- © 2016 Mineralogical Society of America
In high-temperature geochemistry and cosmochemistry, highly siderophile and strongly chalophile elements can be defined as strongly preferring metal or sulfide, respectively, relative to silicate or oxide phases. The highly siderophile elements (HSE) comprise Re, Os, Ir, Ru, Pt, Rh, Pd, and Au and are defined by their extreme partitioning (> 104) into the metallic phase, but will also strongly partition into sulfide phases, in the absence of metal. The HSE are highly refractory, as indicated by their high melting and condensation temperatures and were therefore concentrated in early accreted nebular materials. Within the HSE are the platinum-group elements (PGE), which include the six elements lying in the d-block of the periodic table (groups 8, 9, and 10, periods 5 and 6), i.e., Os, Ir, Ru, Pt, Rh and Pd. These six elements tend to exist in the metallic state, or bond with chalcogens (S, Se, Te) or pnictogens (P, As, Sb, Bi). Rhenium and Au do not necessarily behave as coherently as the PGE, due to their differing electronegativity and oxidation states. For these reasons, a clear definition between the discussion of the PGE and the HSE (PGE, Re and Au) exists in the literature, especially in economic geology, industrial, or bio-medical studies.
The strongly chalcophile elements can be considered to include S, Se, and Te. These three elements are distinguished from other chalcophile elements, such as Cd or Pb, because, like the HSE, they are all in very low abundances in the bulk silicate Earth (Fig. 1). By contrast with the HSE, S, Se, and Te all have far lower melting and condensation temperatures, classifying them as highly volatile elements (Table 1). Moreover, these elements are not equally distributed within chondrite meteorite groups (Fig. 2). Since their initial distribution in the Solar nebula, planetary formation and differentiation process have led to large fractionations of the HSE and strongly chalcophile elements, producing a range of absolute and relative inter-element fractionations.
The chemical properties of the HSE, that set them apart from any other elements in the periodic table (Table 2), have made them geochemical tracers par excellence. As tracers of key processes, the HSE have found application in virtually all areas of the physical Earth sciences. These elements have been used to inform on the nucleosynthetic sources and formation of the Solar System, planetary differentiation, late accretion addition of elements to planets, core-formation and possible core-mantle interaction, crust-mantle partitioning, volcanic processes and outgassing, formation of magmatic, hydrothermal and epithermal ore deposits, ocean circulation, climate-related events, weathering, and biogeochemical cycling. More recently, studies of strongly chalcophile elements are finding a similar range of applications. Their utility lies in the fact that these elements will behave as siderophile or strongly chalcophile elements under reducing conditions, but will also behave as lithophile or atmophile elements under oxidizing conditions, as experienced at the present day Earth’s surface.
A key aspect of the HSE is that three long-lived, geologically useful decay systems exist with the HSE as parent (107Pd–107Ag), or parent–daughter isotopes (187Re–187Os and 190Pt–186Os). This volume is dedicated to some of the processes that can be investigated at high-temperatures in planets using the HSE and strongly chalcophile elements.
While this volume is not dedicated to the practical applications of the HSE and strongly chalcophile elements, it would be remiss not to briefly discuss the importance of these elements in society. All of these elements have found important societal use (Table 3), from the application of Au as a valued commodity in early societies, through to the present-day; the importance of S and Se in biological processes; the discovery and implementation of Pt, Pd, and subsequently other PGE to catalytic oxidation (Davy 1817), and the importance of the anti-cancer drug cisplatin (cis-[Pt(NH3)2Cl2]) to anti-tumour treatments (e.g., Rosenberg et al. 1969). The use of the PGE, most especially Pt, Pd and Rh, in the automotive industry to generate harmless gases has caused some potential collateral effects; the possible environmental impact and human health-risks from available PGE in the environment (see Rauch and Morrison 2008, for a review). An entire volume can (and should!) equally be written on the utility of the HSE and strongly chalcophile elements during low-temperature geochemistry.
