Rare Earth Element Profiles Research Articles

Geology, petrology and geochemistry of the “Americano do Brasil” layered intrusion, central Brazil, and its Ni–Cu sulfide deposits

The “Americano do Brasil” Complex (ABC) is part of a cluster of coeval synorogenic mafic–ultramafic intrusions emplaced during the Brasiliano/Pan-African Orogenic Cycle in Brazil. The medium-sized ABC consists of interlayered dunite, peridotite, websterite, and gabbronorite. High Fo values of olivine (up to Fo88) and the crystallization sequence of the ABC (Ol + Chr ≥ Ol + Opx + Chr ≥ Cpx + Opx ≥ Opx + Pl + Cpx ≥ Opx + Pl + Cpx + Ilm + Mag) suggest crystallization from tholeiitic high-MgO parental magmas. Light rare earth element (REE)-enriched mantle-normalized REE profiles and eNd(T) values of +2.4 for cumulate rocks from the ABC suggest a depleted mantle source for the parental magma. The ABC Ni–Cu sulfide deposit (3.1 Mt at 1.12 wt.% Ni and 1.02 wt.% Cu) consists of three distinctively different orebodies (S1, S2, and G2). The S2 orebody, an unusual occurrence of stratiform massive sulfide hosted by dunite and peridotite in the interior of a layered intrusion, results from sulfides accumulated at the transient base of the magma chamber following a new influx of parental magma. The G2 orebody has an irregular and roughly cylindrical shape, consisting mainly of net-textured sulfides. The G2 orebody is hosted by peridotite and pyroxenite and located stratigraphically below the S1 orebody. S2 and G2 orebodies are characterized by low Cu/Cu + Ni ratios (mainly below 0.4). The S1 orebody, hosted by websterite and gabbronorite in the more fractionated sequence of the ABC, is a cluster of several irregular discontinuous orebodies of Ni–Cu disseminated sulfides. The sulfides of the S1 orebody have high Cu/Cu + Ni ratios (mainly between 0.5 and 0.8) and are highly depleted in PGE. The S1 orebody is interpreted to result from a later event of sulfide segregation in the magma chamber, possibly following the event that originated the G2 orebody. The bulk of δ34S values for sulfides of the ABC orebodies and their host rocks fall in the range of 0 ± 2‰. Higher δ34S values (between 3‰ and 5‰) are restricted to pyrite from xenoliths of gneiss located close to the S1 orebody and sulfides from the S1 orebody. Crustal xenoliths and chemical data (lithogeochemistry and sulfur isotope composition) provide evidence of crustal contamination of the igneous rocks hosting the S1 orebody, suggesting that sulfur saturation was induced by contamination with sulfide-bearing crustal rocks. The ABC deposit is an example of Ni–Cu sulfide mineralization hosted by synorogenic mafic–ultramafic intrusions. The S2 orebody is the first documented example of an economic stratiform massive sulfide orebody located within layered intrusions, expanding the opportunities for exploration of Ni–Cu sulfides in orogenic regions worldwide.

Timescales and mechanisms of REE and Hf uptake in fossil bones

Rare earth element (REE) patterns of fossil bones and teeth are widely used as proxies for provenance, taphonomy, and palaeoenvironment. In order to investigate if fossil bones behave as closed systems over geologic time, REE profiles were analysed by LA-ICPMS along cross sections of 54 bones from various well-characterised and well-dated settings. These include terrestrial and marine diagenetic environments, covering Early Triassic to Holocene ages. In general, all fossil bones exhibit the highest REE concentrations at the outer rim, gradually decreasing by up to four orders of magnitude toward the inner bone cortex. Intra-bone REE concentration gradients decrease significantly from Quaternary via Tertiary to Mesozoic specimens, suggesting long term REE uptake and open system behaviour of fossil bone. This view is further corroborated by 176Lu– 176Hf dating of selected samples, all yielding significantly younger ages than the known chronostratigraphic ages. Hence, there is clear evidence for long term open system behaviour of fossil bones with respect to REE, which is in marked contrast to currently accepted models suggesting that REE uptake is only early diagenetic. Although unexpected, statistically significant four to seven point isochrons are observed for four fossil dinosaur bone samples and one Upper Triassic Mastodonsaurus tooth with MSWDs ranging from 0.083 to 4.5. Notably, mobility of Lu alone cannot account for the observed age patterns. Assuming constant Lu uptake rates over time, the radiometric ages should only be as low as half of the chronostratigraphic age. However, a six-point isochron defined by subsamples of a single Upper Triassic Mastodonsaurus tooth yields an age of 65.2 ± 1.1 Ma (MSWD = 0.68), much younger than half of the stratigraphic age (ca. 234 Ma). Hence, Hf must also undergo late diagenetic exchange. Likely mechanisms to account for the presence of statistically meaningful isochrons as well as for the late diagenetic exchange of both REE and Hf are diffusion, adsorption, and dissolution–reprecipitation processes.

