Showing papers in "Reviews in Mineralogy & Geochemistry in 2004"

Applying Stable Isotope Fractionation Theory to New Systems

Abstract: A basic theoretical understanding of stable isotope fractionations can help researczzzhers plan and interpret both laboratory experiments and measurements on natural samples. The goal of this chapter is to provide an introduction to stable isotope fractionation theory, particularly as it applies to mass-dependent fractionations of non-traditional elements and materials. Concepts are illustrated using a number of worked examples. For most elements, and typical terrestrial temperature and pressure conditions, equilibrium isotopic fractionations are caused by the sensitivities of molecular and condensed-phase vibrational frequencies to isotopic substitution. This is explained using the concepts of vibrational zero-point energy and the partition function, leading to Urey’s (1947) simplified equation for calculating isotopic partition function ratios for molecules, and Kieffer’s (1982) extension to condensed phases. Discussion will focus on methods of obtaining the necessary input data (vibrational frequencies) for partition function calculations. Vibrational spectra have not been measured or are incomplete for most of the substances that Earth scientists are interested in studying, making it necessary to estimate unknown frequencies, or to measure them directly. Techniques for estimating unknown frequencies range from simple analogies to well-studied materials to more complex empirical force-field calculations and ab initio quantum chemistry. Mossbauer spectroscopy has also been used to obtain the vibrational properties of some elements, particularly iron, in a variety of compounds. Some kinetic isotopic fractionations are controlled by molecular or atomic translational velocities; this class includes many diffusive and evaporative fractionations. These fractionations can be modeled using classical statistical mechanics. Other kinetic fractionations may result from the isotopic sensitivity of the activation energy required to achieve a transition state, a process that (in its simplest form) can be modeled using a modification of Urey’s equation (Bigeleisen 1949). Theoretical estimates of isotopic fractionations are particularly powerful in systems that are difficult to characterize experimentally, or when empirical …

The Isotope Geochemistry and Cosmochemistry of Magnesium

Abstract: Magnesium is second only to oxygen in abundance among the rock-forming elements and is an important element in the oceans and in hydrological and biological systems. Differences in the relative abundances of its three stable isotopes, 24Mg (78.99%), 25Mg (10.00%), and 26Mg (11.01%), are expected as a result of physicochemical processes because of the large relative mass differences of 4 and 8% between 25Mg and 26Mg, and 24Mg, respectively. Although isotopes of Mg have been used for many years as tracers in artificially spiked systems (in which the abundance of one isotope is enriched) (Cary et al. 1990; Dombovari et al. 2000), reliable measurements of 25Mg/24Mg and 26Mg/24Mg in natural systems have been limited historically by the 1‰ (one part per thousand) reproducibility imparted by instrumental mass fractionation effects. In order to be useful for many geochemical and cosmochemical applications the isotope ratios of Mg must be resolved to ≤ 200 parts per million (ppm). As a result, with a few exceptions (e.g., Davis et al. 1990; Goswami et al. 1994; Russell et al. 1998), many past studies of Mg isotope ratios focused on detection of non-mass dependent, so-called “anomalous” Mg isotopic effects rather than on investigations of mass-dependent fractionation. The principle outcome of this focus was the discovery of radiogenic 26Mg (26Mg*) in primitive meteorites (Gray and Compston 1974; Lee and Papanastassiou 1974). With the advent of multiple-collector inductively coupled plasma-source mass spectrometry (MC-ICPMS) it is now possible to measure 25Mg/24Mg and 26Mg/24Mg of Mg in solution with a reproducibility of 30 to 60 ppm or better (Galy et al. 2001). What is more, ultraviolet (UV) laser ablation combined with MC-ICPMS permits in situ …

Analytical Methods for Non-Traditional Isotopes

Abstract: This chapter is devoted to the analytical methods employed for making high precision isotope ratio measurements that preserve naturally occurring mass-dependent isotopic variations. The biggest challenge in making these types of measurements is deconvolving mass-dependent isotopic fractionation produced in the laboratory and mass spectrometer, from naturally occurring mass-dependent isotopic fractionation, because the patterns of isotope variation produced by these processes are identical. Therefore, the main theme of this chapter is the description and mathematical treatment of mass-dependent isotopic variations and the possible pitfalls in deconvolving instrumental mass bias from naturally occurring mass-dependent isotopic variations. This chapter will not attempt to catalog methods for isotopic analysis of different elements. These details are better discussed in later chapters where ‘element specific’ analytical issues are covered. Rather, the effort in the chapter will be to focus on those specific items that make mass analysis of non-traditional isotopes challenging and unique, and the methods that can be employed to make precise and accurate isotope ratios. Isotopic analysis of non-traditional isotopes is made using three main types of mass spectrometers. Elements that can easily be introduced as gases, such as Cl or Br, are typically analyzed using a gas source mass spectrometer. In contrast, metal elements are analyzed using either a thermalionization mass spectrometer (TIMS) or a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). The main difference between these three types of instruments is how the sample is introduced into the instrument, and how the sample is ionized. In contrast, the analyzer part of each instrument is similar. All three types of instruments use a stack of lenses with variable potential to focus the ion beam, a magnet to resolve the ion beam into different masses, and a series of collectors to measure ion currents of different isotopes simultaneously. The core of the mass analyzer …

Developments in the Understanding and Application of Lithium Isotopes in the Earth and Planetary Sciences

