Showing papers in "Reviews in Mineralogy & Geochemistry in 2017"

Zircon: The Metamorphic Mineral

Abstract: A mineral that forms under conditions as variable as diagenesis to deep subduction, melt crystallization to low temperature alteration, and that retains information on time, temperature, trace element and isotopic signatures is bound to be a useful petrogenetic tool. The variety of conditions under which zircon forms and reacts during metamorphism is a great asset, but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure–temperature–deformation–fluid context. Under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information. In the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology. With the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high-grade metamorphic rocks. Since then, there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures, its capacity to protect mineral inclusions, zircon thermometry, trace element patterns and their relation to main mineral assemblages, solubility of zircon in melt and fluids, and isotopic systematics in single domains that go beyond U–Pb age determinations. Metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks, but has become a truly indispensable mineral in reconstructing pressure–temperature–time–fluid-paths over a wide range of settings. An obvious consequence of its wide use, is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial: in this chapter, reference to published works are intended as examples and not as a compilation. This chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture, deformation structure and mineral chemistry, including trace element and isotopic systematics. Linking this information together highlights the potential of zircon as a key mineral in petrochronology.

The Isotope Geochemistry of Zinc and Copper

Abstract: Copper, a native metal found in ores, is the principal metal in bronze and brass. It is a reddish metal with a density of 8920 kg m−3. All of copper’s compounds tend to be brightly colored: for example, copper in hemocyanin imparts a blue color to blood of mollusks and crustaceans. Copper has three oxidation states, with electronic configurations of Cu([Ar]3 d 104 s 1), Cu+([Ar]3 d 10), and Cu2+([Ar]3 d 9). Cu does not react with aqueous hydrochloric or sulfuric acids, but is soluble in concentrated nitric acid due to its lesser tendency to be oxidized. Cu(I) exists as the colorless cuprous ion, Cu+. Cu(II) is found as the sky-blue cupric ion, Cu2+. The Cu+ ion is unstable, and tends to disproportionate to Cu and Cu2+. Nevertheless, Cu(I) forms compounds such as Cu2O. Cu(I) bonds more readily to carbon than Cu(II), hence Cu(I) has an extensive chemistry with organic compounds. In aqueous solutions, Cu2+ ion occurs as an aquacomplex. There is no clearly predominant structure among the four-, five-, and six-fold coordinated Cu(II) species (Chaboy et al. 2006). Hydrated Cu(II) ion has been represented as the hexaaqua complex Cu(H2O)62+, which shows the Jahn–Teller distortion effect (Sherman 2001; Bersuker 2006), whereby the two Cu–O distances of the vertical axial bond (Cu–Oax) are longer than four Cu–O distances in the equatorial plane (Cu–Oeq). The Jahn–Teller effect lowers the symmetry of Cu(H2O)62+ from octahedral Th to D2h. The sixfold coordination of hydrated Cu(II) species is questioned by a finding of fivefold coordination (Pasquarello et al. 2001; Chaboy et al. 2006; Little et al. 2014b …

Magnesium Isotope Geochemistry

Abstract: Magnesium (Mg) has an atomic number of 12 and belongs to the alkaline earth element (Group II) of the Periodic Table. The pure Mg is a silvery white metal and has a melting point of 650 °C and boiling point of 1090 °C at 1 standard atmosphere (Lide 1993–1994). The electronic configuration of Mg is [Ne]3s2, with low ionization energies, which makes Mg ionic in character with a common valance state of 2+ and a typical ionic radius of 0.72 A (Shannon 1976). Magnesium is a major element and widely distributed in the silicate Earth, hydrosphere and biosphere (Fig. 1a). It is the fourth most abundant element in the Earth (after O, Fe and Si, MgO = 25.5 wt%) (McDonough and Sun 1995), the fifth most abundant element in the bulk continental crust (MgO = 4.66 wt%) (Rudnick and Gao 2003) and the second most abundant cation in seawater (after Na, Mg = 0.128 wt%) (Pilson 2013). Nonetheless, the mantle has > 99.9% of Mg in the Earth because of its high MgO content (37.8 wt%, McDonough and Sun 1995) and mass fraction. The high abundance of Mg in the silicate Earth makes it a major constituent of minerals (e.g., olivine, pyroxene, garnet, amphibole, mica, spinel, carbonate, sulfate, and clay minerals) in igneous, metamorphic and sedimentary rocks. Magnesium has three stable isotopes, with mass numbers of 24, 25 and 26, and typical abundances of 78.99%, 10.00% and 11.01%, respectively (Berglund and Wieser 2011) (Fig. 1b), and a standard atomic weight of 24.305 (CIAAW 2015). Because of the limitations in the mass spectrometry, many previous Mg isotopic studies have concentrated on either mass independent isotope anomalies to look for the radiogenic 26Mg produced by the decay of short-lived 26Al (Gray and Compston 1974; Lee and …

Iron Isotope Systematics

Abstract: Iron is a ubiquitous element with a rich (i.e., complex) chemical behavior. It possesses three oxidation states, metallic iron (Fe), ferrous iron (Fe2+) and ferric iron (Fe3+). The distribution of these oxidation states is markedly stratified in the Earth.

