Abstract Studies of submarine hydrothermal vents over the past 20 years have confirmed that extensive sulfide chimney fields and mounds are created along active rift zones in open-ocean and back-arc basin settings (see Fornari and Embley, 1995; Hannington et al., 1995). These modern deposits represent the surficial expressions and upflow zones of hydrothermal systems thermally driven by intrusion of magma into the oceanic crust. Comparisons have been made with ancient volcanogenic massive sulfide (VMS) orebodies, some associated with ophiolites, but there are few places on the modern sea floor where the “stockwork” zone has been exposed or studied in detail. In addition, ancient deposits and their associated volcanics are commonly metamorphosed and deformed, so that original structural, mineralogic, and geochemical characteristics have been severely modified or obliterated. Here, we present a review, based on detailed sea-floor observations and sampling, of the geology and petrochemistry of the eastern Galapagos spreading center (EGSC) at ∼86° W in the Pacific Ocean where an extinct hydrothermal system (Galapagos fossil hydrothermal field), including sulfide mounds, chimneys, and an underlying stockwork zone, are intimately associated with fresh and altered lavas. Data for major elements, lithophile trace elements, Sr, Pb, and O isotopes are presented to demonstrate the cogenetic nature of the diverse assemblage of oceanic lavas. Using this framework, the behavior of the trace base metals and sulfur during magmatic evolution is evaluated. We also provide detailed field, mineralogic and chemical descriptions of two areas of massive sulfide mineralization within the Galapagos fossil hydrothermal field (GFHF). The physical and chemical relations between magmatism and hydrothermal mineralization indicate that there is a synergy between these geologic processes, which may be of general application.

Abstract There would be few geological studies in which, at some stage, there did not arise a question of timing. The answer is often to be found through direct observation; the principles of superposition and crosscutting relationships apply in determining the order of events on all scales from the microscopic to the macroscopic, from crystallization history to continental assembly. By augmenting those principles with the means to establish sequence and correlation provided by palaeontology, the geologist has the capability, through observation and logical reasoning alone, to determine the relative ages of a great range of geological processes. However, while these techniques make it possible to place geological events in time order, they do not provide an absolute measure of time itself. The measurement of absolute time in geology— geochronology—requires a quantifiable physical process that takes place continuously at a known rate from the time of the event to be dated to the present day. Some cyclic processes, such as the passage of the seasons, leave their imprint in parts of the geological record and can provide detailed, accurate measurements of elapsed time intervals, but they do not permit the measurement of absolute time (age) unless the record is unbroken to the present day or the age of one of the cycles is known by some independent means. The number of annual growth bands in a fossil coral, for example, tells how long that coral once lived, but not when.

Abstract Multiple-collector inductively coupled plasma source mass spectrometry (MC-ICPMS) is a new technique of considerable relevance to research in the Earth sciences. The very high ionization efficiency attained in ICP sources is combined with the precision of magnetic sector multiple collector isotopic ratio mass spectrometry to permit very accurate isotopic measurements of elements that are normally difficult to ionize by thermal (surface) means. The addition of a laser facilitates studies for which spatial resolution is required. The precision achieved for isotopic ratios (typically ± 0.001-0.01 %) sets this instrument apart from all other forms of ICPMS. With laser ablation, the sample utilization is much faster than in dynamic SIMS (ion probes), but the greatly enhanced ion beam signals and simultaneous measurement of all masses of interest on multiple stable and linear Faraday detectors result in the most precise isotopic ratios for trace elements yet measured in situ. The multifarious applications of this new technique include in situ Sr and Pb isotopic measurements and Lu-Hf and In-Sn geochronology. Although in its infancy, the techniques can clearly be applied to a variety of problems in low temperature geochemistry, including the dating of ancient mineral deposits, the scales of circulation of crustal fluids and the origins of their dissolved constituents.