Trace Element and Rare Earth Element Profiles in Berry Tissues of Three Grape Cultivars

Mineral elements play an important role in the geographic traceability of wine, and understanding mineral element profiles in berry tissues helps to determine the relationship between elemental accumulation and regional flavor formation in grapes and wine. Eighteen trace elements and 15 rare earth elements were investigated in the skin, pulp, and seeds of grape cultivars Cabernet Sauvignon, Marselan, and Italian Riesling using inductively coupled plasma–mass spectrometry (ICP–MS). Most trace elements, except for B, Zr, Tl, and U, presented similar tissue-specific distributions in their concentrations in the order of seeds > skin > pulp, but concentrations varied within the three cultivars. The two red cultivars showed significantly higher concentrations of Cu, Cr, Ba, Mo, Cd, Ga, Ge, and Tl and lower concentrations of B, Mn, Sr, and U than the white cultivar Italian Riesling. However, the tissue distribution of rare earth elements was totally different from that of the trace elements. In Italian Riesling berries, concentrations of most rare earth elements in the skin were greater than or similar to those in the seeds and pulp, except that Yb was not detected. For the two red cultivars, concentrations of 11 rare earth elements (Y, La, Ce, Nd, Pr, Sm, Gd, Dy, Ho, Er, and Yb) in the seeds were not as high as those in the skin and pulp. These findings give new insights into the tissue distribution of trace and rare earth elements in white and red grape cultivars.

An overview of petrochemistry in the regional exploration for volcanogenic massive sulphide (VMS) deposits

ABSTRACT Volcanogenic massive sulphide (VMS) deposits are important global sources of base and precious metals. Igneous geochemistry (petrochemistry) of mafic and felsic rocks associated with VMS deposits is extremely useful in delineating potentially fertile ground for VMS mineralization. In mafic-dominated, juvenile environments (e.g. mafic, bimodal mafic and mafic-siliciclastic VMS-types) VMS deposits are associated with boninite and low-Ti island arc tholeiite, mid-ocean ridge basalt, and back-arc basin basalt. These rocks are ultimately sourced from either depleted arc mantle wedge (e.g. boninite, low Ti island arc tholeiite) or upwelling depleted, mid-ocean ridge or back-arc asthenospheric mantle (e.g. MORB and back-arc basin basalt). In evolved environments, those associated with continental crust and typically dominated by felsic magmatism (e.g. bimodal felsic and felsic-siliciclastic VMS-types), VMS-associated mafic rocks have alkalic (ocean island basalt-like) and/or mid-ocean ridge/back-arc basin basalt-like signatures. In these environments alkalic basalt and mid-ocean ridge/back-arc basin basalt-like mafic rocks overlie felsic rocks and mineralization and represent melts derived from lithospheric and asthenospheric mantle sources, respectively. Felsic rocks in Archean sequences are typically tholeiitic, have elevated high field strength elements (HFSE) and rare earth elements (REE), and FIII affinities (low Zr/Y and La/Yb n , flat chondrite-normalized rare earth element profiles). In post-Archean evolved environments, felsic rocks associated with VMS deposits have HFSE- and REE-enrichment and within-plate signatures on discrimination diagrams, like their Archean counterparts, but are more calc–alkalic in composition and commonly have FII affinities. Felsic rocks associated with VMS deposits in post-Archean mafic-dominated, juvenile substrates are associated with trace element depleted rhyolites with tholeiitic to boninite-like signatures and M-type and FIV affinities on discrimination plots. Using mafic or felsic rocks in isolation may lead to erroneous assignments of prospectivity for terrains; however, when mafic and felsic rocks are used in tandem with geological context they are powerful tools in outlining potentially prospective regions. Within VMS-hosting environments there are specific petrochemical assemblages of mafic and felsic rocks. Petrochemical assemblages are specific lithogeochemical associations between mafic and felsic rocks that are common to VMS forming environments and are useful in identifying two key ingredients required to form prospective VMS belts: (1) rifting; and (2) high temperature magmatism.

Rare earth elements distribution in marine sediments of Malaysia coasts

In the east coast Peninsular Malaysia region, sediments are transported by several rivers from the east Malaysia into the South China Sea estuary. In the vicinity of the five river estuaries core sediments were collected in order to investigate rare earth elements (REEs) profile. Core sediments were divided into strata of between 2 to 4 cm intervals and prepared for analyzing by ICP-AES. REE concentrations of 54.3 μg/gr at 24–26 cm in EC4 increased to 114.1 μg/gr at 20–22 cm in EC5. The measured concentration of LREEs and Fe normalized enrichment factors indicated that sediments were not enriched with light rare earth elements derived from effluents of anthropogenic activities. Results of the total concentration were used to establish baseline data in environmental pollution assessment and to develop the correlations between the Ce/Ce * and Eu/Eu * anomalies and the distribution patterns of some light rare earth elements (LREEs) and the heavy rare earth elements (HREEs). The chondrite-normalized ratios of REEs showed LREEs enrichment and flat HREE depletion.