The Stable Isotope Geochemistry of Molybdenum

Abstract: > “The Answer to the Great Question... Of Life, the Universe and Everything... > > Is... Forty-two,” said Deep Thought, with infinite majesty and calm… > > “I checked it very thoroughly,” said the computer, “and that quite definitely is the answer.” > > — Douglas Adams, The Hitchhiker’s Guide to the Galaxy Molybdenum (Mo)—the element with atomic number 42—possesses unique properties that make it the answer to many questions in the geosciences, life sciences, and industry. In the geosciences, the redox sensitivity of Mo makes it particularly useful for answering questions about environmental redox conditions. In particular, it was first suggested as an ocean paleoredox proxy over 30 years ago (Holland 1984; Emerson and Huested 1991)—an application that finally came to fruition in the late 1990s and 2000s when understanding of Mo geochemical behavior in modern environments improved significantly (e.g., Crusius et al. 1996; Helz et al. 1996, 2011; Morford and Emerson 1999; Erickson and Helz 2000; Barling et al. 2001; Siebert et al. 2003, 2005; Arnold et al. 2004; Vorlicek et al. 2004; Morford et al. 2005; Nagler et al. 2005; Algeo and Lyons 2006; McManus et al. 2006; Poulson et al. 2006; Anbar et al. 2007; Wille et al. 2007; Pearce et al. 2008; Archer and Vance 2008; Neubert et al. 2008; Scott et al. 2008; Gordon et al. 2009; Poulson Brucker et al. 2009). In the life sciences, nature settled on Mo as the answer to the challenge of biological-N2 fixation at least ~ 2 billion years ago (Boyd et al. 2011), with the evolution of the Mo-dependent nitrogenase enzyme. Molybdenum is also at the heart of nitrate reductase enzymes, which are essential for assimilatory and dissimilatory nitrate reduction (Glass et al. 2009). Therefore, Mo is central …

Non-Traditional Stable Isotopes: Retrospective and Prospective

Abstract: Traditional stable isotope geochemistry involves isotopes of light elements such as H, C, N, O, and S, which are measured predominantly by gas-source mass spectrometry (Valley et al. 1986; Valley and Cole 2001). Even though Li isotope geochemistry was developed in 1980s based on thermal ionization mass spectrometry (TIMS) (Chan 1987), the real flourish of so-called non-traditional stable isotope geochemistry was made possible by the development of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) (Halliday et al. 1995; Marechal et al. 1999). Since then, isotopes of both light (e.g., Li, Mg) and heavy (e.g., Tl, U) elements have been routinely measured at a precision that is high enough to resolve natural variations (Fig. 1). The publication of RIMG volume 55 ( Geochemistry of Non-Traditional Stable Isotopes ) in 2004 was the first extensive review of Non-Traditional Stable Isotopes summarizing the advances in the field up to 2003 (Johnson et al. 2004). When compared to traditional stable isotopes, the non-traditional stable isotopes have several distinctive geochemical features: 1) as many of these elements are trace elements, their concentrations vary widely in different geological reservoirs; 2) these elements range from highly volatile (e.g., Zn and K) to refractory (e.g., Ca and Ti); 3) many of these elements are redox-sensitive; 4) many of them are biologically active; 5) the bonding environments, especially for the metal elements, are different from those of H, C, N, O and S; and finally, 6) many of these elements have high atomic numbers and more than two stable isotopes. These features make the different elements susceptible to different fractionation mechanisms, and by extension, make them unique tracers of different cosmochemical, geological and biological processes, as highlighted throughout this volume. Figure 1 Non-traditional stable isotope systems covered in this volume. Figure 2 The terrestrial isotopic variation vs. the relative mass difference for non-traditional …

Lithium Isotope Geochemistry

Abstract: The lithium isotope system is increasingly being applied to a variety of Earth science studies, as the burgeoning literature attests; over 180 papers have been published in the last twelve years that report lithium isotope data, including five review papers that cover different aspects of lithium isotope applications (Elliott et al. 2004; Tomascak 2004; Tang et al. 2007b; Burton and Vigier 2011; Schmitt et al. 2012), and a book (Tomascak et al. 2016). The upswing in lithium isotope studies over the past decade reflects analytical advances that have made Li measurements readily obtainable. These include the use of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for relatively precise solution measurements (Tomascak et al. 1999a) and secondary ion mass spectrometry (SIMS) for high spatial resolution measurements (Chaussidon and Robert 1998; Kasemann et al. 2005; Bell et al. 2009). In addition, lithium isotope studies are motivated by the large variety of problems for which they may provide insight, including crust–mantle recycling, silicate weathering, fluid–rock interaction, as well as geospeedometry. The great interest in the Li system that spurred the development of these new analytical methods was initiated by the pioneering work of Lui-Heung Chan, who demonstrated not only that Li isotopic fractionation can be very large at or near the Earth’s surface (Chan and Edmond 1988), but also that Li isotopes are strongly fractionated during seawater-basalt interaction (Chan et al. 1992). This discovery naturally led to the search for a recycled slab signature in Li isotopes of arc lavas (some of the earlier studies include Moriguti and Nakamura 1998a; Chan et al. 1999, 2002b; Tomascak et al. 2000, 2002; Leeman et al. 2004; Moriguti et al. 2004), as well as more deeply derived intraplate basalts (e.g., Chan and Frey 2003 …

Uranium Isotope Fractionation

Abstract: This review focuses on the rapidly growing field of natural 238U/235U variability, largely driven by the technical advances in the measurement of U isotope ratios by mass spectrometry with increasing precision over the last decade. A thorough review on the application of the U-decay series systems within Earth sciences was published in Reviews in Mineralogy and Geochemistry (RiMG) volume 52 in 2003, and will not be discussed further within this review. Instead, this article will first focus on the basic chemical properties of U and the evolution of 238U/235U measurement techniques, before discussing the latest findings and use of this isotopic system to address questions within geochronology, cosmochemistry and Earth sciences.