Abstract Although K-Ar geochronology is one of the most time- honored methods of measuring the age of geologic materials, its practical application is limited by a fundamental difference in the physical states of potassium (a solid) and argon (a gas). This difference means that K-Ar dating of a sample requires splitting it into two fractions, one destined for K analysis (usually by flame photometry) and the other for Ar analysis (by isotope dilution and rare gas mass spectrometry). This procedure is simple enough for large, homogeneous samples, but it effectively precludes any form of K-Ar microanalysis. In the mid-1960s, the recognition that bombardment of K-bearing minerals and glasses with fast neutrons in a nuclear reactor could convert naturally occurring 39 K to 39 Ar led to the development of a derivative of the K-Ar method: 40 Ar/ 3 9Ar geochronology (Merrihue and Turner, 1966). It became possible to determine 40 Ar/ 39 Ar ages simply by measuring Ar isotope ratios on a rare gas mass spectrometer, dramatically extending the range of geologic problems that could be addressed through the K → Ar decay scheme. Most importantly, 40 Ar/ 39 Ar geochronology does not require knowledge of the mass of the analyzed material, permitting the use of microanalytical procedures that extract Ar from samples so small that measurement of their weight is impractical. One such procedure involves the use of a laser beam to melt, ablate, or gently

Abstract Oxygen, carbon, and hydrogen are the most abundant elements in mineralizing fluids, and their isotope ratios provide a powerful tool for deciphering the complex histories of mineralizing systems (see Ohmoto, 1986; Kerrich, 1987; Taylor, 1987; Hebert and Ho, 1990). In particular, stable isotope ratios can be used to constrain theories of fluid sources, pathways, and fluxes, mechanisms of mineral reaction and exchange, and thermal evolution (see Valley, 1986; Bickle and McKenzie, 1987; Baumgartner and Rumble, 1988; Nabelek, 1991; Eiler et al., 1993; Skelton et al., 1995). Sulfur isotopes have special importance for the genesis of sulfide ores and are discussed in separate chapters of this volume (McKibben and Riciputi, 1998; Shanks et al., 1998). Microanalysis of boron isotope ratios was recently reviewed by Hervig (1996). This chapter will review studies of the stable isotopes of oxygen, carbon, and hydrogen ( 18 O, 17 O, 16O , 13 C, 12 C, 2 H (D), and 1 H (H), referred to collectively as stable isotopes) with emphasis on recent work that has attained accuracy in the 1 per mil range for δ 18 O or δ 13 C (10‰ for δD) that is necessary for research on terrestrial samples. The vast majority of all stable isotope ratio analyses have been made of 1 to 100 mg samples of whole-rock powder or mineral separate. Commonly, analyses have been made of splits, prepared from much larger samples with the implicit assumption that measured compositions are representative

Abstract Laser analysis of minerals for stable isotope ratios is a rapidly growing and rapidly changing branch of geochemistry. Any attempt at reviewing recent developments, including this one, is likely to be outdated before publication. Let the reader beware! The following review consists of separate discussions of (1) laser fluorination of silicate and oxide minerals for 18 O/ 17 O/ 16 O ratios, and (2) laser analysis of carbonate minerals for 18 O/ 16 O and 13 C/ 12 C ratios. Oxygen isotopes are of intrinsic interest to geologists because oxygen is volumetrically the most abundant element in Earth's crust. Microanalysis of oxygen isotopes to map out heterogeneities in minerals is needed to deduce the nature of mass transfer in the crust. Countless studies show the isotopes to sensitively record water-rock interaction history. Such mechanisms as diffusion, solution/ reprecipitation, mineral reactions, and infiltration all impose on minerals characteristic microscopic signatures in texture, chemistry and isotope ratios. Furthermore, mapping patterns of stable isotope heterogeneity in relation to chemical growth zoning, mineral overgrowths and reaction coronas provides the evidence for correlating the geologic age at which isotopic compositions were acquired.

