Flat Rare Earth Element Research Articles

Geochemistry of the oldest MORB and OIB in the Isua Supracrustal Belt, southern West Greenland: Implications for the composition and temperature of early Archean upper mantle

Abstract Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate‐type stratigraphy and a duplex structure. Plate history from mid‐oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid‐oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO2 and Na2O contents. The Isua Cpx show slightly light (L)REE‐depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least‐altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al2O3, and depleted in TiO2, FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE‐depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg♯ and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480°C, indicating that the Early Archean mantle was hotter by at most approximately 150°C than the modern mantle.

Light Rare Earth Elements enrichment in an acidic mine lake (Lusatia, Germany)

The distribution of Rare Earth Elements (REE) was investigated in the acidic waters (lake and groundwater) of a lignite mining district (Germany). The Fe- and SO4-rich lake water (pH 2.7) displays high REE contents (e.g. La∼70 μg/l, Ce∼160 μg/l) and an enrichment of light REE (LREE) in the NASC normalised pattern. Considering the hydrodynamic model and geochemical data, the lake water composition may be calculated as a mixture of inflowing Quaternary and mining dump groundwaters. The groundwater of the dump aquifer is LREE enriched. Nevertheless, the leachates of dump sediments generally have low REE contents and display flat NASC normalised patterns. However, geochemical differences and REE pattern in undisturbed lignite (LREE enriched pattern and low water soluble REE contents) and the weathered lignite of the dumps (flat REE pattern and high water soluble REE contents) suggest that lignite is probably the main REE source rock for the lake water.

Late Paleozoic and Triassic plume-derived magmas in the Canadian Cordillera played a key role in continental crust growth

Two major Late Paleozoic–Triassic oceanic terranes are exposed in the North America Cordillera. Slide Mountain Terrane is made up of dolerites, pillow basalts associated with cumulate gabbros and peridotites ranging in age from Carboniferous to Permian. Cache Creek Terrane consists of tectonic slices of Paleozoic platform carbonates, undated cumulate gabbros intruded by dolerites, foliated ultramafic rocks and mafic volcanic rocks interbedded within Upper Triassic siliceous and volcaniclastic sediments. Among the Permian (Slide Mountain) and Upper Triassic (Cache Creek) volcanic rocks, four types have been distinguished. Type 1 is geochemically similar to N-MORB. Type 2 is mildly to highly enriched in LREE, Ti, Zr, Hf, Nb, Ta and Th, and differs from Type 1 by lower εNd and higher Pb isotopic ratios. It displays alkaline affinities. Types 3 and 4 occur only in the Cache Creek Terrane. Type 3 has flat REE patterns similar to oceanic plateau basalts and isotope compositions intermediate between Types 1 and 2. Type 4 is distinguished from Type 3 by convex REE patterns and higher Pb isotopic ratios. Cumulate peridotites and gabbros are LREE-depleted and their high εNd suggest that they derived from the melting of a depleted MORB-type mantle source. The foliated ultramafic rocks are either serpentinized harzburgites (Trembleur ultramafics) or dunites, harburgites and minor lherzolites (Murray Ridge), which are intruded by undeformed pyroxenite veins. The ultramafic rocks have very low contents of rare earth elements (REE) and incompatible elements, and U-shaped REE patterns. The pyroxene-bearing peridotites are distinguished from the Trembleur harzburgites by an absence of Nb negative anomalies and lower ( 87Sr/ 86Sr) i ratios. Slide Mountain and Cache Creek Terranes formed in different tectonic settings. Slide Mountain basin, fringing the North American margin, was probably floored by oceanic crust locally thickened by oceanic island magmas. Indeed, in the Slide Mountain Terrane, N-MORB type basalts predominate and are associated with minor alkaline volcanic rocks within the same thrust sheet. Correlations between incompatible elements and values of the εNd of the Slide Mountain rocks suggest that the N-MORB-type basalts and alkaline rocks are genetically linked. N-MORB type basalts and alkaline volcanic rocks could derive from the mixing of depleted MORB-type and enriched Oceanic island Basalt-type mantle sources. The Cache Creek Terrane may represent remnants of a Late Triassic oceanic plateau because plume-related volcanic rocks predominate while N-MORB type rocks are represented solely by cumulate gabbros and sheeted dykes. The Murray Ridge peridotites could represent the roots of the oceanic plateau. We conclude that the Slide Mountain and Cache Creek Terranes contributed to the growth of the North American continent as when they were accreted to the craton during the Early Triassic and Jurassic times.