Local Bulk Composition Effects on Metamorphic Mineral Assemblages

Abstract: Plate tectonic forcing leads to changes in the physical conditions that affect the lithosphere. In response to such changes, notably the local temperature ( T ) and pressure ( P ), rocks evolve dynamically. Processes mostly involve mineral transformations, i.e., solid-state reactions, but (hydrous) fluids are often involved, and partial melting may occur in the Earth’s middle and lower crust. While these chemical reactions reflect the tendency of natural systems to reduce their Gibbs free energy, metamorphic rocks commonly preserve textural and mineralogical relics, such as compositionally zoned minerals. Where relics are present, thermodynamic equilibrium clearly was not attained during the evolution of the rock. Petrochronology seeks to establish a temporal framework of petrologic evolution, and for this purpose it is essential to determine the P–T conditions prevailing at several stages. When analyzing a rock sample it is thus critical: 1. to recognize whether several stages of its evolution can be discerned, 2. to document the minerals that formed or were coexisting at each stage, and 3. to estimate at what physical conditions this happened. If (and only if) a chronometer then can be associated to one of these stages—or better yet several chronometers to different stages—then the power of petrochronology becomes realizable. This chapter is concerned with a basic dilemma that results directly from steps (b) and (c) above: P–T conditions are determined on the basis of mineral barometers and thermometers, which mostly rest on the assumption of chemical (or isotopic) equilibrium, yet the presence of relics is proof that thermodynamic equilibrium was not attained. One way out of the dilemma is to analyze reaction mechanisms and formulate a model based on non-equilibrium thermodynamics and kinetics (Lasaga 1998). While this can be fruitful for understanding fundamental aspects of metamorphic petrogenesis, there are more direct ways to address the limited scope needed for petrochronology. The alternative pursued …

Petrochronology Based on REE-Minerals: Monazite, Allanite, Xenotime, Apatite

Abstract: ### REE-minerals Monazite, xenotime, and allanite are REE1-minerals sensu stricto because lanthanides (La…Lu) and yttrium are critical constituents in them Apatite does not require REE, but because it contains substantial REE in many rocks, it is included in this review All four minerals also host unusually high radionuclide concentrations, notably Th and U, forming the basis of their utility as geochronometers This quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic ( P–T ) conditions so can lend petrogenetic context to chronology based on other minerals Part I of this review assembles the basic requisites prior to integrative petrochronologic analysis Individual characteristics of the four REE-minerals are addressed first, ie, their crystal chemistry and stability relations Thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next, and finally their chronology is summarized Part II presents case studies to highlight the specific strengths of REE-minerals used to resolve the dynamics of a broad range of processes, from diagenetic to magmatic conditions Finally, a brief section at the end outlines a few of the current challenges and promising perspectives for future work To introduce the four REE-minerals in style, let us recall the origins of their names The three phosphates have well respected Greek grandparents, and allanite has solid Scottish roots, yet of all four of them show idiosyncracies in etymology or type material Apatite had long puzzled naturalists, as it shows great chemical and physical variability and can resemble other minerals Once properly identified, Abraham Gottlieb Werner named it apatite His reasoning referred to the Greek root ἀπατὰω and giving the precise Latin translation: decipio Taken literally, both mean “I deceive” or “I mislead”, which sounds like an apt confession from this mineral for having fooled …

Recent Developments in Mercury Stable Isotope Analysis

Abstract: The first Reviews in Mineralogy volume on the Geochemistry of Non-Traditional Stable Isotopes was compiled before it was appropriate to include a chapter on mercury (Hg) stable isotope geochemistry. At that time there were only a few papers on this new topic (Jackson 2001; Lauretta et al. 2001; Hintelmann and Lu 2003), and there were still some important analytical issues that needed to be resolved. But the field has come a long way in a decade. Now we have a different problem; at our last count there were well over 100 publications utilizing mercury stable isotopes and it is becoming very difficult to synthesize this vast amount of exciting and rapidly developing research. Experimental studies have expanded our knowledge of the mechanisms of mercury isotope fractionation and applications of mercury isotope measurements have touched virtually every area of research in mercury biogeochemistry. There have been a number of previous reviews of the mercury stable isotope literature as it has developed (Ridley and Stetson 2006; Bergquist and Blum 2009; Yin et al. 2010; Blum 2011; Hintelmann 2012; Blum et al. 2014). It is our view that the field has become too large to comprehensively review the entire literature on mercury stable isotopes. Ten years ago Hg isotope researchers were just beginning to explore the boundaries of natural Hg isotope variation and the mechanisms that cause this variation in the environment. At that time large and relatively easily measured isotope signals were of great interest and mercury isotope researchers were beginning to develop theories to explain mass dependent isotope fractionation (MDF) and mass independent isotope fractionation of the odd mass-numbered isotopes of mercury (odd-MIF). More recently researchers have discovered a wider range of types of isotopic variability (even-MIF), some of which are subtle and …

Garnet: A Rock-Forming Mineral Petrochronometer

Abstract: Garnet could be the ultimate petrochronometer. Not only can you date it directly (with an accuracy and precision that may surprise some), but it is also a common rock-forming and porphyroblast-forming mineral, with wide ranging—yet thermodynamically well understood—solid solution that provides direct and quantitative petrologic context. While accessory phase petrochronology is based largely upon establishing links to the growth or breakdown of key rock-forming pressure–temperature–composition ( P–T–X ) indicators (e.g., Rubatto 2002; Williams et al. 2007), garnet is one of those key indicator minerals. Garnet occurs in a great variety of rock types (see Baxter et al. 2013) and is frequently zoned (texturally, chemically) meaning that it contains more than just a snapshot of metamorphic conditions, but rather a semi-continuous history of evolving tectonometamorphic conditions during its often prolonged growth. In this way, garnet and its growth zonation have been likened to dendrochronology: garnet as the tree rings of evolving tectonometamorphic conditions (e.g., Pollington and Baxter 2010). In some ways, the dream of ‘petrochronology’ all started with garnet (Fig. 1). When Atherton and Edmunds (1965) or Hollister (1966) recognized the chemical zonation in garnet, when Rosenfeld (1968) noted the spiral ‘snowball’ of inclusions in rotated garnet, or when Tracy et al. (1976) drew the first 2-D map of garnet chemical zonation, illuminating those ‘tree-rings’ for the first time, they could only imagine what is now a reality decades later—direct zoned garnet geochronology of those concentric rings of growth. Geoscientists soon thereafter attempted the first garnet geochronology (van Breemen and Hawkesworth 1980), though several factors severely limited the development and wider-spread use of garnet geochronology from that point. These factors included 1) contamination of garnet by micro-mineral inclusions, 2) analytical limitations of small sample size, 3) the requirement of anchoring a garnet age analysis with another point on an isochron, and …