BASIC CONCEPTS AND TERMINOLOGY
Of the eight HSE, only Rh and Au are monoisotopic. Important data on Au and/or Rh abundances can be generated using non-isotope dilution methodologies, but the over-whelming majority of abundance data discussed in this volume for the HSE are for Re, Os, Ir, Ru, Pt, and Pd, which can be measured using isotope-dilution methodologies. Abundances of the strongly chalcophile elements, S, Se, and Te, can also be measured by isotope dilution. Isotope dilution studies to obtain abundances tend to use isotopically enriched tracers (typically 34S, 77Se, 99Ru, 105Pd, 125Te, 185Re, 190Os, 191Ir, 194Pt), followed by inductively coupled plasma mass spectrometry measurement. Isotopic studies of Os are described below and methods for determination of isotopic variations in other HSE and strongly chalcophile elements have been developed, but are not explicitly discussed here. The reader is referred to Meisel and Horan (2016, this volume), and associated references for details of these isotopic methodologies.
In addition to their strongly siderophile tendencies, HSE exhibit contrasting behaviors during melting, with the platinum-PGE (PPGE; Pt, Pd: melting temperature < 2000 °C; Barnes et al. 1985), Re and Au typically being more incompatible during melting and crystallization, relative to the iridium–PGE (IPGE; Os, Ir, Ru: melting temperature > 2000 °C; Barnes et al. 1985). For this reason, studies of the cosmochemical behavior of the HSE will often list the HSE in order of melting temperature of the pure metal, whereas studies using the HSE to investigate mantle melting processes will order the HSE according to relative incompatibility during melting. For mantle peridotites, there is general agreement that bulk peridotite-melt partition coefficients follow the sequence (e.g., Fischer-Gödde et al. 2011; Wang and Becker 2013; König et al. 2014):
The ability of relative and absolute HSE abundances to record recent processes acting on rocks are complemented by the existence of the long-lived 190Pt–186Os (190Pt → 186Os + α + Q; λ = 1.48 × 10−12 a−1; Walker et al. 1997) and 187Re–187Os (187Re − 187Os + β− + ν̄; λ = 1.6668 × 10−11 a−1; Selby et al. 2007) chronometers. Both long-lived radiogenically produced isotopes are minor constituents (186Os = 1.6%; 187Os = 1.5%; Shirey and Walker, 1998) of osmium. In the case of the 187Re–187Os system, where 187Re is a major isotope (62.6%) of rhenium, and has a half-life of 41.6 Ga, the range of natural materials spans several orders of magnitude and 187Os/188Os can reasonably range from a Solar System initial ratio of ~ 0.095 to nearly pure 187Os derived from samples essentially devoid of Os and with high concentrations of Re (e.g., molybdenite; Luck and Allègre 1982). This characteristic means that the percent-level difference of 187Os/188Os between natural samples allows routine analysis of low Os abundance samples to percent precision or better, with the most widely-used method of analysis being negative thermal ionisation mass spectrometry (N-TIMS; Creaser et al. 1991; Völkening et al. 1991).
