Rare Earth Element Systematics Research Articles

Retentiveness of rare earth elements in garnet with implications for garnet Lu‐Hf chronology

AbstractIncorporation of rare earth elements (REE) in garnet enables garnet chronology (Sm‐Nd, Lu‐Hf), and imparts a garnet‐stable signature on cogenetic phases, which allows petrochronology and general petrogenetic tracing of garnet stability in minerals and melts. Constraints on the uptake and redistribution mechanisms, as well as on the diffusive behaviour of REE in garnet are required for allowing accurate interpretation of REE signatures and ages. Garnet REE profiles are often measured to gain insight into the nature and cause of REE zoning. Interpretation of such profiles is nevertheless complicated by poor constraints on the extent of diffusive relaxation. This is especially relevant for Lu, which, according to experiments, has a relatively high diffusivity and thus may re‐equilibrate with possible consequences for Lu‐Hf chronology. To provide new insight into the REE systematics of garnet, we applied quantitative trace‐element mapping of garnet grains from metamorphic rocks that record peak temperatures above 750°C and cooling rates as low as 1.5°C Ma−1. Garnet in all samples preserves Rayleigh‐type or oscillatory growth zoning with sharply defined interfacial angles that match the garnet habit. Re‐equilibration of REE compositions appears restricted to domains with nebulous and patchy zoning, which likely form by interface‐coupled dissolution and re‐precipitation reactions mediated by fluids or melts, rather than REE volume diffusion. The possible effect of Lu diffusion in the analysed grains was investigated by comparing the observations to the results from 2D numerical modelling using Lu diffusivities from recent diffusion experiments. This test indicates that Lu diffuses significantly slower in natural garnet than experiments predict. The retentiveness of REE in garnet demonstrates the reliability of REE signatures in magmatic tracing and petrochronology and establishes Lu‐Hf chronology as a robust means of dating garnet growth and recrystallization in metamorphic rocks, including those that underwent high‐ or ultrahigh‐temperature conditions.

Rare earth element systematics of chimney anhydrite from seafloor hydrothermal vents

Comparative studies of sulphide chemistry have been conducted over the past few decades to characterise seafloor hydrothermal mineralisation, but few studies have been carried out on sulphate minerals. Here we report on a systematic study of the S isotopic composition and rare earth element (REE) chemistry of chimney anhydrite from various types of seafloor hydrothermal vent fields in the Central Indian Ridge, North Fiji Basin, and Tonga intra-oceanic arc. Our results show that sulphate δ34S values of anhydrite cluster around that of seawater sulphate, indicating a significant role for fluid–seawater mixing during anhydrite precipitation. However, some anhydrite has δ34S values lower than that of seawater. This likely reflects sulphate derived from magmatic SO2 disproportionation. Chondrite-normalised REE patterns of the anhydrite are not uniform on the scale of individual grains and/or between each type of hydrothermal vent field. The REE compositions of the anhydrite do not have a clear relationship with the REE compositions of the primary host rocks with which the hydrothermal fluids interacted. This indicates that the REEs are not sensitive indicators of the host rock compositions, even though the host rocks are the dominant source of REEs in the seafloor hydrothermal fluids. In contrast, anhydrite REE concentrations and Eu anomalies are correlated with δ34S values and the redox state of the hydrothermal fluids, respectively. We suggest that relatively REE-enriched anhydrite is formed by efficient leaching of REEs from host rocks by acidic magmatic fluids (i.e., a low pH), whereas a low redox potential of the hydrothermal fluids is the most important factor controlling Eu mobility. The original concentrations of REEs supplied to the hydrothermal fluids can also be modified by fluid boiling and fluid–seawater mixing. The former produces light REE-depleted anhydrite because the availability of chloride ligands decreases in low-density vapor-dominated boiled fluids. In situ geochemical analysis of anhydrite on the intra-grain scale shows that high degrees of fluid–seawater mixing contribute to the relatively low REE concentrations and positive Eu anomalies. As such, our results indicate that the REE systematics of anhydrite are mainly controlled by the combined effects of fluid conditions (i.e., redox state, pH, and Cl concentrations) and sub-seafloor geochemical processes (i.e., magmatic input, fluid boiling, and seawater mixing) rather than the nature of the source materials. In situ analysis of anhydrite grains is a useful tool for constraining variable hydrothermal mineralisation conditions during chimney growth, which cannot be solely inferred from geochemical studies of sulphide minerals, particularly in inactive and relict systems where the hydrothermal fluids cannot be sampled.

