Terrestrial samples elements

Extra-terrestrial samples Excess xenon in meteorites 304 Extinct superheavy elements... 

The molar mass of most elements is that of a normal terrestrial sample, containing a mixture of isotopes. 

Cl meteorites contain about one-third water and other volatile elements. For these reasons, we use volatile-free lanthanide abundances for normalisation. Such values are similar to those in H and L ordinary chondrites, used by many workers previously. Accordingly, the use of volatile-free Cl lanthanide abundances for normalisation does not introduce a substantial variation from previous practices, but has a much better cosmochemical justification. The values selected are given in table 4. Since there is no apparent relative fractionation of the lanthanides during planetary accretion processes, these abundances are generally taken to be simply related to bulk earth compositions. Accordingly, their use as normalising abundances for terrestrial samples provides a measure of the degree of fractionation from the primitive terrestrial abundance patterns. 

The concentrations of both elements have been determined in iron meteorites and these plus the metallic grains of chondrules were found to fit the same isochron, implying that they formed from the solar nebula (thought to have contained isotopically homogeneous osmium) in a very short time interval estimated to be about 90 Ma. The primordial [ Os/ Os]p ratio has been given as 0.807 0.006. And the isotopic compositions of both rhenium and osmium in meteorites are compatible with terrestrial sample values. 

Planets and other celestial bodies have been used to name some elements. For example, palladium, which was discovered in 1803, is named after Pallas, or the second asteroid that was itself discovered just one year earlier in 1802. Hehum is named after helios, the Greek name for the sun. It was first observed in the spectrum of the sun in 1868, and it was not until 1895 that it was first identified in terrestrial samples. 

Only now did it become clear why such elements as tellurium and iodine, and other pair reversals, had caused so much trouble for the pioneers of the periodic system. Tellurium has a lower atomic number than iodine and so should genuinely be placed before iodine, as Mendeleev and others had guessed. In addition, it was now clear that the higher atomic weight of tellurium was due to a higher average mass of the various isotopes that made up a terrestrial sample of this element. 

Element for which known variation in isotopic abundance in terrestrial samples limits the precision of the atomic weight given. 

The extraterrestrial materials that are analyzed by cosmochemists commonly feature isotope anomalies for a much broader range of elements than terrestrial samples and this variability is generally ascribed to five principle sources, which are introduced below (see also Chapter 1). 

Confidence in elemental assignment can be assured by comparison of the intensities of signals that can be ascribed to other isotopes of the element of interest, i.e. these intensity ratios should match the natural abundances isotopes listed in Appendix A.2, unless, of course, isotopic enrichment of one form or another has occurred (as is the case in nuclear reactions, extra-terrestrial samples, and so on) or has been applied (isotopic implantation). In the case of mono-isotopic elements such as Fluorine and Aluminum, molecular patterns, or even multiply charged ions, can be used to confirm assigmnents. 

The abundance of an isotope in normal terrestrial samples of an element, listed in atom percent. 

Isotopic abundances are said to be anomalous if the measured isotopic ratios cannot be related to the terrestrial (and by inference solar) isotopic composition of elements through a mass fractionation relationship resulting from a natural physiochemical process, and/or the mass spectrometer measurement itself. The only variations in the isotope abundances in terrestrial materials occur by radioactive decay and by the physiochemical processes mentioned above. Thus terrestrial samples serve as a good example of homogenized Solar System material against which anomalous isotopic material can be compared. 

Gr. technetos, artificial) Element 43 was predicted on the basis of the periodic table, and was erroneously reported as having been discovered in 1925, at which time it was named masurium. The element was actually discovered by Perrier and Segre in Italy in 1937. It was found in a sample of molybdenum, which was bombarded by deuterons in the Berkeley cyclotron, and which E. Eawrence sent to these investigators. Technetium was the first element to be produced artificially. Since its discovery, searches for the element in terrestrial material have been made. Finally in 1962, technetium-99 was isolated and identified in African pitchblende (a uranium rich ore) in extremely minute quantities as a spontaneous fission product of uranium-238 by B.T. Kenna and P.K. Kuroda. If it does exist, the concentration must be very small. Technetium has been found in the spectrum of S-, M-, and N-type stars, and its presence in stellar matter is leading to new theories of the production of heavy elements in the stars. 

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

D. W. Allen, C.C. Britt, D.T. (2000) Mineralogy, composition, and alteration of Mars Pathfinder rocks and sods Evidence from multispectral, elemental, and magnetic data on terrestrial analogue, SNC meteorite, and Pathfinder samples. J. Geophys. Res. 105 1757-1817... 

Why can one assume that the isotopic compositions of elements on the Earth are representative of the bulk solar system This was not obvious in the beginning. Isotope-ratio measurements of meteorites made in the late 1950s appeared to show significant variations. However, within a few years, it was shown that those variations were due almost entirely to problems with the measurements, not to real variations in the samples. By the early 1960s enough measurements had been made to convince most scientists that in the material to which they had access, terrestrial rocks and meteorites, each element always had the same isotopic composition (except for small fractionations of stable isotopes and shifts... 

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. 

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