The generally accepted ‘chondritic composition’ for 187Os/188Os is 0.127 (Shirey and Walker 1998), although there are clear differences between carbonaceous chondrites (187Os/188Os = ~ 0.1262), relative to ordinary (187Os/188Os = ~ 0.1284), or enstatite chondrites (187Os/188Os = ~ 0.1280; Day et al. 2016, this volume). Chondritic evolution is established from the most primitive initial 187Os/188Os defined from early Solar System iron meteorites (initial 187Os/188Os = 0.09531) to the average chondritic composition for the present day. For these parameters, the average 187Re/188Os of chondrites is 0.40186. To calculate the 187Os/188Os of chondrites at any time in the past—or future—the following equation can be used:
where λ is equal to 1.6668 × 10−11 a−1 (Selby et al. 2007). For ease of reference, studies will often report the percentage difference between the Os isotope composition of a samples and the average ‘chondritic’ composition for a specified time, γOs. Samples with positive γOs are often described as ‘enriched’, because it implies long-term elevated 187Re/188Os with respect to chondrites. Samples with negative γOs are often described as ‘depleted’, due to the opposite implication of long-term low 187Re/188Os, in the following way:
Model ages (MA) and relative rhenium depletion ages (RD) ages can all be calculated using the Re–Os isotope system (Fig. 3). Model ages (TMA) represent the timing of separation from chondritic evolution and can be estimated for low Re/Os mantle materials, as well as high Re/Os melts or crustal materials. The assumption with this method is that the Re/Os measured in the sample is an accurate reflection of its long-term history and has not been affected by later processes:
By contrast, time of relative Re depletion ages (TRD), which apply to low Re/Os mantle peridotites, does not rely on the Re/Os measured in the sample, which can be affected by recent Re addition. Instead this method uses sample compositions at the time of eruption and assumes that all of the Re in the sample was removed during melt-depletion. In reality, this method provides a minimum age for samples that have experienced melt-depletion:
Due to potential disturbance from terrestrial weathering, or from cosmic-ray exposure affecting Re isotopic composition, studies of meteorites and planetary rocks have used Re*, which is the concentration of Re calculated assuming chondritic 187Os/188Os at the assumed time of sample crystallization (Day et al. 2010). This notation can be calculated as:
By contrast with the 187Re–187Os decay system, 190Pt is a minor isotope of Pt (0.01292%) and has a longer half-life (~ 450 Ga), so 186Os/188Os variations in the mantle are small and of the order of ~ 0.00015%, with an ‘average’ mantle value of 0.119837 ± 5 (2σ). The typically minor variations of 186Os/188Os in volcanic settings require external analytical precision of better than 30 ppm. To obtain sufficient analytical precision, large quantities of Os are needed (typically 50–75 ng of Os) to generate sufficient signals on 186Os given the ionization efficiency of Os by N-TIMS (~ 2–6%; Creaser et al. 1991). Inevitably, the analytical challenge of measuring 186Os/188Os means that there is far less data currently available than there is for 187Os/188Os.
The now-extinct 107Pd was also the parent for 107Ag in the early Solar System. This extinct radionuclide system had a half-life of 6.5 Ma (107Pd – 107Ag + β− + ν̄; λ = 1.06638 × 10−7 a−1; Parrington et al. 1996). The 107Pd–107Ag parent–daughter isotopic decay system is a candidate for use in both constraining the timing of early planetary fractionation events, for potentially determining whether Earth’s core material is incorporated into mantle plumes, and for investigating the timing of volatile-element depletion in planets. The relatively short half-life renders the system sensitive to fractionation events occurring within the first 40 million years of Solar System history (i.e., Kelly and Wasserburg 1978). Because Pd is more siderophile than Ag, planetary differentiation should result in an enrichment of Pd relative to Ag in planetary cores. If this happened during the lifetime of 107Pd, a correspondingly high 107Ag core signature would develop. If Earth’s differentiation occurred within 40 million years (approximately five half-lives) of the beginning of the Solar System, an isotopic excess of 107Ag should exist within the core. Equally, because Ag is a moderately volatile element, whereas Pd is more refractory than Ag, large ranges in Pd/Ag have been observed in volatile-depleted iron meteorites (up to 100,000), compared with a Solar Pd/Ag of ~ 3, leading to 107Ag/109Ag ratios > 9, compared with the solar value of 1.079 (Chen and Wasserburg 1996). For the Pd–Ag isotope system, the initial 107Pd/108Pd has been determined as 5.9 ± 2.2 × 10−5 (Schönbächler et al. 2008), with 107Ag/109Ag typically reported in parts per ten thousand notation relative to the NIST SRM978a silver standard (107Ag/109Ag = 1.07976):
FUNDAMENTAL PROCESSES AND OUTLINE OF THE VOLUME
In this volume, a number of key areas are reviewed in the use of the HSE and strongly chalcophile elements to investigate fundamental processes in high-temperature geochemistry and cosmochemistry. It is divided into five parts. The first part of the volume concerns measurements and experiments. Chapter 1, by Brenan et al. (2016), provides an comprehensive overview of experimental constraints applied to understanding HSE partitioning under a range of conditions, including: liquid metal–solid metal; metal–silicate; silicate–melt; monosulfide solid solution (MSS)–sulfide melt; sulfide melt–silicate melt; silicate melt–aqueous fluid–vapor. Chapter 2, by Meisel and Horan (2016) provides a summary of analytical methods, issues specifically associated with measurement of the HSE, and a review of important reference materials.