Geochemical benchmarks in the Phanerozoic LIPs constrained from well-cores in the Deccan Volcanic Province, India

The ∼66 Ma Deccan flood basalt province is one of the largest in the World. Here, we present mineral chemistry, bulk-rock geochemistry and NdSr isotopes in the ∼1000 m thick basalt lava sequence extracted from the deep bore-hole well-cores KBH-8 and KBH-7 from the main Deccan Plateau in the southwestern Deccan Volcanic Province. On the basis of trace and rare earth element systematics, we have statistically grouped the accumulated lava into fifteen Eruptive Phases (EPs). The basalts show extreme variation in their initial 87Sr/86Sr (0.7038–0.7166) and εNd (+5.9 to −12.1) isotopic compositions consistent with the involvement of depleted and enriched mantle sources in their genesis. The lavas sourced from the enriched subcontinental lithospheric mantle has been categorized as EP4-type, and EP11-type represents the isotopically depleted lavas derived from a plume asthenospheric source. The remaining EPs resulted from mixing of the components from these two endmembers in some proportion. This essentially reflects upon source contamination rather than crustal assimilation enroute to eruption or emplacement. The unique geochemical and isotopic fingerprint of EP4 and EP11 is traceable in the Phanerozoic LIPs, as well. Therefore, they potentially qualify as “geochemical benchmarks”. The systematic “pinch and swell” pattern observed in the trace element and isotopic compositions throughout the depth profile cannot be reconciled with continuous outpouring of lava. Instead, the EPs erupted in discreet batches in a quick span of time from a periodically replenished tapped fractionated magma chamber fed by a mantle plume at the asthenosphere-lithosphere interface. The results of this study enunciate the critical role of subcontinental lithospheric mantle in the evolution of a continental flood basalt province, irrespective of space and time.

Equilibrium fractionation of REE isotopes in nature: Insights from NRIXS and DFT+U studies of Eu and Dy phonon density of states

Rare Earth Elements (REEs) are commonly utilized in Earth and environmental sciences to study a variety of geological processes due to their distinct patterns and radioactive-radiogenic decay systems (147Sm-143Nd, 146Sm-142Nd, 138La-138Ce). Advances in analytical techniques now enable the use of REE stable isotopic fractionations to clarify lingering ambiguities in REE systematics. In this study, we employed Nuclear Resonant Inelastic X-ray Scattering (NRIXS) to study the phonon density of states of 151Eu and 161Dy in several pure compounds, as well as in synthetic basalt and andesite glasses, and rhyolite glasses produced under various redox conditions, to determine equilibrium isotopic fractionation factors for the REEs. We additionally utilized Density Functional Theory with a Hubbard U correction (DFT+U) to calculate these factors. Our findings indicate that the directionally averaged mean force constant of Dy3+ is ∼270 N/m across various compounds, while those of Eu2+ and Eu3+ are ∼83 and 214 N/m, respectively, in geologically relevant glasses and other pure compounds. These force constants were then used to estimate those of all REEs using scaling arguments. The results suggest that equilibrium isotopic fractionation should be limited for REEs in igneous rocks, allowing for the interpretation of REE isotopic fractionation in these rocks and minerals as a result of kinetic effects. This could facilitate understanding the role of diffusion in igneous rocks and ore formation. Additionally, our results imply that significant Eu isotopic fractionation could exist in hydrothermal fluids, which could aid in understanding the formation of ore deposits and REE cycling in the oceans.

Complex REE systematics of carbonatites and weathering products from uniquely rich Mount Weld REE deposit, Western Australia

Carbonatite intrusive complexes are important hosts for Rare Earth Elements (REE) deposits and are commonly affected by surficial weathering processes. Mount Weld REE deposit of Western Australia is one of the world’s richest REE deposits and here we present whole-rock and REE mineral geochemical data on compositions of primary and weathered carbonatites. The REE concentrations in magmatic carbonatites at Mount Weld vary by two orders of magnitude, suggesting a significant role of magmatic processes in REE enrichment in the carbonatite complex. The Th-Pb age 2056±67 (2σ) Ma of monazite-(Ce) from the carbonatite obtained by in situ LA-ICP-MS analyses confirm Paleoproterozoic age of the mineralization. The regolith samples preserve a detailed record of evolution from carbonatite to products of intensive weathering. The silica-cemented regolith (silcrete) contains monazite-(Ce) and apatite chemically identical to their carbonatite-hosted equivalents. The high-REE regolith containing 51.8 wt% ∑REE2O3 has some of the highest REE concentrations found to date, negative Ce anomaly and contains lithogenic monazite within matrix of florencite-(Ce) and rhabdophane-(Nd) and rhabdophane-(Nd) tubes resembling casts of plant material. The pronounced negative Ce anomalies in the ferruginous cap rock (ferricrete) and the high-REE regolith indicate intensive weathering, where Ce4+ was preferential removed relative to the REE3+. The presence of paleo-plants in the high-REE regolith implicates bio-assisted processes involved in the extreme REE fractionation. The diverse REE enrichment processes, which occurred in supergene environment of Mount Weld, have implications for the understanding of the genesis of REE deposits worldwide.

Nickel isotopes and rare earth elements systematics in marine hydrogenetic and hydrothermal ferromanganese deposits