REE geochemistry of surface sediments in the Chukchi Sea

Analyses of rare earth elements (REEs) in 26 surface sediment samples obtained from the Chukchi Sea were conducted using ICP-MS. In general, REEs are relatively rich in fine-grained sediments and deplete in coarse-grained sediments in the Chukchi Sea although REEs have large concentration spans in different types of sediments. Except that a few samples have weak en-richments of light or heavy REEs?most samples exhibit flat shale-normalized REE pattern, indi-cating that surface sediments in the Chukchi Sea are composed dominantly of terrigenous com-ponents experiencing weak chemical weathering. In terms of REE concentrations and other char-acteristic parameters, we inferred that sediments on the eastern and western sides of the Chukchi Sea are derived from landmasses of Alaska and Siberia, respectively; the midsouth sediments are possibly related to northward dispersion of the Yukon River materials. The Herald Shoal in the center of the study area is covered with relict sediment, which has large ratios of light-to-heavy REEs (ΣCe/ΣY ratio) and lacks evident negative Ce anomaly; cerium enrichment is possibly re-lated to manganese transfer under oxidizing conditions in early diagenesis.

Geochemistry and tectonic implications of Proterozoic amphibolites in the northeastern part of the Yeongnam massif, South Korea

Abstract Amphibolites in the Haenggongni area (Haenggongni amphibolite) and the Okbang area (Okbang amphibolite) in northeastern Yeongnam massif, South Korea occur as a sill‐like body or inclusions within the metasedimentary sequences of the Proterozoic Wonnam Group. Major and trace element characteristics demonstrate that both amphibolites have tholeiitic chemical affinity. They are characterized by nearly flat rare earth element (REE) patterns, and low contents of immobile incompatible elements and have low values of Zr/Y, Ti/Y, La/Nb and Ta/Yb ratios, indicating enriched (E)‐type mid‐oceanic ridge basalt (MORB) affinities for their protoliths. This suggests that amphibolite protoliths formed in an extensional rift setting leading up to ocean opening. In combination with the previous studies in Yeongnam massif, three protolith types of amphibolites are assumed (E‐type MORB, within‐plate basalt and volcanic arc basalt). They could have been originated in different tectonic settings and/or different episodes. These characteristics are clearly different from the amphibolites in the Gyeonggi massif and Okcheon belt, in which most of the amphibolites show a within‐plate basalt affinity that developed in continental rift zone.

The Petrogenesis of A-type Magmas from the Amram Massif, Southern Israel

The (550‐530Ma) alkaline magmatic suite of the Amram Massif, southern Israel, was emplaced during the transition from an orogenic to an intra-plate tectonic setting in the northeastern Arabian‐Nubian Shield (ANS). The suite ranges from 45 6t o 788 wt % SiO2, and consists of rhyolites, alkali quartz syenites, quartz syenites, monzonites, and co-magmatic mafic to felsic alkaline dikes. These rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content. The felsic members of the suite display A-type characteristics and are genetically linked through fractionation to the more mafic ones. Moderately positive initial eNd values (a2 05), low initial 87 Sr/ 86 Sr values (07036 2), high MgO and Fe2O3 concentrations (410‐895 and 100‐125 wt %, respectively) and relatively flat rare earth element patterns [(La/Yb)na 64 09] in the Amram mafic dikes (456‐495 wt % SiO2), suggest their derivation from the sub-continental lithospheric mantle, above the garnet stability zone. The MELTS program was used to quantitatively model the chemical evolution of the suite. Extensive anhydrous fractionation (490%), of plagioclase, alkali-feldspar, clinopyroxene, olivine, and minor Timagnetite and apatite from parental mafic magmas, represented by the Amram mafic dikes, produced the rhyolitic compositions as well as the intermediate members of the suite. This suggests the presence of a large unexposed body of cumulate rocks at depth, as well as fusion of a large source-region (equivalent to an 5km layer) in the lithospheric mantle. Regarded as a representative example for similar A-type outcrops in this region, this petrogenetic model further suggests that Neoproterozoic‐Early Cambrian A-type magmatism in the northeastern ANS represents a significant post-orogenic addition of mantlederived material to the juvenile crust. This magmatic episode was of a similar magnitude to that of the Cenozoic, extensionrelated, alkaline volcanism of the Arabian plate.