Silicon Isotope Geochemistry

Abstract: In contrast to many other stable isotopes of the elements discussed in this book, those of silicon are not strictly speaking “Non-Traditional Stable Isotopes” because they have been studied for more than 60 years. After the pioneering works of Reynolds and Verhoogen (1953) and Allenby (1954), a steady increase in silicon isotope studies of geological materials has led to a substantial corpus of data. These data were compiled by Ding et al. (1996) alongside new measurements that, collectively, included over a thousand samples of rocks, minerals, waters and biological materials. Most of these data were produced using the well established method of gas source mass spectrometry after sample decomposition and silicon purification via fluorination techniques. As for many non-traditional stable isotopes, silicon isotope research has flourished with the advent of second generation of multicollector plasma source mass spectrometers (MC–ICP–MS). These instruments eliminated the requirement of hazardous gaseous fluorine sample preparation methods while permitting improved analytical precision in both wet plasma (De La Rocha 2002) and in dry plasma (Cardinal et al. 2003). Subsequent analytical developments involving high mass resolution MC–ICP–MS combined with improved silicon purification methods (Georg et al. 2006) made this analytical technique more robust and precise enough to study even the subtle silicon isotope variations produced during high temperature geological processes (Savage et al. 2014). Silicon is the fourteenth element of the Periodic Table. Its atomic mass was precisely determined to be 28.08553 ± 0.00039 in atomic mass units (a.m.u.) on a pure silicon reference material (NIST SRM–990, Barnes et al. 1975). This 95% confidence limit error includes the overall natural isotopic variation range for 30Si/28Si known by the time, estimated to be about 5‰ from the analysis of biological, meteoritic and terrestrial materials (Tilles 1961). As detailed below, the current database suggests …

Petrology and Geochronology of Rutile

Abstract: Rutile (TiO2) is an important accessory mineral that, when present, offers a rich source of information about the rock units in which it is incorporated It occurs in a variety of specific microstructural settings, contains significant amounts of several trace elements and is one of the classical minerals used for U–Pb age determination Here, we focus on information obtainable from rutile in its original textural context We do not present an exhaustive review on detrital rutile in clastic sediments, but note that an understanding of the petrochronology of rutile in its source rocks will aid interpretation of data obtained from detrital rutile For further information on the important role of rutile in provenance studies, the reader is referred to previous reviews (eg, Zack et al 2004b; Meinhold 2010; Triebold et al 2012) Coarse rutile is the only stable TiO2 polymorph under all crustal and upper mantle conditions, with the exception of certain hydrothermal environments (Smith et al 2009) As such, we will focus on rutile rather than the polymorphs brookite, anatase and ultrahigh-pressure modifications In this chapter, we first review rutile occurrences, trace element geochemistry, and U–Pb geochronology individually to illustrate the insights that can be gained from microstructures, chemistry and ages Then, in the spirit of petrochronology, we show the interpretational power of combining these approaches, using the Ivrea Zone (Italy) as a case study Finally, we suggest some areas of future research that would improve petrochronologic research using rutile Rutile is a characteristic mineral in moderate- to high pressure metapelitic rocks, in high pressure metamorphosed mafic rocks, and in sedimentary rocks (eg, Force 1980; Frost 1991; Zack et al 2004b; Triebold et al 2012) Rutile also occurs rarely in magmatic rocks, eg, anorthosites, as well as in some hydrothermal systems Coarse-grained …

Significant Ages—An Introduction to Petrochronology

Abstract: Question : Why “Petrochronology”? Why add another term to an already cluttered scientific lexicon? Answer : Because petrologists and geochronologists need a term that describes the unique, distinctive way in which they apply geochronology to the study of igneous and metamorphic processes Other terms just won’t do Such evolution of language is natural and well-established For instance, “Geochronology” was originally coined during the waning stages of the great Age-of-the-Earth debate as a means of distinguishing timescales relevant to Earth processes from timescales relevant to humans (Williams 1893) Eighty-eight years later, Berger and York (1981) coined the term “Thermochronology,” which has evolved as a branch of geochronology aimed at constraining thermal histories of rocks, where (typically) the thermally activated diffusive loss of a radiogenic daughter governs the ages we measure Thermochronology may now be distinguished from “plain vanilla” geochronology, whose limited purpose, in the words of Reiners et al (2005), is “…exclusively to determine a singular absolute stratigraphic or magmatic [or metamorphic] formation age, with little concern for durations or rates of processes” that give rise to these rocks Neither of these terms describes what petrologists do with chronologic data A single date is virtually useless in understanding the protracted history of magma crystallization or metamorphic pressure–temperature evolution And we are not simply interested in thermal histories, but in chemical and baric evolution as well Rather, we petrologists and geochronologists strive to understand rock-forming processes, and the rates at which they occur, by integrating numerous ages into the petrologic evolution of a rock It is within this context that a new discipline, termed “Petrochronology”, has emerged1 In some sense petrochronology may be considered the sister of thermochronology: petrochronology typically focuses on the processes leading up to the formation of igneous and metamorphic rocks—the minerals and textures we observe …