Study on vertical profiles of rare earth elements by using an ocean general circulation model

Observed vertical profiles of rare earth elements (REEs) in the North Pacific Ocean show a systematic change from lighter to heavier REEs. Their source and sink consist of external fluxes coming from the surface and the bottom of the ocean and internal recycling within the ocean. In order to evaluate the role of each source and sink in controlling the vertical profiles of REEs, we conduct numerical simulations of REEs by using an ocean general circulation model. These simulations demonstrate that the shape of their vertical profiles is not directly related to the external sources from river runoff, atmospheric dust deposition, and coastal regions but is sensitive to the internal recycling processes within the ocean. Nutrient‐like treatment where REEs are removed at the surface and released in the deep ocean is suitable for simulating the vertical profiles of lighter REEs but not for heavier REEs. When the irreversible scavenging is considered as a sink term and particle dissolution is taken into consideration as a source term, the vertical profiles of heavier REEs can be reproduced but those of lighter REEs cannot. The observed difference in the vertical profiles among REEs can be reproduced only when the reversible scavenging process is incorporated into the model. Only the reversible scavenging process accounts for the observed vertical profiles of different REEs in comprehensive manner.

Geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of Borborema Province, NE Brazil: Geodynamic implications

The Borborema Province, northeastern Brazil, occupies a central position in pre-drift reconstructions of western Gondwana, making an understanding of its geological evolution crucial for Neoproterozoic reconstructions. In recent years, it has been proposed that the Borborema Province grew by accretion of distinct tectonic terranes. In order to test this hypothesis, we compare here the geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of the province. Orthogneiss samples show smooth trends in Harker diagrams and similar rare earth element (REE) patterns, characterized by sharp decreases from La to Sm (chondrite-normalized La/Sm = 3–6) and flat heavy REE profiles (chondrite-normalized Tb/Yb = 1.5–2.5), with small or no Eu anomalies. In primitive mantle-normalized multi-element diagrams, all samples show parallel patterns characterized by sharp negative anomalies of U, Ta, Nb, P and Ti. The metasedimentary samples show little scatter of the major elements in Harker diagrams, suggesting that their chemistry was little affected by post-depositional diagenesis and metamorphism. They have indistinguishable chondrite-normalized REE patterns, characterized by light REE enrichment, flat heavy REE (normalized Tb/Y = 1–2) and small or no negative Eu anomalies, and similar ratios of immobile trace elements (e.g., Th/Sc, Zr/Sc). The geochemistry of the metasedimentary samples is comparable in many ways to those of the orthogneisses, suggesting that these may have been an important source of the precursor sedimentary rocks. These data do not support the terrane accretion hypothesis, rather suggesting the existence of a continuous basement that became available for erosion during intraplate continental extension in the late Neoproterozoic. Comparisons of the studied sequences with those present in the Northern Domain suggest that most, if not all of the Neoproterozoic geodynamic evolution of Borborema Province, occurred in an intracontinental setting.

REE and Sr isotope characteristics of carbonate within the Cu–Co mineralized sedimentary sequence of the Nchanga Mine, Zambian Copperbelt

Carbonate, largely in the form of dolomite, is found throughout the host rocks and ores of the Nchanga mine of the Zambian Copperbelt. Dolomite samples from the hanging wall of the mineralization show low concentrations of rare-earth elements (REE) and roof-shaped, upward convex, shale-normalized REE patterns, with positive Eu*SN anomalies (1.54 and 1.39) and marginally negative Ce anomalies (Ce*SN 0.98,0.93). In contrast, dolomite samples associated with copper and cobalt mineralization show a significant rotation of the REE profile, with HREE enrichment, and La/LuSN ratios <1 (0.06–0.42). These samples also tend to show variable but predominantly negative Eu*SN and positive cerium anomalies and an upwardly concave MREE distribution (Gd-Er). Malachite samples from the Lower Orebody show roof-tile-normalized REE patterns with negative europium anomalies (Eu*SN 0.65–0.80) and negative cerium anomalies (Ce*SN 0.86–0.9). The carbonate 87Sr/86Sr signature correlates with the associated REE values. The uppermost dolomite samples show Neoproterozoic seawater-like 87Sr/86Sr ratios ranging from 0.7111 to 0.7116, whereas carbonate from Cu–Co mineralized samples show relatively low concentrations of strontium and more radiogenic 87Sr/86Sr, ranging between 0.7136–0.7469. The malachite samples show low concentrations of strontium, but give a highly radiogenic 87Sr/86Sr of 0.7735, the most radiogenic 87Sr/86Sr ratio. These new data suggest that the origin and timing of carbonate precipitation at Nchanga is reflected in the REE and Sr isotope chemistry. The upper dolomite samples show a modified, but essentially seawater-like signature, whereas the rotation of the REE profile, the MREE enrichment, the development of a negative Eu*SN anomaly and more radiogenic 87Sr/86Sr suggests the dolomite in the Cu–Co mineralized samples precipitated from basinal brines which had undergone significant fluid–rock interaction. Petrographic, REE, and 87Sr/86Sr data for malachite are consistent with the original sulfide Lower Orebody being subject to a later oxidizing event.

