REE Depletion Research Articles

Fine-scale LA-ICP-MS study of redox oscillations and REEY cycling during the latest Devonian Hangenberg Crisis (Moravian Karst, Czech Republic)

Trace elementsand REEY distribution of thinly laminated limestone (laminite) and underlying pre-event calciturbidite (Siphonodella praesulcata conodont Zone) deposited in association with the Hangenberg Black Shale Event were analyzed using laser ablation-inductively coupled plasma-mass spectrometry. These deposits were also analyzed by carbonate petrography and scanning electron microscopy. Our study documents the origin of authigenic microbial limestones during the latest Devonian Hangenberg extinction event, when unusual oceanic conditions appear to have enhanced precipitation of calcite within microbial carbonates. Detrital contamination of the calciturbidite and laminated limestone is very low, as demonstrated by low contents of Al, Zr, Ti and Th. These deposits display only limited degree of early diagenetic overprint of limestones, and some of them fall within or very close to the seawater field. Elevated U and Mo enrichments and covariation patterns reflect the episodic establishment of oxic to euxinic bottom condition during deposition of the laminite. The oxic conditions that existed during accumulation of the underlying calciturbidite were likely the result of turbidity current introduction of oxygen into the bottom environment. REE contents of both the laminite and calciturbidite are nearly ten times higher than those of most modern carbonates and similar to Devonian microbialites from the Canning Basin. Calciturbidite display a prominent MREEN bulge. Shale-normalized REE patterns of the overlying laminite are similar to those of modern seawater and Devonian microbialites from the Canning Basin (LREE depletion and progressive enrichments in heavier REE) reflecting the dominance of authigenic carbonate phases. Minor contamination is bound to apatites, siliciclastic minerals, pyrites and organic matter. The element content of authigenic phases benefited from Fe and Mn oxyhydroxide redox cycling. Negative Ce and positive Eu anomalies, low Y/Ho ratios, and very low Al/(Al+Fe+Mn) ratios suggest an influence of diluted high- temperature hydrothermal fluids perhaps related to latest Devonian rift magmatism in the eastern part of the Rhenohercynian zone. The proliferation of anachronistic facies following the Devonian-Carboniferous (D-C) extinction event recognized in Moravia and traced worldwide seems to share some features in common with anachronistic carbonate sequences documented in the aftermath of the end-Permian mass extinction.

Geochemistry, U-Pb zircon geochronology and Sm-Nd isotopes of the Xincai banded iron formation in the southern margin of the North China Craton: Implications on Neoarchean seawater compositions and solute sources

Banded iron formations (BIFs) that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition. Here, we discuss the major and trace elements and Sm-Nd isotopic geochemistry of the poorly-studied Xincai BIF, located west of the Superior-type Huoqiu BIF along a NW-SE trending supracrustal belt in the southern margin of the North China Craton, to understand the compositions and solute sources of seawater during the Xincai BIF precipitation. U-Pb zircon dating of granitic intrusive rock and biotite schist interlayer of the BIF constrains the depositional age of the Xincai BIF to 2.5–2.7 Ga for the first time, indicating that the Xincai BIF could be part of the Huoqiu BIF or consist of contemporaneous sediments formed in a similar tectonic-depositional environment. Shale-normalized rare earth element and yttrium (REY) patterns of the bulk and individual band samples display features that are characteristic of other Archean BIFs with light REE depletion relative to heavy REE, positive LaSN, GdSN and YSN anomalies, indicative of pure chemical sedimentation. The marine origin of these samples is further supported by their super-chondritic Y/Ho ratios (average Y/Ho = 47). The very low Al, Ti and high field strength element contents of the Xincai BIF indicate an essentially detritus-free component. The bulk BIF samples show a narrow range of εNd(t) values from −1.5 to +1.0, and most of the samples have negative εNd(t) values, −0.3 on average, which is different from Archean BIFs with mantle-derived depleted Nd isotopes. Deviations toward negative εNd(t) values of the iron bands (−0.5 to −0.1) and the silica bands (−4.7 to −0.5) further indicate a continental Nd contribution. In addition, positive EuSN anomalies that were found in all of the bulk and individual band samples indicate that a high-T hydrothermal input also contributed to their REY signatures. It is considered that both high-T hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their REY signatures.

Geochemistry of rare earth elements in a neutral mine drainage environment, Anjir Tangeh, northern Iran