Abstract: The significant relative mass difference (c. 16%) between the two stable isotopes of Li (approximately 6Li 7.5%, 7Li 92.5%), coupled with broad elemental dispersion in Earth and planetary materials, makes this a system of considerable interest in fingerprinting geochemical processes, determining mass balances, and in thermometry. Natural mass fractionation in this system is responsible for c. 6% variation among materials examined to date (Fig. 1⇓). Although the “modern era” of Li isotope quantification has begun, there are still many questions about the Li isotopic compositions of fundamental materials and the nature of fractionation by important mechanisms that are unanswered (e.g., Hoefs 1997). Figure 1. Summary of lithium isotopic compositions of Earth and planetary materials. Filled bars are solid samples, open bars are liquids. See text for references and details. The purpose of this chapter is to summarize the current understanding of Li isotopes in geo- and cosmochemical systems and to indicate (1) where Li isotopes have a high probability of adding new understanding of these systems; (2) where some of the more significant deficits in knowledge exist. The small but burgeoning Li isotope community has not yet compiled the volume of peer-reviewed literature needed to adequately assess even that which has been studied to date. As a result, significant portions of this chapter are based on data reported in abstracts, and as such are more than normally subject to revisions over time. This chapter is anticipated to serve as a starting point for those interested in research incorporating Li isotope geochemistry, or in understanding the state of extant research. ### Experiments in Li isotope fractionation Knowledge of significant Li isotopic fractionation during basic chemical processes is long standing. The early experiments by Taylor and Urey (1938), in which Li isotopes were fractionated by incomplete extraction of an aqueous solution from a zeolite exchange …

Calcium Isotopic Variations Produced by Biological, Kinetic, Radiogenic and Nucleosynthetic Processes

Abstract: Study of the isotopic variations of calcium is of interest because Ca is important in geochemical and biochemical processes, and is one of few major cations in rocks and minerals with demonstrated isotopic variability. Calcium is critical to life and a major component of the global geochemical cycles that control climate. Studies to date show that biological processing of calcium produces significant isotopic fractionation (4 to 5‰ variation of the 44Ca/40Ca ratio has been observed). Calcium isotopic fractionation due to inorganic processing at high (e.g., magmatic) temperatures is small. There are few studies of calcium isotope fractionation behavior for low-temperature inorganic processes. The Ca isotopic variations observed in nature, both biological and inorganic, are mostly attributed to kinetic effects, but this inference cannot be confirmed until equilibrium Ca isotope fractionation is more thoroughly investigated. Evaporation of silicate liquids into vacuum at high temperature is expected to produce kinetic Ca isotopic fractionation, as is diffusion of calcium in silicate liquids and aqueous solutions. The evaporation effects have been observed in meteorite samples. Diffusion effects have been observed in the laboratory but not yet in natural samples. The potential value of Ca isotopic studies has barely been tapped. Improvements in measurement precision would increase the attractiveness of Ca isotopes as a geochemical tool, but such improvements have been slow in coming. Calcium is composed mainly of the isotope 40Ca, which is a highly stable, doubly magic nuclide (both the number of neutrons and the number of protons represent closed nuclear shells). There are a total of six stable isotopes covering a mass range from 40 to 48. Ca has been studied for isotopic variations in three ways. The major isotope, 40Ca is the primary radioactive decay product of radioactive 40K. About 89.5% of the 40K …

Allanite and Other REE-Rich Epidote-Group Minerals

Abstract: Epidote-group minerals rich in rare earth elements (REE), in particular allanite, are common accessory phases in igneous, metamorphic, metasomatic, and sedimentary rocks. Small amounts of REE are present in most epidote-group minerals, but in allanite—and the related minerals dissakisite, ferriallanite, dollaseite, khristovite and androsite—the REE are essential structural constituents. An important characteristic of REE-rich epidote-group minerals is that their octahedrally coordinated M sites contain major amounts of divalent cations. This paper summarizes literature data for these minerals and discusses their chemistry, occurrence, phase relations, and petrologic and geologic significance. The chapter emphasizes allanite, because it is the most common and best-studied of the REE-rich epidote-group minerals. Epidote-group minerals contain isolated silicon tetrahedra and corner-sharing groups of two tetrahedra, and are thus assigned to the disilicate or sorosilicate structural family (for a detailed description of the structure, see Franz and Liebscher 2004). The epidote-group structural formula is A2M3(SiO4)(Si2O7)(O,F)(OH), or in a simplified form A2M3Si3O11(O,F)(OH), in which A = Ca, Sr, Pb2+, Mn2+, Th, REE3+, and U, and M = Al, Fe3+, Fe2+, Mn3+, Mn2+, Mg, Cr3+, and V3+ (Deer et al. 1986). There are two structurally different A sites, A(1) and A(2), with different coordination numbers, and there are three different M sites, M(1), M(2), and M(3), which are all octahedrally coordinated (Ueda 1955; Dollase 1971). In epidote-group minerals, trivalent REE are accommodated in the A sites, which in endmember epidote both contain Ca. The incorporation of REE3+ is commonly charge balanced by a divalent cation (Fe2+, Mn2+, Mg) substituted for a trivalent one in the M sites (Table 1 …

