REE Fractionation Research Articles

Comprehensive geochemical identification of highly evolved marine carbonate rocks as hydrocarbon-source rocks as exemplified by the Ordos Basin

This study reversed the developing environments of hydrocarbon-source rocks in the Ordos Basin and evaluated carbonate rocks as hydrocarbon-source rocks and their distributions on account of the fact that China’s marine carbonate rocks as hydrocarbon-source rocks are characterized by intensive thermal evolution and relatively low abundance of organic matter, by taking the Lower Paleozoic of the Ordos Basin for example and in light of the calculated enrichment coefficients of trace elements, the REE distribution patterns, the mathematical statistics analysis of elements and carbon isotopes and their three-dimensional diagrammatization in combination with the necessary organic parameters. As for the Ordos Basin, TOC=0.2% is an important boundary value. Studies have shown that in the strata where TOC is greater than 0.2%, Ba is highly enriched with positive δ13Ccarb shifts and δ13Corg less than −28‰, reflecting a thigh paleo-productivity, high burial amounts of organic matter, relatively good hydrocarbon-generating potentiality and intensive REE fractionation. All these indicated that the settlement rates are low and the geological conditions are good for the preservation of organic matter, hence favoring the development of hydrocarbon-source rocks. The Klimory and Wulalik formations show certain regularities in those aspects and, therefore, they can be regarded as the potential effective hydrocarbon-source rocks. In the strata where total TOC is less than 0.2%, the contents of Ba are low, δ13Ccarb values are mostly negative, and δ13Corg values range from −24‰–−28‰, demonstrating low burial amounts of organic matter, poor potentialities of hydrocarbon generation, weak REE fractionation and rapid settlement rates. These facts showed that most of the hydrocarbon-source rocks were formed in shallow-water, high-energy oxidizing environments, thus unfavoring the development of hydrocarbon-source rocks. It is feasible to make use of the geochemical method to comprehensively assess the highly evolved marine carbonates rocks as potential hydrocarbon-source rocks and their distributions.

Discovery of the Early Paleozoic post-collisional granites in northern margin of the Erguna massif and its geological significance

The Luoguhe intrusion, located in Mohe County, Heilongjiang Province, is mainly composed of monzogranite, quartz diorite and granodiorite, with minor diorite, tonalite, quartz monzodiorite, quartz monzonite, syenogranite and alkali-feldspar granite. The intrusion can be divided into two lithological units, i.e. quartz diorite and monzogranite units, with affinities to high-K calc-alkaline series. The quartz diorite unit (SiO2: 54.79%–58.30%, Na2O/CaO: 0.79–1.53 and Shand index: 0.77–0.82) belongs to metaluminous rocks. And the monzogranite unit (SiO2: 65.29%–66.45%, Na2O/CaO: 1.73–3.43 and Shand index < 1.05) can be considered as weakly peraluminous rocks. The intrusion is characterized by high REE abundance (ΣREE = 180.2-344.3 µg/g), medium-strong negative Eu anomalies (δEu = 0.33-0.82), weak REE fractionation [(La/Yb)N = 4.12-10.45], enrichments in Rb, Th, U, K, La, Ce, Nd, Hf, Zr and Sm, but strong depletions of Ba, Nb, Ta, Sr, P and Ti. These characteristics of major, REE and trace elements indicate that the intrusion was formed in a transitional tectonic setting from compressional to extensional regime, which can be classified as post-collisional granitoids. SHRIMP U-Pb zircon analyses yield ages of 517±9 and 504±8 Ma for the quartz diorite and monzogranite units, respectively. The discovery of Early Paleozoic post-collisional granites in the northern margin of the Erguna massif indicates that the northern branch of Paleo-Asian Ocean between Siberian plate and Erguna massif was closed in the Early Paleozoic and the Salair orogeny ended ca. 500 Ma ago.

Disrupted peridotites and basalts from the Neoproterozoic Araguaia belt (northern Brazil): Remnants of a poorly evolved oceanic crust?

Fault-bound slices of serpentinized peridotites and pillow basalts interbedded with the metasedimentary rocks of the Tocantins Group in the central western Araguaia belt are probably remnants of poorly developed oceanic crust dismembered into a melange. The hydrothermally altered basalts display a fairly well-defined T-MORB, whereas the serpentinite lenses appear to be derived from moderately depleted upper mantle rocks, such as harzburgites and dunites. Neither sheeted dykes nor gabbros, which characterize a normal ophiolitic sequence, have been identified. The restricted volume of mafic volcanics, their moderate REE fractionation, the LREE enrichment of the serpentinized upper mantle peridotites, and the lack of other ophiolitic lithologies suggest that no appropriate geotectonic conditions were reached for normal oceanic crust development. Instead, it appears that the rifting of a large cratonic block resulted in local mantle exhumation and the formation of a partially mantle-floored basin, with sporadic basaltic magma supply. Thus, only a thin and discontinuous oceanic crust formed, somewhat similar to the Alpine–Apennine poorly evolved oceanic basins. Basin inversion occurred during the Neoproterozoic Brasiliano tectonic event before it reached an advanced ocean stage. The sedimentary sequence underwent intense deformation and greenschist facies metamorphism and was transformed into a complex system of westward to northwestward, overthrusted, imbricated slabs with slivers of basalt and serpentinized peridotites.