Constraints on early Archean crustal extraction and tholeiitic-komatiitic volcanism in greenstone belts of the Northern Superior Province

Numerous small remnants of Archean greenstone belts in the Northern Superior Province (ca. 2875–2710 Ma) have chemical characteristics similar to those of the larger greenstone belts of the Southern Superior Province, and preserve direct evidence of crustal conditions prior to the major volcanic events of the late Archean (Wawa–Abitibi subprovinces; ca. 2760–2700 Ma). Three of the best preserved belts are engulfed in tonalite intrusions of the Faribault-Thury Complex (FTC) and exhibit common chemical characteristics, which may imply a similar origin. The dominant tholeiitic basalts typically have MgO contents > 7 wt.%, TiO2 < 1 wt.% and nearly flat rare-earth element (REE) patterns (La/Smn = 0.77–1.22; Gd/Ybn = 0.86–1.20). Associated komatiites have flat to depleted REE patterns (La/Smn = 0.45–0.95), high Al2O3/TiO2 (>15), low CaO/Al2O3 (<1.2), and chondritic Gd/Yb ratios similar to 2.7 Ga Al-undepleted komatiites. The trace-element ratios of komatiitic rocks are indistinguishable from those of the associated tholeiites, suggesting either a derivation from similar mantle sources or a comagmatic relationship (Nb/Thpm = 0.8–1.1; La/Cepm = 0.9–1.3; Nb/Ce = 0.7–0.9; Y/Hopm ~1; and Th/Lapm = 0.7–1.1). Numerical modelling of trace and major elements during low-pressure crystal fractionation reproduces the spectrum of both inferred liquid and cumulate compositions and is consistent with a comagmatic origin between the komatiites and tholeiites. The relatively low Nb/Th ratios of these mid-Archean volcanic rocks relative to both modern day basalts and late Archean basalts may indicate that they were derived from a mantle source that had not lost its crustal components, nor seen significant recycled oceanic crust (high Nb/Th). The extraction of continental crust from this Archean mantle source might then postdate the FTC volcanism, and may be associated with the generation of the voluminous tonalites that engulf the belts.

Elemental mobility in phosphatic shales during concretion growth and implications for provenance analysis

Shale-normalized rare-earth element (REE) patterns record variable redox conditions in coprolitic phosphorite concretions and their host shales from the Pennsylvanian-age cyclothems in the U.S. midwestern states of Oklahoma, Kansas, Nebraska, Iowa, and Ohio as well as the Mississippian-age Fayetteville Shale of Arkansas. Shale-normalized enrichments in middle REE (MREE) developed as the phosphatic concretions lithified under reducing conditions in pore waters of organic-rich muds. Depletions in MREE (Sm–Ho) in host shales correspond to the MREE enrichments in some nearby concretions. Rare and weak negative cerium depletions in both reworked and unreworked phosphorite concretions suggest exposure to oxygenated waters. Positive europium anomalies in thick phosphorite coatings on coprolitic phosphorite concretion cores suggest extreme reduction during lithification of the phosphorite coating. Flat REE patterns in both host shales and phosphorite concretions probably reflect detrital influence. Phosphate diagenesis can affect the trace element chemistry of black shale enough to alter provenance signals. Overall enrichment in REE in phosphorite relative to host shale suggests movement of REE as does the MREE-enriched phosphorite near the MREE-depleted host shale. Diagenetically mobile REE can affect La–Th–Sc relationships used in provenance analysis. The extent of the diagenetic modification of La–Th–Sc distribution is related to the amount of P 2O 5 present. Typical La–Th–Sc distribution in host shale results when P 2O 5 values are below 0.5%. High La abundance substantially skews La–Th–Sc distribution when P 2O 5 is above 5.0%, but when P 2O 5 is between 0.5% and 5.0% the extent of the effect is unclear. Oxides of aluminum and other major elements may be useful in recording detrital influence and proximity to the shoreline in phosphorite concretions. Most of the concretions nearest to shoreline sources of detrital input have both flat REE patterns and elevated values of Al 2O 3/(Al 2O 3+Fe 2O 3).