Fe Isotope Variations in the Modern and Ancient Earth and Other Planetary Bodies

Abstract: Iron, the fourth most abundant element in the Earth’s crust, has four naturally occurring stable isotopes: 54Fe (5.84%), 56Fe (91.76%), 57Fe (2.12%), and 58Fe (0.28%), and the natural, mass-dependent isotope variations of Fe in the rock record span a range of ~4 per mil (‰) in 56Fe/54Fe ratios (Fig. 1⇓). The field of Fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how Fe is cycled in different environments. Iron typically occurs as either reduced ferrous Fe in oxygen-poor environments, or as oxidized ferric iron in oxygen-rich environments. Notably, only the reduced species is soluble in oxygenated aqueous solutions, unless the pH is low. In the Archean and Early Proterozoic, the earth may have been relatively oxygen-poor (e.g., Kasting et al. 1979; Grandstaff 1980; Holland 1994), suggesting that there may have been significant quantities of Fe (0.9 millimolar) dissolved in the oceans as Fe(II)aq (e.g., Ewers 1983; Sumner 1997). The extensive iron formations of Archean to Early Proterozoic age may have been deposited from such Fe(II)-rich oceans (e.g., Beukes and Klein 1992). In the modern oxic oceans, however, Fe contents are exceedingly low, <1 nanomolar in the open oceans (e.g., Martin and Gordon 1988; Bruland et al. 1991; Martin 1992; Johnson et al. 1997), and it is now recognized that marine productivity is Fe-limited in parts of the open oceans (e.g., Martin and Fitzwater 1988; Martin et al. 1989, 1994). The differences in the behavior of Fe with redox state, and the significant isotope fractionations (1‰ or more in 56Fe/54Fe) that are associated with redox conditions, suggests that Fe isotope studies will be extremely useful for tracing …

Molybdenum Stable Isotopes: Observations, Interpretations and Directions

Abstract: The unusual chemistry of molybdenum (Mo) makes this trace element interesting to both geochemists and biochemists. Geochemically, Mo is relatively unreactive in oxygenated, aqueous solutions, and hence is a nominally conservative element in the oceans. In fact, Mo is removed so slowly from seawater that it is the most abundant transition metal in the oceans despite being a ppm-level constituent of the crust. In contrast, Mo is readily removed from solution in anoxic-sulfidic (“euxinic”) settings, so that Mo enrichments in sediments are considered diagnostic of reducing depositional conditions. Few elements possess such bimodal redox behavior at the Earth’s surface. Biochemically, Mo draws attention because it is an essential enzyme cofactor in nearly all organisms, with particular importance for nitrogen fixation, nitrate reduction and sulfite oxidation. Such biochemical ubiquity is surprising in view of the general scarcity of Mo at the Earth’s surface. Isotopically, Mo initially catches the eye because it has seven stable isotopes of 10–25% abundance, covering a mass range of ~8% (Fig. 1⇓). Thus, from an analyst’s perspective, Mo offers both an unusually large mass spread and a number of options for isotope ratio determination. Combined with rich redox chemistry and covalent-type bonding, both of which tend to drive isotope fractionation, these factors make the Mo isotope system a particularly promising target for stable isotope investigation. Figure 1. The average natural abundances of the stable isotopes of Mo as recommended by IUPAC, based on (Moore et al. 1974). In the environment, Mo isotope research began in earnest with the application of multiple-collector inductively coupled plasma mass spectrometry. While much work remains to be done, this early research points to promising applications in paleoceanography, and beyond. This review is intended to provide an overview of this emerging stable isotope system in the context of Mo environmental biogeochemistry. Special attention …

The Stable Isotope Geochemistry of Copper and Zinc

Abstract: Copper and zinc are the last two elements of the first row of transition metals (d-block). Interest in these elements arises because they are both strongly chalcophile and, thanks to a rich coordination chemistry, participate in a large number of important biological compounds and reactions. Copper has two isotopes, 63Cu and 65Cu with respective abundances of 69.174% and 30.826% in the reference metal SRM-NIST 976 (Shields et al. 1964). Zn has fi ve stable isotopes 64Zn, 66Zn, 67Zn, 68Zn, and 70Zn with average natural abundances of 48.63, 27.90, 4.10, 18.75, and 0.62%, respectively (Rosman and Taylor 1998). The first attempts at describing the stable isotope geochemistry of copper go back to Walker et al. (1958) and Shields et al. (1965) who identified isotopic variations in the range of several per mil but the analytical difficulties were then such that these pioneering investigations remained isolated. With the exception of a paper by Rosman (1972) who determined the isotopic abundances of Zn isotopes and concluded that there is no noticeable isotopic fractionation in terrestrial samples, the stable isotope geochemistry of this element remained essentially unexplored. It was not until the advent of inductively-coupled plasma mass spectrometry (ICP-MS) instruments equipped with a magnetic sector and multiple collection that precise isotopic measurements became possible and that the isotope geochemistry of these two elements took off. Marechal et al. (1999) published the first measurements of Cu and Zn isotope compositions in a variety of minerals and biological materials. Marechal et al. (2000) and Pichat et al. (2003) demonstrated the variability of Zn isotopes in ferromanganese nodules, sediment trap material, and marine carbonates. Marechal et al. (1999) and Zhu et al. (2000) confirmed the broad range of isotopic variations in copper ores observed by the earlier workers. The same …