This study investigates the geochemical characteristics of rare earth elements and yttrium (REE+Y) in the neutral mine drainage (NMD) and river water in the vicinity of the Anjir Tangeh coal washing plant, Mazandaran province, northern Iran. The NMD water had a neutral pH along with high sulphate and bicarbonate concentrations. The total concentrations of REE in the NMD water were lower than in the river water. According to the North American Shale Composite (NASC) normalised concentrations, the patterns of NMD showed heavy REE (HREE) enrichment while the river water represented middle REE (MREE) enhancement. Both negative europium (Eu) and positive gadolinium (Gd) anomalies were observed in the drainage and river water within the study area. The sum of REE concentration increased gradually downstream of the discharging point of the plant reject water, in contrast to decreasing of Al, Fe, Mn and Cu+Pb+Zn concentrations. The river sediment with high Fe concentrations was enriched with lanthanum (La) and cerium (Ce). The overall REE pattern of the NMD approximates those of coal except for the depletion in light REE (LREE) and the enrichment of Eu. Dominant inorganic REE species in the river water included Ln (CO3)2− and LnCO3+, while LnCO3+, Ln (CO3)2− and Ln (SO4) + were the main complexes in the NMD. The saturation index indicated that Al and Fe were both oversaturated in the drainage and river water, which can control the fractionation of REE by the sorption process. According to both hierarchical cluster analysis (HCA) and principal component analysis (PCA), pH, Al, and P were the significant parameters in the REE distribution.

Geochemistry and origin of alkaline granites at Wadi Umm Adawi-Yahmid area, south Sinai- Egypt

The alkaline granites of the study area are characterized by the predominance of alkali feldspars, quartz minor amounts of riebeckite, plagioclase and biotite. Accessories are represented by zircon, apatite, titanite, allanite, chevkinite with secondary chlorite. They have peralkaline to slightly metalumminous character and were emplaced during post orogenic stages. The chondrite-normalized REE patterns display enrichment of light REE against depletion of heavy REE with significant Eu anomaly indicating extreme feldspar fractionation. They belong to A-type granites and have mantle derived source characterized by high K2O, extremely Low MgO and CaO contents, the pronounced Eu anomaly and depleted in Ba, Sr and Ta against enrichment of (HFSE) Zr, Y, Rb,Nb and ƩREE. They have formed under low oxygen fugacity (fO2) conditions with high magmatic temperature up to 896 °C with fairly low water pressures. Intrusions of alkali feldspar granite followed by riebeckite granite (alkaline granite) are consistent with fractional crystallization model. This was supported by the presence of syenitic microgranular enclaves with felsic alkaline granite. The most plausible source of these alkaline granites is mantle derived potassium rich source, that may accompanied by further magma mixing or assimilations.

Platinum-group elemental and Sr-Nd-Os isotopic geochemistry of the ∼635 Ma mafic intrusions in the northern margin of the Yangtze Block: A link of metasomatized subcontinental lithospheric mantle and Ni-Cu-(PGE) sulfide mineralization

A number of ∼635Ma mafic-ultramafic intrusions occur in the Nanyang region and Suizhou-Zaoyang (Suizao) region along the northern margin of the Yangtze Block. The intrusions in the Suizao region are mainly composed of troctolite, olivine gabbro and gabbronorite. The parental magmas of the intrusions are estimated based on the non-modal melting using two fresh samples with the lowest REE concentrations and the results show that the parental magmas may have La/Sm of ∼2.5, Sm/Yb of ∼2.5 and La/Yb of ∼6.2 and could be produced by 5–10% of partial melting of the mantle composed of spinel-garnet (50:50) lherzolite. The estimated compositions of the parental magmas also display the enrichment of light REE and weak depletion of HFSE (Th, U, Nb and Ta) on the primitive mantle-normalized trace element patterns and have Ba/Nb, Ba/La and Ba/Th distinctly higher than MORB and OIB, consistent with the derivation from metasomatized subcontinental lithospheric mantle (SCLM) rather than convecting mantle. The rocks of the intrusions have radiogenic 187Os/188Os(i) of 0.137–0.247, which are likely attributed to low degrees of crustal contamination and no involvement of convecting mantle-derived magmas. They consistently have εNd(t) values of −5.4 to −10.4, much lower than the value of +2.5 for the mantle inferred from Neoproterozoic evolutionary trend and could be interpreted as the mixing of the parental magmas with 7–10% of Archean basement of the Yangtze Block. The rocks of the intrusions commonly have <0.1wt% S and 0.08–17.5ppb PGE (Os, Ir, Ru, Rh, Pt and Pd). They have Cu/Pd (7900–6,500,000) much higher than the value of the primitive mantle (∼7000), which is likely resulted from the retention of sulfide in the mantle source due to the low degrees of partial melting of the mantle. We conclude that the low PGE concentrations of the rocks can be attributed to the sulfide retention in metasomatized SCLM source and no substantial interaction of metasomatized SCLM with the melts from convecting mantle. The ∼635Ma mafic intrusions in the Suizao region may thus have little potential for economically important Ni-Cu-(PGE) sulfide mineralization.