Trace element degassing and enrichment in the eruptive plume of the 2000 eruption of Hekla volcano, Iceland

During its last eruption in February 2000, Hekla volcano (Iceland) emitted a sub-Plinian plume that was condensed and scavenged down to the ground by heavy snowstorms, offering the unique opportunity to study the chemistry of the gaseous plume released during highly explosive eruptions. In this paper, we present results on trace element and minor volatile species (sulfates, chlorides, and fluorides) concentrations in snow samples collected shortly after the beginning of the eruption. The goal of this study is to better constrain the degassing and mobility of trace elements in gaseous emissions. Trace element volatility at Hekla is quantified by means of enrichment factors (EF) relative to Be. Well-known volatile trace elements (e.g., transition metals, heavy metals, and metalloids) are considerably enriched in the volcanic plume of Hekla. Their abundances are governed by the primary magmatic degassing of sulfate and/or halide compounds, which are gaseous at magmatic temperature. Their volatility is, however, slightly lower than in basaltic systems, most likely because of the lower magma temperature and higher magma viscosity at Hekla. More surprisingly, refractory elements (e.g., REE, Th, Ba, and Y) are also significantly enriched in the eruptive plume of Hekla where their apparent volatility is two orders of magnitude higher than in mafic systems. In addition, REE patterns normalized to the Hekla 2000 lava composition show a significant enrichment of HREE over LREE, suggesting the presence of REE fluorides in the volcanic plume. Such enrichments in the most refractory elements and REE fractionation are difficult to reconcile with primary degassing processes, since REE fluorides are not gaseous at magma temperature. REE enrichments at Hekla could be attributed to incongruent dissolution of tephra grains at low temperature by F-rich volcanic gases and aerosols within the eruptive plume. This view is supported by both leaching experiments performed on Hekla tephra and thermodynamic considerations on REE mobility in hydrothermal fluids and modeling of glass dissolution in F-rich aqueous solutions. Tephra dissolution may also explain the observed enrichments in other refractory elements (e.g., Th, Y, and Ba) and could contribute to the degassing mass balance of some volatile trace elements, provided they are mobile in F-rich fluids. It thus appears that both primary magmatic degassing and secondary tephra dissolution processes govern the chemistry of eruptive plumes released during explosive eruptions.

Experimental sorption of Ni 2+, Cs + and Ln 3+ onto a montmorillonite up to 150°C

The effect of temperature on the sorption of cations onto a dioctahedral smectite was investigated by running batch experiments at 25, 40, 80 and 150°C. We measured the distribution coefficient (Kd) of Cs +, Ni 2+ and 14 lanthanides (Ln 3+) between solutions and the montmorillonite fraction of the MX80 bentonite at various pH and ionic strengths. Up to 80°C we used a conventional experimental protocol derived from Coppin et al. (2002). At 150°C, the experiments were conducted in a PTFE reactor equipped with an internal filter allowing the sampling of clear aliquots of solution. The results show a weak but measurable influence of the temperature on the elements sorption. Kd’s for Ni 2+ and Ln 3+ increase by a factor 2 to 5 whereas temperature raises from 25 to 150°C. This effect seems higher at high ionic strength. The estimated apparent endothermic sorption enthalpies are 33 ± 10 kJ.mol −1 and 39 ± 15 kJ.mol −1 for Ni 2+ and Eu 3+, respectively. On the other hand, the temperature effect on Cs + sorption is only evidenced at low ionic strength and under neutral conditions where the Kd decreases by a factor 3 between 25 and 150°C. Apparent exothermic sorption enthalpy for Cs + on the montmorillonite is −19 ± 5 kJ.mol −1. Experiments conducted at the four temperatures with the coexistence of all of the cations in the reacting solution (100 ppb of each element in the starting solution) or only one of them, produced similar values of Kd. This suggests the absence of competition between the sorbed cations, and consequently a low degree of saturation of the available sites. A fractionation of the lanthanides spectrum is also observed at high pH and high ionic strength whatever the temperature. The conclusion of this study is that the temperature dependence on sorption reflects, as the fractionation of REE or the pH and ionic strength effects, the chemical process which controls the overall reaction. In the case of an exchange dominated reaction (low pH and low ionic strength), the temperature effect is negligible. In the case of surface complexation (high pH and high ionic strength), the observed increase of Kd with temperature reflects either an increase of the sorption equilibrium constant with temperature or an endothermic property for reactions describing the montmorillonite surface chemistry.

The use of vein fluorite as probe for paleofluid REE and Sr–Nd isotope geochemistry: The Santa Catarina Fluorite District, Southern Brazil

Fluorite can be used as a probe for the source of Sr and REE, as well as for the Sr and Nd isotope systematics of mineralizing solutions, allowing characterization of the composition, oxidation state and sources of the fluids. The 87Sr / 86Sr ratios in vein fluorite from the Santa Catarina Fluorite District, southern Brazil, are low (0.720 to 0.745) relative to those of the majority of host granites at the time of mineralization (90 Ma), but are similar to those of less abundant and less evolved Sr- and Ca-rich granites and plagioclases of the heterogeneous Pedras Grandes granite association. Major contributions of Sr from the unradiogenic Parana Basin rocks ( 87Sr / 86Sr 90 Ma = 0.705 to 0.718) are unlikely, considering the radiogenic character of the lower 87Sr / 86Sr end-member in fluorite mixing lines. Estimated fluorite fluid partition coefficients ( K d Sr–Ca = 0.019 and D Sr ≈ 600) indicate a Sr / Ca ratio in the fluorite-forming solution of 0.012, and Sr contents of 0.05 to 0.25 ppm, which are similar to those of present-day granitic geothermal waters. Initial Nd isotopic compositions of the vein fluorites (0.5120 to 0.512) are similar to those of the Pedras Grandes granites. The 143Nd / 144Nd 90 Ma of the evolved granites of the Tabuleiro granite association, their accessory fluorites and the Parana Basin rocks are considerably more radiogenic (0.5120 to 0.5127) and these are thus considered to be unlikely sources of the fluids. The REE patterns of vein fluorites, normalized to upper continental crust, show a range of LREE-depleted patterns, with highly variable positive and negative Eu anomalies. The host Pedras Grandes granites show flat to slightly depleted UCC normalized LREE patterns with strong negative Eu anomalies. Depletion of the LREE in fluorites resulted from the mobility of HREE fluoride complexes during fluid migration. A REE fractionation model based on ionic potential ratios indicates that Eu 3+ was stable during fluid migration and fluorite precipitation. The coexistence of pyrite and Eu 3+ in the mineralizing fluids is consistent with low pH and oxygen fugacities near the hematite–magnetite buffer.