Petrogenesis of A-type granitoids from the Wallagga area, western Ethiopia: constraints from mineralogy, bulk-rock chemistry, Nd and Sr isotopic compositions

Abstract A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplaced at the contact between low- and high-grade terranes, constitute a significant proportion of the granitoid rocks in the Precambrian of western Ethiopia. These granitoids are characterized by Fe-rich biotite, ferro-hornblende, alkali-amphiboles, and alkali pyroxenes. High total alkalis, high FeO T /MgO (particularly in the peralkaline varieties and syenite), enriched rare earth element (REE), Y, Nb, Ta and low CaO, MgO, and Sr abundance characterize these granitoids. Chondrite-normalized REE patterns show enriched light REE, moderate to strong negative Eu anomalies, and more or less flat heavy REE patterns. Wide ranges of major and trace element compositions among these different A-type granitoids are attributed to variations of source compositions, crystal fractionation, and assimilation. Aluminum-in-hornblende barometric and coexisting amphibole-plagioclase thermometric estimates show that the Ganjii monzogranite crystallized at 600–630 °C and 5–6 kbar. Compared to A-type magmas elsewhere, this temperature estimate is low, suggesting that it might represent conditions of crystallization close to the solidus. Major and trace element modelling of the Ganjii monzogranite suggests derivation by crystal fractionation dominated by plagioclase, hornblende, and biotite from monzodioritic magma. A binary mixing model involving monzodiorite marginal facies and microgranitic dyke end members could not explain the chemical variation in the Ganjii monzogranite, suggesting that the effect of magma mixing on the petrogenetic evolution of the granite was insignificant. The peralkaline to mildly peraluminous Homa gneissic granite and the Tuppii granite, characterized by variable mineralogy, major, and trace element compositions are formed as a result of combined effects of fractionation and assimilation. The initial 87 Sr / 86 Sr of 0.70281 and positive e Nd (625 Ma) values (>+4) for the Ganjii monzogranite are consistent with generation of parental monzodioritic magma from the mantle. The depleted mantle Nd model age ( T DM ) of 0.86 Ga for this granite is coeval with early magmatic stages in the Arabian Nubian shield elsewhere. Despite the occurrence of Mesoproterozoic xenocrystic zircons in the Ganjii monzogranite, the Sr and Nd isotopic compositions indicate insignificant contribution of older crustal materials. The Tullu Kapii quartz syenite yielded e Nd (625 Ma) of +2.4 and +3, consistent with the results obtained from the Ganjii monzogranite, suggesting generation from a juvenile source. However, the older T DM (1.1 Ga) estimated for the Tullu Kapii quartz syenite indicates derivation from a relatively older source or contribution of pre-Pan-African crustal material, whose existence is indirectly substantiated by Mesoproterozic zircon inheritance and T DM ≅1.5 Ga obtained from a sample of calc-alkaline Ujjukka granitoids. Overall, A-type magmas in the area may have derived as a result of orogenic collapse that caused decompression melting of subcontinental lithospheric mantle during late Neoproterozoic. The abundance of comparable mantle-derived granitoids in other parts of the Arabian Nubian shield indicates considerable new crustal addition in Neoproterozoic time.

Accreted fragments of the Late Cretaceous Caribbean–Colombian Plateau in Ecuador

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous (≈123 Ma) oceanic plateau accreted around 85–80 Ma (San Juan–unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit). Picrites have LREE-depleted patterns, high εNd i and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Galápagos HIMU component; their εNd i are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower εNd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan–Multitud Unit by higher Pb ratios and lower εNd i. The Ecuadorian and Gorgona 88–86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92–86 Ma Mg-rich basalts of the Caribbean–Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOP. The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (≈68–65 Ma).