Morphology and impurity elements of zircon in the oceanic lithosphere at the Mid-Atlantic ridge axial zone (6°–13° N): Evidence of specifics of magmatic crystallization and postmagmatic transformations

The paper presents newly obtained original data on the morphology, internal structure (as seen in cathodoluminescence images, CL), and composition of more than 400 zircon grains separated from gabbroids and plagiogranites (OPG) sampled at the axial zone of the Mid-Atlantic Ridge (MAR). The zircons were analyzed for REE by LA-ICP-MS and for Hf, U, Th, Y, and P by EPMA. Magmatic zircon in the gabbroids crystallized from differentiating magmatic melt in a number of episodes, as follows from systematic rimward increase in the Hf concentration, and also often from the simultaneous increase in the (U + Th) and (Y + P) concentrations. These tendencies are also discernible (although much less clearly) in zircons from the OPG. Zircon in the OPG is depleted in REE compared to the least modified zircons in the gabbro, which suggests that the OPG were derived via partial melting of gabbro in the presence of seawater-derived concentrated aqueous salt fluid. Another reason for the REE depletion might be simultaneous crystallization of zircon and apatite. The CL-dark sectors, which are found in practically all of the magmatic zircon grains, have Y/P (a.p.f.u.) ≫ 1 which most likely resulted from OH accommodation in the zircon structure, a fact suggesting that the OPG parental melt contained water. High-temperature hydrothermal processes induced partial to complete recrystallization of zircon (via dissolution-reprecepitation), a process that was associated with ductile and brittle deformations of the zircon-hosting rocks. The morphology of the hydrothermal zircons varies depending on pH and silica activity in the fluid from weakly corroded subhedral crystals with typical vermicular microtopography of the crystal faces to completely modified grains of colloform structure. Geochemically, the earlier hydrothermal transformations of the zircons resulted in their enrichment in La and other LREE, except only Ce, whose concentration, conversely, decreases compared to that of the unmodified magmatic zircons. The hydrothermal zircon displays a reduced Ce anomaly and its most altered domains typically host minute inclusions of xenotime, U and Th oxides and silicates, and occasionally also baddeleyite, which suggests that the hydrothermal fluid was reduced and highly alkaline. These features were acquired by the seawater-derived fluid when it circulated within the axial MAR zone area due to phase separation in the H2O–NaCl system and particularly as a result of fluid interaction with the abyssal peridotites of oceanic core complexes. Our data demonstrate that zircon is a sensitive indicator of tectonic and physicochemical processes in the oceanic crust.

Petrogenesis of the mineralized granitoids from the Kounrad and Borly porphyry Cu deposits and the East Kounrad porphyry Mo deposit in Kazakhstan: Implication for tectonic evolution and mineralization of the western part of the Central Asian Orogenic Belt

The Kounrad region, located in the western part of the Central Asian Orogenic Belt, hosts the Kounrad porphyry Cu, the Borly porphyry Cu–Mo, and the East Kounrad porphyry Mo deposits. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that the mineralized granitoids from the Kounrad, the Borly and the East Kounrad deposits emplaced at 331.7±2.2Ma, 311.6±2.6Ma, and 295.4±2.9Ma, respectively. The mineralized granodiorite porphyries at Kounrad show a geochemical affinity to adakitic rocks with high Sr (357–670ppm), Sr/Y (40–68) and Mg numbers (Mg#=molar Mg/(Mg+Fe2+)) from 0.43 to 0.51, low Yb (0.97–1.1ppm) and Y (8.3–11.1ppm). They have variable Sr–Nd–Hf–O isotopic compositions ((87Sr/86Sr)i=0.7046 to 0.7051, εNd(t)=−0.1 to +1.1, εHf(t)=+5.2 to +9.0, δ18O=+5.7 to +6.8). These features indicate that the Kounrad adakitic magmas derived from the MASH (melting, assimilation, storage, homogenization) zone at depth of ~40km with 5–15% ancient basement rocks contamination. The mineralized granodiorite porphyries at Borly have a normal arc magma geochemical signature (e.g., enrichment of light rare earth elements (LREE) and depletion of heavy REE, Nb and Ti) and experienced fractional crystallization. They also have variable Sr–Nd–Hf–O isotopic compositions ((87Sr/86Sr)i=0.7047 to 0.7053, εNd(t)=0 to −1.3, εHf(t)=−0.6 to +7.4, δ18O=+5.5 to +6.7) and Mg# (0.45 to 0.51), indicating that they were generated by melting of juvenile basaltic lower crust with normal thickness, followed by 10–30% ancient crustal contamination. The East Kounrad mineralized intrusions, consisting of granite and leucogranite, have experienced advanced degrees of fractional crystallization and have similar Sr–Nd–Hf–O isotopic compositions ((87Sr/86Sr)i=0.7048, ɛNd (t)=+1.0 to +2.8, εHf(t)=+4.8 to +8.9, δ18O=+5.6 to +6.4) and low Mg# (0.18–0.37), indicating a juvenile lower crust source at depths of <27km with 10–20% ancient crustal contamination.The link between geochemistry and mineralization indicate that the subduction of the Junggar-Balkhash Ocean at 332Ma resulted in the formation of the Kounrad adakitic rocks and assocatied Cu deposit, and continuing subduction at around 311Ma resulted in the formation of the Borly granodiorite porphyry and assocatied Cu–Mo deposit, but entered a collisional stage by the Early-Permian (~295Ma) and resulted in the formation of the East Kounrad granite and leucogranite and assocatied Mo deposit. It is likely that evolution from porphyry Cu to Mo mineralization in the Kounrad region is owing to tectonic evolution from subduction to collision settings, crust depth variation from thickness to thinness, and magma evolution from adaktic and normal arc magma to highly fractionated I-type granitic magma.