Influence of slab thermal structure on basalt source regions and melting conditions: REE and HFSE constraints from the Garibaldi volcanic belt, northern Cascadia subduction system

Garibaldi volcanic belt (GVB) basalts were erupted above the relatively young (≤ 24 Ma) Juan de Fuca plate, which comprises the subducted oceanic lithosphere that becomes progressively younger (22–13 Ma), and presumably hotter, northward along the northern Cascadia convergent margin. Primitive and near-primitive mafic lavas of the 15-km-wide volcanic belt change from high-alumina olivine tholeiites and magnesian andesites near Glacier Peak, northwestern Washington, through transitional basalts to alkali-olivine basalts and basanites in the Bridge River-Salal Glacier areas, southwestern British Columbia. The distribution of different basalt types is consistent with varied source conditions imposed by differences in the thermal structure of the underlying subducted plate. Significant arc-parallel variations characterize REE and HFSE contents in GVB basalts and suggest that source enrichment processes and melting conditions vary within the mantle wedge as the age and thermal state of the underlying subducted plate changes. More northerly GVB basaltic suites tend to have higher TiO 2, Nb, Ta, total REE, La, Sm / Yb, Nb / Yb, Ti / V, Y / Sc and Zr / Yb and lower Th / U, Zr / Ti and Zr / Nb than their southern counterparts. The basalts have sub-chondritic to chondritic Nb / Ta (6–21) and super-chondritic Zr / Hf (up to 55.90) ratios that exhibit positive correlation. Only Mount Baker and Glacier Peak basalts exhibit the distinctive negative Nb–Ta anomalies associated with arc lavas. Inter-HFSE and REE fractionations (including La / Yb, La / Nb and Ce / Pb) show significant correlations with the inferred age of the underlying subducted plate. Proportions of slab-derived HFSE-REE components (SC) transferred to basalt sources in the Cascadia mantle wedge appear to vary from negligible (Ti, Nb, Ta, Zr, Hf, Y, Sm, Eu and Tb: less than 15% SC) to perceptible (Nd: up to 35% SC) through moderate (La: up to 75% SC) to substantial (U, Th and Pb: up to 95% SC). Arc-parallel HFSE-REE variations in primitive GVB basalts cannot be explained by variable degrees of depletion produced during prior episodes of melt generation in the mantle wedge. Instead, these differences in basalt chemistry probably reflect different extents of melting of a regionally homogeneous, locally heterogeneous, mantle wedge under conditions influenced by the thermal structure of the underlying subduction zone. Phase relationships and REE systematics of the primitive to near-primitive basalts argue that conditions of magma generation beneath the Bridge River, Salal Glacier, Meager Mountain and Cheakamus Valley lava fields involved lower degrees of melting, higher pressures, and mantle sources richer in garnet than those beneath Mount Baker and Glacier Peak. In addition, Nb / Ta in the Glacier Peak basalts exhibits slight positive correlations with Nb, Ta, La / Yb, and Th / Yb but not Nb / La or Nb / Th, implying that a residual mineral, most likely rutile, controlled extremely low HFSE partitioning into subduction-related fluids that equilibrated with mantle source regions above older, colder portions of the subducted plate.

Constraints in using Cerium-anomaly of bulk sediments as an indicator of paleo bottom water redox environment: A case study from the Central Indian Ocean Basin

The total rare earth elements (∑REE) abundance along with major and a range of trace element chemistry of twenty-eight sub-sections in a 5 m long sediment core (AAS-05/GC-02) from the Central Indian Ocean Basin shows a distinct REE fractionation. Sediments from the top ∼1 m of the core have comparatively low ∑REE abundance (close to average shale), with a sample / shale REE ratio very close to 1 strongly suggesting a dominantly terrigenous source. The down-core ∑REE abundance shows a nearly two fold increase (∑REE = 167–341 ppm) with depth and the REE strongly co-varies with Mn, Ti, P and smectite. Inter-element correlations and the shale-normalized REE patterns suggest that light, middle and heavy REE are selectively carried by terrigenous, Mn oxide and authigenic phosphate phases respectively suggesting a distinct REE fractionation. These fractionation effects become more pronounced with increasing depth in the core. The Cerium (Ce)-anomaly in marine sediments is used as one of the promising tools to trace paleo bottom water redox conditions and this varies in the sediments from 0.10 to − 0.08. The Ce-anomaly in general is positive in the top 4 m but the top ∼1 m have higher Ce-anomaly, Mn / Ti and Ce / Ti suggest sediment deposition relatively under more oxic bottom water condition. The negative Ce-anomaly between 4 and 5 m depth, which could be interpreted to indicate a suboxic/anoxic environment. The redox environment inferred by the Ce-anomaly is compared to a number of other redox sensitive parameters to test its reliability. These include Mn content, total organic carbon, U / Th, authigenic Uranium, Cu / Zn and V / Cr ratio that all suggestive of deposition of sediments under oxygenated bottom water condition. Thus, the negative Ce-anomaly observed between 4 and 5 m core depth does not represent a suboxic/anoxic environment, but is more likely due to the retention of a negative Ce-anomaly caused by authigenic phosphate and authigenic Fe-rich smectite which is formed by the reaction between iron oxyhydroxide and biogenic opal suggests an early diagenetic process and is in equilibrium with seawater REE. Early diagenetic process is responsible for the part of negative Ce-anomaly observed here. Therefore, we suggest that the Ce-anomaly of bulk sediments as an indicator of paleo-ocean bottom water redox conditions needs to be used with a caution.