Dating of zircon from Ti-clinohumite–bearing garnet peridotite: Implication for timing of mantle metasomatism

Garnet peridotites from the Kokchetav ultrahigh-pressure (UHP) massif contain abundant volatile and high field strength element (HFSE) bearing minerals, such as Ti-clinohumite and zircon. These characteristics are interpreted to be related to fluid-infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area. The zircons from the peridotites were dated by using sensitive high-resolution ion microprobe (SHRIMP) and yielded apparent U-Pb ages of 554–494 Ma (weighted mean age, 528 Ma) that are mostly consistent with the timing of UHP metamorphism deduced from diamond-bearing country rocks in this massif. These zircons have an almost flat rare earth element (REE) pattern and very low REE concentrations; these characteristics are similar to those observed in kimberlitic zircons. Inherited zircon cores, although only rarely preserved, yielded apparently Proterozoic ages and have different trace element characteristics compared to the overgrowth rims. These features indicate that the mantle metasomatic events and the recrystallization of Ti-clinohumite and zircon were due to HFSE-enriched fluid infiltration during the UHP metamorphism at great depths. The metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts, because these rocks have characteristically high volatile and HFSE concentrations such as those of the Ti-clinohumite–bearing garnet peridotites.

Geochemistry of xenolithic eclogites from West Africa, part 2: origins of the high MgO eclogites

Oxygen isotope, mineral trace element, and measured and reconstructed whole-rock compositions are reported for the high MgO eclogite xenolith suite (16 to 20 wt% MgO in the whole rock) from the Koidu Kimberlite complex, Sierra Leone. In contrast to the previously published data for low MgO eclogites (6 to 13 wt% MgO) from this area, high MgO eclogites equilibrated at higher temperatures (1080 to 1130°C vs. 890 to 930°C) have only mantlelike δ 18O and show variable degrees of light rare earth element (REE) enrichment. Analyses of multiple mineral generations suggest that the heterogeneous REE patterns of the high MgO eclogites reflect variable degrees of metasomatic overprinting. High MgO and Al 2O 3 contents of the eclogites suggest a cumulate origin, either as high-pressure (2 to 3 GPa) garnet–pyroxene cumulates or low-pressure (<1 GPa) plagioclase–pyroxene–olivine cumulates. Trace element modeling suggests a low-P origin for eclogites with flat heavy REE patterns and a high-P origin for eclogites with fractionated heavy REE. Flat heavy REE patterns, the presence of Sr anomalies, and low to moderate transition element contents in the low-P group are consistent with a low-pressure origin as metamorphosed olivine gabbros and troctolites. These metagabbroic high MgO eclogites either could represent the basal section of subducted oceanic crust or foundered mafic lower continental crust. In the former case, the metagabbroic high MgO eclogites may be genetically related to the Koidu low MgO suite. Crystal fractionation trends suggest that the metapyroxenitic high MgO eclogites formed at lower pressures than their current estimated equilibrium pressures (>4 GPa).

The basaltic shergottite Northwest Africa 856: Petrology and chemistry

Abstract— We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa (NWA) 856 with Djel Ibone as a synonymous name. It is a fresh, fine‐grained basaltic rock consisting mainly of two pyroxenes (total ˜68 vol%: 45% pigeonite, En61‐16Wo9–22Fs26–68; 23% augite, En46‐26Wo34‐29Fs21–43) and plagioclase converted to maskelynite (˜23 vol%, Ab43–57Or1–5An54‐36). Accessory minerals include merrillite, Cl‐apatite, pyrrhotite, ilmenite, ulvöspinel, silica (stishovite and glass), amorphous K‐feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (˜2 vol%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and submicrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of ˜50 vol% maskelynite. The meteorite is highly fractured at all scales.The bulk composition of NWA 856 has been measured for 44 elements. It is an Al‐poor ferroan basaltic rock which strongly resembles Shergotty and Zagami in its major and trace element composition.The nearly flat rare earth element (REE) pattern (La/Lu)n = 0.9, is similar to that of Shergotty or Zagami and differs significantly from NWA 480, another Moroccan shergottite recently described. According to the U, Ba and Sr abundances, NWA 856 is not significantly weathered. The oxygen isotopes (δ18O = +5.03%, δ17O = +3.09%, and Δ17O = +0.47%) are in agreement with the martian origin of this meteorite.On the basis of grain size, pyroxene zoning and composition, abundance of silica inclusions associated with maskelynite, trace element abundances, REE pattern and oxygen isotopes, pairing with NWA 480 is excluded. The similarity with Shergotty and Zagami is striking. The only significant differences are a larger grain size, a greater abundance of silica and melt pockets, a slightly more restricted range of pyroxene compositions and the absence of significant mesostasis.