Isotopic Constraints on Biogeochemical Cycling of Fe

Abstract: Cycling of redox-sensitive elements such as Fe is affected by not only ambient Eh-pH conditions, but also by a significant biomass that may derive energy through changes in redox state (e.g., Nealson 1983; Lovely et al. 1987; Myers and Nealson 1988; Ghiorse 1989). The evidence now seems overwhelming that biological processing of redox-sensitive metals is likely to be the rule in surface- and near-surface environments, rather than the exception. The Fe redox cycle of the Earth fundamentally begins with tectonic processes, where “juvenile” crust (high-temperature metamorphic and igneous rocks) that contains Fe which is largely in the divalent state is continuously exposed on the surface. If the surface is oxidizing, which is likely for the Earth over at least the last two billion years (e.g., Holland 1984), exposure of large quantities of Fe(II) at the surface represents a tremendous redox disequilibrium. Oxidation of Fe(II) early in Earth’s history may have occurred through increases in ambient O2 contents through photosynthesis (e.g., Cloud 1965, 1968), UV-photo oxidation (e.g., Braterman and Cairns-Smith 1987), or anaerobic photosynthetic Fe(II) oxidation (e.g., Hartman 1984; Widdel et al. 1993; Ehrenreich and Widdel 1994). Iron oxides produced by oxidation of Fe(II) represent an important sink for Fe released by terrestrial weathering processes, which will generally be quite reactive. In turn, dissimilatory microbial reduction of ferric oxides, coupled to oxidation of organic carbon and/or H2, is an important process by which Fe(III) is reduced in both modern and ancient sedimentary environments (Lovley 1991; Nealson and Saffarini 1994). Recent microbiological evidence (Vargas et al. 1998), together with a wealth of geochemical information, suggests that microbial Fe(III) reduction may have been one of the earliest forms of respiration on Earth. It therefore seems inescapable that biological redox cycling of Fe has occurred for at least several billion years of Earth's history.

Physical and chemical properties of the epidote minerals: An introduction

Abstract: Epidote minerals are known since the 18th century, but at that time the greenish to dark colored varieties were termed actinolite or schorl and not distinguished from the minerals to which these names apply today. Hauy defined the mineral species and introduced the name “epidote” in 1801, whereas Werner in 1805 used the term pistacite (quoted from Hintze 1897). Epidote is derived from greek epidosis = to increase, because the base of the rhombohedral prism has one side larger than the other and pistacite refers to its green color (all references for names after Luschen 1979; Blackburn and Dennen 1997). Weinschenk (1896) proposed the name clinozoisite from its monoclinic symmetry and zoisite-like composition for those monoclinic members of the epidote family that are Fe poor, optically positive and have low refractive indices and birefringence. Zoisite was probably confused with tremolite until the beginning of the 19th century. In 1804, Siegmund Zois, Baron von Edelstein 1747–1819, an Austrian sponsor of mineral collections, found and described a new mineral in a handspecimen from the Saualpe Mountains in Carinthia that was named zoisite by Werner. Hauy (1822) interpreted zoisite as a variety of epidote and included it in his “epidote spezies.” Weiss (1820) presented a theory of the epidote system and also discussed crystal morphological features of the epidote minerals (Weiss 1828). Rammelsberg (1856) studied the relationship between epidote and zoisite and presented a compilation of chemical analyses of zoisite. He already noticed that the relative concentrations of di-, tri- and tetravalent cations are identical in zoisite and epidote but that the Fe content in zoisite (about 2–3.5 wt% Fe2O3) is generally less than in epidote (about 9–12 wt% Fe2O3). Piemontite (see Bonazzi and Menchetti 2004) was probably first described in 1758 by Cronstedt …

Overview and general concepts

Abstract: Of the eighty-three naturally occurring elements that are not radioactive or have half lives long enough to be considered stable (≥109 yrs), nearly three-quarters have two or more isotopes. Variations in the isotopic ratios of a number of these elements, including H, C, N, O, and S, provide the foundation for the field of stable isotope geochemistry . Investigations of variations in the isotopic compositions of these traditional elements have provided important constraints on their sources in natural rocks, minerals, and fluids. These studies have focused on a range of problems including planetary geology, the origin and evolution of life, crust and mantle evolution, climate change, and the genesis of natural resources. Much less attention, however, has been paid to stable isotope variations of other elements that are also geochemically important such as certain metals and halogens. In part this has been due to analytical challenges, although first-order variations for several systems have been constrained using long-standing analytical methods such as gas- and solid-source mass spectrometry. With the advent of analytical instrumentation such as multi-collector, inductively-coupled plasma mass spectrometry (MC-ICP-MS), large portions of the Periodic Table are now accessible to stable isotope studies. In this volume, the geochemistry of a number of non-traditional stable isotopes is reviewed for those elements which have been studied in some detail: Li, Mg, Cl, Ca, Cr, Fe, Cu, Zn, Se, and Mo. This volume is intended for the non-specialist and specialist alike. The volume touches on the multiple approaches that are required in developing new isotopic systems, including development of a theoretical framework for predicting possible isotopic fractionations, perfecting analytical methods, studies of natural samples, and establishment of a database of experimentally-determined isotope fractionation factors to confirm those predicted from theory. In addition to the systems discussed in this volume, we expect that …

Mass-Dependent Fractionation of Selenium and Chromium Isotopes in Low-Temperature Environments

Abstract: Selenium (masses 74, 76, 77, 78, 80, and 82; Table 1⇓) and chromium (masses 50, 52, 53 54; Table 1⇓) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. View this table: Table 1. Compositions of natural Se and Cr and currently used double spikes (atom %). There are only about a dozen published studies on Se isotopes and only two on Cr isotopes. This chapter summarizes what has been learned thus far, and almost all of this work concentrates on aqueous reactions at earth surface temperatures. It also attempts to provide some geochemical background and reviews some relevant points from the sulfur isotope literature, which provides insight into the isotopic systematics of Se and Cr. ### Se geochemistry Se is chemically similar to sulfur, which is immediately above it in the periodic table. Its concentration in the earth’s crust is small, with most rocks containing less than 0.1 ppm Se except for shales, which span a wide range of concentration and average roughly 1ppm (Faure 1991). Coal is also relatively rich in Se, averaging 3 ppm (Cooper et al. 1974). Se can substitute extensively for S in pyrite (Coleman and Delevaux 1957). However, Se concentrations of many sulfide minerals are quite small, suggesting strong decoupling of Se from S in some systems. …

