Ion microprobes

Scanning Auger Electron Spectroscopy (SAM) and SIMS (in microprobe or microscope modes). SAM is the most widespread technique, but generally is considered to be of lesser sensitivity than SIMS, at least for spatial resolutions (defined by primary beam diameter d) of approximately 0.1 im. However, with a field emission electron source, SAM can achieve sensitivities tanging from 0.3% at. to 3% at. for Pranging from 1000 A to 300 A, respectively, which is competitive with the best ion microprobes. Even with competitive sensitivity, though, SAM can be very problematic for insulators and electron-sensitive materials. 

Reid MR, Coath CD, Harrison TM, McKeegan KD (1997) Prolonged residence time for the youngest rhyolites associated with Long Valley Caldera, °Th- U ion microprobe dating of young zircons. Earth Planet Sci Lett 150 27-39... 

There are two principal sources of reliable partitioning data for any trace element glassy volcanic rocks and high temperature experiments. For the reasons outlined above, both sources rely on analytical techniques with high spatial resolution. Typically these are microbeam techniques, such as electron-microprobe (EMPA), laser ablation ICP-MS, ion-microprobe secondary ion mass spectrometry (SIMS) or proton-induced X-ray emission (PIXE). 

There are surprisingly few microbeam studies of zircon-melt partitioning in natural systems and none in experimental systems. Recently Thomas et al. (2002) have derived zircon-melt partition coefficients from rehomogenised glass inclusions in zircons from an intrusive tonalite, while Hinton et al. (R. Hinton, S. Marshall and R. Macdonald, written comm.) have used an ion-microprobe to measure zircon-melt partition coefficients from a Kenyan peralkaline rhyolite, with an estimated eruption temperature of 700°C (Scaillet and Macdonald 2001). We have used the lanthanide partition coefficients from these two studies to derive best-fit values for and for the large Vlll-co-ordinated site. In total there are 13 individual sets of partition coefficients. All of these yield broadly consistent values of, in the range 0.968-1.018 A, but very variable, in the range 373-1575 GPa. Because Lu is comparable in size to cannot be well... 

It is possible to measure nearly any type of sample for almost any element with little or no preparation. Only a few mg of sample is required, and the measurements are non-destructive in that the sample is generally undamaged. Measurements take only 1-20 min of beam time. Elemental mapping showing the variations in elemental concentrations can be measured over the surface of a sample using the ion microprobe for an area as large as 5 x 5 mm. 

The Mg isotopic measurements were performed with a modified AEI IM-20 ion microprobe [13,14]. Secondary ions were generated by bombarding the sample with a focussed ion beam to excavate a small volume of the sample. A fraction of the sputtered material is ionized during the sputtering process and is drawn off into the mass spectrometer. A duoplasmatron ion source produces a... 

Figure 2. Schematic drawing of the AEl IM-20 ion microprobe. The magnetic analyzer and ion counting system are linked to an on-line computer for automated...

Since the IM-20 had not previously been used for high precision isotopic measurements and since previous isotopic measurements with other ion microprobes [24,25] were characterized by percent level relative errors, we carried out an extensive series of Mg isotopic analyses of terrestrial samples. Terrestrial standards were especially emphasized prior to the study of meteoritic samples, but were also periodically interspersed with later meteoritic analyses as a check on the performance of the ion probe. Terrestrial standards included Ceylon spinel (MgAl204), Madagascar hibonite (CaA112019), a suite of olivines (Fo 100 to Fo 5) used in... 

A unique feature of the ion microprobe is the potential to measure both elemental concentrations and isotopic ratios in the same spot. This capability was particularly valuable in the present study since the possible correlations between the 26Mg/24Mg and Al/Mg ratios are central to the interpretation of Mg isotopic anomalies. The Al/Mg ratio is calculated from the... 

Fig. 3.32. Diagram of 26Al-26Mg evolution for a Ca-Al inclusion from Allende containing minerals with a wide range of Al/Mg. Data taken by an ion microprobe. After Lee, Papanastassiou and Wasserburg (1977). Courtesy G. J. Wasserburg.

Hart, S.R., Shimizu, N. and Sverjensky, D.A. (1981). Lead isotope zoning in galena an ion microprobe study of a galena crystal from the Buick Mine, Southeast Missouri. Economic Geology 76 1873-1878. 

The relatively small mass differences for most of the elements discussed in this volume requires very high-precision analytical methods, and these are reviewed in Chapter 4 by Albarede and Beard (2004), where it is shown that precisions of 0.05 to 0.2 per mil (%o) are attainable for many isotopic systems. Isotopic analysis may be done using a variety of mass spectrometers, including so-called gas source and solid source mass spectrometers (also referred to as isotope ratio and thermal ionization mass spectrometers, respectively), and, importantly, MC-ICP-MS. Future advancements in instrumentation will include improvement in in situ isotopic analyses using ion microprobes (secondary ion mass spectrometry). Even a small increase in precision is likely to be critical for isotopic analysis of the intermediate- to high-mass elements where, for example, an increase in precision from 0.2 to 0.05%o could result in an increase in signal to noise ratio from 10 to 40. 

Fortier SM, Cole DR, Wesolowski DJ, Riciputi LR, Paterson BA, Valley JW, Horita J (1995) Determination of the magnetite-water equilibrium oxygen isotope fractionation factor at 350°C a comparison of ion microprobe and laser fluorination techniques. Geochim Cosmochim Acta 59 3871-3875 Friedman I, O Neil JR (1977) Compilation of Stable Isotope Fractionation Factors of Geochemical Interest. US Geol Surv Prof Paper 440-KK... 

Other meteorite classes like C2, CO and ordinary chondrites contain much smaller inclusions less than 1 mm (MacPherson et al. 1988) and require ion microprobe techniques to evaluate the isotopic compositions. On the least metamorphosed side. Cl have very few inclusions or oxide grains, but are the carrier of the greatest amounts of stellar nanodiamond and other carbides (Anders and Zirmer 1993). As will be shown for Cr anomalies in carbonaceous chondrites, the survival of the mineral carriers of the anomalies also depends on the metamorphic grade of the meteorites. Nevertheless, isotopic anomalies have also been formd in higher metamorphic grade from other classes, especially in the reduced enstatite chondrites. 

Hidaka H, Ohta Y, Yoneda S, DeLaeter JR (2001) Isotopic search for live Cs in the early solar system and possibility of Cs- Ba chronometer. Earth Planet Sci Lett 193 459-466 Hinton RW, Bischofif A (1984) Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites. Nature 308 169-172... 

Giletti B. J., Sennet M. R, and Yund R. A. (1978). Studies in diffusion. III Oxygen in feldspars—an ion microprobe determination. Geochim. Cosmochim. Acta, 42 45-57. 

Steele I. M., Hutcheon I. D. and Smith J. V. (1980b). Ion microprobe analysis of plagioclase feldspar (Cai xNaxAl2 xSi2+x08)- for major, minor and trace elements. VIII Int. Congr. X-ray Optics Micro analysis, Pendell Pub. Co, Midland, Michigan. 

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