Meteorites isotope production

Studies Based on Isotope Production in Meteorites and on the Lunar Surface... 

Figure 7.4 shows the reduction in sulfates and the corresponding growth of both the parent carbonates and the offspring methane with subbottom depth. The methane production is parallel but lower in isotope production than the carbonates. In Figure 7.4 the sulfur isotope (< 34S) content is defined in an identical manner to Equation 7.2 with the replacement of the fraction 13C/12C by 34S/32S in both the numerator and the denominator, using Canon Diablo meteoritic troilite as a standard. The < 34S value increases from 20-60%c before substantial biogenic methane is produced. 

A chemically based, mass-independent fractionation process was first observed during ozone formation through the gas-phase recombination reaction (Thiemens Heidenreich 1983) O + O2 + M - O3 + M. The product ozone possesses equally enriched heavy-oxygen isotopes I7 IS0. by approximately lOO /oo with respect to the initial oxygen, with a slope value of unity in a three-isotope oxygen plot. This discovery led to the conclusion that a symmetry-dependent reaction can produce meteoritic isotopic anomalies (Thiemens 1999, 2006). Recently, theoretical calculations of Gao Marcus (2001) established the major role of symmetry in isotopolog-specific stabilization of vibrationally excited ozone molecules that give rise to the mass-independent compositions. 

A detailed interpretation of the meteoritic isotopic anomalies involving p-nuclides still eludes us to a large extent. This concerns in particular the Xe-HL case. It is also concluded that no reliable p-process chronometry can be built yet on the p-radionuclides 92Nbg or 146Sm in view of the uncertainties still affecting their production in each of the considered p-process sites and of the possible variations of their yields from one site to the other. 

Bhandari N, Bonino G, Cirri Castagnoli G (1994) The 11-year solar cycle variation of cosmogenic isotope production rates in chondrites. Meteoritics 29 443-444... 

The Zag meteorite fell in the western Sahara of Morocco in August 1998. This meteorite was unusual in that it contained small crystals of halite (table salt), which experts believe formed by the evaporation of brine (salt water). It is one of the few indications that liquid water, which is essential for the development of life, may have existed in the early solar system. The halite crystals in the meteorite had a remarkably high abundance of 128Xe, a decay product of a short-lived iodine isotope that has long been absent from the solar system. Scientists believe that the iodine existed when the halite crystals formed. The xenon formed when this iodine decayed. For this reason, the Zag meteorite is believed to be one of the oldest artifacts in the solar system. In this lab, you will use potassium-argon radiochemical dating to estimate the age of the Zag meteorite and the solar system. 

Whereas isotopes produced in the solid matter of meteorites or the lunar surface material remain at their production sites, isotopes produced mainly in the earth s atmosphere are separated according to the geochemical properties of the different elements. 

The penetration depth of cosmic radiation is of the order of 1 m and therefore isotopes are produced by spallation only in the surface layers of meteorites and the moon. After collisions of meteorites with each other or with the moon, newly formed surfaces get exposed to cosmic radiation and production of stable and radioactive isotopes starts. If P is the production rate of a... 

In the 81Kr-83Kr dating method, which is discussed in the next paragraph, the production ratio of radioactive and stable Kr isotopes can be evaluated directly from the Kr spallation spectrum in a meteorite according to the relation [1] ... 

A fundamental assumption made for most dating with atmospheric radionuclides is that the cosmic radiation flux and hence, the natural production of the radionuclides has been constant with time. Various studies of this problem using 14C and tree-ring calibration have been made. Isotopic studies of meteorites have also been useful [17]. Considering the probable lack of basic accuracy of dating water, the problem of changes in cosmic ray flux is not serious. 

The assignment of the earth/moon system to one precursor nebular reservoir, and meteorites to a second, while still logically possible, was shown experimentally to be unnecessary by Thiemens and coworkers (reading list). In the early 1980s these workers studied isotope fractionation during the synthesis of ozone from molecular oxygen in an electric discharge operated at low pressure. The product ozone was... 

Leya I, Wider R, Halliday AN (2000) Cosmic-ray production of tungsten isotopes in lunar samples and meteorites and its implications for Hf-W cosmochemistry. Earth Planet Sci Lett 175 1-12 Loss RD, Lugmair GW (1990) Zinc isotope anomalies in Allende meteorite inclusions. Astrophys J 360 L59-L62... 

Laser ablation combined with LA-MC-ICPMS provides a new dimension to the analysis of Mg isotopes in calcium aluminum-rich inclusions from primitive meteorites. Dispersion in 26Mg - Al/ Mg evolution lines can be correlated with mass-dependent variahons in 5 Mg that distinguish open-system from closed-system processes. The ultimate product of such studies will be a better understanding of the chronological significance of variations in Mg in these objects. 

The half-life of 244Pu (8.2 X 107 years) is short compared with the age of the earth (4.5 X 109 years), and hence this nuclide is now extinct. However, the time interval (a) between the element synthesis in stars and formation of the solar system may have been comparable with the half-life of 244Pu. It has been found recently in this laboratory that various meteorites contain excess amounts of heavy xenon isotopes, which appear to be the spontaneous fission decay products of 244Pu. The value of H calculated from the experimental data range between 1 to 3 X 108 years. The process of formation of the solar system from the debris of supernova is somewhat analogous to the formation of fallout particles from a nuclear explosion. 

Lead is of interest us being the terminal product of radioactive decay. Thus, while ordinary lead has the atomic wcighi 207.11 (being composed of I. 47 r oJPb. 26.205, - " Ph. 2(1.H > " Pb and 51.55. . - " Phi. the isotopic composition, and hence the atomic weight, varies somewhat it) lead from meteorites, from deep-sealed rocks and from uranium ores (the last being sonn-vvhat less dense, as would be expected from the fact that ""Ph is the end produei ol the uranium series I. These variations in isotopic composition of lead pcnnii of calculations. it the age ol ihe eanh laud (he metcoritesi. 

Ag2S was mixed with cuprous oxide and converted to SO2 by heating at 900 C (2). BaSC>4 was directly converted to SO2 by thermal decomposition in quartz at 1600 C (3). Product SO2 was freed from water and COo> before isotopic measurement on a Micromass 602D mass spectrometer. S/ S ratios are reported to a precision of 0.2%o relative to troilite from the Canyon Diablo meteorite using the normal 8 S%o notation. 

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