Genesis of metapelitic migmatites in the Adirondack Mountains, New York

AbstractOxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ18O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (∼1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ18O(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ18O(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ18O values (&gt;10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ18O(Zrc) &lt; 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ18O(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ18O metasedimentary rocks.

Lower Silurian subduction-related volcanic rocks in the Chaleurs Group, northern New Brunswick, CanadaGeological Survey of Canada, Contribution No. 2008-0166.Contribution to Natural Resources Canada’s Targeted Geoscience Initiative 3 (2005–2010).

Early Silurian volcanic and subvolcanic rocks are preserved in the lower part of the Chaleurs Group at two locations in northern New Brunswick. At Quinn Point, mafic to intermediate rocks are hosted by sedimentary rocks of the Weir Formation, and at Pointe Rochette, a bed of felsic tuff occurs near the base of the Weir. These rocks are interpreted as the first evidence in New Brunswick of magmatism associated with Late Ordovician – Early Silurian subduction of Tetagouche–Exploits back-arc oceanic crust. At Quinn Point, mafic rocks include a thick basaltic flow or sill and intermediate to mafic cobbles in overlying conglomerate beds. The in situ mafic rocks and the conglomerate clasts are chemically alike and display subduction-related affinities on tectonic discrimination diagrams. At Pointe Rochette, fine-grained felsic tuff contains elevated Th and U and depleted high-field-strength elements, consistent with a subduction-influenced setting, although rare-earth element (REE) abundances are low and the REE profile is relatively flat. A U–Pb (zircon) age of 429.2 ± 0.5 Ma was obtained from the tuff, consistent with the late Llandovery to early Wenlock age of the overlying La Vieille Formation and coinciding with the latter stages of development of the Brunswick subduction complex. Volcanic rocks were emplaced in the arc to arc-trench gap region, probably reflecting local step-back of the magmatic axis due to accretion of continental back-arc ribbons. The low volume of Early Silurian subduction-influenced rocks is probably related to the relatively narrow width of the back-arc basin and the young, “warm” character of back-arc crust.

Dissolved rare earth elements tracing lithogenic inputs over the Kerguelen Plateau (Southern Ocean)

Abstract Dissolved rare earth elements (REEs) were measured in the water masses upstream from and over the Kerguelen Plateau (Indian sector of the Southern Ocean). Upstream from the plateau, the REE profiles were consistent with published data from the Southern and Indian Oceans. Over the plateau ( A one-box model yields a lithogenic-Nd flux, from the dissolution of lithogenic material, of 89±20 t Nd y−1, and a corresponding dissolved-iron input larger than 100,000 t Fe y−1. However, despite evidence of this significant lithogenic input, dissolved Fe is strongly depleted in the upper layers, contrasting with the enriched REE concentrations. This contrast stresses the different behaviours of REE and iron in the surface waters and the fact that strong fertilization does not imply dissolved Fe-replete waters.

Age and duration of ultrahigh‐temperature metamorphism in the Anápolis–Itauçu Complex, Southern Brasília Belt, central Brazil – constraints from U–Pb geochronology, mineral rare earth element chemistry and trace‐element thermometry

AbstractIntegrated petrographic and chemical analysis of zircon, garnet and rutile from ultrahigh‐temperature (UHT) granulites in the Anápolis–Itauçu Complex, Brazil, is used to constrain the significance of zircon ID‐TIMS U–Pb geochronological data. Chondrite‐normalized rare earth element (REE) profiles of zircon cores have positive‐sloping heavy‐REE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively sloping heavy‐REE patterns. However, in one sample, a core of zircon interpreted as having formed prior to garnet crystallization and a metamorphic zircon formed within microstructures involving garnet breakdown both display elevated heavy‐REE (and Y) with positive‐sloping patterns. DREE(zrc/grt) partition coefficients suggest an approximation to equilibrium between zircon and garnet cores, although progressive enrichment in heavy REE towards garnet rims occurs in two of the samples investigated. Titanium‐in‐zircon thermometry indicates zircon growth during both the prograde and post‐peak evolution, but not at peak temperatures of the UHT metamorphism. By contrast, zirconium‐in‐rutile thermometry of inclusions armoured by garnet records crystallization temperatures, based on the upper end of the interquartile range of the data, of ∼890 to 870 °C and maximum temperature around 980 °C, indicating prograde and retrograde growth close to and after peak conditions. Rutile located in retrograde microstructures records crystallization temperatures of 850 to 820 °C. Rutile intergrown with ilmenite and included within orthopyroxene, which is associated with exsolved zircon, records temperatures ∼760 °C, consistent with Ti‐in‐zircon crystallization temperatures. ID‐TIMS U–Pb geochronological data from two of the four samples investigated define upper intercept ages of 641.3 ± 8.4 Ma (MSWD 0.91) and 638.8 ± 2.5 Ma (MSWD 1.03) that correlate with periods of zircon growth along the prograde segment of the P–T path. Individual zircon U–Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of c. 15 Myr for the UHT event. This period is interpreted as recording modest thickening of hot backarc lithosphere located behind the Arenópolis Arc at the edge of the São Francisco Craton consequent upon terminal collision of the Parána Block with the arc during the amalgamation of West Gondwana.