Coupled evolution of back-arc and island arc-like mafic crust in the late-Neoproterozoic Agardagh Tes-Chem ophiolite, Central Asia: evidence from trace element and Sr-Nd-Pb isotope data

We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#≥0.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-normalised (La/Yb)n=14.6–5.1) and negative Nb anomalies (Nb/La=0.37–0.62), but low Zr/Nb ratios (7–14). Initial eNd values are around +5.5, initial Pb isotopic compositions are 206Pb/204Pb=17.39–18.45, 207Pb/204Pb=15.49–15.61, 208Pb/204Pb=37.06–38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La=11–130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8–15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare-earth element patterns [(La/Yb)n=0.6–2.4] and higher eNd values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La=0.81–1.30), but Zr/Nb is high (21–48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island-like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (~6.5), a low eNd value (around 0), and radiogenic 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb; (2) a back-arc basin component similar to N-MORB with a flat rare-earth element pattern and a high eNd value (around +8.5); and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-arc system and an associated back-arc basin.

Rare earth element contents of Jurassic fish and reptile teeth and their potential relation to seawater composition (Anglo-Paris Basin, France and England)

The rare earth element (REE) chemistry of Jurassic shelf seawater from western Europe (Anglo-Paris Basin) was investigated by analyzing the fish and reptile teeth deposited in shallow to deeper water (<200 m) environments. REE patterns in apatites are controlled by the host sediments. Vertebrate teeth sampled from the siliciclastic sediments (calcareous sandstones and shales) show flat shale-normalized REE patterns that reflect the dominant influence of the continental source from which the REE were derived. Carbonate deposits, protected from the clastic sources, contain fish and reptile teeth whose REE patterns reflect more accurately the REE composition of the overlying water column. The REE patterns are similar through the Bajocian to the Oxfordian and are characterized by a depletion in heavy rare earth element (Dy/Yb N =1.8–5.0) compared to modern seawater compositions (Dy/Yb N =0.8–0.9). The fractionations among HREE in the Jurassic seawater are more efficient than in modern oceans and increases with depth instead of decreasing. These HREE removal processes operated as early as the Paleozoic and disappeared during the Late Cretaceous. The fractionation of HREE relative to LREE is correlated with the increasing depth of the basin as indicated by two independent criteria that are the sedimentary structures and the oxygen-isotope composition of coexisting brachiopods. The Dy/Yb N ratio of the marine biogenic phosphates can be used as a proxy of paleo-water depths at the scale of a water mass. Ce anomalies ( Ω Ce) in biogenic apatite-bearing limestones are variable and systematically negative. Relative to modern surface seawater and Bathonian values (mean Ω Ce=−0.63±0.10), weak Ce anomalies during the Callovian (mean Ω Ce=−0.21±0.09) suggest the onset of more reducing conditions. This redox change coincided with an increase of seawater temperatures as suggested by the oxygen-isotope compositions measured on the same teeth. We speculated that the Callovian low Ω Ce could result from a decrease in the p O 2 due to the warming of seawater.

Mass balance during gabbro-amphibolite transition, Bamble Sector, Norway: implications for petrogenesis and tectonic setting of the gabbros

Mafic intrusions of mid-Proterozoic ages are widespread in the Bamble Sector, southern Norway, and elsewhere in the Baltic Shield. They vary from subordinate ultramafic rocks to troctolitic gabbros, olivine–ferrogabbro, olivine-free gabbros and norites. Ni–Cu sulfide ores locally occur in the marginal parts of the intrusions. Post-solidus retrograde reactions under prolonged high P– T conditions led to serial changes from corona development around ferromagnesian minerals, to replacement of olivine and pyroxenes by hornblende and to the formation of amphibolites. Variations of elements during the gabbro-amphibolite transformation are evaluated using a statistical approach that takes into consideration both initial magmatic differentiation effects, and metasomatic changes imprinted during metamorphism. This indicates that rare earth elements (REE), high field strength elements (HFSE), and transition metals were immobile. Large ion lithophile elements (LILE) and halogens were significantly introduced into the amphibolites. Sulfur, Se, Cu, Au, and As remained constant, or locally remobilized and reprecipitated; Sb was enriched in the amphibolites. The Bamble gabbros are tholeiitic, enriched in Fe (Mg#=35–70); on MORB-normalized plots, they show features typical of destructive margins setting (i.e. enriched LILE and LREE, low Th abundance, and Nb trough). They are further characterized by low Ce/Yb ratios (Ce/Yb<10) and nearly flat chondrite-normalized REE patterns (La N/Yb N=1–2). The absence of major Mg-rich cumulates, such as peridotites, argues against fractional crystallization as the main cause of the enrichment in LILE–LREE. Lack of crustal contamination is shown by the REE patterns as well as radiogenic and stable isotope data [Precambrian Research 64 (1993) 403; Chemical Geology 181/1–4 (2001) 23]. A model consistent with the geochemical data favors an early fractionation of olivine in a source area that had been metasomatized by LILE–LREE bearing fluids.