Chromium Isotope Geochemistry

Abstract: Chromium consists of four stable isotopes (50Cr, 52Cr, 53Cr and 54Cr) with natural abundances of 4.35%, 83.79%, 9.50% and 2.36%, respectively (Rossman and Taylor 1998). Among these four isotopes, 50Cr, 52Cr and 54Cr are non-radiogenic, whereas 53Cr is a radiogenic product of the extinct nuclide 53Mn, which has a half-life of 3.7 Myr (Honda and Imamura 1971). Chromium isotope systems have a wide range of applications in geochemistry and cosmochemistry. They have been used to study early solar system processes (e.g., Rotaru et al. 1992); the oxidation/reduction (redox) potential of underground systems, which governs the transport and fate of many contaminants (e.g., Ellis et al. 2002); and more recently, the redox evolution of Earth’s early ocean-atmosphere system, which is intimately linked to the evolution of life (Frei et al. 2009; Crowe et al. 2013; Planavsky et al. 2014; Cole et al. 2016). ### Chemical properties of Cr Chromium is redox-sensitive. In Earth’s near-surface environments, Cr has two main valence states, +3 and + 6, which are expressed as Cr(III) and Cr(VI), respectively. The valence state of Cr is controlled by the prevailing redox potential (Eh) and pH conditions (Fig. 1). Cr(VI) is always bound with O2− to form the oxyanion species CrO42− (chromate), HCrO4− (bichromate), and Cr2O72−(dichromate), all of which are water-soluble. In contrast, Cr3+ usually forms oxyhydroxides or oxides, which are insoluble and immobile in the natural pH range. During oxidative weathering, Cr(III) in minerals can be oxidized by O2 to Cr(VI), a process that is catalyzed by manganese oxides (Fendorf and Zasoski 1992; Economou-Eliopoulos et al. 2014). The Cr(VI) migrates to rivers and eventually to the ocean. In the modern ocean, Cr occurs as both Cr(VI) and …

Phase Relations, Reaction Sequences and Petrochronology

Abstract: At the core of petrochronology is the relationship between geochronology and the petrological evolution of major mineral assemblages. The focus of this chapter is on outlining some of the available strategies to link inferred reaction sequences and microstructures in metamorphic rocks to the ages obtained from geochronology of accessory minerals and datable major minerals. Reaction sequences and mineral assemblages in metamorphic rocks are primarily a function of pressure ( P ), temperature ( T ) and bulk composition ( X ). Several of the major rock-forming minerals are particularly sensitive to changes in P–T (e.g., garnet, staurolite, biotite, plagioclase), but their direct geochronology is challenging and in many cases not currently possible. One exception is garnet, which can be dated using Sm–Nd and Lu–Hf geochronology (e.g., Baxter et al. 2013). Accessory mineral chronometers such as zircon, monazite, xenotime, titanite and rutile are stable over a relatively wide range of P–T conditions and can incorporate enough U and/or Th to be dated using U–Th–Pb geochronology. Therefore, linking the growth of P–T sensitive major minerals to accessory and/or major mineral chronometers is essential for determining a metamorphic P–T–t history, which is itself critical for understanding metamorphic rocks and the geodynamic processes that produce them (e.g., England and Thompson 1984; McClelland and Lapen 2013; Brown 2014). Linking the ages obtained from accessory and major minerals with the growth and breakdown of the important P–T sensitive minerals requires an understanding of the metamorphic reaction sequences for a particular bulk rock composition along a well-constrained P–T evolution. Fortunately, the phase relations and reaction sequences for the most widely studied metamorphic protoliths (e.g., pelites, greywackes, basalts) can be determined using quantitative phase equilibria forward modelling (e.g., Powell and Holland 2008). Comprehensive activity–composition models of the major metamorphic minerals in large chemical systems (e.g., White et al. 2014a) allow …

Kinetic Fractionation of Non-Traditional Stable Isotopes by Diffusion and Crystal Growth Reactions

Abstract: Natural variations in the isotopic composition of some 50 chemical elements are now being used in geochemistry for studying transport processes, estimating temperature, reconstructing ocean chemistry, identifying biological signatures, and classifying planets and meteorites. Within the past decade, there has been growing interest in measuring isotopic variations in a wider variety of elements, and improved techniques make it possible to measure very small effects. Many of the observations have raised questions concerning when and where the attainment of equilibrium is a valid assumption. In situations where the distribution of isotopes within and among phases is not representative of the equilibrium distribution, the isotopic compositions can be used to access information on mechanisms of chemical reactions and rates of geological processes. In a general sense, the fractionation of stable isotopes between any two phases, or between any two compounds within a phase, can be ascribed to some combination of the mass dependence of thermodynamic (equilibrium) partition coefficients, the mass dependence of diffusion coefficients, and the mass dependence of reaction rate constants. Many documentations of kinetic isotope effects (KIEs), and their practical applications, are described in this volume and are therefore not reviewed here. Instead, the focus of this chapter is on the measurement and interpretation of mass dependent diffusivities and reactivities, and how these parameters are implemented in models of crystal growth within a fluid phase. There are, of course, processes aside from crystal growth that give rise to KIEs among non-traditional isotopes, such as evaporation (Young et al. 2002; Knight et al. 2009; Richter et al. 2009a), vapor exsolution (Aubaud et al. 2004), thermal diffusion (Richter et al. 2009a, 2014b; Huang et al. 2010; Dominguez et al. 2011), mineral dissolution (e.g., Brantley et al. 2004; Wall et al. 2011; Pearce et al. 2012 …

Petrochronology of Zircon and Baddeleyite in Igneous Rocks: Reconstructing Magmatic Processes at High Temporal Resolution