Sr–Nd isotopic evidence for crustal contamination in the Niquelândia complex, Goiás, Central Brazil

The Niquelândia complex is a Neoproterozoic mafic-ultramafic intrusion resulting from fractional crystallization of primary picritic basalt intrusions. It consists of two layered sequences: a lower and larger one (LS), where four stratigraphic units exhibit an upward decrease of ultramafic layers and increase of gabbroic layers; an upper, smaller sequence (US), separated from LS by a high-temperature shear zone and consisting of two stratigraphic units (gabbros + anorthosites and amphibolites). Nd and Sr isotopic analyses and rare earth element (REE) profiles provide evidence that the complex suffered important crustal contamination. The LS isotopic array trends from a DM region with positive εNd and moderately positive εSr towards a field occupied by crustal xenoliths, especially abundant in the upper LS (negative εNd and large, positive εSr). Each LS stratigraphic unit is distinct from the next underlying unit, showing lower εNd and higher εSr, suggesting inputs of fresh magma and mixing with the contaminated, residual magma. The US is characterised by a relatively high variation of εNd and constant εSr. REE patterns vary within each unit from LREE depleted to LREE enriched in the samples having lower εNd and higher εSr. The contamination process has been modelled by using the EC-AFC algorithms from [Spera, F.J., Bohrson, W.A., 2001. Energy-constrained open-system magmatic processes I: general model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation. J. Petrology 42, 999–1018]. The differences between the LS and US isotopic arrays are consistent with contamination by the same crustal component, provided that its melting degree was higher in LS than in US. The different degrees of anatexis are explained by the heat budget released from the magma, higher in LS (because of its larger mass) than in US. Comparison of the correlations between isotopes and incompatible trace element ratios of the models and of the gabbros shows some differences, which are demonstrably related with the variable amount of cumulus phases and trapped melt in the gabbros.

The metasomatic record in the shallow peridotite mantle beneath Grenada (Lesser Antilles arc)

The composition and geochemical signatures of the mantle wedge beneath the Lesser Antilles arc are documented by the ultramafic xenoliths included in alkali basalts (M-series) on Grenada. Xenoliths consist of harzburgites, lherzolites, dunites and subordinate wehrlites and pyroxenites. Primary minerals phases are olivine, low-Al and high-Al orthopyroxene, clinopyroxene and Cr-Spinel. In addition to the primary assemblage, Grenada xenoliths contain metasomatic phases such as Al-rich clinopyroxene, plagioclase, Al-rich spinel, pargasitic amphibole and Si- and Al-rich glasses. The trace-element signatures of pyroxenes and glasses have been determined on selected samples by LA-ICP-MS. Pyroxenes from both lherzolite and harzburgite xenoliths have U-shaped rare earth element (REE) profiles, unusually high Th, U and Sr concentrations and large negative Nb, Ta and Zr, and Hf anomalies. The geochemical signatures of metasomatic clinopyroxene are different from those reported for clinopyroxene from fluid-metasomatised mantle wedge, and are clearly distinct from those of clinopyroxene in equilibrium with host lavas. Si-rich glasses show a narrow compositional range, with trace-element characteristics closely similar to those of reacted pyroxenes. This, along with the general lack of chemical gradients of LILE and LREE over more compatible elements suggests dacitic glasses represent the products of in-situ melting caused by temperature increase before and during the uptake of xenoliths by host lavas. Dacitic melts are believed to represent local re-melts of regions metasomatically enriched by earlier arc magmas that had stalled, fractionated, and solidified in the upper mantle. These local re-melts thus reflect the metasomatic component formed by earlier arc-related metasomatic agents and liable to be re-mobilised. This also appears to be the easiest way to explain the compositional similarities between erupted arc lavas and the metasomatised peridotites. The results of this study suggest that the mantle wedge beneath the Lesser Antilles underwent complex peridotite–melt reaction processes operated by sub-arc melts and, later on, by magmas similar in compositions to the host alkali basalts. The majority of the compositional range of erupted Grenada magmas, but adakites found at surface, can be obtained by the interaction of basalts, possibly formed by hydrous melting of MORB-source mantle, with the overlying mantle wedge.