Rare earth element abundances in the CK chondrites including the Kobe meteorite.

The Kobe (CK4) meteorite and other CK chondrites are characterized with respect to rare earth element (REE) abundances using 12 bulk samples from 10 CK chondrites including Ningqiang (CK3), Karoonda (CK4), Kobe (CK4), Y-693 (CK4), A-882113 (CK4), Maralinga (CK4), Y-82102 (CK5), Y-82105 (CK5), Y-82191 (CK6) and A-881551 (CK6). REE, Ba, Sr, Ca, Mg, Fe, Rb and K were analyzed by high-precision isotope dilution. The CK chondrites examined exhibit systematically higher REE abundances compared to ordinary chondrites, comparable to CV and CO, flat REE patterns with minor negative anomalies up to -15% on Ce and -25% on Eu. These chondrites also exhibit an appreciable light/heavy REE gap and Yb anomaly compared to Cl chondrites. CK3-4 chondrites in general are found to exhibit a larger negative Ce anomaly (mean anomaly -9.6 ′3.8%) compared to CK5-6 chondrites (3.3 ′ 1.0%), and some CK5-6 show a slightly light-REE depleted pattern with less-pronounced negative Ce anomaly. It is suggested that these CK chondrites had a larger negative Ce anomaly prior to early thermal metamorphism, which yielded the minor redistribution of light REEs. No clear correlation is found between Sm and Eu/Sm or between Eu and Sr. From these observations, it is suggested that Eu existed in the trivalent state due to higher oxidizing conditions, suppressing redistribution of REEs among minerals compared to metamorphosed ordinary chondrites during the early thermal metamorphic event on the parent body. The systematic REE anomalies observed in CK chondrites are considered to be inherited properties from the refractory precursors produced in the early solar nebula.

Petrogenesis of the Pan-African El-Bula Igneous Suite, central Eastern Desert, Egypt

The Precambrian rocks of the El-Bula area include syn-orogenic intrusive granitoids associated with gneisses, metavolcanics and metagabbro-diorite complexes. The metavolcanics range in composition from basalt to basaltic andesite. They show tholeiite affinity and provide unfractionated, flat rare earth element patterns and have geochemical features suggesting their generation in a back-arc environment. Low and variable Mg# of the basic metavolcanics suggest that they were derived from the upper mantle with subsequent modification by fractional crystallisation. The least evolved basaltic sample could be generated by 25% batch melting of a spinel Iherzolite source followed by 35% fractionation of clinopyroxene, olivine and plagioclase in proportions of 45%, 30% and 25%, respectively. The most evolved basaltic rock could be generated by 55% fractional crystallisation (65% hornblende, 30% plagioclase and 5% titanomagnetite) of the least evolved basaltic sample liquid.The El-Bula syn-orogenic granitoids show a normal igneous evolution from metaluminous tonalites to weakly peraluminous granodiorites. They exhibit geochemical features of I-type calc-alkaline granites emplaced in a volcanic arc environment. The least evolved tonalitic sample could be generated by about 30% batch melting of the most evolved basic metavolcanic sample. The ElBula tonalites and granodiorites have geochemical and petrological characteristics indicating that they might be genetically related by fractional crystallisation. The chemical variation in the tonalites is dominantly controlled by about 45% fractional crystallisation of hornblende, plagioclase and K-feldspar in proportions of 55%, 44% and 1%, respectively. The least evolved granodioritic sample could be produced from the most evolved tonalitic sample by about 20% crystallisation of plagioclase, biotite, hornblende and K-feldspar phases in proportions of 54%, 43%, 2.5% and 0.5%, respectively. The variations in the granodiorites could be modelled by about 30% fractional crystallisation of plagioclase, K-feldspar and biotite (62%, 24% and 14%, respectively).