Abstract: Zircon (ZrSiO4) and baddeleyite (ZrO2) are common accessory minerals in igneous rocks of felsic to mafic composition. Both minerals host trace elements substituting for Zr, among them Hf, Th, U, Y, REEs and many more. The excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past, utilizing the outstanding power and temporal resolution of the U–Pb decay schemes. Baddeleyite is a chemically and physically much more fragile mineral. It preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine-grained or non-reactive rock matrix. It offers the potential for dating the solidification of mafic rocks with high-precision through its crystallization in small pockets of Zr-enriched melt, after extensive olivine and pyroxene fractionation. Zircon and baddelelyite U–Pb dates are, for an overwhelming majority of cases and where we can assume a closed system, considered to reflect the time of crystallization. The development of the U–Pb dating tool CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) since 2005 has led to unprecedented precision of better than 0.1% in 206Pb/238U dates (Bowring et al. 2005). Increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine U–Pb age determinations of micrograms of material at sufficiently high radiogenic/common lead ratios (see Schoene and Baxter 2017, this volume). In situ U–Pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub-microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from ~30 μm for laser-ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS) to 10 μm for secondary ion mass spectrometry (SIMS), lateral resolutions of 2–5 μm for NanoSIMS …

Investigation and Application of Thallium Isotope Fractionation

Abstract: This contribution summarizes the current state of understanding and recent advances made in the field of stable thallium (Tl) isotope geochemistry. High precision measurements of Tl isotope compositions were developed in the late 1990s with the advent of multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and subsequent studies revealed that Tl, despite the small relative mass difference of the two isotopes, exhibits substantial stable isotope fractionation, especially in the marine environment. The most fractionated reservoirs identified are ferromanganese sediments with ɛ 205 Tl ≈ +15 and low temperature altered oceanic crust with ɛ 205 Tl ≈ −20. The total isotopic variability of more than 35 ɛ 205 Tl-units hence exceeds the current analytical reproducibility of the measurement technique by more than a factor of 70. This isotopic variation can be explained by invoking a combination of conventional mass dependent equilibrium isotope effects and nuclear field shift isotope fractionation, but the specific mechanisms are still largely unaccounted for. Thallium isotopes have been applied to investigate paleoceanographic processes in the Cenozoic and there is evidence to suggest that Tl isotopes may be utilized as a monitor of the marine manganese oxide burial flux over million year time scales. In addition, Tl isotopes can be used to calculate the magnitude of hydrothermal fluid circulation through ocean crust. It has also been shown that the subduction of marine ferromanganese sediments can be detected with Tl isotopes in lavas erupted in subduction zone settings as well as in ocean island basalts. Meteorite samples display Tl isotope variations that exceed the terrestrial range with a total variability of about 50 ɛ 205 Tl. The large isotopic diversity, however, is generated by both stable Tl isotope fractionations, which reflect the highly volatile and labile cosmochemical nature of the element, and radiogenic decay of extinct 205 Pb to 205 Tl with a half-life of about 15 Ma. The difficulty of deconvolving these two sources of isotopic variability restricts the utility of both the 205 Pb– 205 Tl chronometer and the Tl stable isotope system to inform on early solar system processes.

Equilibrium Fractionation of Non-traditional Isotopes: a Molecular Modeling Perspective

Abstract: The isotopic compositions of natural materials are determined by their parent reservoirs, on the one hand, and by fractionation mechanisms, on the other hand. Under the right conditions, fractionation represents isotope partitioning at thermodynamic equilibrium. In this case, the isotopic equilibrium constant depends on temperature, and reflects the slight change of free energy between two phases when they contain different isotopes of the same chemical element. The practical foundation of the theory of mass-dependent stable isotope fractionation dates back to the mid-twentieth century, when Bigeleisen and Mayer (1947) and Urey (1947) proposed a formalism that takes advantage of the Teller–Redlich product rule (Redlich 1935) to simplify the estimation of equilibrium isotope fractionations. In this chapter, we first give a brief introduction to this isotope fractionation theory. We see in particular how the various expressions of the fractionation factors are derived from the thermodynamic properties of harmonically vibrating molecules, a surprisingly effective mathematical approximation to real molecular behavior. The central input data of these expressions are vibrational frequencies, but an approximate formula that requires only force constants acting on the element of interest can be applied to many non-traditional isotopic systems, especially at elevated temperatures. This force-constant based approach can be particularly convenient to use in concert with first-principles electronic structure models of vibrating crystal structures and aqueous solutions. Collectively, these expressions allow us to discuss the crystal chemical parameters governing the equilibrium stable isotope fractionation. Since the previous volume of Reviews in Mineralogy and Geochemistry dedicated to non-traditional stable isotopes, the number of first-principles molecular modeling studies applied to geosciences in general and to isotopic fractionation in particular, has significantly increased. After a concise introduction to computational methods based on quantum mechanics, we will focus on the modeling of isotopic properties in liquids, which represents a bigger methodological challenge than …

Hadean Zircon Petrochronology

Abstract: The inspiration for this volume arose in part from a shift in perception among U–Pb geochronologists that began to develop in the late 1980s Prior to then, analytical geochronology emphasized progressively lower blank analysis of separated accessory mineral aggregates (eg, Krogh 1982; Parrish 1987), with results generally interpreted to reflect a singular moment in time For example, a widespread measure of confidence in intra-analytical reliability was conformity to an MSWD (a form of χ2 test; Wendt and Carl 1991) of unity This approach implicitly assumed that geological processes act on timescales that are short with respect to analytical errors (eg, Schoene et al 2015) As in situ methodologies (eg, Compston and Pidgeon 1986; Harrison et al 1997; Griffin et al 2000) and increasingly well-calibrated double spikes (eg, Amelin and Davis 2006; McLean et al 2015) emerged, geochronologists began to move away from interpreting geological processes as a series of instantaneous episodes (eg, Rubatto 2002) At about the same time, petrologists developed techniques that permitted in situ chemical analyses to be interpreted in terms of continuously changing pressure–temperature–time histories (eg, Spear 1988) The recognition followed that specific mineral reactions yielded products that could be directly dated or interpreted in terms of protracted petrogenetic processes Part of this shift was due to an appreciation that trace elements in accessory phases could identify the changing nature of modal mineralogy during crystal growth (eg, Pyle et al 2001; Kohn and Malloy 2004) and thus potentially relate petrogenesis to absolute time The transition to petrochronology was complete upon recognition that high MSWDs were in fact the expected case for most metamorphic minerals (Kohn 2009) One of the great frontiers for fundamental discovery in the geosciences is earliest Earth (DePaolo et al 2008) However, investigations of the first five …

