Studies of calcium isotope fractionation found in nature and produced during ion sputtering

• Main Author: Russell, William A.
• Format: Others
• Language: en
• Published: 1979
• Online Access: https://thesis.library.caltech.edu/916/1/Russell_wa_1979.pdf; Russell, William A. (1979) Studies of calcium isotope fractionation found in nature and produced during ion sputtering. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/nxss-sh80. https://resolver.caltech.edu/CaltechETD:etd-03102008-090242 <https://resolver.caltech.edu/CaltechETD:etd-03102008-090242>

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. High precision analytical techniques have been developed for the measurement of calcium isotopic abundances in samples as small as 2x[...] mole. Typical relative uncertainties for measurement of calcium isotope abundance ratios are 1- 4x 10[...], and isotopic differences between samples arising from mass fractionation can be resolved to 1 x 10[...] per unit mass difference. Variation of Ca isotope compositions in the solar system has been studied through analysis of meteorites, lunar samples, and terrestrial samples. Terrestrial, lunar, and meteoritic Ca isotopic fractionation ranges overlap and are about 2.5[...] for [...]. The small range of effects does not permit at present identification of the mechanisms leading to the isotopic fractionation. Measurements were carried out for Ca leached with water from a lunar soil. Large 0, Si, and S isotopic fractionation of components residing near the surfaces of the grains of this soil have been found previously. The Ca was found to be definitely mass fractionated, but only to a small extent. Either only small Ca fractionation is present, or bulk, unfractionated material was also removed and diluted the effects. Sputtering by the solar wind is one mechanism which might have led to the thin, isotopically heavy coatings on lunar soil grains. This might occur either directly by the sputtering process or by escape from the lunar gravitational field of different fractions of the sputtered species. Experiments reported here show that substantial isotopic fractionation arises directly from sputtering. Terrestrial samples were sputtered with relatively low current 130 keV [...] or 100 keV [...] beams. Sputtered Ca was collected over a wide range of angles and later analyzed. It has been found that at 130 keV, the first Ca removed is isotopically light, ranging from [...] = +11 to +13 for the fluorite and apatite samples studied. Further sputtering causes the fractionation of the material being collected to decrease, achieving unfractionated and, for one sample, negative cS values. The initial fractionation behavior using 100 keV [...] has not been thoroughly investigated, although initial sputtering of a plagioclase sample yielded [...] = + 21. Eventual achievement of unfractionated values is viewed as an attainment of equilibrium in which the surface of the target has become isotopically heavy by an amount sufficient to counter the positive fractionation produced by the sputtering. Two attempts have been made to measure the approximate thickness of this [...]< 0 surface layer by changing to a 100 keV [...] beam after extensive sputtering at 130 keV. For a single crystal of CaF[...], the thickness was measured to be at least 1000 A. A polycrystalline CaF[...] sample showed no evidence of such a thick layer. This may be caused by minor differences in target temperatures or by atomic mixing in the targets which proceeded to greater depths in the single crystal. SEM photographs, though, show no conclusive evidence of substantial heating by the beam of large areas of the targets. The data provide various indications that atomic mixing is important in controlling the effects. Mixing appears to be slow, and may not efficiently bring unfractionated material from the end of the ion beam range up into the very near surface region. Measurements show that there are large differences in the fractionation of Ca sputtered in different directions. These variations with ejection angle also change with increasing ion bombardment. The initial material sputtered had [...] > 0 at all angles, with a maximum at intermediate angles. After further sputtering at the same energy, the normally ejected material had [...] > 0, while the obliquely sputtered material had [...] < 0. In this case and in the case of Ca subsequently sputtered with 100 keV N[...], the total range of effects was 16[...] for [...]. It has been proposed that simple kinematic differences in the scattering of light and heavy isotopes off of each other at the target surface could account for similar effects which are seen with very low energy sputtering. Such differences could probably not have been important at the high beam energies used here; another mechanism must be responsible. These sputtering experiments roughly simulate the solar wind conditions in terms of the beam energy per mass unit and in terms of the sputtering yields. The large effects indicate the likelihood that fractionation intrinsic to sputtering was a major mechanism contributing to the lunar soil isotopic fractionation.