorites isochron

The Earth was assumed to have the same age as meteorites, so that the geochron is identical to the meteorite isochron of 4.56 Ga. If the total silicate portion of the Earth remained a closed system involved only in internal (crust-mantle) differentiation, the sum of the parts of this system must lie on the geochron. The reader is referred to textbooks (e.g., Faure, 1986) on isotope geology for fuller explanations of the construction and meaning of the geochron and the construction of common-lead isochrons. 

Lu-Hf isotope systematics provide an important complement to Sm-Nd in the study of the cmst and mantle (e.g., Patchett et al., 1981 Salters and Hart, 1991 Vervoort and Blichert-Toft, 1999). In the crustal context, Lu-Hf is extremely important because of the —1% hafnium content of zircon, and the consequent ability to isotopically characterize the hafnium within grains that have been U-Pb dated (Patchett et al., 1981 Corfu and Stott, 1996 Vervoort et al., 1996 Amelin et al., 1999). However, the Lu-Hf isotopic system is currently overshadowed by a controversy over the decay constant. For many years, a value for the Lu decay constant of 1.94 X 10 yr, based on the eucrite meteorite isochron of Patchett and Tatsumoto (1980) and Tatsumoto et al. (1981) was used. More recent physical determinations reviewed by Begemann et al. (2001) have high dispersion, but do not seem to corroborate the 1.94 X 10 value. At the present time, there is a discrepancy between values based on U-Pb-dated terrestrial Precambrian REE-rich minerals, such... 

Age of Earth. All leads from terrestrial rocks except those from uranium and thorium ores are enclosed in the cross-hatched areas of Fig. 12. If the earth and meteorites were totally unrelated in time of origin or in the isotopic composition of primordial lead, there is no reason why terrestrial lead should cluster about the primary isochron for meteorites. The proximity of terrestrial leads to the primary meteorite isochron has been used by Patterson (Patterson et al, 1955 Patterson, 1956 Murthy and Patterson, 1962) in estimating the age of the earth as well as the isotopic composition of terrestrial primordial lead. Little change has been made in the estimate since Patterson s first work nevertheless, detailed studies on terrestrial materials do indicate that the earth may be as much as 200-m.y.-older (Tilton and Steiger, 1965, 1969) however, if the earth is even that much older, either there must have also been differentiation of U relative to lead in the early history (Patterson and Tatsumoto, 1964) or the values of primordial lead for the earth and for meteorites must differ (Doe et al, 1965). 

Fig. 10.1. Rb-Sr isochrone measured from separated components of the stony meteorite Guarena. The initial 87Sr/86Sr ratio is slightly higher than that inferred in basaltic achondrites (BABI) because of a period of metamorphism. After Wasserburg, Papanastassiou and Sanz (1969), with permission. Courtesy G.J. Wasserburg.

Equation 11.118 finds practical application in cosmological studies and in geology (dating of sulfide deposits and sediments). Figure 11.27A shows, for instance, the Re-Os isochron for iron meteorites and the metallic phase of chondrites, obtained by Luck and Allegre (1983). The fact that all samples fit the same isochron within analytical uncertainty has three important cosmological implications ... 

Because the present-day composition of the earth s mantle falls on the same isochron, the earth and the parent bodies of meteorites must have formed at about the same time from the same primordial source. 

Figure 11.27 Re-Os isochron for iron meteorites and metallic phase of chondrites and earth s mantle. Reprinted with permission from J. M. Luck and C. J. Allegre, Nature, 302, 130-132, copyright 1983 Macmillan Magazines Limited.

The Pb-Pb isochron was made famous by the determination of the age of the Earth Patterson (1956) grouped meteorite samples with a sediment sample that is supposed to represent the bulk silicate Earth in terms of Pb isotopes (Figure 5-8). The assumption is that the Earth formed at roughly the same time as the meteorites. The colinearity of the data in Figure 5-8 is viewed as verification of the assumption. The age given by Patterson (1956) is 4.55 Ga. 

Figure 5-8 A Pb-Pb isochron that determined the age of the Earth to be about 4.55 Ga. Stony and iron meteorites as well as a sediment of the Earth are plotted on a Pb-Pb isochron. The sediment, as a "bulk sample of the silicate Earth in terms of Pb isotopes, plots on the same line as the meteorites, suggesting that the Earth and meteorites formed at the same time and are the same age. Erom Patterson (1956). Later studies reveal a more detailed evolution history of the Earth, including core formation (about 4.53 Ga), atmospheric formation (about 4.45 Ga), and crustal evolution.