Geodynamic evolution of the Sabzevar zone, northern central Iranian micro-continent

The Northern Central Iranian Micro-continent (CIM) represents Neotethys-related oceanic crust remnants, emplaced due to convergence between CIM and Eurasia plates during Eocene. Mafic and ultramafic units are exposed along the northern part of the CIM in the Sabzevar area. The geology and field relation of Sabzevar ophiolite indicate northward subduction of the Sabzevar basin. The average whole rock chemistry of mafic (gabbros) and ultramafic samples (lherzolite, harzburgite and dunite) is characterized by a range of MgO of 11.16–31.88, CaO 5.22–11.53 and Al2O3 2.77–14.57, respectively. Low LREE/HREE ratio of ultramafic samples is accompanied by enrichment of large ion lithophile elements (LILE) such as Sr, Pb and K. Mafic samples show two distinct groups with low and high LREE/HREE ratios. The spider diagram of mafic samples indicates enrichment in Sr, Pb and K and depletion in REE. Petrological and geochemical evidence and field relations show that the mafic rocks formed in a supra-subduction zone setting. Petrological studies reveal the role of fractional crystallization and assimilation effect by released fluids during subduction related generation of the Sabzevar mafic rocks. We suggest that the studied mafic rocks likely represent the basement of an initial island arc, which was generated in a supra-subduction zone setting within the Neotethys branch of the Sabzevar Ocean at the north of CIM. Copper, gold and chromite mineralizations are studied in relation to island arc setting and supra-subduction environment. Similarities in lithology, ophiolite age and mineralization between Sabzevar ophiolite and Bardaskan-Torbat Heydariyeh ophiolites testify for their separation due to rotation (or faulting) of the Central Iranian Micro-continent.

Arc-generated metavolcanic rocks in the Anshan–Benxi greenstone belt, North China Craton: Constraints from geochemistry and zircon U–Pb–Hf isotopic systematics

The Anshan–Benxi granite-greenstone belt (ABGB) in the Eastern Block of the North China Craton mainly comprises supracrustal rocks of amphibole plagioclase gneisses, plagioclase amphibolites, chlorite schists, biotite schists, quartzites and banded iron formations (BIFs), and intrusive monzogranitic to syenogranitic gneisses. Based on geology, spatial distributions, lithologies and zircon U-Pb geochronology, the metavolcanic rocks of the southern ABGB can be divided into the ∼2527Ma southern plagioclase gneiss group, ∼2536Ma southern amphibolite group, ∼2574 to 2560Ma northern chlorite schist group and northern amphibolite group from south to north. These rocks outcrop as stratified and stratoid beds, whereas some occur as enclaves within later intrusive K-rich granitic gneisses. The southern plagioclase gneiss group exhibits calc-alkaline andesite compositions with high MgO, Cr, and Ni, fractionated REE patterns and depletion in Nb–Ta, which resemble high magnesian andesite (HMA) and are derived from the partial melting of sub-arc mantle peridotite that was previously metasomatized by slab melts. The southern amphibolite group has tholeiitic basalt compositions with slight enrichment in LREEs and high Nb–Ti concentrations but low (Nb/La)PM ratios, which coincide with Nb-enriched basalt and are generated by the remelting of residues of sub-arc mantle after the extraction of HMA melts. The northern schist group shows high-K calc-alkaline andesite-dacite compositions and have highest silica and potassium, and obvious enrichment in LREEs and LILEs and depletion in Nb–Ta, which are comparable to the sanukitoid series rocks. These rocks probably formed by low-degree partial melting of a sub-arc mantle wedge that was metasomatized by slab-derived fluids. The northern amphibolite group exhibits tholeiite compositions with unfractionated REE patterns and low (Nb/La)PM ratios, which are consistent with back-arc basin basalts (BABB) and are produced by partial melting of depleted mantle.Combined with lithological associations of the HMA, NEB, sanukitoid-like rocks and BABB-like rocks, the Anshan–Benxi granite-greenstone belt was produced under a convergent plate environment. Both the zircon εHf values and petrogenesis indicate that the four groups of the metamorphic volcanic rocks directly contributed to continental crustal growth. Integrating with the widespread ∼2.5Ga syenogranites and subordinate K-rich granitoids that were derived from reworking of ancient crust, we propose that the ABGB experienced a significant episode of continental growth dominated by a subduction-related lateral accretion accompanied with contemporaneously strong reworking of ancient crustal materials.

Rare earth element characteristics of the carboniferous Huanglong Formation dolomites in eastern Sichuan Basin, southwest China: Implications for origins of dolomitizing and diagenetic fluids

Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks.

