Michael Bau

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

Abstract Shale-normalized rare-earths and yttrium (REYSN; Y inserted between Dy and Ho) patterns for detritus-free samples from both the Kuruman and Penge Iron-Formations (IFs) in the Late-Archaean to Early-Palaeoproterozoic Transvaal Supergroup display pronounced heavy rare-earth element (REE) enrichment, and positive anomalies of LaSN, EuSN, GdSN, YSN, and ErSN, but neither positive nor negative CeSN anomalies. Excepting CeSN and EuSN anomalies, the Transvaal IFs yield all the features that are typical of the REY distribution in Modern seawater. ( Eu Eu ∗ ) SN ratios in the Kuruman IF correspond to ratios observed in other IFs of similar age, whereas the Penge IF is characterized by distinctly higher ratios. Within a sequence of eleven adjacent samples (each comprising less than ten microbands) from the Kuruman IF, ( Eu Eu ∗ ) SN ratios were found to vary significantly. Positive EuSN anomalies reveal the presence of a high-temperature hydrothermal component in Transvaal seawater. The absence of positive CeSN anomalies rules out the existence of an alkaline ‘soda-ocean’ with pH considerably above the Recent value of 8.2. Small-scale variation of ( Eu Eu ∗ ) SN ratios within the Kuruman IF as well as alternation of iron- and silica-dominated layers cannot be due to post-depositional modification of initially homogeneous material showing homogeneous REY distribution, because neither diagenetic nor metamorphic conditions were suitable for decoupling of Eu from the other REY. The observed small-scale variation may indicate short-term variability of ( Eu Eu ∗ ) SN ratios of Transvaal seawater, probably resulting from temporal variation of the activity of high-temperature venting at the seafloor. Preservation of this feature in IF microbands and the presence of positive YSN anomalies suggest that IF precipitation from upwelling marine bottom waters in an oxygenated shelf environment occurred very rapidly. Hence, REY adsorbed on the surface of iron-oxyhydroxide particles that eventually became Fe-rich IF microbands, were not in exchange equilibrium with REY dissolved in ambient seawater. Higher ( Eu Eu ∗ ) SN ratios in the Penge IF compared to the Kuruman IF suggest significantly more important REY input from high-temperature solutions to the REY budget of bottom waters in the Eastern Transvaal than in the Griqualand West sub-basin. The REY distribution in Penge and Kuruman IFs is compatible with a palaeogeographic setting which invokes the existence of a rather small basin in the northeast (the Eastern Transvaal sub-basin) in which spreading-related high-temperature fluid-rock interaction occurred. The basin widened towards the southwest (the Griqualand West sub-basin) where it was connected to the open ocean.

Rare-earth element mobility during hydrothermal and metamorphic fluid-rock interaction and the significance of the oxidation state of europium

Abstract The Eu 3+ Eu 2+ redox boundary for temperatures up to 600°C and pressures up to 4000 bar has been calculated using the “revised HKF model” for the prediction of the molal Gibbs free energy of formation at elevated pressures and temperatures. The results indicate that the oxygen fugacity (fO2) at which trivalent Eu is reduced to Eu2+ increases with increasing temperature. Thus, in high-temperature regimes Eu can be fractionated from the other REE. If in fluid-rock interaction under mildly acidic conditions the REE pattern of the fluid is controlled by sorption processes, the decreasing ionic radius from La to Lu leads to ( La Lu ) cn > 1 , and, provided Eu occurs as Eu2+, to a positive Eu anomaly (compared to the source-rock for the REE) in the fluid. Under nearly neutral to mildly basic conditions the REE pattern of the same fluid is governed by complexation mechanisms with carbonate, fluoride and hydroxide complexes being the most important REE species. This leads to ( La Lu ) cn . Because complexation extends the stability of Eu3+ towards lower fO2, reduction to Eu2+ under mildly basic conditions is limited to more reducing environments. Since the REE content of alteration fluids is extremely low, alteration of whole-rock REE patterns during hydrothermal or metamorphic fluid-rock interaction is rather ineffective, unless the water/rock ratio is ⪢ 102–103 or infiltration metasomatism is severe. Apart from these situations, REE systematics should not be affected by hydrothermal or metamorphic fluid-rock interaction.