Geochemistry, Geochronology and Isotopic Evolution of the Chewore-Rufunsa Terrane, Southern Irumide Belt: a Mesoproterozoic Continental Margin Arc

The southern Irumide Belt (SIB) is an ENE^WSW-trending, late Mesoproterozoic orogenic belt located between the Congo^Tanzania^ Bangweulu (CTB) and Kalahari cratons in central southern Africa. It is separated from the late Mesoproterozoic Irumide Belt (IB) to the north by Permo-Triassic graben, raising the possibility that the younger rifts reactivated a suture between the two belts that has been rendered cryptic as a result of younger Karoo cover. Both belts are dominated by calc-alkaline gneisses, but in addition the SIB contains abundant metavolcanic and metasedimentary rocks. In this study we present detailed geochemical, isotopic and geochronological data for volcanic and plutonic lithologies from the southernmost part of the SIB, the Chewore^Rufunsa Terrane. This terrane comprises a wide variety of supracrustal to mid-crustal rocks that have major- and trace-element compositions similar to magmas formed in present-day subduction zones. Chondrite-normalized rare earth element (REE) profiles and whole-rock Sm^Nd isotope compositions indicate that the parental supra-subduction melts interacted with, and were contaminated by sialic continental crust, implying a continental-margin-arc setting. Secondary ionization mass spectrometry dating of magmatic zircon has yielded crystallization ages between c. 1095 and 1040 Ma, similar to elsewhere in the SIB. U^Pb dating and in situ Lu^Hf isotopic analyses of abundant xenocrystic zircon extracted from the late Mesoproterozoic granitoids indicate that the contaminant continental basement was principally Palaeoproterozoic in age and had a juvenile isotopic signature at the time of its formation. These data are in contrast to those for the IB, which is characterized by younger, c. 1020 Ma, calc-alkaline gneisses that formed by the direct recycling of Archaean crust without significant addition of any juvenile material. We suggest that the SIB developed by the subduction of oceanic crust under the margin of an unnamed continental mass until ocean closure at c. 1040 Ma. Subsequent collision between the SIB and the CTB margin led to the cessation of magmatism in the SIB and the initiation of compression and crustal melting in the IB.

Rare Earth Elements in the hydrothermal system at Okinawa Trough back-arc basin

We present novel data sets of rare earth element (REE) distributions in a hydrothermal vent field at Yonaguni Knoll IV in the Okinawa Trough. Vertical REE profiles in three water columns showed horizontal variation of REE concentrations within 1000‐1200 m. Hydrothermal plumes were discovered by anomalous values of methane, manganese and transmissometry at that site. Europium anomalies in the North Pacific deep water (NPDW) (Nozaki et al., 1999) normalized pattern decreased with distance from the hydrothermal vent site, indicating that the dilution of hydrothermal fluid in the plume can be traced using REE. The horizontal variation of negative Ce-anomalies represents the continuous scavenging of REE by suspended matter in the plume. In addition, we measured nine hydrothermal fluid samples. The REE geochemistry of hydrothermal vent systems had been investigated intensively at sediment-starved mid-oceanic ridges, but few studies had examined sediment-hosted hydrothermal systems like those of the Okinawa Trough. The chondrite-normalized REE patterns of the fluids collected at Yonaguni Knoll IV show typical lighter rare earth elements (LREE) and Eu enrichment similar to at the Mid-ocean Ridge sites. A remarkable characteristic of the Yonaguni Knoll IV fluid pattern is its higher concentrations of heavier rare earth elements (HREE) and La composition than the hydrothermal fluids of the sediment-starved East Pacific Rise and Trans-Atlantic Geotraverse. Such a feature is explainable by influences of covering sediments in the back-arc basin Okinawa Trough. At the hydrothermal vent, lighter REE (LREE) in the fluid was reduced systematically during fluid mixing with seawater within the chimney. Light REE elimination resembles fractionation caused by particle scavenging within the water column. However, the lack of Ce depletion, which is a typical REE feature in the water column, along with distinctive Eu reduction, were unique in the Yonaguni Knoll IV fluid, suggesting that fluid REE fractionation at the vent site was induced predominantly by coprecipitation with hydrothermally originated minerals (e.g. sulfate and carbonate), not by adhesive removal by Fe and/or Mn oxide particles. Previous studies had shown that REE removal and fractionation of the hydrothermal system were observed only in deposit samples. Results of this study elucidated REE fractionation in fluid samples using previous analytical data. We were also able to distinguish REE removal mechanisms occurring at the vent site and water column using REE pattern characteristics.

The Flat Landing Brook Zn-Pb-Ag Massive Sulfide Deposit, Bathurst Mining Camp, New Brunswick, Canada