EVIDENCE FOR PERVASIVE MELT MIGRATION OF EXTREMELY DEPLETED MELTS IN MANTLE PERIDOTITES FROM SUPRA-SUBDUCTION ZONE: AN EXAMPLE FROM THE IZU-BONIN-MARIANA FOREARC

The petrographic, mineralogical and geochemical features of the serpentinites recovered at the top of three seamounts located in the Izu-Bonin-Mariana forearc (South Chamorro, Site 1200, ODP Leg 195; Conical, Site 779A, Leg 125; Torishima, Site 784A, Leg 125) evidence a complex, multistage, evolution of the pristine mantle sequences, involving partial melting, reactive porous flow melt migration, subsolidus metamorphic re-equilibration under decreasing T and open system conditions, and, finally, lowgrade hydrothermal alteration processes. In spite of pervasive serpentinisation, there are areas in which the mantle mineral assemblages are preserved and provide information about the high-T petrologic evolution. The analysed peridotites varied from spinel harzburgite to orthopyroxene-bearing spinel dunite in early modal composition with clinopyroxene modal content mostly in the range of 0 to 2% by volume. Petrologic and geochemical modelling indicates that peridotites experienced up to 25% partial melting. Successively, they were percolated by ascending melts of variable composition. The melt-peridotite interaction is revealed by the following petrochemical features: 1) exsolved and deformed orthopyroxene porphyroclasts are partially replaced by secondary fine-grained assemblage consisting of Ol ± Opx ± Cpx: moreover, secondary clinopyroxene ubiquitously grows in correspondence of fractures across the orthopyroxene porphyroclasts; 2) interstitial clinopyroxene is concentrated locally in clusters or pseudo-veins; 3) the bulk rock major element variation does not match the trends expected for refractory residua after basalt removal; 4) the chemical compositions of the minerals is not correlated with the variation of the modal and chemical composition of bulk rock. Valuable information about the geochemical affinity of the migration melts are furnished by the trace element composition of clinopyroxenes (Cpx). The composition of the Cpx from the Torishima peridotites records the migration of extremely depleted melts with relatively enriched LREE and Sr content. As a whole the trace element fractionation of the putative liquids is consistent with that of boninitic melts. A different story is recorded by the Cpx from the South Chamorro and Conical peridotites. In particular, the geochemical composition of the Cpx from the South Chamorro samples is strongly heterogeneous. Thus, for sake of clarity, three different compositional groups can be recognised: Type1, 2 and 3. Type 1 Cpx are in equilibrium with a calculated liquid characterised by REE composition closely approaching that expected for melt increments produced by 15% fractional melting of spinel-facies depleted mantle. By contrast, Type 3 has Cpx characterised by an exceptional depletion in incompatible trace elements. C1-normalised REE patterns have the maximum at LuN (0.73-1.25). The low REE concentration is accompanied by very low Ti, Zr, Hf, Y, V and Sc concentration (e.g. Ti about 30 ppm). Type 2 Cpx has trace element contents in between those of Type 1 and 3. The REE patterns are characterised by a fractionation in the MREE-HREE region steeper than that expected for liquid produced by fractional melting of spinel facies assemblages (SmN/HoN=0.08-0.14; HoN/YbN = 0.21- 0.46) at the given HREE level (YbN = 2.1-2.7). A stronger MREE-HREE fractionation characterise the REE patterns of the Cpx from the Conical peridotites (SmN/HoN=0.05-0.08; HoN/YbN a 0.55), which show again the YbN in the range of 2.3-2.5. Similar REE fractionations have been documented in Cpx from abyssal peridotites (e.g Johnson, 1998; Hellebrand et al., 2002;), ophiolitic harzburgites (e.g. Kelemen et al., 1995; Dijkstra et al., 2003; Barth et al., 2003)and interstitial Cpx from “replacive” harzurgite-dunite bodies (e.g. Suhr et al., 1998, Suhr, 1999; Zanetti et al., 2005). Geochemical modelling indicates that this compositional character of the percolating melts can be explained by the following three petrologic processes: 1) polybaric evolution of their mantle source, involving an early phase under garnet- facies conditions followed by a second stage under spinel-facies ones; 2) transient composition acquired during the migration of extremely-depleted liquids (e.g. those calculated in equilibrium with Type 3 Cpx through a mantle column with a chemical composition approaching that of residual peridotite after 15% fractional melting. The meltperidotite interaction was mainly controlled by chromatographic- type chemical exchange; 3) transient composition acquired during the migration of less depleted liquids (e.g., those in equilibrium with Type 1 Cpx), through an extremely depleted mantle column. The melt-peridotite interaction involved a significant assimilation of the peridotite orthopyroxene. The model (3) is preferred, because it documents the overall petrochemical features shown by the South Chamorro and Conical peridotites. In conclusion, the strongly refractory characters acquired during the early partial melting episodes affection mantle peridotites from Izu-Bonin-Mariana supra-subduction zone has successively been overprinted by the migration of extremely depleted melts. The boninitic affinity recognised for the melts migrating through the mantle section in the Torishima Seamount area suggests that they were related to island- arc magmatism established in a MOR lithosphere. By contrast, the occurrence of depleted to extremely melts in the South Chamorro and Conical seamounts peridotites is more conceivable with the extinction of the magmatism in a MOR lithosphere.