Scavenging of dissolved yttrium and rare earths by precipitating iron oxyhydroxide: experimental evidence for Ce oxidation, Y-Ho fractionation, and lanthanide tetrad effect

Scavenging experiments were performed at pH 3.6 to 6.2 with synthetic solutions containing dissolved Fe (≈7 mg/L), Rare Earths and Yttrium (ΣREY: ≈61 μg/L) in a matrix of 0.01 M HCl, and with natural water from Nishiki-numa spring, Japan, with the aim to study the fractionation that results from the interaction of dissolved REY with precipitating Fe oxyhydroxide. All patterns of apparent REY distribution coefficients between Fe oxyhydroxide and solution, appDREY, show negative anomalies at Y, La, and Gd, and the M-type lanthanide tetrad effect. These features become more pronounced with increasing pH. At pH ≤ 4.6, positive anomalies of appDCe give evidence for oxidative scavenging of Ce on the Fe oxyhydroxide. A time-series experiment at pH 3.5 suggests that a stationary exchange equilibrium for the REY(III) is reached within less than 6.5 min, whereas the Ce(IV)/Ce(III) redox-equilibrium is not attained before 120 min. Oxidation rates of Ce(III) were found to decrease significantly during the first minutes after Fe oxyhydroxide formation, indicating that the capacity for Ce(III) oxidation is drastically higher in systems in which fresh Fe oxyhydroxides precipitate than in systems in which dissolved REY interact with pre-formed Fe oxyhydroxides. This additionally complicates the use of Ce anomalies of natural precipitates as quantitative paleo-redox-proxies. Radius-independent fractionation of REY(III) is very similar in experiments using synthetic solutions and natural water, despite the additional precipitation of hydrous Al oxides from the latter. Because there is no change of solution-complexation (speciation) along the REY series, radius-independent fractionation of REY(III) is likely due to differences between the stabilities of surface-complexes of the individual members of the REY series. The results presented here are an experimental verification of a natural process that may produce the lanthanide tetrad effect in geological samples.

Comparison of the partitioning behaviours of yttrium, rare earth elements, and titanium between hydrogenetic marine ferromanganese crusts and seawater

In order to evaluate details of the partitioning behaviours of Y, rare earth elements (REEs), and Ti between inorganic metal oxide surfaces and seawater, we studied the distribution of these elements in hydrogenetic marine ferromanganese (Fe-Mn) crusts from the Central Pacific Ocean. Nonphosphatized Fe-Mn crusts display shale-normalized rare earths and yttrium (REYSN) patterns (Y inserted between Dy and Ho) that are depleted in light REEs (LREEs) and which show negative anomalies for Ysn, and positive anomalies for LaSN, EuSN, GdSN, and in most cases, Cesn. They show considerably smaller Y/ Ho ratios than seawater or common igneous and clastic rocks, indicating that Y and Ho are fractionated in the marine environment. Compared to P-poor crusts, REYSN patterns of phosphatized Fe-Mn crusts are similar, but yield pronounced positive Ysn anomalies, stronger positive LaSN anomalies, and enrichment of the HREEs relative to the MREEs. The data suggest modification of REY during phosphatization and indicate that studies requiring primary REY distributions or isotopic ratios should be restricted to non-phosphatized (layers of) Fe-Mn crusts. Apparent bulk coefficients, Kdm, describing trace metal partitioning between nonphosphatized hydrogenetic Fe-Mn crusts and seawater, are similar for Pr to Eu and decrease for Eu to Yb. Exceptionally high values of KDCe, which are similar to those of Ti, result from oxidative scavenging of Ce and support previous suggestions that Ce (IV) is a hydroxide-dominated element in seawater. Yttrium and Gd show lower KD values than their respective neighbours in the REY series. Results of modelling the exchange equilibrium between REY dissolved in seawater and REY sorbed on hydrous Fe-Mn oxides corroborate previous studies that suggested the surface complexation of REY can be approximated by their first hydroxide binding constant. Negative “anomalies” occur for stabilities of bulk surface complexes of Gd, La, and particularly Y. The differences in inorganic surface complex stability between Y and Ho and between Gd and its REE neighbours are similar to those shown by the stabilities of complexes with aminocarboxylic acids and are significantly larger than those shown by stabilities of complexes with carboxylic acids. Hence, sorption of Y and REEs onto hydrous Fe-Mn oxides may contribute significantly to the positive YSN and GdSN anomalies in seawater.

Anthropogenic origin of positive gadolinium anomalies in river waters

Positive Gd anomalies in shale-normalised rare earth element (REESN) patterns of natural waters may provide information on the types of ligands which control surface complexation of REE on particle surfaces. However, REESN patterns of rivers which drain densely populated and industrialised areas in Central Europe and North America are characterised by pronounced positive GdSN anomalies, whereas rivers in thinly populated, non-industrialised areas in Varmland and Dalarna, central Sweden, and Hokkaido, Japan, do not show such anomalies. Acidification experiments suggest that, unlike the other REE, the excess Gd found in German rivers is almost completely related to the ‘dissolved’ REE fraction (< 0.2 μm) in a water sample and not to the acid-soluble particulate fraction, suggesting a negligible particle reactivity of the excess Gd. The positive GdSN anomalies are of anthropogenic origin and are most likely to result from the application of gadopentetic acid, Gd(DTPA)2−, in magnetic resonance imaging (MRI). In MRI, gadopentetic acid, which is an organic aqueous Gd(III) complex with very high stability constant, is used as a paramagnetic contrast agent. Since positive GdSN anomalies in rivers, lakes, semi-closed sea basins, and coastal seas, which receive riverine REE input from industrialised, densely populated areas may (partly) be of anthropogenic origin, the positive GdSN anomaly can no longer be used as a natural geochemical indicator.