Petrology of the Mechanic Settlement Pluton and Related Platinum-Group Element Mineralization

The Mechanic Settlement pluton is a small body of mafic and ultramafic rocks of tholeiitic affinity, occurring in the Late Neoproterozoic volcanosedimentary rocks of the Avalon terrane of the Appalachian Orogen. The lenticular-shaped intrusion is layered and consists of ultramafic and mafic lithologies in at least six cyclic units that are relatively thin or short-lived. The lowermost exposed portion of the intrusion is characterized by the cumulate sequences of olivine and olivine-clinopyroxene and display relatively flat rare earth element profiles. In the overlying lithologies, plagioclase is the dominant cumulus phase following olivine in the crystallization sequence. Olivine ranges in composition from Fo 73 to Fo 85 and plagioclase from An 62 to An 89. Variations in the compositions of orthopyroxene and clinopyroxene are within Wo 0–4 En 52–87 Fs 11–45 and Wo 39–50 En 23–50 Fs 3–19 . These mineral compositional variations in part reflect cryptic patterns and correspond to normal fractionation trends in cyclic units. Elevated Pt and Pd concentrations of up to 2.6 ppm occur in ten zones within the central cyclic series in association with both the peridotitic and gabbronoritic rock types. The mineralized zones are about several meters thick and appear to have lateral continuities over several hundred meters, suggesting that they are stratiform. Platinum-group elements (PGE) occur as discrete platinum-group minerals (PGM) including vysotskite, merenskyite, michenerite, sperrylite, stillwaterite, and hollingworthite that are associated with magmatic sulfides composed of pentlandite, pyrrhotite, and chalcopyrite. PGM grains range in size from a few micrometers to well above 20 μm. Sulfur isotope ratios and the Se/S ratios of the mineralized rocks are typically higher than the mantle values. Sulfide saturation probably occurred as a result of assimilation of crustal rocks and magma mixing during emplacement of subsequent pulses of magma. Repetition of the PGE mineralization in rather thin cyclic units suggests that the replenishing magma probably had high PGE levels and was saturated with respect to sulfide during emplacement. Ingression of the replenishing magma into the chamber in a turbulent plume and its mixing with the resident magma was probably responsible for the formation of the PGE mineralizations in the Mechanic Settlement pluton.

Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada

Rare earth elements (REE) were determined in groundwater samples collected from a thick till and clay-rich aquitard sequence located in southern Saskatchewan, Canada. The groundwaters are Na-Mg-SO 4-type waters that range from highly concentrated brines (e.g., I up to 2 moles/kg) near the ground surface to relatively dilute waters ( I ≈ 0.04 moles/kg) at depth. The majority of these groundwaters have pH values between 7 and 8, although the deepest samples are more alkaline (9 ≤ pH ≤ 9.6). Groundwater REE concentrations are relatively constant in the overlying till but increase by up to a factor of 50 in the underlying clay bedrock. Shallow groundwaters have heavy REE (HREE)—enriched shale-normalized patterns, whereas the REE patterns of the deep groundwaters are relatively flat. Solution complexation modelling indicates that variations in REE patterns reflect differences in solution complexation across the REE suite. In the shallow groundwaters, strongly adsorbed, positively charged carbonate complexes (LnCO 3 +), sulfate complexes (LnSO 4 +), and free metal ion species (Ln 3+) dominate the speciation of light REEs (LREE), whereas HREEs occur chiefly as more stable, negatively charged dicarbonato complexes [i.e., Ln(CO 3) 2 −)]. For the deepest groundwaters, however, all of the REEs are predicted to occur in solution as dicarbonato complexes. The large HREE enrichments of the shallow groundwaters reflect the greater affinity of the positively charged LREE solution species to adsorb to clay minerals or coatings on clay minerals in the aquitard sequence compared to the more stable, negatively charged HREE dicarbonato complexes. On the other hand, the flat REE patterns of the deep groundwaters reflect the dominance of the negatively charged dicarbonato complex for all REEs. The solution complexation model along with the strong positive correlation between REEs and [CO 3 2−] F (0.79 ≤ r ≤ 0.95), and to a lesser extent pH (0.57 ≤ r ≤ 0.72), indicates that carbonate ion concentrations, and thus pH, exert important controls on aqueous REE concentrations in these groundwaters.