Geochemical characteristics of the niobium-rich arfvedsonite granites, Younger Granites province of Nigeria

Arfvedsonite granites are most prevalent in the northern sector of the Nigerian anorogenic ring-complex province wherein they form the main granitic rocks at Kudaru and Fagam and are important components of Kila-Warji, Ririwai and Dutsen-Wai ring-complexes. The albitized variety of these rocks hosts pyrochlore to varying extents depending on the degree of albitization and are, therefore, important targets for niobium investigation. Geochemical data of the granites reveal that niobium has a mean concentration of 111ppm in the arfvedsonite granite, increasing to 168ppm in the aegirine arfvedsonite granite and reaching 1568ppm in the albite arfvedsonite granite. Niobium is thus enriched in the albite arfvedsonite granite by a factor of 8–11 relative to its mean value in the aegirine arfvedsonite and arfvedsonite granites, respectively. Uranium contents show a sympathetic trend with niobium, being also enriched in the albite arfvedsonite granite relative to its abundance in both the aegirine arfvedsonite granite and arfvedsonite granite by a factor of 15. The uranium abundance in the albite arfvedsonite granite is more than 48 times higher than the mean background values in low-calcium granite.The REE fractionation patterns in all three arfvedsonite granite varieties are characterized by enrichment of both the light (La–Sm) and heavy (Gd–Lu) rare earth elements and a significant negative Eu anomaly. The albite arfvedsonite granite is, however, preferentially more enriched in the heavy REE relative to the aegirine arfvedsonite and the arfvedsonite granites. A plot of the ∑REE against Na2O and niobium reveals positive correlation in the arfvedsonite granites. There is also a linear relationship and strongly positive correlation between Nb and Na2O because the pyrochlore is most abundant in the most extensively albitized variety of the arfvedsonite granites.

TTG magmatism in the Congo craton; a view from major and trace element geochemistry, Rb–Sr and Sm–Nd systematics: case of the Sangmelima region, Ntem complex, southern Cameroon

TTG rocks of the Sangmelima region (Ntem complex, Congo craton) in southern Cameroon range in modal composition from rocks of the charnockitic suite, tonalites to granodiorites. They display a wide spectrum of SiO 2 composition (54–76 wt%), including metaluminous intermediate rocks (0.56 < A/CNK < 1) and rocks that are slightly peraluminous (1 < A/CNK < 1.08). Charnockites and tonalites define a trondhjemitic suite while granodiorites portray a slight potassic calc-alkaline affinity. The primordial mantle-normalized spidergrams are characterized by negative Nb–Ta anomalies, suggesting a subduction related signature or melting of crustal rocks that were formed along subduction zones. More siliceous members of the rock suites are enriched in LILE. REE fractionation is shown by 2 < La N/Yb N < 28 in the charnockitic suite while higher ratios of 12 < La N/Yb N < 114 in the tonalitic suite and 18 < La N/Yb N < 99 in granodiorites indicate melting of source rocks with garnet as a restite phase. Samples with overall lowest abundance in REE show a positive Eu anomaly and represent small amounts of magmas derived from a source with much feldspar contribution to the melt. Sr and Nd systematics coupled with Mg#–SiO 2 compositions are consistent with derivation by partial melting in a subduction setting or merely from a thickened Archaean crust of eclogite facies basaltic proto-crust with mantle material input. From Rb–Sr and Sm–Nd whole-rock data, intrusion timing is defined around 2900 Ma for rocks of the charnockitic suite. The Rb–Sr system in biotite portrays post-emplacement reheating during the Eburnean orogeny. TTG intrusions in the Sangmelima region represent a major Archaean accretion event in the Congo craton.

Geology and geochemistry of the Shangmanggang red clay-type gold deposit in West Yunnan