Epidote in Geothermal Systems

Abstract: Early in the 20th Century epidote was readily recognized as a common rock-forming mineral of metamorphic and hydrothermal processes (Becke 1903; Grubenmann 1904; Van Hise 1904; Goldschmidt 1911; Eskola 1915). Its distribution is widespread in the Earth’s crust, including metamorphic environments of pumpellyite-prehnite, greenschist, epidote-amphibolite, and blueschist facies (Seki 1972; Liou 1993). In lower-pressure hydrothermal environments epidote is a common mineral in skarns, in propylitic altered volcanic rocks and in late-stage veins related to silicic intrusions (Lindgren 1933; Coats 1940; Nakovnik 1963). Within obducted segments of oceanic crust (ophiolites) and in large igneous provinces epidote is found in veins and replacement bodies (epidosites, cf. Dana 1875) associated with intrusion of dolerite dikes and gabbros (Coleman 1977). It was not until the 1960’s that epidote was first discovered in drill hole samples from active geothermal systems (Naboko and Piip 1961; Sigvaldason 1963; White et al. 1963; Steiner 1966; Keith et al. 1968; Marinelli 1969). Geothermal drill holes provided the first samples of epidote-altered rocks and coexisting hydrothermal fluids at measured temperatures and pressures (White and Sigvaldason 1963; Naboko 1964). Formation of epidote in such low-pressure geologic environments was initially questioned (Rusinov 1966), due in part to geologic observations (Korzhinskiy 1963) and to the sluggish nature of epidote synthesis at low pressures and temperatures (Fyfe et al. 1958; Coombs et al. 1959; Merrin 1960; Fyfe 1960). Epidote is now recognized as a key index mineral related to temperature, permeability, and fluid composition in geothermal systems worldwide (Browne 1978; Giggenbach 1981; Henley and Ellis 1983; Bird et al. 1984; Reyes 1990; Absar 1991; Reed 1994; Muramatsu and Doi 2000). In general, hydrothermal epidote exhibits a wide range in octahedral …

Trace Element Geochemistry of Epidote Minerals

Abstract: One of the most striking features of epidote minerals is their ability to incorporate significant amounts of geochemically important trace elements such as large ion lithophile elements (LILE), especially Sr and Pb, transition metals, actinides, and rare earth elements (REE). Epidote minerals are common in a broad range of whole rock compositions and they can be the most important reservoir for these elements in a variety of crustal rocks. We summarize the available trace element data of epidote minerals including zoisite from the literature and discuss their geochemical significance. Additionally, we present a set of new data from a wide range of geological environments. We focus on the orthorhombic polymorph zoisite [Ca2Al3Si3O11O(OH)], which shows a very limited variation in major element chemistry, and the monoclinic epidote minerals along the join Ca2Al3Si3O11O(OH)–Ca2Fe3+3Si3O11O(OH), which is typically constrained to the Al-rich part, i.e., the Fe3+ content rarely exceeds one cation per formula unit (pfu). The term “trace element” is problematic and ambiguous for the epidote minerals because they form solid solutions with actual end members whose components are usually abundant only as minor or trace elements, such as piemontite Ca2Al2( Mn 3+,Fe3+)Si3O11O(OH), mukhinite Ca2Al2 V 3+Si3O11O(OH), tawmawite Ca2Al2 Cr 3+Si3O11O(OH), niigataite Ca Sr Al3Si3O11O(OH), hancockite Ca Pb Al2 (Al, Fe3+)Si3O11O(OH), allanite Ca REE Al2Fe2+Si3O11O(OH) dissakisite Ca REE Al2MgSi3O11O(OH), dollaseite Ca REE Al2MgSi …

An Overview of Isotopic Anomalies in Extraterrestrial Materials and Their Nucleosynthetic Heritage

Abstract: Isotopic anomalies are expected in primitive meteorites since astronomical observation and astrophysical modeling of stars predict a great variety of stellar processes. Protostellar clouds should partially preserve the memory of this diversity in solid grains. Since 1970, high precision mass spectrometry and high resolution ion probes have led to the discovery of numerous isotopic anomalies, which were rapidly associated with nucleosynthetic processes. A general rule is that small isotopic effects (parts in 103–104) are observed in centimeter size samples, whereas order of magnitude variations are observed at the micron scale in circumstellar grains. Refractory materials in primitive meteorites were investigated first as they have the best chance of escaping homogenization in the early solar system. Inclusions in C3 carbonaceous chondrites exhibit widespread anomalies for oxygen and the iron group elements. Only a few members, dubbed “FUN” (for “Fractionated and Unknown Nuclear” effects), also display anomalous compositions for the heavy elements. Anomalies in inclusions have generally been connected with explosive or supernova nucleosynthesis. Several types of presolar circumstellar grains have been separated from the matrix of chondrites: diamonds, silicon carbide, graphite, oxides. The isotopic ratios of the light elements (C-N-O) vary over several orders of magnitude in these grains. Only a few measurement have been performed for heavier elements with generally s-process signatures. AGB stars at different stages of their evolution are thought to be the sources of most circumstellar grains. Nevertheless grains with supernova signatures have also been found. For Cr and Mo in bulk primitive carbonaceous chondrites (C1, C2), large isotopic differences exist between the different major mineral phases of the bulk rock. A number of now extinct radioactive isotopes have existed in the early solar system. This is shown by the variations that they induce in the abundances in their daughter nuclides. Their main use …