Abstract Over the last decade, secondary ionization mass spectrometry (SIMS, or the ion microprobe) has been applied to a wide variety of areas in the geosciences. In this technique, a collimated beam of primary ions is accelerated onto the sample, and the secondary ions that result from the sputtering of the sample are extracted for analysis. It offers the advantages of in situ analysis of both elemental concentrations and isotope ratios, with high spatial resolution (typically 10-50 pm) and excellent sensitivity (often ppb level). This technique has been used to study a variety of different light stable isotope systems, including H, B, C, N, and O. However, application of the ion microprobe to sulfur isotope studies has, to this point, been more common than for any of the other stable isotopes, particularly in terrestrial systems. In comparison with other commonly analyzed light stable isotopes (H,C,O), analysis of sulfur isotopes by ion microprobe has several advantages. The abundance of the important minor isotope 34 S is high (∼4.5%) relative to the major isotope 32 S. This allows favorable counting statistics for the minor isotope, resulting in good precision during isotopic analysis. Many sulfide phases are electrically conductive, which eliminates problems associated with surface charging. In many natural low-temperature systems, the variations in δ 34 S values are large, so that meaningful results can be obtained even if precision is limited to 1 to 2 per mil, as is the case in many sulfur isotope studies using ion microprobes. The spatial resolution (typically a crater 15-30 pm wide by 2-5 pm deep)

Abstract Since the first studies of sulfur isotope variations in natural materials (Thode, 1949), it has been apparent that there are large and dramatic variations of 34 S/ 32 S ratios and that sulfur isotope studies are a powerful tool for interpreting the origins of sulfur-bearing minerals. However, sulfur is such a common element in the Earth's crust (sixteenth most abundant, averaging 0.03 wt %; Mason, 1966), and is involved in so many igneous, hydrothermal, biological, and surficial processes that a simple measurement of δ 34 S, without constraining geological, biological, and geochemical data, is often unenlightening. In many sedimentary and hydrothermal systems, geologists are confronted with multiple sulfur sources, large fractionations of sulfur isotopes during oxidation-reduction reactions that sometimes produce disequilibrium effects, and strong chemical and physical gradients at the site of mineral deposition. Despite significant advances in the understanding and utilization of sulfur isotopes to characterize ore-forming processes (Ohmoto, 1972; Ohmoto and Rye, 1979; Shanks et al., 1981; Janecky and Shanks, 1988), interpretations may be ambiguous and, in ancient ore deposits, difficult to test. Part of this difficulty has been due to an inability to resolve fine-scale spatial variations in isotopic fractionation between successive zones or between coexisting minerals. The development of laser and ion microprobes for high spatial-resolution stable isotopic analyses has opened new research frontiers

Abstract SIMS (secondary-ion mass spectrometry) is an analytical technique for the surface, near-surface, and bulk characterization of solids. The technique uses a primary beam of ions to sputter a sample surface, producing a secondary beam of ionized particles (secondary ions) that are passed through a mass spectrometer. Acquired data may be presented as mass spectra, depth profiles showing element concentrations or isotope ratios, and ion images. SIMS has a number of advantages over electron-beam and X-ray analytical techniques. Secondary-ion intensities can be measured over a dynamic range of nine orders of magnitude (vs. two orders of magnitude for AES and XPS). All elements may be detected, and their isotopes distinguished. Detection limits range from ppm to ppb. Depth profiling and ion imaging are possible, with excellent depth and lateral resolution. Because of the low detection limits that may be obtained with SIMS, this technique is invaluable for the quantification of precious metals, which commonly occur in very low concentrations. In addition, the depth- profiling and imaging capabilities of SIMS reveal whether metals are present as submicroscopic inclusions or are dispersed throughout the matrix; this information is important for maximizing the efficiency of mineral processing. Figure 1 shows a schematic of the ion optical system for the Cameca ims 4f ion microprobe. Primary ions are produced in a duoplasmatron or cesium source, then extracted, mass filtered and accelerated by a high voltage (10-30 kV).