The Shangmanggang red clay-type gold deposit was discovered in West Yunnan in the early 1990s, which is strictly controlled by the Shangmanggang fault, belonging to a new type of gold deposits. Its red clay profile, about 20 m thick, is divided into six zones: topsoil zone, slope-wash zone, travertine zone, eluvium zone, saprolite zone, and unweathered bedrocks. The degree of laterization is relatively high in the eluvium zone where gold is very rich. Ore minerals mainly consist of quartz, goethite, and clay minerals. Sand and silt fractions are mainly composed of quartz and goethite with minor feldspar; clay fractions comprise illite, kaolinite, quartz, a small quantity of goethite, feldspar, and hydrargillite. The variation of gold content in the red clays is similar to that of sand fractions. Gold is probably closely associated with iron nodules developed in the red clays. At the same time, gold is well correlated with clay minerals and Fe, Mn oxides. Gold exists mostly in the form of dissociated gold and microfine gold, which is adsorbed on the surface or in the cracks of quartz, clay minerals, and goethite. Geochemistry characteristics of the major elements, trace elements, and REEs in the red clays of the Shangmanggang red clay-type gold deposit in West Yunnan were studied in detail. The results show that Al 2O 3, Fe 2O 3, and K 2O were migrated and precipitated together in the red clay profile, but the case is not true for SiO 2. As a whole, SiO 2 is high in content, desilication is incomplete, and the degree of laterization is low in the red clays, except for a high degree of laterization in the eluvium zone. In contrast with the sedimentary rocks and altered rocks, the red clays come from the Mengga Formation mudstone with laterization. Au, Hg, As, Sb, Cu, Pb, Zn, and Mo are coextensive trace elements in the red clay profile and are significantly enriched in the eluvium zone where laterization is relatively complete. The crust-normalized large-ion-lithophile element patterns in the red clays are similar to those of both altered rocks and the Mengga Formation. The chondrite-normalized REEs patterns in the red clays are also similar to those of both altered rocks and the Mengga Formation. The LREEs are enriched, and gold is concentrated in the location where REE fractionation is strong. Our work reveals that gold comes mainly from the altered rocks, and the red clays are mainly derived from the Mengga Formation and altered rocks.

Geology and geochemistry of paleoproterozoic magnesite deposits (∼1.8Ga), State of Ceará, Northeastern Brazil

Five important magnesite mines and small occurrences in the state of Ceará form a sequence of lenses that extend, discontinuously, for over 140km. The magnesite rocks are hosted by metadolomites with lutecite, sulphate nodules pseudomorphosed by fiber-radiated quartz, scapolite and dissolution breccias. This metacarbonate sequence is more voluminous and more calcic in the southwestern extremity of the belt, but less voluminous and more magnesian towards the northeastern portion of the belt, the highest Mg contents being observed in the Alencar-Orós region. Magnesite deposits pass gradually into metadolomites and then to almost pure calcitic marbles westward the belt. This group is hosted in a greenschists to amphibolite-facies metavolcanic-sedimentary sequence crosscut by basic sills and granitic intrusions of variable size, form, composition and age (middle to late Proterozoic ∼550Ma). Two types of magnesite marble may be distinguished: (1) medium-grained (1 to 9mm) and (2) sparry-magnesite marble (1 to 15cm). The latter displays wide textural variation (porphyric, rosette, layered and palisadic types) and exhibits remnants of the original sedimentary features. The sparry crystals, in spite of their deformation, are hypidiomorphic and pinolitic. The colors vary from white to light or dark gray, or even red. The dark banded term presents traces of microfossils and stromatolites structures. The medium-grained magnesite marble, on the other hand, shows more homogeneous texture and color and anhedral crystals are more abundant. The sparry-magnesite marble is less enriched in SiO2, Fe2O3, Al2O3 and CaO and richer in MgO than the medium-grained ones. The magnesitic marbles display positive Ce, and negative Eu anomalies. This suggests a shallow marine platform environment with continental, reduced influence (lagunar). These magnesitic marbles also show positive Ce and Eu anomalies in relation to the present seawater, suggesting compositional differences, materialized mainly by an enrichement of these elements in the past seawater. Fluid inclusions in the medium-grained magnesites of the Riacho Fundo deposit are aqueous inclusions, while in the sparry magnesite deposits, particularly in Cabeça de Negro, aquo-carbonic inclusions dominate. Hydrocarbons, however, are found in both cases. The calcitic marble, deposited on open-marine environment, displays positive13C (+0.5 to 2.1 ‰ PDB), while dolomitic to dolostone marbles, deposited on less open marine environment, display negative shift (0.7 to −3.2‰ PDB). This13C fluctuation is within the range for carbonates deposited around 1.8Ga. The depositional environment of these carbonates is close to a paralic system, lagoon, with strong evaporitic conditions. The depressions were of variable depth and lengths and could be isolated, in particular by stromatolitic barriers, dried and flooded by seawater with continental inflows. The largest and deepest ones correspond to the sparry magnesite deposits. These magnesites are located in a lateral way to almost pure calcitic marbles, through dolomitic marbles. This points to a process of chemical differentiation within a paralic system, from open sea to landwards, more calcic in the first to more magnesian in the confined environment, in which lagoon waters are progressively purified from Ca through preferential precipitation of calcic carbonates and secondly of gypsum/anhydrite, which increases the Mg/Ca ratio, and lead carbonate deposits towards the magnesite pole. These metacarbonates also reveal a fractionation of the light REE, starting from the calcitic, through the dolomitic to magnesitic marbles. In the calcitic marbles, REE are in association with the clay fraction, while in magnesitic marbles, they seem to be in association with organic and/or soluble complexes. The magnesitic marbles are of sedimentary origin and underwent important diagenetic evolution before being metamorphosed during the Brasiliano orogenic cycle.