Experimental Subsolidus Studies on Epidote Minerals

Abstract: Despite the fact that epidote group minerals are very typical for metamorphism at very low pressure, e.g., in geothermal fields (Bird and Spieler 2004), the first successful synthesis of zoisite and epidotess was reported by Coes (1955) in a paper in the Journal of American Ceramic Society entitled “High pressure minerals.” Synthesis conditions were 1 GPa at 800°C; zoisite was obtained from a mixture of kaolin, SiO2, CaO, and CaCl2, whereas epidote was formed by adding FeCl2•H2O to the previous mixture. Once experimental facilities enabled pressures exceeding a few hundred MPa, zoisite and epidote minerals were easily obtained from a variety of starting materials, made of oxides, gels and glasses. Historically, early experimental studies on epidote focused on the formation at low pressure conditions, and then ventured into the simple system CaO-Al2O3-SiO2-H2O at conditions attainable by piston cylinder equipment (Newton and Kennedy 1963; Boettcher 1970) in which zoisite was found to have an extremely large temperature stability. Then, the role of Fe3+ was investigated systematically at pressures typical for the middle and lower continental crust (Holdaway 1972; Liou 1973). Epidote minerals in bulk compositions directly applicable to natural rocks were not investigated experimentally until the early 70’s (Liou et al. 1974; Apted and Liou 1983). Subsequent studies in the context of the very popular hydrous phase stabilities at subduction conditions in the 90’s extended the experimentally determined stability of epidotess in natural compositions to 3.5 GPa. With the relatively easy access to multi-anvil machines, the pressure stability of zoisite was defined (Poli and Schmidt 1998). The increasing number of experimental studies on epidote minerals reveals that the members of this group of ubiquitous rock forming minerals have …

Epidote Minerals in High P/T Metamorphic Terranes: Subduction Zone and High- to Ultrahigh-Pressure Metamorphism

Abstract: Epidote minerals—the monoclinic epidote group minerals together with the orthorhombic polymorph zoisite—are important Ca-Al-silicates in many metabasites, metapelites and metacherts that are characterized by high P / T ratios. Such high P / T ratios are typical for subduction zones and the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism during continent-continent collisions (e.g., Liou 1973, 1993). All of these P-T conditions can be described by geothermal gradients between 5 and 20°C/km, that therefore provide a rough framework for the P-T conditions covered by this review (Fig. 1⇓). Depending on the actual thermal structure of a subduction zone, the subducting plate will encounter subgreenschist, greenschist, blueschist, epidote-amphibolite, amphibolite, HP granulite, and/or eclogite facies conditions during its travel down into the mantle (Fig. 1⇓). The P-T regime of the eclogite facies can further be subdivided into amphibole eclogite, epidote eclogite, lawsonite eclogite, and dry eclogite facies (Fig. 1⇓). HP metamorphism refers to metamorphic pressure in excess of ~1.0 GPa and includes parts of the blueschist, epidote-amphibolite, and HP granulite facies as well as the eclogite facies (Fig. 1⇓). UHP refers to the metamorphism of crustal rocks (both continental and oceanic) at P high enough to crystallize the index minerals coesite and/or diamond. HP and UHP metamorphism are separated conveniently by the quartz-coesite equilibrium which implies a minimum P > 2.7 GPa at T > 600°C for UHP metamorphism (Fig. 1⇓). The equilibrium boundary for the graphite-diamond transition can be used to further subdivide the UHP region into diamond-grade and coesite-grade. The stability of coesite and other UHP minerals in a metamorphic regime requires abnormally low temperatures at depths greater than 100 km. Such environments can be attained only by the subduction of cold oceanic crust-capped lithosphere ± pelagic sediments or of continental crust. Figure 1. P-T regimes of UHP …

Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes

Abstract: Epidote group minerals are common in metamorphosed mafic to intermediate igneous rocks, quartzofeldspathic sediments and calc-alumina silicate (marl) rocks of higher grade zeolite to medium grade amphibolite facies of low–medium pressure contact and regional metamorphic terranes (i.e., pressure and temperature conditions below the calcite-to-aragonite transition). Within any one rock, epidote composition in terms of Fe3+/(Fe3+ + Al) can be variable, but in general, is limited by whole-rock composition, such that epidote group minerals in metabasite lithologies are more Fe-rich than those in marls that tend to be more Al-rich and typically include zoisite. Because of their wide range of P - T stability, epidote group minerals of variable composition may form in a single rock during several stages of metamorphic re-equilibration. Slow rates of intra-crystalline Fe3+-Al exchange, especially at low temperatures, preserve complex zonation patterns in individual grains that can serve as a “tape-recorder” providing evidence for continuous or discontinuous prograde and retrograde reactions and the P - T -fluid-redox conditions of metamorphism. Thus, relic lower grade epidote (typically Fe-rich) often form cores over which new (typically less Fe-rich) higher grade epidote rims form. In such a case, the boundary between the two generations of epidote may be sharp or gradational depending on the temperature history of the rock and Fe3+-Al diffusivity. Often compositional differences are blurred across the boundary. In addition to zoning is the spread of individual epidote grain compositions within a rock (even on a thin-section scale). This is related to variation in the composition of coexisting phases or reactants (e.g., quartz; Ca-Al silicates such as plagioclase, margarite, lawsonite; mafic silicates such as chlorite, pumpellyite, amphiboles; other Ca-Fe3+ silicates such as prehnite, andraditic garnet; carbonates; Fe-oxides; relict volcanic glass), which serve as compositional micro-domains in which epidote may form. …