The second part of the volume concerns the cosmochemical importance of the HSE and strongly chalcophile elements. In their assessment of nucleosynthetic isotopic variations of siderophile and chalcophile elements in Solar System materials, Yokoyama and Walker (2016, Chapter 3) discuss some of the fundamentals of stellar nucleosynthesis, the evidence for nucleosynthetic anomalies in pre-Solar grains, bulk meteorites and individual components of chondrites, ultimately providing a synthesis on the different information afforded by nucleosynthetic anomalies of Ru, Mo, Os, and other siderophile and chalcophile elements. Chapter 4 concerns the HSE in terrestrial bodies, including the Earth, Moon, Mars and asteroidal bodies for which we have materials as meteorites. Day et al. (2016) provide a summary of HSE abundance and 187Os/188Os variations in the range of materials available and a synthesis of initial Solar System composition, evidence for late accretion, and estimates of current planetary mantle composition.
The third part of the volume concerns our understanding of the Earth’s mantle from direct study of mantle materials. In Chapter 5, Aulbach et al. (2016) discuss the importance and challenges associated with understanding HSE in the cratonic mantle, providing new HSE alloy solubility modelling for melt extraction at pressures, temperatures, fO2 and fS2 pertaining to conditions of cratonic mantle lithosphere formation. Luguet and Reisberg (2016) provide similar constraints on non-cratonic mantle in Chapter 6, emphasizing the importance of combined geochemical and petrological approaches to fully understand the histories of mantle peridotites. The information derived from studies of Alpine peridotites, obducted ophiolites and oceanic abyssal peridotites are reviewed in Chapter 7 by Becker and Dale (2016).
The fourth part of the volume focusses on important minerals present in the mantle and crust. Chapter 8 provides a broad overview of mantle chalcophiles. In this chapter, Lorand et al. (2016) emphasise that chalcophile and siderophile elements are important tracers that can be strongly affected by host minerals as a function of sulfur-saturation, redox conditions, pressure, temperature, fugacity of sulfur, and silicate melt compositions. Along a similar theme in Chapter 9, O’Driscoll and Gonzalez-Jimenez (2016) provide an overview of platinum-group minerals (PGM), pointing out that, where present PGM dominate the HSE budget of silicate rocks. Finally in this section, Harvey et al. (2016) examine the importance of Re–Os–Pb isotope dating methods of sulfides for improving our understanding of mantle processes (Chapter 10).
The fifth and final part of the volume considers the important of the HSE for studying volcanic and magmatic processes. In Chapter 11, Gannoun et al. (2016) provide a synthesis of the most abundant forms of volcanism currently operating on Earth, including mid-ocean ridge basalts, volcanism unassociated with plate boundaries, and subduction zone magmatism. The volume is completed in Chapter 12 by Barnes and Ripley (2016), by an appraisal of the obvious importance of magmatic HSE ore formation in Earth’s crust.
We are grateful to all of the authors involved in the production of this Reviews in Mineralogy and Geochemistry volume. The task of putting together the volume would not have been so easy were it not for the diligence of the reviewers, and most especially, the Series Editor, Ian Swainson.