Electron Microprobe Petrochronology

Abstract: The term petrochronology has increasingly appeared in publications and presentations over the past decade The term has been defined in a somewhat narrow sense as “the interpretation of isotopic dates in the light of complementary elemental or isotopic information from the same mineral(s)” (Kylander-Clark et al 2013) Although complementary isotopic and elementary information are certainly a central and critical part of most, if not all, petrochronology studies, the range of recent studies that might use the term covers a much broader scope The term “petrochronology” might alternatively be defined as the detailed incorporation of chronometer phases into the petrologic (and tectonic) evolution of their host rocks, in order to place direct age constraints on petrologic and structural processes As noted by Kylander-Clark et al (2013), the linkage between geochronology and petrology can involve a variety of data including mineral textures and fabrics, the distribution of mineral modes or volume proportions, compositional zoning, mineral inclusion relationships, and certainly major element, trace element, and isotopic composition of the chronometer and all other phases Electron probe micro-analysis (EPMA) has a central and critical role to play in establishing the linkage between chronometer phases and their host assemblage The basic instrument is an electron microscope which can be used in either scanning or fixed beam modes, with integrated wavelength dispersive spectrometers (WDS), energy dispersive spectrometers (EDS), electron detectors (to image secondary and backscattered signals) a light optical system, and optionally cathodoluminescence (CL) detection The electron microprobe is used to investigate the distribution, composition, and compositional zonation of all mineral phases, the data that underpin thermobarometric analysis and modeling of P–T histories The microprobe, with μm-scale spatial resolution, can also characterize compositional zonation in very small accessory phases including monazite, xenotime, zircon, allanite, titanite, apatite, and others This, as discussed below, can be a …

Chlorine Isotope Geochemistry

Abstract: Chlorine played a prominent role in the discovery of isotopes. The famous Cavendish Laboratory scientists were fascinated with the atomic mass of Cl. Most elements have a mass that is a close approximation of the multiple of hydrogen (e.g., Aston 1927). By 1920, it was recognized that the atomic weight of Cl was ~35.5, which appeared to violate Francis Aston’s whole number rule. Sir Joseph J. Thomson started the famous “Discussion on Isotopes” (Thomson et al. 1921) with the following: “I will plunge at once into the most dramatic case of the isotopes—the case of chlorine”. The discussion that followed between three Nobel Prize winners pitted Thomson against Aston and Frederick Soddy, the latter two in defense of multiple isotopes of a single element. And so the game began. Aston (1919, 1920) argued that the mass spectra of Cl-bearing compounds (e.g., HCl, COCl) supported the existence of at least two isotopes of Cl, 35Cl and 37Cl. However, Thomson contended that the spectra may be the result of different compounds of Cl and not necessarily different isotopes of Cl (Thomson et al. 1921). Ultimately, Aston was proven correct (e.g., Harkins and Hayes 1921; Harkins and Liggett 1923) and is now credited with the discovery of the two stable isotopes of Cl, which is notable for the unusually large abundance of its “rare” isotope. The relative abundances of 35Cl and 37Cl are currently accepted to be 75.76% and 24.24%, respectively (Berglund and Wieser 2011). It was not until ~75 years after the discovery of the stable isotopes of Cl that they become more “routinely” analyzed and the chlorine isotope compositions of various chlorine reservoirs were beginning to be determined. Here we summarize the current state of chlorine isotope standards, analytical methods, and fractionation, as well …

Petrochronology by Laser-Ablation Inductively Coupled Plasma Mass Spectrometry

Abstract: Petrochronology is a field of Earth science in which the isotopic and / or elemental composition of a mineral chronometer is interpreted in combination with its age, thus yielding a more synergistic combination of petrology and chronology that can be used to interpret geologic processes It has recently attracted renewed interest as technologies for mineral analysis have improved Examples are many, and continue to grow, from the early adoption of U / Th ratios in zircon as an indicator for magmatic vs igneous crystallization (eg, Ahrens 1965), to using the Nd isotopic composition in titanite to track source contribution over time (see Applications ; B R Hacker, personal communication) Age and chemical information can be obtained by a variety of techniques: electron microprobe (age; major and minor elements; see Williams et al 2017), secondary ion mass spectrometry (SIMS; age; trace elements; isotopic ratios; see Schmitt and Vazquez 2017), and laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS; age; trace elements; isotopic ratios) Laser-ablation ICPMS instrumentation and techniques, the focus of this chapter, have been employed as a petrochronologic tool for decades, starting with separate analyses of ages and elemental and / or isotopic compositions, which were then combined and interpreted For example, Zheng et al (2009) employed LA-ICPMS to analyze the trace-element (TE) chemistry, Hf isotopic composition, and age of zircons from kimberlites by using three spots on each zircon grain, one for each type of analysis This work was relatively time consuming and expensive, given the required number of analytical sessions, but yielded far better confidence in the conclusions, because of the link between physical conditions (petrology) and time (chronology) Instrumentation and techniques which employ LA-ICPMS have continued to improve, particularly in the ease with which petrochronologic data can be obtained A single LA-ICPMS instrument can now measure both the …