Abstract A quarter-century ago, Johansson et al. (1970) demonstrated qualitative multi-element analysis using Si(Li) detectors and a combination of (1) X-ray excitation by accelerator-derived, MeV-energy proton beams and (2) energy-dispersive X-ray spectroscopy. Other Particle beams from accelerators have also been used to Induce X-ray Emission (hence PIXE), and other X-ray detection systems have been used on occasion, but essentially all geologic applications remain based on the original combination. PIXE is now a fully quantitative, analytical technique for trace and major heavy elements (Z ≥ 22), often with parts-per- million limits of detection (LODs, Johansson et al., 1995). Other than as a bulk-sampling technique for fused, powdered, or otherwise-homogenized whole rocks, there are few reasons to use the unfocused proton beam of conventional PIXE. Nearly all PIXE work in Earth science is now done with proton beams that are focused to spot sizes of a few micrometers (hence micro-PIXE) and employed for nondestructive, in situ analysis of individual mineral grains or quantification of fine-scale, trace-element zonation. Micro-PIXE analysis is a close analog of electron-probe microanalysis (EPMA). The two methods share the same physics, i.e., charged-particle penetration of matter, ionization, generation of both characteristic X-rays and brems- strahlung radiation, secondary fluorescence (in modest amounts), and attenuation of X-rays within the specimen (Johansson et al., 1995). However, there are two important differences:

Abstract Accelerator mass spectrometry (AMS) is a variant of secondary-ion mass spectrometry (SIMS). The latter is now a well-established branch of microanalysis that utilizes such well-known ion microprobes as the Cameca ims series and the large-magnet SHRIMP machines. The intention here is to provide a primer for the AMS technique, to aid potential users in (1) interpreting existing results, (2) evaluating the utility of the method versus other techniques, and (3) selecting appropriate samples for analysis. Reprinting of published results is minimized: a compact bibliography is provided (a further list is available in Wilson, 1994a).

Abstract The predictable behavior of trace elements over a range of temperatures and pressures applicable to the Earth has played an important role in formulating models of a variety of geologic processes. These processes include mag- matic and metamorphic differentiation, mantle and crustal anatexis, and ore-formation. However, testing of these types of models requires accurate and precise determination of trace-element abundances in rocks and minerals, an effort that has been a major focus of geochemistry over the last several years. There has evolved an awareness that well-chosen, small geochemical systems on the scale of individual minerals, for example, provide geochemical information that can be scaled to larger systems such as lava flows, magma reservoirs, batholiths, hydrothermal and regional flow regimes, and even planetary bodies. The continuing effort in trace-element analysis has also been encouraged by innovative technical developments in microanalysis. New generations of established instruments have improved spatial and spectral resolution for in situ analysis, and improved methodologies have expanded the analytical range of instruments. These developments are particularly applicable to the electron microprobe and secondary ion mass spectrometer (SIMS). Geochemists also agree that the most useful analytical datasets should include element groups displaying disparate geochemical behavior, e.g., siderophilic, magmaphilic, and volatile elements. Such datasets result in robust modeling. However, these requirements place substantial strain on current analytical capabilities, particularly in regard to elements that commonly are present in the low-ppm or ppb range in nature.

Abstract Although it has long been appreciated that some fluid inclusions (FI) trapped in minerals are valuable samples of ore-forming solutions that provide information not readily obtained by other methods, many attempts to measure the isotopic and elemental composition of FI directly have not been entirely successful because more than one fluid type may be present in the volume of sample required for the analysis (see Roedder, 1984, 1990 for reviews). In some rocks there are obvious, visible differences in the composition of FI within the same mineral grain; in other cases FI that do not appear to be significantly different in petrography or phase equilibria are quite different in halogen and noble gas chemistry (see Irwin and Roedder, 1995; Irwin and Reynolds, 1995). It is usually essential to measure abundances on a microscopic scale in order to determine the composition of one Fl population. Minerals can be sampled on a micro scale similar to the distribution of homogeneous natural FI assemblages by targeting a laser through an optical microscope, which, in tandem with a highly sensitive mass spectrometer, form the basis of the laser microprobe noble gas mass spectrometry technique (abbreviated as LMNGMS). Three distinct types of information can be obtained by analysis of noble gases in Fl: 1. Abundances of Cl, K, Ca, Br, Se, I, Ba, Te and U are determined by measuring isotopes of Ar, Kr, and Xe produced synthetically in a nuclear pile. Low natural abundances of some noble gases, combined