Attention is now being given to Ni isotope systematics in hydrogenetic marine ferromanganese (Fe-Mn) crusts as paleoceanographic proxies. Previous work focused on identifying both mineralogy (post-depositional) and source effects (Gall et al. 2013; Gueguen et al. 2016), in particular regarding hydrothermal inputs in the oceans and the response of Ni isotope biogeochemical cycling through time. The most important sink for Ni in the oceans is the Fe-Mn oxides sink, but estimation of its Ni isotope composition is only based on hydrogenetic Fe-Mn crusts. In this study, we investigated a range of Fe-Mn deposits including Fe-Mn deposits variably affected by hydrothermal inputs, including hydrothermal deposits from the Lau back-arc basin (South West Pacific) and Lo'ihi seamount (Hawaii), hydrogenetic crust and nodules from the Bauer Basin (Pacific Ocean). Nickel isotope ratios were measured by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using a double-spike (61Ni and 62Ni) correction method. The combination of Ni isotopes and rare earth element (REE) geochemistry show that Ni isotope fractionation in Fe-Mn deposits is essentially controlled by formation processes of the deposits (such as the rate of formation, the initial Mn-phase and sorption processes) which are also related to the depositional environment. Consistent with previous studies, pure hydrogenetic crusts are characterized by isotopically heavy Ni isotope signatures (δ60/58Ni values range from ‰ 0.9 and 2.5‰) and well-developed positive Ce anomalies. In contrast, mixed hydrothermal‑hydrogenetic crust and nodules from the Bauer Basin (East Pacific) display negative Ce anomaly and lighter δ60/58Ni values (0.3‰ to 0.4‰), which are interpreted as the result of far-field hydrothermal inputs of Fe-Mn precipitates from the East Pacific Rise.Nickel in hydrothermal deposits from the Lau Basin (0.5 and 1.1‰) and Lo'ihi seamount (−0.8 to −1.5‰) is isotopically lighter than in hydrogenetic Fe-Mn crusts. Light δ60/58Ni values in Lo'ihi deposits is due to the removal of Ni during Ni adsorption from seawater and from the hydrothermal fluid (between 0 and 1.4‰) on Fe-oxides followed by isotope fractionation between the fluid and the mineral phase. Results suggest that Ni isotopes in hydrothermal Fe-rich deposits are strongly fractionated relative to the seawater/fluid source due to partial removal of Ni on Fe-phases. Hydrothermal Mn-oxides deposits from the Lau Basin acquired their Ni isotope signature through Ni adsorption and continuous exchange of Ni with seawater. We propose that the systematic difference in Ni isotope signatures between hydrogeneous and hydrothermal Fe-Mn deposits is related to the mechanisms of Ni uptake into oxide minerals (e.g., birnessite vs. todorokite; Fe-oxides vs. Mn-oxides) which depend on the rate of formation and the source of Mn and Fe to marine ferromanganese deposits (i.e., depositional environment) rather than Ni sources.

Microbial utilization of rare earth elements at cold seeps related to aerobic methane oxidation

A major breakthrough in the field of rare earth element (REE) geochemistry has been the recent discovery of their utility to microbial life, as essential metalloenzymes catalyzing the oxidation of methanol to formaldehyde. Lanthanide-dependent bacteria are thought to be ubiquitous in marine and terrestrial environments, but direct field evidence of preferential microbial utilization of REE in natural systems is still lacking. In this study, we report on the REE and trace element composition of the tube of a siboglinid worm collected at a methane seep in the Gulf of Guinea; a tube-dwelling annelid that thrives in deep-sea chemosynthetic ecosystems. High-resolution trace element profiles along the chitin tube indicate marked enrichments of lanthanum (La) and cerium (Ce) in its oxic part, resulting in REE distribution patterns that depart significantly from the ambient seawater signature. Combined with various geochemical and microbiological evidence, this observation provides direct support for an active consumption of light-REE at cold seeps, associated with the aerobic microbial oxidation of methane. To further evaluate this hypothesis, we also re-examine the available set of REE data for modern seep carbonates worldwide. While most carbonate concretions at cold seeps generally display REE distribution patterns very similar to those for reduced pore waters in marine sediments, we find that seafloor carbonate pavements composed of aragonite commonly exhibit pronounced light-REE enrichments, as inferred from high shale-normalized La/Gd ratio (>~0.8), interpreted here as possibly reflecting the signature of lanthanide-dependent methanotrophic activity. This finding opens new perspectives for revisiting REE systematics in ancient seep carbonates and other microbialites throughout the Earth's history. In particular, the geochemical imprint of aerobic methane oxidation could be possibly traced using REE in Archaean stromatolites and other archives of Precambrian seawater chemistry, potentially providing new insights into the oxygenation of early Earth's oceans and associated microbiogeochemical processes.

REE, Y, Th, U and Mn systematics of Upper Devonian conodonts in the West Uralian Folded Zone (Southern Urals)

Research subject. This article presents the results of a study undertaken to investigate the systematics of rare-earth elements (REE), Y, Th, U and Mn in the Upper Devonian conodonts of the Western Ural region of the foreland fold of the Southern Urals (Askyn and Makarovo horizons, Kukkarauk, Ryauzyak and Lemezinsky sections).Methods. The conodonts were isolated from carbonate rocks using the conventional method of their dissolution in organic acids, mainly formic acid. Sample preparation and mass spectrometric analysis were carried out in the class 1000 and 10 000 cleanroom facilities of the Zavaritsky Institute of Geology and Geochemistry. A PerkinElmer ELAN 9000 quadrupole ICP mass-spectrometer was used to determine the concentration of the trace elements under study. Sm and Nd isotope ratios were measured from a 3% nitric acid solution by a Thermo Fischer Neptune Plus multicollector inductively coupled plasma mass-spectrometer. The long-term reproducibility and accuracy of the measurement procedure were evaluated using a standard Merck Nd solution based on the NIST Nd2O3 and yielded 143Nd/144Nd = 0.511720 ± 15 (1 SD, n = 40).Results. The established features of PAAS-normalised lanthanide distributions in the conodont bulk samples (10.4–21.8 mg), Ce-anomalies, high REE values (173–1211 ppm) and a number of other parameters indicate the leading role of late diagenetic processes in the formation of conodont REE systematics. This is also evidenced by the Y/Ho values (≈26–32) specific for the studied conodont samples.Conclusions. In general, the distribution of lanthanides in the conodonts suggests that this process was mainly controlled by a lithogenic (from 90 to more than 99%) REE source. The εNd (t) (–4.0…–2.8) values characteristic of the cono donts of the Askyn and Makarovo horizons suggest that their Nd isotopic composition was either formed under the influence of the open ocean (island arc basin), characterised by a significant share of radiogenic Nd, or due to the entry of radiogenic waters of the ocean into the shelf zone at the peak of the marine transgression that took place in the region under consideration in the Famennian.

Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts

AbstractWe examine REE (Rare-Earth Element) and isotopic (Sr–Hf–Nd–Pb) signatures in OIB (Ocean Island Basalts) as a function of lithospheric thickness and show that the data can be divided into thin- (<12 Ma) and thick-plate (>12 Ma) sub-sets. Comparison to geophysically constrained thermal plate models indicates that the demarcation age (∼12 Ma) corresponds to a lithospheric thickness of about 50 km. Thick-plate OIB show incompatible element and isotopic enrichments, whereas thin-plate lavas show MORB-like or slightly enriched values. We argue that enriched signatures in thick-plate OIB originate from low-degree melting at depths below the dry solidus, while depleted signatures in MORB and thin-plate OIB are indicative of higher-degree melting. We tested quantitative explanations of REE systematics using melting models for homogeneous fertile peridotite. Using experimental partition coefficients for major upper mantle minerals, our equilibrium melting models are not able to explain the data. However, using a new grain-scale disequilibrium melting model for the same homogeneous lithology the data can be explained. Disequilibrium models are able to explain the data by reducing the amount of incompatible element partitioning into low degree melts. To explore new levels of detail in disequilibrium phenomena, we employ the Monte-Carlo Potts model to characterize the textural evolution of a microstructure undergoing coarsening and phase transformation processes simultaneous with the diffusive partitioning of trace elements among solid phases and melt in decompressing mantle. We further employ inverse methods to study the thermochemical properties required for models to explain the OIB data. Both data and theory show that OIB erupted on spreading ridges contain signatures close to MORB values, although E-MORB provides the best fit. This indicates that MORB and OIB are produced by compositionally indistinguishable sources, although the isotopic data indicate that the source is heterogeneous. Also, a posteriori distributions are found for the temperature of the thermomechanical lithosphere-asthenosphere boundary (TLAB), the temperature in the source of OIB (Tp, oib) and the extent of equilibrium during melting (i.e. grain size). TLAB has been constrained to 1200–1300°C and Tp, oib is constrained to be <1400°C. However, we consider the constraints on Tp, oib as a description of all OIB to be provisional, because it is a statistical inference from the global dataset. Exceptional islands or island groups may exist, such as the classical ‘hotspots’ (Hawaii, Reunion, etc) and these islands may originate from hot sources. On the other hand, by the same statistical arguments their origins may be anomalously hydrated or enriched instead. Mean grain size in the source of OIB is about 1–5 mm, although this is also provisional due to a strong dependence on knowledge of partition coefficients, ascent rate and the melting function. We also perform an inversion in which partition coefficients were allowed to vary from their experimental values. In these inversions TLAB and Tp, oib are unchanged, but realizations close to equilibrium can be found when partition coefficients differ substantially from their experimental values. We also investigated bulk compositions in the source of OIB constrained by our inverse models. Corrections for crystallization effects provided ambiguous confirmations of previously proposed mantle compositions, with depleted mantle providing the poorest fits. We did not include isotopes in our models, but we briefly evaluate the lithospheric thickness effect on isotopes. Although REE data do not require a lithologically heterogeneous source, isotopes indicate that a minor enriched component disproportionately contributes to thick-plate OIB, but is diluted by high-degree melting in the generation of thin-plate OIB and MORB.

Rare earth element systematics in boiled fluids from basalt-hosted geothermal systems