The Flat Landing Brook Zn-Pb-Ag deposit of the Bathurst Mining Camp occurs within a narrow thrust-bound nappe containing felsic volcanic and volcaniclastic rocks of the Tetagouche Group. Within the host nappe, the Tetagouche Group is represented by the Nepisiguit Falls Formation and the overlying Flat Landing Brook Formation. The Nepisiguit Falls Formation is divided into two members: quartz- and quartz-feldspar-phyric volcaniclastic rocks ± minor lavas (Grand Falls member), and aphyric, fine-grained volcaniclastic rocks (Little Falls member). The Flat Landing Brook Formation consists of aphyric rhyolite flows and interbedded pyroclastic rocks. Several gabbroic intrusions occur in both the footwall and hanging-wall sequences. These gabbros locally cut out the mineralized horizon at shallow levels, and are considered to be feeders to tholeiitic basaltic flows (Forty Mile Brook member) of the Flat Landing Brook Formation. The Flat Landing Brook deposit has many of the characteristics typical of volcanogenic massive sulfide deposits occurring within the highly productive Nepisiguit Falls Formation in the eastern part of the Bathurst Mining Camp. Mineralization occurs within or at the top of the Grand Falls member and comprises four or more massive to semi-massive sulfide lenses that vary in thickness between 3 and 5 m. Massive lenses are laterally gradational to, or underlain by, zones of disseminated sulfides up to 38 m thick. The deposit contains an estimated resource of 1.7 Mt grading 4.9% Zn, 0.94% Pb, and 19.54 g/t Ag to a depth of approximately 150 m. From 150 to 300 m below surface, mineralization is low grade and mostly disseminated. However, below 300 m, ore-grade (>10% Pb+Zn) massive sulfide lenses have been intersected over mineable widths. Oxide facies iron formation overlies and (or) grades laterally into the sulfide lenses. The oxide facies has strong positive Eu anomalies and gently sloping rare earth element (REE) profiles suggesting that it was formed from relatively hot acidic fluids that had interacted with felsic volcanic rocks in the footwall. In contrast, the silicate facies iron formation that is more distal to sulfide accumulations has very weak positive Eu anomalies and gently sloping REE profiles, suggesting either cooler hydrothermal fluids or dilution of the hydrothermal component by detrital material. Hydrothermal alteration has affected most footwall rocks. Most notably, albite-destructive alteration has resulted in Na2O depletion, whereas mass addition of K2O is manifested in the formation of sericite (white mica). In more intensely altered quartz- and feldspar-phyric volcaniclastic rocks of the Grand Falls member, feldspar destruction is accompanied by chlorite alteration, producing quartz-phyric rocks similar to those in the footwall of many Bathurst Camp deposits.

Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins

Banded iron-formations (BIFs) occur in the Precambrian geologic record over a wide time span. Beginning at 3.8 Ga (Isua, West Greenland), they are part of Archean cratons and range in age from about 3.5 until 2.5 Ga. Their overall volume reaches a maximum at about 2.5 Ga (iron-formations in the Hamersley Basin of Western Australia) and they disappear from the geologic record at about 1.8 Ga, only to reappear between 0.8 and 0.6 Ga. The stratigraphic sequences in which BIFs occur are highly variable. Most Archean iron-formations are part of greenstone belts that have been deformed, metamorphosed, and dismembered. This makes reconstruction of the basinal setting of such BIFs very difficult. The general lack of metamorphism and deformation of extensive BIFs of the Hamersley Range of Western Australia and the Transvaal Supergroup of South Africa allow for much better evaluations of original basinal settings. Most Archean iron-formations show fine laminations and/or microbanding. Such microbanding is especially well developed in the Brockman Iron Formation of Western Australia, where it has been interpreted as chemical varves, or annual layers of sedimentation. BIFs ranging in age from 2.2 Ga to about 1.8 Ga (e.g., those of the Lake Superior region, U.S.A., Labrador Trough, Canada, and the Nabberu Basin of Western Australia) commonly exhibit granular textures and lack microbanding. The mineralogy of the least metamorphosed BIFs consists of combinations of the following minerals: chert, magnetite, hematite, carbonates (most commonly siderite and members of the dolomite-ankerite series), greenalite, stilpnomelane, and riebeckite, and locally pyrite. Minnesotaite is a common, very low-grade metamorphic reaction product. The Eh-pH stability fields of the above minerals (and/or their precursors) indicate anoxic conditions for the original depositional environment. The average bulk chemistry of BIFs, from 3.8 through 1.8 Ga in age, is very similar. They are rich in total Fe (ranging from about 20 to 40 wt%) and SiO2 (ranging from 43 to 56 wt%). CaO and MgO contents range from 1.75 to 9.0 and from 1.20 to 6.7 wt%, respectively. Al2O3 contents are very low, ranging from 0.09 to 1.8 wt%. These chemical values show that they are clean chemical sediments devoid of detrital input. Only the Neoproterozoic iron-formations (of 0.8 to 0.6 Ga in age) have very different mineralogical and chemical make-ups. They consist mainly of chert and hematite, with minor carbonates. The rare-earth element profiles of almost all BIFs,with generally pronounced positive Eu anomalies, indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The prograde metamorphism of iron-formations produces sequentially Fe-amphiboles, then Fe-pyroxenes, and finally (at highest grade) Fe-olivine-containing assemblages. Such metamorphic reactions are isochemical except for decarbonation and dehydration. The common fine lamination (and/or microbanding) as well as the lack of detrital components in most BIFs suggest that such are the result of deposition, below wave base, in the deeper parts of ocean basins. Those with granular textures are regarded as the result of deposition in shallow water, platformal areas. Carbon isotope data suggest that for a long period of time (from Archean to Early Proterozoic) the ocean basins were stratified with respect to δ13C (in carbonates) as well as organic carbon content. In Middle Proterozoic time (when granular BIFs appear) this stratification diminishes and is lost. The Neoproterozoic BIFs occur in stratigraphic sequences with glaciomarine deposits. These BIFs are the result of anoxic conditions that resulted from the stagnation in the oceans beneath a near-global ice cover, referred to as “Snowball Earth.” All of the most “primary” mineral assemblages appear to be the result of chemical precipitation under anoxic conditions. There are, as yet, no data to support that BIF precipitation was linked directly to microbial activity. The relative abundance of BIF throughout the Precambrian record is correlated with a possible curve for the evolution of the O2 content in the Precambrian atmosphere.