The Origin and Tectonic Setting of Precambrian Greywacke of Ribandar‐Chimbel, Goa, India: Petrological and Geochemical Evidence

AbstractThe Precambrian greywacke of Ribandar‐Chimbel belonging to the Sanvordem Formation of the Goa Group, India, has been studied for petrography and analyzed for major trace elements. The greywacke is characterized by angular to sub‐round grains of quartz, feldspar, biotite, chlorite and clay minerals. The abundance of clay in the matrix seems to have influenced the Al2O3 content and the K2O/Al2O3 ratio. The variation diagrams indicate a decreasing trend of TiO2, Al2O3, Fe2O3 and MgO; whereas Na2O and CaO exhibit a scatter which could be a result of the variable presence of feldspar within the sediments. The immobile elements, vanadium (25 to 144 ppm), nickel (up to 107 ppm) and chromium (up to 184 ppm), reflect abundance of clay minerals. The greywacke shows strongly fractionated REE patterns with LaN/YbN = 8 to 26 and with higher total REE abundances (up to 245 ppm). The low REE enrichment and depletion in heavier REE with prominent negative Eu anomaly (Eu/Eu*= 0.54 to 0.79) suggest a derivation of the greywacke from an old upper continental crust composed chiefly of felsic components. Petrological evidence and geochemical data suggest that the deposition of the greywacke largely took place in a deep to shallow basin that progressively changed from that of a continental island arc to an active continental setting.

A petrographic and geochemical study of carbonate and silica phases from the Ediacaran Doushantuo Formation in the Three Gorges area of South China: Implications for diagenetic conditions

The carbonate of the Ediacaran Doushantuo Formation contains various silica phases in the Three Gorges area of South China. To better understand the diagenetic conditions under which these silica phases formed, we carried out a petrographic and geochemical study of the silica phases and the coadjacent carbonate phases at the Xiaofenghe section (XFH), in the Three Gorges area of South China. Five types of silica phases can be distinguished based on petrographic characteristics: S1 and S2 are composed of microcrystalline quartz, in chert nodules and disseminated in the host dolostone, respectively. S3 is composed of the megaquartz, while S4 and S5 are microcrystalline quartz and fibrous silica, respectively, both replacing calcite cement. Six distinct types of carbonate can be recognized, including the host matrix carbonate (C1), sparry calcite nodules (C2), calcite around chert nodules (C3), dispersed calcite cement within chert nodules (C4), fine micropeloidal calcite cements (C5) and calcite veins (C6). Sedimentary structures suggest that formation of the S1 through S5 silica phases and C2 through C5 carbonate phases occurred during early burial diagenesis, and therefore their formation recorded early diagenetic conditions.Assuming that Ediacaran surface seawater temperature ranged from 28 to 39°C, surface seawater δ18O values can be calculated based on the oxygen isotopic composition of the host dolostone matrix (C1), which range from −6 to 0‰, −4 to 0‰, and −2 to 0‰, for the D2, D3 and D4 subunits of the Doushantuo Formation, respectively. Based on the δ18O values of chert nodules and beds (mainly phase S1), diagenetic temperatures can be constrained to between 27 and 60°C. Under such low diagenetic temperatures, the primary geochemical signatures of the host dolostone, such as REE+Y patterns, δ13CCarb and δ34SPy values, are likely to be preserved.Those silica phases without evidence of contamination of continental silicate detritus show characteristic REE+Y patterns, with minor light REE depletions, positive La, Eu and Gd anomalies, negative Ce anomalies and slightly super-chondritic Y/Ho ratios, implying that the source of silica may be derived from seawater. Negative Ce anomalies in the silica phases in D2 through D4 subunits indicate that the bottom seawater or early diagenetic porewater was oxic at the time of silica precipitation.

Mineralogy, geochemistry, and radiocarbon ages of deep sea sediments from the Gulf of Mexico, Mexico

The mineralogy, geochemistry, and radiocarbon ages of two sediment cores (GMX1 and GMX2) collected from the deep sea area of the Southwestern Gulf of Mexico (∼876–1752 m water depth) were studied to infer the sedimentation rate, provenance, heavy metal contamination, and depositional environment. The sediments are dominated by silt and clay fractions. The mineralogy determined by X-Ray diffractometry for the sediment cores reveals that montmorillonite and muscovite are the dominant clay minerals. The sections between 100 and 210 cm of the sediment cores GMX1 and GMX2, respectively, are characterized by the G. menardii group and G. Inflata planktonic foraminiferal species, which represent the Holocene and Pleistocene, respectively. The radiocarbon-age measurements of mixed planktonic foraminifera varied from ∼268 to 45,738 cal. years B.P and ∼104 to 25,705 cal. years B.P, for the sediment cores GMX1 and GMX2, respectively. The variation in age between the two sediment cores is due to a change in sediment accumulation rate, which was lowest at the location GMX1 (0.006 cm/yr) and highest at the location GMX2 (0.017 cm/yr).The chemical index of alteration (CIA), chemical index of weathering (CIW), and index of chemical maturity (ICV) values indicated a moderate intensity of weathering in the source area. The total rare earth element concentrations (∑REE) in the cores GMX1 and GMX2 vary from ∼94 to 171 and ∼78 to 151, respectively. The North American Shale Composite (NASC) normalized REE patterns showed flat low REE (LREE), heavy REE (HREE) depletion with low negative to positive Eu anomalies, which suggested that the sediments were likely derived from intermediate source rocks.The enrichment factor of heavy metals indicated that the Cd and Zn concentrations in the sediment cores were impacted by an anthropogenic source. The redox-proxy trace element ratios such as V/Cr, Ni/Co, Cu/Zn, (Cu + Mo)/Zn, and Ce/Ce* indicated that the sediments were deposited under an oxic depositional environment. The similarity in major element concentrations, REE content, and the NASC normalised REE patterns between the cores GMX1 and GMX2 revealed that the provenance of sediments remained relatively uniform or constant during deposition for ∼4.5 Ma. The major and trace element based multidimensional discrimination diagrams showed a rift setting for the core sediments, which is consistent with the geology of the Gulf of Mexico.