Hydrothermal processes that lead to REE fractionation and redistribution are important for understanding water-rock interactions in geothermal energy resources and mineral deposits, and for determining how submarine hydrothermal activity affects the composition of oceanic crust. Much previous work on REE transport and deposition has focused on submarine hydrothermal vents. We report REE concentrations in boiled fluids sampled from five subaerial, basalt-hosted geothermal fields, and explore controls on aqueous REE concentrations by ligand complexation and mineral supersaturation. Samples that boiled at pressures between 0.8 and 2.83 MPa were obtained from the Reykjanes, Svartsengi, Hellisheidi, and Nesjavellir geothermal systems in Iceland, and the Puna geothermal system in Hawaii. For comparison, we also report REE concentrations in hydrothermal fluids from the sediment hosted submarine Middle Valley hydrothermal system, which boiled at >250 MPa. The pH(25°C) values of the sampled subaerial geothermal fluids range from 3.94 to 6.77, and Cl concentrations range from near seawater (502 mmol/kg) to dilute (1.9 mmol/kg). La, Ce and Eu are the only REE present at levels above 5 picomole/kg (pmol/kg) in the boiled geothermal fluids; and there are notable CI chondrite normalized La and Eu anomalies in the saline fluids. REE concentrations in Middle Valley hydrothermal fluids fall within the typical range reported for submarine hydrothermal fluids and have around two orders of magnitude higher REE than the boiled subaerial geothermal fluids. Bulk samples of precipitates in pipes from the Reykjanes geothermal system have detectable REE, confirming that downhole fluids have lost REE during boiling and production of fluids for geothermal energy. Isenthalpic boiling models show that the proportions of La and Eu chloride complexes increase relative to other aqueous species as boiling progresses, attenuating the incorporation of La and Eu into precipitated well scale solids. Fluorapatite is calculated to precipitate on boiling of low pH and saline fluids and calcite is calculated to precipitate from dilute and near-neutral pH fluids, and these minerals likely sequester REE in boiled subaerial fluids. Submarine hydrothermal fluids are constrained to boiling at higher temperatures than subaerial geothermal fluids owing to pressure from overlying cold seawater, therefore secondary minerals and solids that incorporate REE are not extensively precipitated and REE concentrations in the fluids are higher.

Element mobility during regional metamorphism in crustal and subduction zone environments with a focus on the rare earth elements (REE)

This paper explores bulk-rock geochemical data for a wide array of metamorphosed mafic, quartzo-feldspathic, pelitic, and metacarbonate rocks using a quantitative mass-balance approach to assess fluid-driven element mobility—particularly of the rare earth elements (REE)—in regional metamorphic and some high-pressure subduction zone environments (40 examples; over 240 individual analyses). Most examples are from focused fluid flow settings, such as veins and lithologic contacts, where fluxes are large and metasomatic signals are thus strong. A variety of REE behaviors are observed, including little or no REE mobility (roughly a third of the data set); light REE (LREE), mid-REE (MREE), and/or heavy (HREE) mobility; europium “anomalies”; overall REE losses; and local REE redistribution. The REE are typically fractionated by mass transfer, with the exception of several examples that underwent fairly uniform overall losses of REE. The fractionation reflects strong mineralogical controls on REE uptake/loss by a comparatively small number of phases. Examples include: HREE mass changes associated with garnet, xenotime, and sphene; LREE and MREE changes associated with apatite, monazite, and allanite; and Eu changes associated with plagioclase and lawsonite. As mineralogy is a dominant control, the nature of the metasomatism is not strongly correlated with metamorphic grade, other than obvious mineralogical differences between settings (e.g., plagioclase in Barrovian metamorphism, lawsonite in subduction zones). Extensive mobilization of non-REE major and trace elements can happen without significant open-system transport of the REE. If REE mobility occurs, it is always accompanied by mobilization of other non-REE. When mobile, neighboring REE (e.g., Sm and Nd) typically have strongly correlated mass changes indicating that both were mobilized to about the same degree. Although individual examples of metasomatism can show correlations between patterns of mass transfer for the REE and the non-REE, little such correlation is evident across the entire data set, with the exception of P. Once again, this highlights the importance of individual minerals in controlling REE systematics. Broad correlations of REE and P mobility suggest REE transport by P complexes, or REE and P transport together by some other complexing agent. Mass changes for REE and Y are more strongly coupled, reflecting the geochemical similarity of these elements and perhaps indicating a role for Y complexing as well. The REE are the most mobile of the high field strength elements (HFSE). On a percentage mass basis, the amount of HFSE mobility decreases roughly in the order: REE > U > Nb > Ti > Th ~ Zr. Th mobility is rare but when present is positively correlated with U and REE mobility. The mobility of the more refractory HFSE is low in aqueous fluids, but is a larger concern in more extreme environments such as magmatic/magmatic-hydrothermal systems, charnockite metamorphism, and supercritical fluids in high-pressure/ultrahigh-pressure settings. The mobility of certain HFSE (e.g., Nb) can be large enough to affect rock plotting positions on petrotectonic discrimination diagrams. Potassium and related (Rb, Ba, Cs) large-ion lithophile elements (LILE) are typically lost in association with mica breakdown. As the data set focuses on high fluid flow environments, LILE changes will likely be smaller for rocks that undergo less fluid-rock interaction. Gains of K and related LILE are coupled to volatile gain and Na loss, consistent with fluid flow down temperature gradients, or infiltration from K-rich sources (e.g., pegmatites). In Barrovian settings, two other mass transfer trends are evident in addition to K mass transfer. First, silica loss is coupled to volatile loss, illustrating the dependence of silica mass transfer on devolatilization. Most of the silica loss reflects local mass transfer into adjacent vein fluid flow conduits were additional silica is precipitated; indeed, wallrock inclusions in veins can be highly silicified. Second, Na and Ca mass gains and losses are well correlated in the Barrovian examples, reflecting control of plagioclase growth or destruction. Strontium and Pb behaviors also appear to be largely related to plagioclase behavior (in subduction settings, this role can be played by phases such as lawsonite or epidote group minerals). Carbon dioxide mass transfer by conventional devolatilization as well as near-stoichiometric CaCO 3 dissolution or precipitation is represented in the data set. Determining the relative roles of these processes as functions of metamorphic grade, intensity of fluid-rock interaction, and tectonic setting will be an important challenge for future research.