Abstract As is true of the other microsampling analytical techniques addressed in this book, laser Raman microprobe (LRM) spectroscopy can be applied to a wide variety of geological materials. The goal of this chapter is to demonstrate the usefulness of LRM analysis to economic geology in general, and to fluid inclusions in particular. The chapter begins with a brief consideration of the analytical challenge presented here, namely the physical nature of fluid inclusions and the types of information desired from their analysis. As a means of convincing the economic geologist of the applicability of Raman analysis, an overview is then presented of the types of direct and indirect information that can be obtained via Raman spectroscopy. After a brief discussion of the physical and chemical principles behind Raman spectroscopy and the typical instrumentation used in modern laboratories, numerous short summaries are presented on the successful application of Raman spectroscopy to different types of ore deposits. Owing to the breadth of the literature on LRM spectroscopy and the page limitations of this chapter, no attempt is made to cover the field completely. For additional summaries of Raman (microprobe) applications in geology, the reader is referred to McMillan (1989), Burke (1994), and McMillan et al. (1996).

Abstract Studies of individual fluid inclusions can provide important information on the origin and evolution of mineralizing fluids and the processes of fluid-rock interaction. The bulk composition of inclusion fluids is often inferred from microthermometric studies; determinations of major elements in individual inclusions provide a test of such inferences, and can lead to insight into the mineralogical controls on fluid composition. Furthermore, analyses of trace elements allow the characterization of fluids to a degree not realizable from microthermometric or bulk leachate studies. Synchrotron-source X-ray fluorescence microprobe (SXRFM) analysis, generally a non-destructive technique, provides chemical data for major and trace elements within individual fluid inclusions. Synchrotrons provide an intense source of exciting radiation in the X-ray region (e.g., Winick and Doniach, 1980; Chen et al., 1990) that can be used to probe individual fluid inclusions (Frantz et al., 1988; Lowenstern et al., 1991; Rankin et al., 1992; Vanko et al., 1993b; Cline and Vanko, 1995; Mavrogenes et al., 1995). In a microprobe set-up (Fig. 1; Rivers et al., 1991; Smith and Rivers, 1995), the X-ray beam generates fluorescence X-rays in a targeted inclusion of interest and the resulting X-ray spectrum includes peaks of characteristic radiation with the potential for quantitative chemical analyses (Fig. 2). Present limitations of SXRFM based on energy-dispersive X-ray analysis include only those elements with atomic numbers greater

Abstract There would be few geological studies in which, at some stage, there did not arise a question of timing. The answer is often to be found through direct observation; the principles of superposition and crosscutting relationships apply in determining the order of events on all scales from the microscopic to the macroscopic, from crystallization history to continental assembly. By augmenting those principles with the means to establish sequence and correlation provided by palaeontology, the geologist has the capability, through observation and logical reasoning alone, to determine the relative ages of a great range of geological processes. However, while these techniques make it possible to place geological events in time order, they do not provide an absolute measure of time itself. The measurement of absolute time in geology— geochronology—requires a quantifiable physical process that takes place continuously at a known rate from the time of the event to be dated to the present day. Some cyclic processes, such as the passage of the seasons, leave their imprint in parts of the geological record and can provide detailed, accurate measurements of elapsed time intervals, but they do not permit the measurement of absolute time (age) unless the record is unbroken to the present day or the age of one of the cycles is known by some independent means. The number of annual growth bands in a fossil coral, for example, tells how long that coral once lived, but not when. To measure absolute geologic time, one needs a process that is continuous and unidirectional. The most widely utilized of such processes is natural radioactivity. The concept behind radioisotope geochronology is quite simple. Some of the elements in rocks and minerals have isotopes (atoms of the same atomic number but different mass numbers) that are naturally radioactive-the nuclei of those isotopes are unstable, and liable to break down spontaneously (decay) to an isotope of a different element. If the newly-formed isotope is also unstable, the process continues until a stable nucleus forms. Radioactive decay occurs at rates characteristic of each element and isotope. As far as is known, those rates are independent of any chemical or physical parameters (e.g., pressure, temperature, chemical state etc. ) . The probability that a given nucleus of a given isotope will decay in any given time period is a constant, so the number of decays occurring per unit time is proportional to the number of atoms of that