Petrological and tectono-magmatic significance of ophiolitic basalts from the Elba Island within the Alpine Corsica-Northern Apennine system

Two distinct ophiolitic units, which represent remnants of the Jurassic Ligurian-Piedmont Ocean, crop out in the Elba Island. They are the Monte Strega unit in central-eastern Elba and the Punta Polveraia-Fetovaia unit in western Elba. Ophiolitic rocks from the Monte Strega unit are commonly affected by ocean floor metamorphism, whereas those from the Punta Polveraia-Fetovaia unit are affected to various extent by thermal metamorphism associated with the Late Miocene Monte Capanne monzogranitic intrusion. Both ophiolitic units include pillow lavas and dykes with compositions ranging from basalt to basaltic andesite, Fe-basalt, and Fe-basaltic andesite. Basaltic rocks from these distinct ophiolitic units show no chemical differences, apart those due to fractional crystallization processes. They display a clear tholeiitic nature with low Nb/Y ratios and relatively high TiO2, P2O5, Zr, and Y contents. They generally display flat N-MORB normalized high field strength element patterns, which are similar to those of N-MORB. Chondrite-normalized rare earth element patterns show light REE / middle REE (LREE/MREE) depletion and marked heavy (H-) REE fractionation with respect to MREE. This HREE/MREE depletion indicates a garnet signature of their mantle sources. Accordingly, they can be classified as garnet-influenced MORB (G-MORB), based on Th, Nb, Ce, Dy, and Yb systematics. We suggest that the Elba Island ophiolitic basalts were generated at a magma starved, slow-spreading mid-ocean ridge. REE, Th, and Nb partial melting modelling shows that the compositions of the relatively primitive Elba Island ophiolitic basalts are compatible with partial melting of a depleted MORB mantle (DMM) source bearing garnet-pyroxenite relics. Hygromagmatophile element ratios suggest that basalts from both ophiolitic units were originated from chemically very similar mantle sources. A comparison with basalts and metabasalts from Alpine Corsica and northern Apennine ophiolitic units shows that the composition of the inferred mantle source for the Elba Island basalts is similar to that of some Lower Schistes Lustres metabasalts of Alpine Corsica ophiolites, and some basalts from the Internal Ligurian units of northern Apennine. In contrast, it slightly differs from those of other ophiolitic units of Alpine Corsica and northern Apennine. The chemical differences observed between basalts and metabasalts from different Ligurian-Piedmont ophiolitic units were likely associated with different partial melting degrees of either DMM source or garnet-pyroxenite relics and/or different mixing proportions of melts derived from them, as well as to different compositions of garnet-pyroxenite relics.

Constraints on formation and evolution of the lunar crust from feldspathic granulitic breccias NWA 3163 and 4881

Lunar granulitic meteorites provide new constraints on the composition and evolution of the lunar crust as they are potentially derived from outside the Apollo and Luna landing sites. Northwest Africa (NWA) 3163, the focus of this study, and its paired stones NWA 4881 and NWA 4483, are shocked granulitic noritic anorthosites. They are petrographically and compositionally distinct from the Apollo granulites and noritic anorthosites. Northwest Africa 3163 is REE-depleted by an order of magnitude compared to Apollo granulites and is one of the most trace element depleted lunar samples studied to date. New in-situ mineral compositional data and Rb–Sr, Ar–Ar isotopic systematics are used to evaluate the petrogenetic history of NWA 3163 (and its paired stones) within the context of early lunar evolution and the bulk composition of the lunar highlands crust. The NWA 3163 protolith was the likely product of reworked lunar crust with a previous history of heavy REE depletion. The bulk feldspathic and pyroxene-rich fragments have 87Sr/86Sr that are indistinguishable and average 0.699282±0.000007(2σ). A calculated source model Sr TRD age of 4.340±0.057Ga is consistent with (1) the recently determined young FAS (Ferroan Anorthosite) age of 4.360±0.003Ga for FAS 60025, (2) 142Nd model ages for the closure of the Sm–Nd system for the mantle source reservoirs of the Apollo mare basalts (4.355–4.314Ga) and (3) a prominent age peak in the Apollo lunar zircon record (c. 4.345Ga). These ages are ∼100Myr younger than predicted timescales for complete LMO crystallization (∼10Myrs after Moon formation, Elkins-Tanton et al., 2011). This supports a later, major event during lunar evolution associated with crustal reworking due to magma ocean cumulate overturn, serial magmatism, or a large impact event leading to localized or global crustal melting and/or exhumation. The Ar–Ar isotopic systematics on aliquots of paired stone NWA 4881 are consistent with an impact event at ⩾3.5Ga. This is inferred to record the event that induced granularization of NWA 3163 (and paired rocks). A later event is also recorded at ∼2Ga by Ar–Ar isotopes is consistent with an increase in the number of impacts on the lunar surface at this time (Fernandes et al., 2013). Northwest Africa 3163 and its paired stones therefore record a c. 2.4Gyr record of lunar crustal production, metamorphism, brecciation, impacts and eventual ejection from the lunar surface.