Систематика Cr, Co и редкоземельных элементов в осадочном материале дрейфующих льдов северной части круговорота Бофорта

Систематика Cr, Co и редкоземельных элементов в осадочном материале дрейфующих льдов северной части круговорота Бофорта

OIB signatures in basin-related lithosphere-derived alkaline basalts from the Batain basin (Oman) — Constraints from 40Ar/39Ar ages and Nd–Sr–Pb–Hf isotopes

Tertiary rift-related intraplate basanites from the Batain basin of northeastern Oman have low SiO2 (<45.6wt.%), high MgO (>9.73wt.%) and moderate to high Cr and Ni contents (Cr >261ppm, Ni >181ppm), representing near primary magmas that have undergone fractionation of mainly olivine and magnetite. Rare earth element systematics and p-T estimates suggest that the alkaline rocks are generated by different degrees of partial melting (4–13%) of a spinel-peridotite lithospheric mantle containing residual amphibole. The alkaline rocks show restricted variations of 87Sr/86Sr and 143Nd/144Nd ranging from 0.70340 to 0.70405 and 0.51275 to 0.51284, respectively. Variations in Pb isotopes (206Pb/204Pb: 18.59–18.82, 207Pb/204Pb: 15.54–15.56, 208Pb/204Pb: 38.65–38.98) of the alkaline rocks fall in the range of most OIB. Trace element constraints together with Sr–Nd–Pb isotope composition indicate that assimilation through crustal material did not affect the lavas. Instead, trace element variations can be explained by melting of a lithospheric mantle source that was metasomatized by an OIB-type magma that was accumulated at the base of the lithosphere sometimes in the past. Although only an area of less than 1000km2 was sampled, magmatic activity lasted for about 5.5Ma with a virtually continuous activity from 40.7±0.7 to 35.3±0.6Ma. During this period magma composition was nearly constant, i.e. the degree of melting and the nature of the tapped source did not change significantly over time.

Cr, Co and rare earth elements systematics in ice-rafted sediments of northern part of the Beaufort gyre

Cr, Co and rare earth elements systematics in ice-rafted sediments of northern part of the Beaufort gyre

A new approach to laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) using the flexible map interrogation tool ‘Monocle’

A new flexible interrogation tool for 2-dimensional LA-ICP-MS element and isotope maps is presented. Called Monocle, the tool works as an add-on in the software Iolite. Initial inspection of map data is undertaken with a size and shape adjustable loupe that is moved across the map. As the loupe is panned over the map, histograms and density/probability plots of the element concentration (or ratio, or age) contained within the area under the loupe are displayed. The data of any loupe position can be saved as a region of interest (ROI) in a table. For maps with rare earth element (REE) data a live normalised REE graph can be shown. Similarly, live graphs (e.g. Wetherill or Tera-Wasserburg concordiae) can be displayed for U/Pb data.The second step of map interpretation refines the initial data selection, for example by drawing polygons over irregular zones of similar chemistry. Where areas are separated (e.g. by cracks) several polygons can be nested into one ROI to improve counting statistics. Additional means of data pooling exist. Firstly, by growing a ROI from a seed, whereby the data distribution in the seed is used to find adjacent pixels that are within a user-defined statistical boundary (e.g. 1 standard deviation). Secondly, multiple criteria can be set (e.g. element A>0.1ppm and element B<40wt%) to determine which pixels (connected or unconnected) are included in a ROI.This new approach is illustrated with several examples. A Mariana Arc oceanic Mn crust is used to show how elemental maps resolve alternating periods of precipitation from variably intense hydrothermal input and how their REE systematics reflect mixing between two distinct sources. A map of a garnet from a lherzolite illustrates that with modern LA-ICP-MS instruments it is possible to obtain reproducible REE data from strongly depleted mantle minerals (e.g. La≈14ppb) with a 20×20μm2 beam and how mineral inclusions, metasomatism and serpentinisation can be avoided to obtain data from the most pristine parts of a grain. A map of a zoned augite crystal from Mt. Etna was used to examine the distribution of Ni and Cr, whose interrelationship in the strong oscillatory zoning can be modelled with extreme fractional crystallisation. A U/Pb map of a complex zircon was used to separate data from the complex inherited core and the much younger magmatic rim.Extraction of data from 2-dimensional maps (<1μm deep) avoids limitations of conventional LA-ICP-MS analysis that typically ablates much deeper. In combination with fast aerosol transfer systems that permit high repetition rate mapping (50–80Hz), extraction of quantitative data with map analysis tools heralds a new era of in situ chemical and isotopic analysis.