The origin of clay minerals in active and relict hydrothermal deposits

Samples of Fe-oxide-rich hydrothermal sediments were collected from active and inactive portions of the TransAtlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge. Clays separated from TAG metalliferous sediments in this study all consist of Al-poor nontronite. Oxygen isotope thermometry of the clays yields formation temperatures of 54–67°C for samples from the inactive Alvin mound compared with 81–96°C for samples from the active TAG site. The latter are the highest recorded temperatures for authigenic hydrothermal clays. Sr isotope analysis of the clays from the active mound suggests that they precipitated from seawater-dominated fluids, containing less than 15% hydrothermal end-member fluid. In contrast, nontronite from the inactive Alvin mound has 87Sr/ 86Sr values that closely resemble that of detrital North Atlantic clays, suggesting a dominantly continental source for the Sr. Rare earth element data are consistent with a significant detrital input to the inactive site but also demonstrate the extent of hydrothermal input to the low temperature fluid. Crystallographic fractionation of the trivalent REE is apparent in the heavy REE enrichments for all nontronite samples. The inferred formation-mechanism for nontronite-rich Fe-oxyhydroxide deposits at the surface of the active mound is by direct precipitation from low temperature fluids. At the inactive Alvin site, in contrast, the deposits form during alteration of pelagic sediments by diffuse fluids and replacement of biogenic carbonate with nontronite and Fe-oxyhydroxide. These two modes of formation are both important in seafloor hydrothermal settings where clay minerals are a significant component of the hydrothermal deposit.

Geochemistry of early Paleozoic alkali dyke swarms in south Qinling and its geological significance

The early Paleozoic alkali dyke swarms in the Ziyang-Langao areas of south Qinling are characterized by bimodal volcanic association. They show the patterns of enriched LREE with high fractionation of light and heavy REE and enrichment of incompatible elements in the primitive man-tle-normalized spider-diagrams with peaks of Ba and Nb and valleys of Pb and Sr. The ratios of Ta/Hf are larger than 0.3 and those of Th/Ta less than 1.6, indicating that they were derived from enriched mantle sources closely related to plume. The occurrence of alkali dyke swarm demon-strates an extensional event of crust resulting from deep plume activity in the Qinling orogeny dur-ing early Paleozoic. The rifting extension could not continue due to the short activity of plume. Consequently, no new oceanic basin was formed.

15N-enriched Gondwana lamproites, eastern India: crustal N in the mantle source

A comagmatic suite of lamproites, ultramafic lamprophyres, and minettes was intruded at ∼100 Ma into thick siliciclastic sequences deposited in five intracratonic basins which developed during Gondwana rifting in eastern India. Lamproites are dominated by microphenocrysts of Ti-rich phlogopite, and olivine pseudomorphed by serpentine and talc, with variable modal abundances of aegirine, amphibole, apatite, carbonates, Cr-spinel, fluorite, perovskite, priderite, and titanomagnetite. Compositionally, lamproites are characterized by extremely high contents of TiO2 (4.2–10 wt%), K2O, and P2O5, over a range of Mg# from 80 to 75, in keeping with lamproites worldwide. They have high abundances of Ba, Th, U, and REE (1019–5400 ppm), with pronounced fractionation of REE, where La/Smcn=3.5–5.6, and Gd/Ybcn=7.3–18, consistent with residual garnet in the mantle source. Nitrogen isotope compositions range from +1.6 to +8.7‰ with an average of 3.8‰, and N contents average 107 ppm. Mantle δ15N averages −5‰, with a N content of 2 ppm, whereas continental crust varies from 2 to 6‰ with 30–90 ppm N. Atmospheric N2 is sequestered by N-fixing microorganisms, and stored as kerogen with an average δ15N of +3‰ in sedimentary rocks. Some of the organic N is converted to NH4+ which substitutes for K in crustal K-silicates. The 15N-enriched lamproites are interpreted to have formed from partial melting of enriched harzburgitic mantle lithosphere by decompressional melting accompanying extension. Enrichment resulted from subduction–erosion of continental crust, or subduction of sediments, low-degree partial melting, hybridization of the melts with mantle lithosphere and incubation, prior to decompressional melting. Consequently, some crustal N is recycled to the mantle.

Petrogenetic and metallogenetic responses to Miocene slab flattening: new constraints from the El Indio-Pascua Au–Ag–Cu belt, Chile/Argentina

The Eocene–Miocene volcanic and hypabyssal rocks of the El Indio-Pascua Au–Ag–-Cu belt in the southern central Andean flat-slab region are medium–high-K calc-alkaline arc suites, ranging in composition from andesite to rhyolite. A significant transition in magmatic trace element chemistry, coinciding with a pronounced reduction in magma output, occurred in the late-Middle Miocene as documented by 40Ar–39Ar geochronology. The upper Eocene–lower-Middle Miocene rocks exhibit low Sr/Y ratios (<50), minor heavy REE fractionation with Sm/Yb ratios not exceeding 3.5 and, in some cases, minor negative Eu anomalies. In contrast, the largely dacitic rocks erupted after ca. 13 Ma are depleted in Y (≤10 ppm), have generally high, but variable Sr/Y ratios (30–200), exhibit moderate middle and heavy REE fractionation (Sm/Yb: 3.7–5.9) and lack negative Eu anomalies. The latter features are characteristic of adakitic suites (i.e. slab-melts), but the regional temporal and spatial distribution of arc magmatism precludes a major magma source in the downgoing slab. This evolution is interpreted as reflecting a progressive increase in pressure and the availability of water in the lower-crustal site of magma generation, establishing both garnet and hornblende as major stable phases in the residuum. The pressure in the lower crust increased in response to episodic crustal thickening related to the shallowing of the slab, a process recorded by the incision of three regional pediplains over the period 17–6 Ma. Elimination of the subarc asthenospheric mantle and much of the lithospheric mantle by ca. 10 Ma permitted direct incursion of slab-derived, highly oxidised metal- and volatile-rich supercritical fluids into the lower crust, stimulating melting of mafic, garnet amphibolitic and eclogitic assemblages. The igneous suites emplaced from 36–11 Ma were associated with widespread, and locally intense, epizonal hydrothermal activity, but this was barren of base and precious metals. The shallow-crustal availability of abundant water highlighted in earlier models was therefore not a metallogenetic determinant. Moreover, economic Au–Ag–Cu mineralization, associated with small volumes of dacitic magma, was restricted to the interval 9.5–5 Ma, and was not initiated until at least 3.5 my after the inception of high-pressure magma generation. In contrast to previous metallogenetic studies, we therefore suggest that this petrochemical transition was not inherently favourable for ore formation. We propose that the incursion of highly oxidized supercritical fluids from the slab into the lower crust was ultimately responsible for the brief Late Miocene metallogenetic episode.