Atmospheric oxygenation three billion years ago

It is widely assumed that atmospheric oxygen concentrations remained persistently low (less than 10−5 times present levels) for about the first 2 billion years of Earth’s history. The first long-term oxygenation of the atmosphere is thought to have taken place around 2.3 billion years ago, during the Great Oxidation Event. Geochemical indications of transient atmospheric oxygenation, however, date back to 2.6–2.7 billion years ago. Here we examine the distribution of chromium isotopes and redox-sensitive metals in the approximately 3-billion-year-old Nsuze palaeosol and in the near-contemporaneous Ijzermyn iron formation from the Pongola Supergroup, South Africa. We find extensive mobilization of redox-sensitive elements through oxidative weathering. Furthermore, using our data we compute a best minimum estimate for atmospheric oxygen concentrations at that time of 3 × 10−4 times present levels. Overall, our findings suggest that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event and some 300–400 million years earlier than previous indications for Earth surface oxygenation.

Origin of anomalous rare-earth element and yttrium enrichments in subaerially exposed basalts: Evidence from French Polynesia

Abstract Basalts from French Polynesian islands occasionally display extremely high abundances and anomalous distributions of rare-earth elements (REE) and yttrium, whereas other incompatible element concentrations and O, Sr, Nd and Pb isotopic ratios do not differ from those of “normal” basalts from the same area. The REE- and Y-enriched basalts contribute up to 15% of the sample set, suggesting that this feature is more widespread than previously thought. REE-Y enrichment is related to the presence of rhabdophane-type REE-Y-phosphate identified through electron microprobe analyses in the most enriched sample and inferred from leaching experiments in the others. This phenomenon is confined to subaerially exposed basaltic sequences, indicating a close relationship to supergene processes. This is supported by negative Ce anomalies in these basalts, since decoupling of Ce from the other REE is restricted to oxidizing, low-temperature, aqueous environments. Similar Nd isotopic ratios for enriched and normal basalts allow us to exclude the possibility that additional REE and Y are derived from marine sediments or guano, but rather suggest an origin from the local basalts. Moreover, light REE enrichment in the REE-Y-phosphates suggests short migration distances of the fluids, supporting the conclusion that additional REE and Y were mobilized from weathered basalts and transported by descending meteoric waters.

Rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere-hydrosphere-lithosphere system

Irrespective of provenance, age and metamorphic grade, the chemically precipitated component in Early Precambrian (>2.3 Ga) iron formations (IFs) displays (SmYb)CN 1 which reflects the corresponding ratios of contemporaneous seawater. In conjunction with eNd-IF >eNd-shale (Jacobsen and Pimentel-Klose, 1988a,b) this rare earth element (REE) signature reveals that the REE distribution in Early Precambrian IFs must be explained by mixing between a marine bottom and a surface water component and that the REEs (and by analogy the Fe) cannot be derived from weathering of a continental source. (EuSm)N ratios of detritus-free IFs are controlled by the marine bottom water component, where (EuSm)CN > 1 results from REE input from high-temperature high-T hydrothermal fluids which received their REE signature during high-T alteration of oceanic crust at spreading centres. Decreasing (EuSm)CN of IFs with decreasing depositional age reflect the vaning importance of high-T compared to low-T (<200°C) hydrothermal fluids, hence oceanic crust alteration, for the REE budget of marine bottom waters and are related to decreasing upper mantle temperatures. (SmYb)N ratios of detritus-free IFs are controlled by the marine surface water component, whose REE distribution is affected by the same processes which operate today in the entire ocean. Mixing calculations reveal that (SmYb)CN in Early Precambrian marine surface waters was significantly lower than it is today. To explain this difference, two mechanisms are discussed on the basis of higher PCO2 and lower PO2 levels of the Precambrian atmosphere: 1. (1) higher [CO32−] in Precambrian river and coastal waters and 2. (2) redox cycling of the REEs at an oceanwide chemocline which separated anoxic and oxic water masses. However, these processes are synergetic and could have operated in unison. The REE distribution in Precambrian IFs is described as a result of mixing in a multicomponent system, where high-T and low-T hydrothermal fluids contributed to the marine bottom water, and REE input from the dissolved REE pool in river waters, after some modification in estuaries, dominated the REE distribution in marine surface waters. After mixing of bottom and surface waters, their combined REE load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component. After addition of variable amounts of clastic detritus, this eventually resulted in the REE distribution observed in Precambrian IFs.

