Recycling of rare earths: a critical review

The parameters which control the behaviour of isovalent trace elements in magmatic and aqueous systems have been investigated by studying the distribution of yttrium, rare-earth elements (REEs), zirconium, and hafnium. If a geochemical system is characterized by CHArge-and-RAdius-Controlled (CHARAC) trace element behaviour, elements of similar charge and radius, such as the Y-Ho and Zr-Hf twin pairs, should display extremely coherent behaviour, and retain their respective chondritic ratio. Moreover, normalized patterns of REE(III) should be smooth functions of ionic radius and atomic number. Basic to intermediate igneous rocks show Y/Ho and Zr/Hf ratios which are close to the chondritic ratios, indicating CHARAC behaviour of these elements in pure silicate melts. In contrast, aqueous solutions and their precipitates show non-chondritic Y/Ho and Zr/Hf ratios. An important process that causes trace element fractionation in aqueous media is chemical complexation. The complexation behaviour of a trace element, however, does not exclusively depend on its ionic charge and radius, but is additionally controlled by its electron configuration and by the type of complexing ligand, since the latter two determine the character of the chemical bonding (covalent vs electrostatic) in the various complexes. Hence, in contrast to pure melt systems, aqueous systems are characterized by non-CHARAC trace element behaviour, and electron structure must be considered as an important additional parameter. Unlike other magmatic rocks, highly evolved magmas rich in components such as H2O, Li, B, F, P, and/or Cl often show non-chondritic Y/Ho and Zr/Hf ratios, and “irregular” REE patterns which are sub-divided into four concave-upward segments referred to as “tetrads”. The combination of non-chondritic Y/Ho and Zr/Hf ratios and lanthanide tetrad effect, which cannot be adequately modelled with current mineral/melt partition coefficients which are smooth functions of ionic radius, reveals that non-CHARAC trace element behaviour prevails in highly evolved magmatic systems. The behaviour of high field strength elements in this environment is distinctly different from that in basic to intermediate magmas (i.e. pure silicate melts), but closely resembles trace element behaviour in aqueous media. “Anomalous” behaviour of Y and REEs, and of Zr and Hf, which are hosted by different minerals, and the fact that these minerals show “anomalous” trace element distributions only if they crystallized from highly evolved magmas, indicate that non-CHARAC behaviour is a reflection of specific physicochemical properties of the magma. This supports models which suggest that high-silica magmatic systems which are rich in H2O, Li, B, F, P, and/or Cl, are transitional between pure silicate melts and hydrothermal fluids. In such a transitional system non-CHARAC behaviour of high field strength elements may be due to chemical complexation with a wide variety of ligands such as non-bridging oxygen, F, B, P, etc., leading to absolute and relative mineral/melt or mineral/aqueous-fluid partition coefficients that are extremely sensitive to the composition and structure of this magma. Hence, any petrogenetic modelling of such magmatic rocks, which utilizes partition coefficients that have not been determined for the specific igneous suite under investigation, may be questionable. But Y/Ho and Zr/Hf ratios provide information on whether or not the evolution of felsic igneous rocks can be quantitatively modelled: samples showing non-chondritic Y/Ho and Zr/Hf ratios or even the lanthanide tetrad effect should not be considered for modelling. However, the most important result of this study is that Y/Ho and Zr/Hf ratios may be used to verify whether Y, REEs, Zr, and Hf in rocks or minerals have been deposited from or modified by silicate melts or aqueous fluids.

Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect

Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa

The presence of pharmaceuticals in the environment due to human use--present knowledge and future challenges.

Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia.

Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations.

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Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

Normalized REE patterns of aqueous solutions and their precipitates bear information on the physico-chemical environments a fluid experienced during REE mobilization, fluid migration and minerogenesis. Positive Eu and Yb anomalies indicate REE mobilization by a F−-, OH−- and CO32−-poor fluid in a high-temperature regime, but are only retained by a precipitating mineral if precipitation occurs in a low-temperature environment. Negative Ce anomalies are typical of oxidizing conditions and are unlikely to develop during siderite precipitation. LREE/HREE fractionation is controlled by fluid composition and “mineralogical control”. REE patterns of Ca minerals allow to class the reacting fluids in “normal” (Ca/ligand ≫ 1) and “ligand-enriched” (Ca/ligand ≈ 1), the latter being characteristic for remobilization processes.

Peter Möller

Papers

Rare earth element fractionation in metamorphogenic hydrothermal calcite, magnesite and siderite

Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling

Rare-earth patterns with positive cerium anomaly in alkaline waters from Lake Van, Turkey

Yttrium and holmium in South Pacific seawater: vertical distribution and possible fractionation mechanisms

Anthropogenic gadolinium as a conservative tracer in hydrology