Evidence for Archean inheritance in the pre-Panafrican crust of Central Cameroon: Insight from zircon internal structure and LA-MC-ICP-MS U[sbnd]Pb ages

The main geological feature of Central Cameroon is the wide spread occurrence of granitoids emplaced in close association with transcurrent regional shear zones. The basement of this vast domain is a Paleoproterozoic ortho-and para-derivative formation, which has been intensely reworked, together with subsequent intrusions and sediments, during the Panafrican orogenesis in the Neoproterozoic. As consequence, the area underwent pervasive metamorphism and intense deformation. This makes it difficult to distinguish between Panafrican metasediments or syntectonic plutonites and their respective basement. Our study presents zircon features (CL-BSE-SE) and in-situ U–Th–Pb LA-MC-ICP-MS geochronology of a meta-sedimentary pyroxene-amphibole-bearing gneiss of the Méiganga area in Central Cameroon. Based on the Internal structures of the zircon four characteristic zonation patterns can be deciphered: 1) cores with magmatic oscillatory zonation 2) zircons with oscillatory or sector zonation, 3) zircons with sector zoning or blurred zoning, and 4) narrow bright un-zoned rims. These groups suggest that the rock experienced a number of geological events. Considering this zircon characteristic, the U–Th–Pb data allow to distinguish four ages: 2116 ± 57 Ma, consistent with ages from the Paleoproterozoic West Central African Belt; 2551 ± 33 Ma which marks a late Neoarchean magmatic event; 2721 ± 27 Ma related to a Neoarchean magmatic even in Central Cameroon, similar to one found in the Congo Craton. A zircon core gives ages around 2925 Ma which provides some evidence of the presence of the Mesoarchean basement prior to the Neoarchean magmatism. A weighted average of lower intercepts ages gives a value of 821 ± 50 Ma, representing the age of later metamorphism event. The various characteristic group and related ages reflect not only the complexity of the history of the pyroxene amphibole gneiss, but also show that the meta-sediment has at least three zircon contributing sources. It is likely that erosion, transport and deposition took place between 2116 and 821 Ma. Geochemical data show that the REE, Y, Yb, Sr/Y of some samples are similar to the known Archean craton formations (depletion in REE, Y ≤ 10 ppm, Yb ≤ 1 ppm, Sr/Y ≥ 30). These characteristics are known as specific for the Archean TTG (Tonalite—Trondhjemite—Granodiorite). It means that: i) Archean TTG contribute significantly to the detritus of the sedimentary basin, ii) The depositional basin and the source rock were close and the detritus was immature.Our results show that the Pre-Panafrican history of central Cameroon includes Meso- to Neo-Archean crustal accretion and associated magmatism prior to the Paleoproterozoic event of the West Central African Belt. In respect to this new insight, any evolutionary reconstruction of the area should integrate the presence of Archean crust.

Seepage carbonate mounds in Cenozoic sedimentary sequences from the Las Minas Basin, SE Spain

A number of carbonate mounds composed of indurate, strongly folded and/or brecciated calcite and dolomite beds occur interstratified in Cenozoic sedimentary sequences from the Las Minas Basin. Part of the fabric of the rock forming the carbonate mounds is composed of laminated to banded dolostone similar to the host rock but showing contrasted lithification. Moreover, the carbonate deposits of the mounds display aggrading neomorphism of dolomite, partial replacement of dolomite by calcite, calcite cementation, and extensive silicification, locally resulting in box-work fabric. Eight main lithofacies were distinguished in the carbonate mound deposits. In some lithofacies, chert is present as both microcrystalline to fibro-radial quartz and opal, the latter occurring mainly as cement whereas the former replace the carbonate and infill voids. Yet one of the carbonate mounds shows distinctive petrography and geochemical features thus suggesting a distinctive growth pattern. The carbon isotope compositions of calcite from the mound samples range from −11.56 to −5.15 δ‰ whilst dolomite is depleted in 13C, with values of −12.38 to 3.02 δ‰. Oxygen isotopic compositions vary from −9.42 to −4.64 δ‰ for calcite and between −6.68 and 8.19 δ‰ for dolomite. Carbonate in the mounds shows significant enrichment in Co, Cr, Ni and Pb content, especially in the strongly deformed (F-2-2 lithofacies) and brecciated carbonate (F-4). The carbonate deposits show depletion in REE and Y in contrast to that determined in lutite. The formation of the carbonate mounds was related to local artesian seepage thermal water flows of moderate to relative high temperatures. Pressure differences between the low permeability host rock and the circulating fluids accounted for dilational fracturing and brecciation of the host sediment packages, which combined with precipitation of new carbonate and silica mineral phases. Locally, some carbonate mounds developed where groundwater intersected the lake floor, this resulting in bedded carbonate deposits composed of organic constituents.

