Rocks and Meteorites

It is estimated that the earth s age is in the neighborhood of 4 to 7 billion years. These estimates are basically derived from carbon-14, potassium-40, uranium-235, and uranium-238 dating of earth rocks and meteorites. The meteorites give important data as to the age of our solar system. Geologic time is felt to be represented by the presence of rock intervals in the geologic column (layers of rock formations in vertical depth) or by the absence of equivalent rocks in correlative columns in adjacent locations [25,26]. The two basic factors that are used to determine geologic time are ... 

Titanium occurs in nature in the minerals rutile( Ti02), ilmenite (FeTiOs), geikielite, (MgTiOs) perovskite (CaTiOs) and titanite or sphene (CaTiSi04(0,0H,F)). It also is found in many iron ores. Abundance of titanium in the earth s crust is 0.565%. Titanium has been detected in moon rocks and meteorites. Titanium oxide has been detected in the spectra of M-type stars and interstellar space. 

A popular method used to date rocks is the potassium-argon method. Potassium is abundant in rocks such as feldspars, hornblendes, and micas. The K-Ar method has been used to date the Earth and its geologic formations. It has also been applied to determine magnetic reversals that have taken place throughout the Earth s history. Another method used in geologic dating is the rubidium-strontium, Rb-Sr, method. Some of the oldest rocks on Earth have been dated with this method, providing evidence that the Earth is approximately 5 billion years old. The method has also been used to date moon rocks and meteorites. 

The distributions of trace elements between minerals and within a suite of related rocks provide powerful tools for constraining the origin and history of rocks and meteorites. Trace-element abundances for rocks typically are part of the data set collected when determining bulk compositions. Trace element compositions of minerals require more powerful techniques such as the ion microprobe or the laser-ablation inductively coupled plasma mass spectrometer (ICPMS). 

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... 

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. 

Plot of nickel versus magnesium in Martian rocks and meteorites. GRS-estimated Mg content is also shown. After McSween et al. (2009). 

Information on the current arsenic content of the solar system is largely limited to spectrographic analyses of the Sun, Saturn, and Jupiter measurements on available Moon rocks and meteorites and analyses of terrestrial materials. Spectrographic analyses indicate that the arsenic concentration of the Sun is about 0.004 mg kg-1 (Matschullat, 2000, 299 Table 3.1). Arsenic is moderately volatile in the vacuum of space (McDonough, 2004, 555) and should be preferentially concentrated on Jupiter and other planets... 

Recently, Takeda et al. (2007) and Minowa et al. (2007) introduced a complex RNAA procedure for the determination of trace siderophil elements and REEs in rock and meteorite samples. After irradiation, samples were subjected to alkaline oxidative fusion. Osmium and Ru were removed sequentially by evaporation and the remaining siderophil elements were retained on the chelating resin SRAFION NMRR REEs, Au, and Pt, and finally. Re and Ir were sequentially stripped from the column with 0.05 mol/1 HCl, 5% thiourea, and 7 mol/1 NH4OH, respectively. Although chemical yields determined by the reactivation method widely varied, the method seems to be promising for the sensitive determination of several PGEs (Ru, Os), Re, and lanthanides. 

Granite and to a lesser extent granodiorite are coarse grained plutonic igneous rocks (see Rocks and Meteorites for a precise petrological definition) primarily composed of silicate minerals such as quartz and feldspars often with small amount of accessory minerals such as... 

For the age of rocks and meteorites, othCT similar methods of dating have been nsed. One method depends on the radioactivity of naturally occurring potassium-40, which decays by positron emission and electron capture (as well as by beta emission). 

Variations in isotopic compositions that are generated by isotope fractionation associated with chemical, physical, or (on Earth) biological processes are generally of mass-dependent nature. This implies that the magnitude of an isotope effect is proportional to the mass difference of the respective isotopes. Such mass-dependent isotope effects are hence generally most prevalent for lighter elements, which feature the largest relative differences in isotopic masses, and classic stable isotope studies were therefore focused on the elements H, C, N, O, and S. However, more recent studies, often conducted by MC-ICP-MS, have shown that natural isotope fractionation is also common for many heavier elements in both terrestrial rocks and meteorites [26, 27]. 

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