The Stable-Chlorine Isotope Compositions of Natural and Anthropogenic Materials

Abstract: The use of stable-isotopic ratios of a number of elements (e.g., H, Li, B, C, N, O, and S) is well established in the study of a broad spectrum of geological and environmental problems such as alteration of the oceanic crust, magmatic-crustal interactions, global chemical fluxes, the nature of the Precambrian crust and identifying the source and fate of pollutants (e.g., Taylor 1968; Muelenbachs and Clayton 1976; Muelenbachs 1980; Gregroy and Taylor 1981; Poreda et al. 1986; Spivack and Edmond 1986; Tanaka and Rye 1991; Mojzsis et al. 2001; Wilde et al. 2001; Numata et al. 2002). The fractionation of these light stable isotopes is a function of the relative mass differences between isotopes (e.g., Richet et al. 1977; Schauble, et al. 2003; Schauble 2004). The two stable isotopes of chlorine are 35Cl and 37Cl with a natural relative abundances of approximately 76% and 24%, respectively, and a relative mass difference of 5.7%, similar in magnitude to the relative mass differences between the isotopes of C and N. Hence, by analogy with these elements it is expected that stable isotopes of chlorine significantly fractionate and can be similarly exploited to understand and solve geological and environmental problems. Early attempts at reproducibly determining 37Cl/35Cl ratios in natural samples were largely unsuccessful (e.g., Curie 1921; Owen and Schaeffer 1955; Hoering and Parker 1961), primarily, because of the limited effectiveness of the extraction and sample preparation techniques, the relatively poor precision of mass spectrometers at the time, and possibly the 37Cl/35Cl of the samples selected for analyses were inappropriate for the precision possible (e.g., Eggenkamp and Schuiling 1995). Taylor and Grimsrud (1969) developed a method for precisely measuring chlorine isotope ratios by mass spectrometry of methyl …

Spectroscopy of Epidote Minerals

Abstract: Numerous spectroscopic techniques have been applied to the epidote minerals to characterize their structure and crystal chemistry. The substitution of transition metal ions Mn, Cr, and V, besides Fe, in the different crystallographic sites of epidote minerals and with different valence states has been studied by optical absorption spectroscopy. These studies mainly focused on the determination of (i) the site preferences of the different transition metal ions within the epidote minerals (e.g., Burns and Strens 1967; Tsang and Ghose 1971), (ii) the physical and structural characteristics of these sites as a function of composition, temperature and/or pressure (e.g., Taran and Langer 2000; Langer et al. 2002), (iii) their crystal field stabilization energy (e.g., Burns and Strens 1967; Langer et al. 2002), and (iv) the cause of the color and pleochroism in some epidote minerals (e.g., Faye and Nickel 1971). Major topics of infrared spectroscopic studies have been the proton environment and its changes with composition, temperature, and pressure (e.g., Langer and Raith 1974; Winkler et al. 1989; Della Ventura et al. 1996; Liebscher et al. 2002) and the phase transition within the orthorhombic solid solution series (e.g., Liebscher and Gottschalk 2004). Mossbauer spectroscopy has been used (i) to resolve the valence state of Fe in the different epidote minerals and its site location (e.g., Dollase 1973; Kartashov et al. 2002) and (ii) to study the intracrystalline Al-Fe partitioning between the different octahedral sites and the kinetic of this ordering process (e.g., Patrier et al. 1991; Fehr and Heuss-Asbichler 1997). This chapter reviews the different spectroscopic studies and techniques applied to epidote minerals with emphasis given to the crystal chemical results. An in-depth presentation and discussion of the different spectroscopic techniques and their theoretical framework is beyond the scope of this chapter. …

Manganese in Monoclinic Members of the Epidote Group: Piemontite and Related Minerals

Abstract: According to Mayo (1932), who provided a brief historical review of the names used for piemontite, the first researcher who described this mineral has been Cronstedt in 1758 who named it “rod Magnesia.” In 1790 Chevalier Napione analyzed the sample described by Cronstedt and termed it “Manganese rouge.” On the basis of his chemical data Hauy designated the substance as “Manganese oxide violet silicifere” in 1801. Later, in his Traite de Mineralogie, Hauy (1822) adopted the name proposed by Cordier (1803) who first recognized the mineral as an “Epidote manganesifere.” The name piedmontite was proposed in 1853 by Kenngott the basis of the type locality and more recently transformed into piemontite. According to the standard guidelines for mineral nomenclature, the name piemontite should be reserved to members of the ternary solid solution Ca2Al2(Mn,Fe,Al)(Si2O7)(SiO4)O(OH) that basically contains Mn3+ dominant at one site. Nonetheless, the use of this name for any monoclinic manganiferous epidote-group members showing the characteristic strong red-yellow-violet pleochroism is very common and probably convenient with special regard to petrographic purposes. Indeed, the color of manganian (i.e., Mn3+ bearing) epidote or clinozoisite ranges to red to pinkish, while manganoan (i.e., Mn2+ bearing) members do not exhibit the characteristic reddish hue. The discredited name “withamite” was used to describe poorly manganiferous piemontite (Hutton 1938; Yoshimura and Momoi 1964) but corresponds, on the basis of the current nomenclature, to a manganian clinozoisite. The name “thulite,” sometimes erroneously used for pinkish clinozoisite, should be reserved to Mn3+ bearing orthorhombic members. In this chapter we focus on piemontite sensu stricto with the ideal formula Ca2Al2Mn3+(Si2O7)(SiO4)O(OH), but also include for the reasons explained above those members of the clinozoisite-epidote-piemontite …