Geochemistry, Nd, Pb and Sr isotope systematics, and U–Pb zircon ages of the Neoarchean Bad Vermilion Lake greenstone belt and spatially associated granitic rocks, western Superior Province, Canada

The ca. 2720Ma Neoarchean Bad Vermilion Lake (BVL) greenstone belt, in the western Superior Province, Canada, is composed of a suite of tholeiitic to calc-alkaline basalts to rhyolites, volcaniclastic rocks, gabbros, and Timiskaming-type siliciclastic sedimentary rocks. The greenstone belt was intruded by Neoarchean granitic rocks, and underwent greenschist facies metamorphism and intense deformation, resulting in mobilization of many elements (e.g., Rb, Ba, Sr, K, U, Pb).The high-field strength element and rare earth element systematics of the volcanic and volcaniclastic rocks, and gabbros are consistent with subduction zone geochemical signatures, suggesting that the BVL greenstone belt formed in a magmatic arc setting. On the basis of lithological associations and trace element systematics, the BVL greenstone belt is defined as a fragment of a Neoarchean subduction-related ophiolite. Three rhyolite samples from the belt have yielded 2722±18Ma, 2706±13Ma and 2710±28Ma U–Pb zircon ages, representing the approximate age of the arc volcanism in the study area and development of a subduction zone between the western Wabigoon terrane to the north and the Wawa-Abitibi terrane to the south. The intrusion of the ca. 2664±15Ma late- to post-tectonic, potassic Ottertail Lake granite marks the end of tectonic accretion in the study area.Both the volcanic rocks and gabbros display large ranges of Nd (143Nd/144Nd=0.511600–0.512849; εNd (2720Ma)=+0.8 to+4.0), Pb (206Pb/204Pb=13.80–60.67) and Sr (87Sr/86Sr=0.701481–1.01154) isotopic compositions, suggesting that these isotope systems were variably affected by post-magmatic element mobility. Neither the Sm–Nd (2921±200Ma) nor Rb–Sr (2130±610Ma) system has yielded reliable regression (isochron) ages, reflecting the open-system behavior of these systems during metamorphism. Despite large uncertainties, Pb–Pb regression ages yielded by all rock types (2661±60Ma), and basalts and gabbros (2725±83Ma) agree with the zircon U–Pb ages of the rhyolites, suggesting that the U–Pb system was the most robust among all three systems.

Paleoenvironments revealed by rare-earth element systematics in vertebrate bioapatite from the Lower Devonian of Svalbard

In situ rare-earth element (REE) compositions have been measured in early vertebrate microremains from the Lower Devonian basin of Andrée Land (Svalbard), with the aim of obtaining information about their early depositional environment and potential reworking. Vertebrate microremains with different histology were used for the analyses, sourced from two different localities of marginal marine to freshwater sediments from geographically distant parts of the Grey Hœk Formation (Skamdalen and Tavlefjellet members). We selected thelodont and undescribed ?chondrichthyan scales, which allowed us to define potential taxonomic, histological, and taphonomic variables of the REE uptake. Results showed REE concentrations to be relatively uniform within the scales of each taxon, but apparent discrepancies were visible between the studied localities and separate taxa. The compilation of REE abundance patterns as well as REE ratios have revealed that thelodont and ?chondrichthyan originating from the same locality must have had different burial and early diagenetic histories. The shapes of the REE profiles, together with the presence and absence of the Eu and Ce anomalies, equally suggested different depositional and diagenetic environments for these two sympatric taxa resulting from either stratigraphical or long-distance reworking. The REE concentrations appear to have visible differences between separate dental tissues, particularly between enameloid and dentine of thelodonts, emphasizing the importance of in situ measurements in microfossil biomineral geochemistry.

Petrogenesis of Tertiary continental intra-plate lavas between Siebengebirge and Westerwald, Germany: Constraints from trace element systematics and Nd, Sr and Pb isotopes

New 39Ar/40Ar ages and major- and trace-element and radiogenic isotope data are presented for basanites and alkali basalts from the transition area between the Westerwald and Siebengebirge volcanic fields (Germany) that belongs to the Central European Volcanic Province (CEVP). The 39Ar/40Ar ages indicate ages of c. 24 and c. 5Ma which are fully compatible with previous K/Ar ages indicating that the evolution of this volcanic field belongs to the Westerwald area (28–22Ma and 5Ma) rather than to the Siebengebirge area (26–23Ma). Based on the occurrence of >30 isolated volcanic plugs with a simple igneous history, this volcanic field can be viewed as a monogenetic volcanic field. Compositions of some basanites are primitive, whereas others and the alkali basalts show decreasing Cr and Ni contents and CaO/Al2O3 ratios. However, increasing TiO2, Al2O3 and incompatible elements (Sr, Zr, Y, Hf, Ta) concentrations with decreasing MgO indicating fractionation of mainly olivine with minor amounts of clinopyroxene and spinel can be noticed. Rare earth element systematics suggest that most of the alkaline rocks are generated by different degrees of melting (5%–10%) of a garnet-bearing peridotite containing some residual amphibole. Negative anomalies of Rb and K in primitive mantle-normalized diagrams and a lack of Ba/Rb fractionation suggest that amphibole was the major OH-bearing mineral phase in the mantle. The alkaline rocks have a restricted range in 87Sr/86Sr and 143Nd/144Nd ratios ranging from 0.7033 to 0.7044 and from 0.51275 to 0.51285, respectively. Lead isotope compositions (206Pb/204Pb: 19.21–19.65; 207Pb/204Pb: 15.62–15.67; 208Pb/204Pb: 39.10–39.46) of the alkaline rocks are within the range of most OIB in which the higher values approach the composition of the European Asthenospheric Reservoir (EAR). The correlation between Sr and Nd isotopes and trace element constraints (Ce/Pb; Nb/U) indicates that for some samples interaction with crustal rocks during fractionation has occurred. Miocene intraplate basaltic volcanism in the area probably occurred as a result of minor “baby plume” activity. Each volcanic plug records evidence of a specific stage of fractionation with or without assimilation; however, in summary the lavas plot on a single fractionation path. This implies that during evolution of the volcanic field initial melting took place in the asthenosphere or at the lithosphere–asthenosphere interface. The melts moved through the lithospheric mantle and stagnated at crustal levels, however the observed fractionation paths suggest that they were fed from a single reservoir. This model, which involves small-scale plume impact followed by asthenosphere–lithosphere interaction together with minor crustal contamination, should also be applicable to other intra-continental rift-related areas.