Another coupled system is the Sm-Nd system, with two Sm isotopes ( Sm and Sm) undergoing ot-decay to become two Nd isotopes ( Nd and Nd). The half-life of Sm is 106 billion years and that of is 103 million years. In principle, the concepts for the U-Pb system (such as concordia and discordia, Nd-Nd isochron) can also be applied to the Sm-Nd system. However, the Sm-Nd coupled system has not found many applications. One reason is that the half-life of " Sm is so short that it is an extinct nuclide. Secondly, the half-lives of Sm and " Sm are very different, by a factor of 1000 (in contrast, the half-lives of and 235 differ only by a factor of 6.3). Hence, the coupled system has found only limited applications to very old rocks, such as meteorites and very old terrestrial rocks. 

If an internal isochron cannot be generated, a model age can be determined from the measured 207pb /206Pb of the sample and the assumed initial lead isotopic ratios. For studies of the early solar system, this initial lead composition is assumed to be that measured in troilite (FeS) from the Canyon Diablo meteorite. Troilite is a uranium-free mineral and its host meteorite formed very early in the history of the solar system. Because the U/Pb ratio of the solar system is low, the lead incorporated into the troilite should not have evolved significantly from the initial composition in the solar system. 

The Pb-Pb isochron used by Patterson (1956) to determine the age of the Earth. The isochron was constructed from troilite (FeS) from two iron meteorites and three bulk chondrites. Because troilite contains essentially no uranium, the lead in troilite is almost unchanged from the time the meteorite formed - it is "primordial" lead. Modern terrestrial sediments fall on the same isochron, indicating that the Earth and the meteorites are of essentially the same age. The slope of the isochron gives an age of T = 4.55 0.07 Ga using the decay constants used by Patterson (1956). 

Equation (8.47), with t = 0 and the composition of lead from meteoritic troilite used for the initial isotopic ratio of lead, was used by Clair Patterson (1955,1956) to determine the age of the Earth. In the 1950s, the largest uncertainty in determining the age of the Earth was the composition of primordial lead. In 1953, Patterson solved this problem by using state-of-the-art analytical techniques to measure the composition of lead from troilite (FeS) in iron meteorites. Troilite has an extremely low U/Pb ratio because uranium was separated from the lead in troilite at near the time of solar-system formation. Patterson (1955) then measured the composition of lead from stony meteorites. In 1956, he demonstrated that the data from stony meteorites, iron meteorites, and terrestrial oceanic sediments all fell on the same isochron (Fig. 8.20). He interpreted the isochron age (4.55+0.07 Ga) as the age of the Earth and of the meteorites. The value for the age of the Earth has remained essentially unchanged since Patterson s determination, although the age of the solar system has been pushed back by —20 Myr. 

The Re- Os method was first applied to extraterrestrial samples in the early 1960s when Hirt et al. (1963) reported a whole-rock isochron for 14 iron meteorites that gave an age of 4 Ga. Further development of this system was hindered by several technical difficulties. Rhenium and osmium each exist in multiple oxidation states and can form a variety of chemical species, so complete digestion of the samples, which is required to chemically separate rhenium and osmium for mass spectrometry, is difficult. In addition, accurate determination of rhenium abundance and osmium isotopic composition requires spiking the samples with isotopically labeled rhenium and osmium, and equilibration of spikes and samples is challenging. A third problem is that osmium and, particularly, rhenium are very difficult to ionize as positive ions for mass spectrometry. These problems were only gradually overcome. 

The Re- Os system does not lend itself to the determination of internal isochrons for most meteorites. Chen et al. (1998) produced an internal isochron for the St. Severin (LL6) chondrite using metal separates and whole-rock samples (Fig. 8.21). The slope of the isochron gives a date of 4.60-0.15 Ga (X = 1.666 x 10 11 yr 1), consistent with expectations based on other chronometers, but not precise enough to improve upon other techniques. 

The Pd- Ag system has been applied most effectively to dating iron meteorites and pallasites. The best examples are the internal isochron for the Gibeon IVA iron (Chen and Wasserburg, 1990) and for the Brenham pallasite (Carlson and Hauri, 2001). New measurements of carbonaceous chondrites (Schonbachler et al., 2008) have provided a reasonably... 

Patchett, P. J. and Tatsumoto, M. (1980) Lu-Hf total-rock isochron for eucrite meteorites. Nature, 288, 571-574. 

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