Medical applications of isotope metallomics

Abstract: One may wonder how a paper discussing medical applications of metal isotopes got lost in a review journal dedicated to mineralogy and geochemistry. The justifications are multiple. First, the coming of age of metal isotopic analysis in the mid ‘90s is largely due to the analytical creativity of the geochemical community and to corporate technical skills allowing the rise of new technologies. Second, many concepts, which can be imbedded in quantitative models testable from their predictions, are common to geochemistry, biochemistry, physiology, and nutrition: a cell, with its organelles, a body with its organ and body fluids, are systems liable to treatments similar to those used to model a lake, the ocean–atmosphere, and the mantle–crust systems. Of course, time scales and length scales differ, the complexity of biology is immense compared to that of the mineral world. Geological systems lack the hallmarks of life, genes and cell signaling. In spite of the overall complexity of the biological systems, pathways, kinetics, and chemical dynamics are better understood than their counterpart in earth sciences. Like in many fields of engineering, comparing the records of inputs and outputs is a powerful tool to identify the internal ‘knobs’ controlling a given system and learn how to tweak them. Third, although some of the most sophisticated techniques such as ab initio calculations of molecular configurations, energetics, and isotopic properties are still limited to molecules with less than a few dozens of atoms, the time is getting closer to when simulations of large molecules will become available for application to ‘real’ proteins with large molecular weights. The present article reviews some of the basic features of what is now known as Metallomics and the preliminary applications of stable isotopes to some medical cases, a discipline for which we suggest the simple term of Isotope Metallomics . …

The Isotope Geochemistry of Ni

Abstract: Nickel is an iron-peak element with 5 stable isotopes (see Table 1) which is both cosmochemically abundant and rich in the information carried in its isotopic signature. Significantly, 60Ni is the radiogenic daughter of 60Fe, a short-lived nuclide (t1/2 = 2.62 Ma; Rugel et al. 2009) of a major element. 60Fe has the potential to be both an important heat source and chronometer in the early solar system. 60Ni abundances serve to document the prior importance 60Fe and this is a topic of on-going debate (see Extinct 60 Fe and radiogenic 60 Ni ). The four other stable Ni nuclides span a sizeable relative mass range of ~10%, including the notably neutron-rich nuclide 64Ni. The relative abundances of these isotopes vary with diverse stellar formation environments and provide a valuable record of the nucleosynthetic heritage of Ni in the solar system (see Nucleosynthetic Ni isotopic variations ). Ni occurs widely as both elemental and divalent cationic species, substituting for Fe and Mg in common silicate structures and forming Fe/Ni metal alloys. The Ni isotope chemistry of all the major planetary reservoirs and fractionations between them can thus be characterized (see Mass-Dependent Ni isotopic Variability ). Ni is also a bio-essential element and its fractionation during low-temperature biogeochemical cycling is a topic that has attracted recent attention (see Mass-Dependent Ni isotopic Variability ). ### Notation Much of the work into Ni has been cosmochemical, focussing on the nucleosynthetic origins of different meteoritic components. Such studies have primarily investigated mass-independent isotopic variations, both radiogenic and non-radiogenic, which require choosing a reference isotope pair for normalization. Throughout this work we use 58Ni–61Ni as the normalizing pair, in keeping with current practice in the field. An alternative 58Ni–62Ni normalization scheme has previously been used for bulk …

Selenium Isotopes as a Biogeochemical Proxy in Deep Time

Abstract: Funding during the compilation of this manuscript was provided by the NASA postdoctoral program.

Chronometry and Speedometry of Magmatic Processes using Chemical Diffusion in Olivine, Plagioclase and Pyroxenes

Abstract: The magmatic processes that fuel volcanism, crustal growth, ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales, from millions of years to just a few seconds. For example, the production of new oceanic crust at mid-ocean ridges is a near-continuous process that can operate in any one ocean basin on timescales of more than 100 m.y. However, the driving force for such processes is the spreading of the ocean plates that happens on a cm/yr timescale. At the other end of the spectrum, explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m/s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes. In this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure. At intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit, while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years. Although each of these processes has a characteristic, time-averaged timescale on which it operates, this is typically the end result of one or more natural processes that operate on much shorter timescales. For example, mid-ocean ridges do not extrude magma continuously onto the ocean floor, mineralising fluids do not discharge continuously through the shallow crust, and granitic magmas do not dribble continuously into evolving batholithic chambers. In some cases it is the long-term timescales that are important, for example the spreading rate of ocean basins, in others it is the short-term timescales that are important, for example the episodic growth of lava domes at active volcanoes. Although …

Germanium Isotope Geochemistry

Abstract: Germanium (Ge) is a trace element in the Earth’s crust and natural waters, averaging about 1.6 ppm in rocks and minerals (El Wardani 1957; Bernstein 1985) and 75 picomol/L in seawater (Froelich and Andreae 1981). The naturally occurring oxidation states of Ge are +2 and +4, with the +4 state forming the principal common and stable compounds. Germanium has outer electronic structure 3 d 10 4 s 2 4 p 2 and mainly occurs in the quadrivalent state, although in some minerals it is octahedrally coordinated. Germanium is chemically similar to silicon (Si), both belonging to the IVA group in the periodic table, with Ge immediately above Si. Germanium is classified as a semimetal, whereas Si is a nonmetal element. Because of nearly identical ionic radii and electron configurations for Ge and Si, the crustal geochemistry of Ge is dominated by a tendency to replace Si in the lattice sites of minerals (Goldschmidt 1958; De Argollo and Schilling 1978b). These two elements exist in seawater as similar hydroxyacids, i.e., Ge(OH)4 and Si(OH)4 (Pokrovski and Schott 1998a) and the concentration profile of Ge is similar to that of Si (Froelich and Andreae 1981), thus making Ge/Si ratio an interesting tracer for biogenic silica cycling in the ocean. Although Ge and Si are geochemically similar, their behavior is different enough so that decoupling of Ge and Si can occur. Germanium commonly occurs in 4-fold (tetrahedral) coordination but in contrast to Si, Ge has a stronger tendency for the 6-fold coordination. Unlike Si, Ge also forms methylated compounds, and high concentrations of monomethyl- and dimethyl-germanium have been detected in ocean waters, accounting for > 70% of the total Ge (Lewis et al. 1985). Germanium is a particularly interesting element for geochemists since it exhibits siderophile, lithophile, chalcophile and …