Kyanite/corundum eclogites from the Kaapvaal Craton: subducted troctolites and layered gabbros from the Mid- to Early Archean

An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and δ 18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4–6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.

Tourmaline as a recorder of magmatic–hydrothermal evolution: an in situ major and trace element analysis of tourmaline from the Qitianling batholith, South China

Four types of tourmaline in the Qitianling granitic batholith, South China, were identified in this study: euhedral disseminated tourmaline (DT type) in the early stage of granite consolidation; typical interstitial nodular tourmaline (NT type) formed late in the crystallization history; radial tourmaline veinlet (RT type) and vein tourmaline (VT type) of hydrothermal origin. We performed major and trace element analysis by in situ electron microprobe and laser ablation inductively coupled plasma mass spectrometry on these four tourmaline types. Compositionally, these tourmalines fall into the alkali group and schorl–dravite solid solution series. There is clear correlation between trace and major elements in VT-type tourmalines, likely related to co-variations of these elements in the hydrothermal fluid from which the tourmaline precipitated. Tourmaline from granites displays low REE abundances and negative LREE trends, positive Eu anomalies, and HREE below or close to their detection limits. The DT-type tourmalines show a positive Eu anomaly, which is probably due to a preferential incorporation of Eu2+ over Eu3+. REE depletion in NT-type tourmalines may reflect co-crystallization of REE-rich minerals. The positive Eu anomaly in NT-type tourmalines is due to late-stage release of Eu2+ caused by tourmaline replacement of early feldspar. Vein tourmalines (RT and VT type) display similar slopes from La to Nd, without an Eu anomaly. The chondrite-normalized REE patterns in tourmalines from veins display a concave upward-shaped, MREE-depleted pattern, with minimum values at Ho increasing steadily to Lu, and may reflect the REE characteristics of the precipitated medium. HREE enrichment in hydrothermal fluid can be attributed to the formation of REE-fluoride complexes during the internal fluid evolution.

Petrology and geochemistry of the banded iron-formations from Ntem complex greenstones belt, Elom area, Southern Cameroon: Implications for the origin and depositional environment

Banded iron-formations (BIFs) form an important part of the Archaean to Proterozoic greenstone belts in the Southern Cameroon. In this study, major, trace and REE chemistry of the banded iron-formation are utilized to explore the source of metals and to constraint the origin and depositional environment of these BIFs. The studied BIF belongs to the oxide facies iron formations composed mainly of iron oxide (mainly magnetite) mesobands alternating with quartz mesobands. The mineralogy of the BIF sample consists of magnetite and quartz with lesser amount of secondary martite, goethite and trace of gibbsite and smectite. The major element chemistry of these iron-formations is remarkably simple with the main constituents being SiO2 and Fe2O3 which constitute 95.6–99.5% of the bulk rock. Low Al2O3, TiO2, and HFSE concentrations show that they are relatively detritus-free chemical sediments. The Pearson’s correlation matrix of major element reveals that there is a strong positive correlation (r=0.99) of Al with Ti and no to weak negative correlation of Ti with Mn, Ca and weak positive correlation of Si with Ca, suggesting the null to very minor contribution of detrital material to chemical sediment. The trace elements with minor enrichments are transition metals such as Zn, Cr, Sr, V and Pb. This is an indicator of direct volcanogenic hydrothermal input in chemical precipitates. The studied BIF have a low ΣREE content, ranging between 0.41 and 3.22ppm with an average of 0.87ppm, similar to that of pure chemical sediments. The shale-normalized patterns show depletion in light REE, slightly enrichment in heavy REE and exhibit weak positive europium anomalies. These geochemical characteristics indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The absence of a large positive Eu anomaly in the studied BIF indicates an important role of low-temperature hydrothermal solutions. The chondrite-normalized REE patterns are characterized by LREE-enriched (Mean LaCN/YbCN=8.01) and HREE depletion (Mean TbCN/YbCN=1.61) patterns and show positive Ce anomalies. With the exception of one sample (LBR133), all of the BIF samples analyzed during this study have positive Ce anomalies on both chondrite- and PASS-normalized plots. This may indicate that the BIFs within the Elom area were formed within a redox stratified ocean. The positive Ce anomalies in the studied samples likely suggest that the basin in which Fe formations were deposited was reducing with respect to Ce, probably in the suboxic or anoxic seawaters.