Nd isotopic, petrologic and geochemical investigation of the Tulawaka East gold deposit, Tanzanian Craton

New isotopic (Sm–Nd) and geochemical data are presented for volcano-sedimentary and granitic lithotypes of the Tulawaka East gold deposit in the Late Archean Sukumaland Greenstone Belt of the Tanzanian Craton. The data help to characterize the geological environment of the greenstone belt/gold deposit and the crustal evolution of the Tanzanian Shield. The volcano-sedimentary units were intruded by leucogranites and aplitic dykes that were deformed and metamorphosed to amphibolite facies grades. The Au mineralization occurs in thrust faults at the contact between the aplite dykes and the volcano-sedimentary units. The metavolcanic rocks are largely basaltic to intermediate in composition with negative Nb anomalies and flat rare earth element (REE) profiles varying from 10 to 100× chondritic values. Initial ɛNd 2.8 Ga values vary from +0.9 to +5.3 and Nd model ages ( T dm) range from 2.8 to 3.0 Ga. The metasedimentary rocks are generally enriched in light REE (100× chondrite) with weak negative Eu anomalies and ɛNd 2.8 Ga values vary from 0.0 to +2.6. The leucogranites are garnet-rich and peraluminous with strong negative anomalies in Eu and Ti and with flat REE profiles (10–100× chondrite). The aplite dyke is also peraluminous with similar negative Eu and Ti anomalies and a flat light REE, but is depleted in heavy REE. The similarity in composition with the leucogranite suggests that the two are cogenetic and formed by fractionation of a more mafic parental magma. The combination of the flat REE element patterns and negative Nb anomalies among the mafic volcanic rocks suggests formation as an immature (island?) arc at ca. 2.8 Ga. However, Nd isotope signatures in the metasedimentary rocks provide evidence of an older crustal sedimentary provenance (ca. 3.0–3.1 Ga) and proximity of a continental influence. The age of the leucogranite and aplite dyke are constrained by a Sm–Nd garnet-whole-rock age of ca. 2.5 Ga, indicating that the shear zone-hosted gold mineralization is younger than 2.5 Ga. The range of isotopic compositions found in the Tanzanian Shield overlap with those of the Zimbabwe Craton to the south.

Diel behavior of rare earth elements in a mountain stream with acidic to neutral pH

Diel (24-h) changes in concentrations of rare earth elements (REE) were investigated in Fisher Creek, a mountain stream in Montana that receives acid mine drainage in its headwaters. Three simultaneous 24-h samplings were conducted at an upstream station (pH = 3.3), an intermediate station (pH = 5.5), and a downstream station (pH = 6.8). The REE were found to behave conservatively at the two upstream stations. At the downstream station, REE partitioned into suspended particles to a degree that varied with the time of day, and concentrations of dissolved REE were 2.9- to 9.4-fold (190% to 830%) higher in the early morning vs. the late afternoon. The decrease in dissolved REE concentrations during the day coincided with a corresponding increase in the concentration of REE in suspended particles, such that diel changes in the total REE concentrations were relatively minor (27% to 55% increase at night). Across the lanthanide series, the heavy REE partitioned into the suspended solid phase to a greater extent than the light REE. Filtered samples from the downstream station showed a decrease in shale-normalized REE concentration across the lanthanide series, with positive anomalies at La and Gd, and a negative Eu anomaly. As the temperature of the creek increased in the afternoon, the slope of the REE profile steepened and the magnitude of the anomalies increased. The above observations are explained by cyclic adsorption of REE onto suspended particles of hydrous ferric and aluminum oxides (HFO, HAO). Conditional partition coefficients for each REE between the suspended solids and the aqueous phase reached a maximum at 1700 hours and a minimum at 0700 hours. This pattern is attributed to diel variations in stream temperature, possibly reinforced by kinetic factors (i.e., slower rates of reaction at night than during the day). Estimates of the enthalpy of adsorption of each REE onto suspended particles based on the field results averaged +82 kJ/mol and are similar in magnitude to estimates in the literature for adsorption of divalent metal cations onto clays and hydrous metal oxides. The results of this study have important implications to the use of REE as hydrogeochemical tracers in streams.