Thermodynamic Properties of Zoisite, Clinozoisite and Epidote

Abstract: The natural occurrence of epidote minerals is widespread over a large variety of geological settings Thus epidote minerals are part of numerous phase equilibria, which need to be evaluated to understand the geological processes in general There are two principal approaches to evaluate phase equilibria in the deep earth The first uses direct experimental investigations, while the second is by thermodynamic calculations and modeling Performing and evaluating experiments is often a tedious procedure and by far not all systems can be studied at the required physical and chemical conditions, considering all of the possible variables Therefore experimental investigations are in many instances only case studies in simplified systems, but the thermodynamic framework provides a powerful tool to perform calculations of complex phase equilibria, if the required parameters are available However, these two approaches are not necessarily independent, because experimental results are often used to evaluate and to calibrate physical-chemical parameters for such calculations Many physical-chemical textbooks treat the principles of thermodynamics, and in addition some texts (eg, Anderson and Crerar 1993; Nordstrom and Munoz 1994) introduce its application to the geological sciences Therefore only the fundamental equations and their relationship to the required parameters are treated here briefly The evaluation of phase equilibria and/or stable phase assemblages involves the calculation of the apparent chemical potential μ i ( P , T ) for each component i present at the P and T of choice according to \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[{\mu}\_{\mathit{i(P,T)}}\ =\ {\Delta}\_{\mathit{f}}\mathit{h^{o}\_{i(P\_{r},T\_{r})}}\ {-}\ \mathit{TS\_{i(P\_{r},T\_{r})}}^{o}\ +\ {{\int}\_{\mathit{T\_{r}}}^{\mathit{T}}}\mathit{c}^{o}\mathit{\_{Pi(T)}dT\ {-}\ T}{\_{\mathit{T\_{r}}}^{\mathit{T}}}\frac{\mathit{c}^{o}\_{\mathit{Pi(T)}}}{\mathit{T}}\mathit{dT}\ +\ {{\int}\_{\mathit{P\_{r}}}^{\mathit{P}}}{ u}^{o}\_{\mathit{i(P,T)}}\mathit{dP}\ +\ \mathit{RT}\ ln\mathit{a\_{i}}\] \end{document}(1) The calculation involves the following molar standard state properties (note small letters designate molar quantities): absolute enthalpy of formation from the elements Δ f h i ° and third law entropy s i ° at reference conditions P r and T r (ie, 01 MPa, 29815 K), the heat capacity c P ° i at constant pressure as function of temperature, and the …

Fluid Inclusions in Epidote Minerals and Fluid Development in Epidote-Bearing Rocks

Abstract: The widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies. Apart from high- to very high-temperature and low- to intermediate-pressure conditions, epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust (e.g., Poli and Schmidt 1998). Yet fluid inclusion studies on epidote minerals are surprisingly scarce, even in fluid-saturated environments like certain vein-type deposits or hydrothermal-volcanic vugs and druses. For example, epidote minerals are not mentioned in the subject index of Roedder’s (1984) outstanding summary and review of fluid inclusion studies and occurrences, which lists more than sixty different host minerals for fluid inclusions. Nonetheless, more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems, ore deposits and high-pressure to ultra-high pressure rocks. The aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes, which are based on temperature ( T ), pressure ( P ), molar volume ( V ) and composition ( X ) data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments. The review starts with a chapter on some typical mixed volatile solid-fluid equilibria involving epidote minerals, which are relevant for the here discussed environments. This is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals. The next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low-grade metamorphic rocks and constraints on the P - T - X properties of the fluids present in these systems. This is followed by a short introduction …

Stable and Radiogenic Isotope Systematics in Epidote Group Minerals

Abstract: Epidote minerals (the epidote group together with the orthorhombic polymorph zoisite) occur in a wide range of igneous, metamorphic and sedimentary lithologies. Although often present in only small quantities, epidote group minerals can nonetheless be used to generate important quantitative constraints on processes such as regional metamorphism, deep-seated pluton emplacement and uplift, hydrothermal fluid flow, and sedimentary provenance (e.g., Zen and Hammarstrom 1984; Brandon et al. 1996; Cartwright et al. 1996; Keane and Morrison 1997; Spiegel et al. 2002). Petrologic studies on epidote group minerals formed by these and other processes are reviewed in Bird and Spieler (2004), Grapes and Hoskin (2004), Enami et al. (2004), and Schmidt and Poli (2004). In this chapter, stable and radiogenic isotope studies of epidote group minerals will be reviewed. Although stable and/or radiogenic isotope data exist for epidote group minerals from a number of different geologic settings, such data have produced particularly important insights into 1) fluid flow associated with variable grades of metamorphism, particularly ultra-high-pressure (UHP) metamorphism, 2) intrusion of deep-seated plutonic systems, 3) the nature of hydrothermal alteration in geothermal systems, 4) age relations in hydrothermal systems, and 5) sedimentary provenance. Based on the chemical composition of naturally occurring epidote minerals, oxygen, hydrogen, and chlorine stable isotopes all have the potential to be important petrogenetic indicators in systems involving epidote minerals. Oxygen is present in epidote minerals bound in isolated TO4 tetrahedra, T2O7 groups, and in the hydroxyl site as both OH− and O−2. Hydrogen and chlorine are present in the hydroxyl site as OH− and Cl−, respectively, although Cl− is present in only very small quantities (Frei et al. 2004). To date, oxygen and hydrogen isotope systematics have been studied both …