Comparative study of yttrium and rare-earth element behaviours in fluorine-rich hydrothermal fluids

The mineral ‘fluorite’ is utilized as a probe to investigate the behaviour of the pseudolanthanide yttrium with respect to the lanthanides (rare-earth elements, REE) in fluorine-rich hydrothermal solutions. Hydrothermal vein fluorites are characterized by the close association of Y and REE, but in contrast to igneous and clastic rocks they show variable and nonchondritic Y/Ho ratios of up to 200. This suggests that Y and Ho, although similar in charge and size, may be fractionated in fluorine-rich medium-temperature aqueous fluids. In such solutions Y acts as a pseudolanthanide heavier than Lu. Y/Ho ratios of hydrothermal siderites are slightly below those of chondrites, suggesting that in (bi)carbonate-rich siderite-precipitating solutions Y may act as a Sm-like light pseudolanthanide. This indicates that Y-Ho fractionation is not a sourcerelated phenomenon but depends on fluid composition. Based on these results it is strongly recommended that discussions of normalized REE patterns in general should be extended to normalized Rare-Earth-and-Yttrium (REY) patterns (Y inserted between Dy and Ho), because the slightly variable behaviour of the pseudolanthanide yttrium with respect to the REE may provide additional geochemical information. Available thermodynamic data suggest a negative correlation between Y/Ho and La/Ho during migration of a fluoriteprecipitating hydrothermal solution. Cogenetic fluorites, therefore, should display either similar Y/Ho and similar La/Ho ratios, or a negative correlation between these ratios. This criterion may help to choose samples suitable for Sm-Nd isotopic studies prior to isotope analysis. However, in cogenetic hydrothermal vein fluorites the range of Y/Ho ratios is often almost negligible compared to the range of La/Ho ratios. This may be explained by modification of REE distributions by post-precipitation processes involving (partial) loss of a separate LREE-enriched phase. The presence of variable amounts of such an accessory phase in most fluorite samples is revealed by experiments employing stepwise incomplete fluorite decomposition. Fluorites derived from and deposited near to igneous rocks apparently display chondritic Y/Ho ratios close to those of their igneous source-rocks. However, a positive YSN anomaly is likely to develop as the distance between sites of REY mobilization and deposition increases.

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

SummaryNormalized 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.The Radenthein magnesite and Hüttenberg siderite deposits, both Carinthia, Austria, are discussed and shown to be of non-sedimentary, non-metamorphic, but metamorphogenic metasomatic origin.ZusammenfassungNormierte Lanthaniden-Verteilungsmuster wässeriger Lösungen und deren Präzipitate enthalten Informationen über die verschiedenen physiko-chemischen Bedingungen, denen die Fluidphase während der Mobilisierung der Lanthaniden, der Migration und der Minerogenese ausgesetzt war. Positive Eu- und Yb-Anomalien weisen auf eine Mobilisierung der Lanthaniden bei erhöhten Temperaturen durch eine F−-, OH−-und CO32−-arme Lösung. Die positiven Anomalien der Lösung werden jedoch nur dann auf ein Mineral übertragen, wenn dessen Präzipitation in einem niedrigen Temperaturbereich erfolgt. Negative Ce-Anomalien sind Indikatoren oxischer Bedingungen, weshalb ihre Entwicklung im Verlauf einer Siderit-Präzipitation weitgehend ausgeschlossen werden kann. Die Fraktionierung von leichten und schweren Lanthaniden wird von der chemischen Zusammensetzung der Fluidphase und der “mineralogischen Kontrolle” bestimmt. Die Lanthaniden-Verteilungsmuster von Ca-Mineralen erlauben es, deren Mutter-Lösungen in “normal” (Ca/Liganil ≫1) und “Liganden-reich” (Ca/Liganil ≈ 1) zu untergliedern, wobei letztere für Remobilisierungsprozesse typisch sind.Verschiedene minerogenetische Modelle für Spatmagnesite aus der Lagerstätte Radenthein und Siderite aus der Lagerstätte Hüttenberg, beide Kärnten, Österreich, werden vor dem Hintergrund deren Lanthaniden-Verteilung diskutiert. Es wird gezeigt, daß sowohl für die Radenthein-Magnesite als auch für die Hüttenberg-Siderite nur ein nicht-sedimentäres, nicht-metamorphes, wohl aber metamorphogen-metasomatisches minerogenetisches Modell mit der Lanthaniden-Verteilung kompatibel ist.