Chemical composition of waters in a former Cu-As mine: water–rock interaction, REE systematics and stability of secondary copper minerals

Waters in the vicinity of natural geochemical anomalies such as hydrothermal ore deposits potentially mobilize heavy metals by interaction with primary hydrothermally formed minerals. On the other hand, secondary alteration minerals can retain some of these heavy metals and effectively immobilize them. In order to understand the chemical evolution of waters in such a system, the best way is to compare different waters from various sample locations in a well-defined locality. Thereby one can compare the importance of parameters such as variable flow paths, flow velocities, fluid-host rock interaction and the various sources and sinks of major and trace elements. This was done for the Cu and As deposit of Neubulach in the northern Schwarzwald, Germany. Flowing and dripping waters from three sites in the abandoned mine were sampled monthly over 14 months. The waters were analyzed with respect to their major and trace element concentrations including rare earth elements (REE) to evaluate their seasonal and spatial variation. The three sample types collected from different depths in the mine have different flow paths and are influenced to different extents by the sandstone host rocks, by overlying carbonates and by the hydrothermal barite quartz vein with Cu-Bi-As ores, mainly tennantite, exploited from the former mine. The sample site in the upper part of the mine (“HGS water”) is about 30 m below surface and the amount of water dripping from the roof of the adit is strongly dependent on the precipitation. By contrast, the water collected in the lower part of the mine (about 100 m below surface) is dripping constantly throughout the year and forms abundant calcite sinters (“sinter water”). A third sample was collected monthly from the mine drainage adit (again about 100 m below surface), that collects water from the entire mine (“flowing water”). Elevated Cl and Na concentrations and increased Cl/Br ratios in the HGS waters taken in the winter months during snow melt indicate input of road salt and therefore show the direct influence of surface runoff. By contrast, the sinter water compositions do not show any variation during the year, which suggests that these waters are unaffected by short-time input from the surface; their chemical composition is determined by water-rock interaction only. Rare earth element patterns and Rb/Cs values indicate interaction with clay minerals whereas Cu and As systematics show that tennantite dissolution is incongruent. Although inverse modelling shows that reaction of surface water with the observed phases in the host rocks and in the hydrothermal vein nicely explains the water compositional variations, speciation modeling and calculations of thermodynamic stabilities of various Cu arsenates suggest that secondary alteration minerals may form metastably.

Specific features of the distribution of trace and rare earth elements in recent bottom sediments in the lower course of the Severnaya Dvina River and White Sea

The paper discusses results of the lithogeochemical examination of recent bottom sediments in the lower course of the Severnaya Dvina River and White Sea. It has been established that the average con� centration of several trace elements (Hf, Sc, Co, Y, Ni, V, Cr, Zr, Ba, and others) therein correlates with the content of the silt-pelite fraction. Maximal concentrations of the majority of above elements are confined to the silty-clayey sediments at the Basin/Dvina Bay boundary. They localized near the coastal zone only for some clastophile (Zr, Cr, and others). Typical values of the hydrolyzate module, chemical index of alteration, and Al2O3/SiO2 ratio in the aleuropelitic and pelitic sediments of the Severnaya Dvina River delta, Dvina Bay, and the Dvina Bay Basin boundary suggest that these sediments are confined to sufficiently cold climate set� tings. Data points of sediment composition in discriminant paleotectonic diagrams are scattered over a large field probably due to high contents of the weakly weathered plagioclases, micas, and amphiboles, as well as the hydrogenic process promoting the accumulation of Fe and Mn. The PAASnormalized spectra of rare earth elements (REE) in bottom sediments of the Pinega and Severnaya Dvina rivers, marginal filter of the latter river, Dvina Bay, and the Dvina Bay Basin boundary are similar to the REE distribution in clayey rocks of the ancient platform cover (except for a slight positive Eu anomaly). The REE systematics and distribution pattern of compositional data points of recent bottom sediments in the GdN/YbN-Eu/Eu* and Eu/Eu*- Cr/Th diagrams and values of several indicator ratios of trace elements suggest that the studied rocks were formed by the mixing of clastic materials from geochemically contrast provenances: northwestern provenance (Kola-Karelia geoblock), which is mostly composed of the Archean and Early Proterozoic crystalline com� plexes, and the southeastern provenance (northwestern periphery of the Mezen syncline), which is almost totally composed of Phanerozoic sedimentary rocks. The latter provenance likely played a crucial role in the geochemical signature of recent bottom sediments over a significant area of the White Sea.