In Situ Analysis of Radiogenic Isotopes with Emphasis on Ion Microprobe Techniques and Applications*
Richard A. Stern, 1999. "In Situ Analysis of Radiogenic Isotopes with Emphasis on Ion Microprobe Techniques and Applications", Application of Radiogenic Isotopes to Ore Deposit Research and Exploration
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For the scientist interested in conducting in situ micrometer-scale radiogenic isotope ratio measurements, the two principal analytical tools available are the ion microprobe and various forms of the laser microprobe. The ion microprobe utilizes the technique of secondary ion mass spectrometry (SIMS), whereby a high energy oxygen or cesium ion (primary) beam excavates and ionizes (secondary) particles of the target mineral, which are then electrostatically extracted and mass analyzed using a double-focusing mass spectrometer. The laser microprobes employ a laser as a sampling device coupled to either a plasma-source or gas-source mass spectrometer depending on the isotopes being considered. For example, laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) utilizes a laser to extract particles of a mineral, which are subsequently transported in argon gas to an ICP chamber for ionization and then mass analysis in a quadrupole- or double-focusing mass spectrometer.
These microprobe techniques allow isotope ratio measurements to be done directly on sectioned raw minerals or other solids, either from prepared mineral separates or in rock samples directly, which distinguishes them from conventional bulk chemical methods. Nevertheless, the reader should be aware that it is possible, using saws, drills, and other tools, to isolate small mineral samples (e.g., <100 μm to hundreds of microns in dimension) from solid matrices mechanically, and subsequently, to conduct bulk chemical radiogenic isotope analyses. Such small sample analytical techniques may be entirely appropriate in cases of low sample complexity or large relative size of the sample domains. However, the sample context is destroyed in the process of extraction, and there exists the possibility of mixing of sample domains. In situ techniques provide unambiguous sampling of complex targets and are substantially faster, typically requiring only several minutes to an hour to acquire data from a raw mineral. The high throughput gives a feeling of real-time problem solving. To varying extents the microprobe techniques more easily allow nonexperts to participate directly in the acquisition of data, although all require considerable expertise and experience with the particular methods and instrumentation. Other operational differences between
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Lead (Pb) isotope compositions of sulfide minerals coupled with rocks associated with an ore deposit provide critical constraints on the source of metals and fluid pathways in a fossil hydrothermal system (Heyl et al., 1966; Stacey et al., 1968; Gulson, 1986; Sanford, 1992). Lead isotope compositions of sulfide minerals also provide chronologic information, either absolute or relative, for ore deposition (for example, Carr et al., 1995) and can also be used as an exploration tool during prospect evaluation (Gulson, 1986; Young, 1995). These varied applications of Pb isotopes to achieve an understanding of the ore genesis process are too diverse to be adequately discussed in a single overview chapter. Instead, this chapter focuses attention on what Pb isotopes tell us about (1) the sources of Pb nd other metals in ore deposits, (2) the interaction between hydrothermal fluids and wall rocks, (3) the influence of basement rocks and tectonic setting on Pb sources in ore deposits in magmatic arcs, and (4) the application f crustal-scale Pb isotope variations to an understanding of regional controls on ore deposition Before Pb isotopes pertinent to understanding ore genesis can be examined, we must review some basic principles of Pb isotope geochemistry (Fig. 1). Elegant discussions of U-Th-Pb geochemistry are presented by Doe (1970), Faure (1977), Zartman and Haines (1988), Garipy and Dupr (1991), and Dickin (1995). The following discussion is simplified from these sources. Three isotopes, 208Pb, 207Pb, and 206Pb, are partly the radiogenic daughter products from the radioactive decay of one isotope of thorium (232Th 208Pb*) and two isotopes of uranium (238U 206Pb* and 235U 207Pb*). (Note that an asterisk (*) after an isotope denotes that it is the product of radioactive decay of a parent isotope over time and is not the total abundance of the isotope in a sample.) The abundance of radiogenic isotopes has grown since the earth formed some 4.56 billion years ago (Fig. 1), building upon an initial concentration. The fourth isotope of Pb, 204Pb, is stable and has no long-lived parent isotope nordoes it decay to another isotope. Time-integrated growth of radiogenic Pb isotopes from an arbitrary starting time, t1, to an ending time, t1, in an environment where there has been no migration of U, Th, and their daughter products, is described by standard decay equations: These equations simply show that the measured present-day Pb isotope composition is equal to the sum of the initial