Primordial terrestrial

To resolve the primordial terrestrial noble gas, it would be useful to examine major noble gas reservoirs in the early solar system, which could have supplied noble gases to the Earth. As we discussed in Chapter 3, two major noble gas components occur very widely in the solar system and can be a potential source for the terrestrial noble gas. They are solar noble gas (representative of the sun), which is generally assumed to be best represented by solar wind noble gas implanted on Al-foil target plates on the moon (elemental ratio) and on lunar breccia (isotopic ratio) (e.g., Ozima et al., 1998), and Q phase noble gas (see Wieler, 1994, for a review), which occurs very widely in various chondrites. Next we will compare the bulk Earth noble gas, which we assume to be represented by atmospheric noble gas with these two major noble gas components in the solar system. 

In summary, we may reasonably conclude that except for Ne, and apart from radio-and nucleogenic components, the isotopic composition of atmospheric noble gases well represent the bulk Earth and can be assumed to approximate a primordial terrestrial noble gas component. Thus defined, primordial terrestrial noble gases are... 

Sekiya, M., Nakazawa, K., Hayashi, C. (1980) Dissipation of the Primordial terrestrial atmosphere due to irradiation of the solar EUV Progr. Theoret. Phys., 64, 1968-85. 

There were times on our planet when the barren dryness of uninhabited continents sharply contrasted with the densely populated sea. The continental lithosphere was then essentially represented by rock surfaces of different types. Sedimentary rocks were rare, if not absent. As rock materials became exposed to the subaerial environment at the Earth s surface, they encountered a whole range of environmental challenges such as temperature fluctuations, water, unbuffered cosmic and solar irradiation and atmospheric gases and solids instead of dissolved species. These influences resulted in rocks undergoing alterations in material properties leading to erosion and breakdown into ever-smaller particles and constituent minerals, formation of sandy sediments, and mineral soils (Ehrlich, 1996). Primordial terrestrial environments can therefore be visualized as a freshly exposed and only slightly physically pre-weathered rock surface. 

However, the contribution of fissiogenic heavy isotopes are more difficult to calculate fractionated chondritic and solar Xe have greater proportions of " Xe and Xe than is actually seen in the atmosphere and so cannot serve as the primordial terrestrial composition (see Pepin 2000). While no other suitable common solar system compositions have been found that provide the nonradiogenic heavy isotope composition, multi-dimensional isotopic correlations of chondrite data have been used to define a composition, U-Xe, that when mass-fractionated yields the light-isotope ratios of terrestrial Xe and differs from atmospheric Xe by a heavy isotope component that has the composition of " " Pu-derived fission Xe (Pepin 2000). The fractionated U-Xe ratios of Xe/ °Xe = 6.053 and Xe/ °Xe = 2.075 are the present best estimates of the isotopic composition of nonradiogenic terrestrial Xe (see further discussion in Pepin and Porcelli... 

Plutonium occurs in natural ores in such small amounts that separation is impractical. The atomic ratio of plutonium to uranium in uranium ores is less than 1 10 however, traces of primordial plutonium-244 have been isolated from the mineral bastnasite (16). One sample contained 1 x 10 g/g ore, corresponding to a plutonium-244 [14119-34-7] Pu, terrestrial abundance of 7 x 10 to 2.8 x 10 g/g of mineral and to <10g of primordial Pu on earth. The content of plutonium-239 [15117 8-3], Pu, in uranium minerals is given in Table 2. 

Helium is the second most abundant element in the universe (76% H, 23% He) as a result of its synthesis from hydrogen (p. 9) but, being too light to be retained by the earth s gravitational field, all primordial helium has been lost and terrestrial helium, like argon, is the result of radioactive decay ( He from a-decay of heavier elements, " °Ar from electron capture by... 

Extraterrestrial origins of life Terrestrial origins of life Life was delivered to the Earth (or any planet) by meteorites of cometary material, leading to the idea of panspermia The molecules of life were built on Earth, perhaps in the primordial soup or little warm pool... 

An interesting possibility raised by this experiment is that of spontaneous resolution in chiral monolayers. If this were a reasonably common phenomenon, it would give yet another possible answer to the perennial question of the chiral environment for the primordial stereospecific condensation reaction that produced the first chiral biopolymers. As our knowledge of chiral monolayers develops, we should have a better perspective on the likelihood of a racemic film spontaneously unmixing to produce patches of enantiomeric film at lower surface energy. The relevance of such a result to the origin of terrestrial life problem will have to remain eternally speculative and untestable. 

Extraterrestrial materials consist of samples from the Moon, Mars, and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on the Earth or on the Moon, because they have become obliterated during high-temperature processes over geologic time. In primitive meteorites, however, components that acquired their isotopic compositions through interaction with constituents of the solar nebula have remained unchanged since that time. 

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. 

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