Zircon U/Pb dating of Cretaceous adakitic volcanic rocks in the eastern part of North Dabie Mountains

Mesozoic volcanic rocks in the eastern part of the North Dabie Mountains are rich in Na (Na2O = 4.03%, Na2O/K2O = 1.31), Sr (865μg/g) and Ba (1361μg/g), and high in Sr/Y ratio (66.1) but low in Nb, Y and HREE. They have experienced strong fractionation of REE [(La/Yb)N = 26.6], and are similar to adakite in geochemical characteristics. The U-Pb dating of zircon from the volcanic rocks is (129.2 ± 2.6) Ma, belonging to Early Cretaceous. These rocks are similar to the volcanic rocks of North Huaiyang not only in age and rare-earth element and trace element geochemistry, but also in the formation temperature and pressure of the minerals. The results indicated that the delamination of mountain root and underplating of maficultramafic magma had happened in the Dabie orogen before Early Cretaceous. Mesozoic magmatism was intense in the North Dabie Mountains, including the intrusion of mafic-ultramafic magma, uplifting of gneiss dome, explosion of volcanic rocks and intrusion of granitic magma. The Mesozoic volcanic rocks in the eastern part of the North Dabie Mountains may be one part of the Mesozoic volcano-intrusive complex belt of North Huaiyang. The existence of Mesozoic volcanic remnant cap means the denudation of the Dabie orogenic belt was not very strong since Early Cretaceous.

Distribution patterns of rare-earth elements and uranium in tertiary sedimentary phosphorites of Hahotoé–Kpogamé, Togo

Seventeen samples have been used to study the distribution behaviour of rare-earth elements and uranium in four profiles of the exploited phosphatic layer C1 of Hahotoé–Kpogamé deposits in Togo. Compared with shale values (NASC: North American Shale Composite) these phosphorites are enriched with REE by factors of 4–7 and with uranium by a factor of 43. REE display seawater-like distribution patterns with a pronounced negative cerium anomaly and an enrichment of light REE relative to shales. The phosphorite deposits of Togo are altered in a tropical climate that led to the formation of crandallite, kaolinite and goethite at the expense of francolite and clay minerals. Horizons rich in crandallite, kaolinite and goethite accumulate high amounts of REE and uranium released during the weathering of phosphorites. The alteration does not considerably affect the Ce-anomalies which remain negative for all samples but does affect the REE fractionation whereby crandallite, kaolinite and goethite rich samples show an enrichment of heavy REE compared with the light ones.

Non-Henry’s Law behavior of REE partitioning between fluorapatite and CaF 2-rich melts: Controls of intrinsic vacancies and implications for natural apatites

The partitioning of rare-earth elements (REEs: Gd and multiple REEs), Sr, and Mn between fluorapatite and CaF 2-rich melts was investigated over a wide range of REE concentrations (i.e., from 0.8 ± 0.1 to 25,000 ± 2600 ppm Gd in fluorapatite) in two different sample assemblies (i.e., tightly covered Pt crucibles and sealed Pt capsules) at 1220 °C and atmospheric pressure. Attainment of equilibrium is indicated by selected reversal experiments. The partition coefficient D(Gd) decreases from ∼2 to ∼0.5 with increasing Gd in fluorapatite, hence a marked non-Henry’s Law behavior, but becomes independent of composition at and above ∼5000 and ∼1000 ppm Gd for experiments in Pt crucibles and Pt capsules, respectively. Non-Henry’s Law behavior is also observed in experiments involving multiple REEs. All REE patterns are convex upward in shape with maxima between Nd and Gd, and D(La)/D(Nd) and D(Nd)/D(Yb) decrease systematically with increasing total REEs in fluorapatite, suggesting that REE fractionations are partly related to non-Henry’s Law behavior. These experimental results and local structural data from previous electron paramagnetic resonance spectroscopic studies suggest that the non-Henry’s Law behavior of REE partitioning between fluorapatite and melt is controlled by intrinsic Ca 2+ vacancies in the c-axis channels. The D(Sr) and D(Mn) values are independent of composition and, therefore, do not deviate from the Henry’s Law in their respective compositional ranges investigated in this study. Nonstoichiometry, such as Ca 2+ and F − vacancies in the c-axis channels, is well known in natural apatites, particularly in biogenic apatites. Therefore, the observed non-Henry’s Law behavior of REE partitioning is expected to have important implications for REE geochemical modeling involving apatites and for the uptake of REEs by natural apatites. Particularly, the non-Henry’s Law behavior of REE partitioning is at least partly responsible for the commonly observed, bell-shaped REE patterns in fossil biogenic apatites.