Light Enrichment Research Articles

Origin of microbial–hydrothermal bedded dolomites in the Permian Lucaogou Formation lacustrine shales, Junggar Basin, NW China

Dolomite is widely developed in the shale sequences of saline lakes and has significance in paleoenvironmental, paleoclimatic, and petroleum geology research. In this study, a new end-member type of dolomite is discovered from the Permian Lucaogou Formation in the Junggar Basin, northwest China, along with its geological significance. The microbial–hydrothermal origin of the bedded dolomite is evident from the development of hydrothermal minerals (including barite and cubic pyrite) and methanogenic micro-organisms. In terms of geochemistry, the bedded dolomite has high (Ba/La)N ratios (~2.8) and Sr contents (~594.9 ppm) and positive Sr anomalies (Sr/Sr* ~3.8). Chondrite-normalized rare earth element (REE) patterns exhibit light REE enrichment, heavy REE depletion, slightly positive Ce anomalies, and negative Eu anomalies. The bedded dolomite has relatively low 87Sr/86Sr ratios (~0.705283), high δ13C values (~6.63‰ ), and low δ18O values (~−9.92‰ ). All these indicate that the dolomitizing fluid is a mixed source of hydrothermal fluids and lake waters, and the hydrothermal fluids and microbial methanogenesis provided the Mg2+ and C, respectively. The combination of microbial and hydrothermal activity overcame the limitation of dolomitization hydration shells around Mg2+. In terms of geological significance, the studied Lucaogou Formation was deposited in an extensional tectonic setting under hot–arid climatic conditions, which controlled the high-frequency interbedding and meter-scale cyclicity of sediment deposition of dolomites and shales. The combined effects of tectonism and paleoclimate may be important factors for the formation of bedded dolomite, enrichment of organic matter and hydrocarbons in saline lacustrine shales.

Nature and evolution of diagenetic fluids in the deeply buried Cambrian Xiaoerbulake Formation, Tarim Basin, China

Significant amounts of petroleum are found in the deeply buried Cambrian Xiaoerbulake Formation in the Tarim Basin. However, limited understanding of origin of diagenetic fluids hampers petroleum exploration. In this study, petrological and high-resolution geochemical methods have been performed to understand the origin and evolution of diagenetic fluids, and their effects on the formation of dolostone reservoir. The Xiaoerbulake Formation is characterised by microbialites. The matrix is characterised by modern seawater-like rare earth element and yttrium (REY) patterns and δ18O values of −6.02 ± 2.11‰ Vienna Pee Dee Belemnite (VPDB). Fine-crystalline dolomite cements have similar δ13C values and heavy rare earth element enrichment patterns to that of matrix dolomite, yet with lower δ18O values of −7.47 ± 1.2‰ VPDB. In the deep-burial regimes, medium- to coarse-crystalline dolomite cements and saddle dolomites were precipitated at temperatures of 90–120 and 130–190 °C, respectively, lower δ18O values, distinct middle rare earth element enrichment, and high Fe and Mn contents compared with matrix and fine-crystalline dolomite cements. These imply thermochemical reduction of Fe- or Mn-rich oxides and/or hydrothermal fluids. Compared with other types of dolomite cements, calcite cements are characterised by low δ13C values (as low as −6‰ VPDB), light rare earth elements enrichment and significantly elevated Y/Ho ratios (86 ± 59.3), likely related to the oxidation of organic matter during thermochemical sulfate reduction (TSR). Consequently, diagenetic fluid may have evolved from: Cambrian seawater, meteoric water, shallow-buried seawater-dominated pore water, medium-deep-buried ferrous-rich pore water, lately high-salinity hydrothermal fluid-dominated pore water and finally to acidic fluids related to TSR dissolution. Reservoir quality is enhanced by hydrothermal pulses and TSR in deep-burial settings. This study is significant for those concerned with the formation of deep-buried carbonate reservoirs.

Discovery of antiskarn-hosted strategic metal mineralization in the Upper Cretaceous Twihinate carbonatite intrusion (West African Craton Margin, Moroccan Sahara)

The Upper Cretaceous Twihinate carbonatite in Moroccan Sahara, which is the focus of ongoing exploration for Nb-P-Fe ± LREE ± U-Th resources, consists of a crescent-like intrusion made of a central sövite body encircled by an annular ring of vuggy siliceous breccia. Drill core logging coupled with petrographic observation, whole-rock and mineral chemistry reveal that the metasomatic interaction between the carbonatite melt and the silicate wall-rocks led to development of calc-silicate paragenesis within the sövite itself at depths ranging from ∼60 to >200 m from the surface for which the term antiskarn is attributed. We provide the first petrographic and textural descriptions, and report the whole-rock major- and trace-element geochemistry and chemical compositions of the main antiskarn-forming minerals and related strategic metal-bearing ore. Geochemically, the antiskarn lithotypes are characterized by a compositional range of 7–17 wt% SiO2, 25–72 wt% CaO, 1–2 wt% Al2O3, and 12–26 wt% loss on ignition, and enrichment in large-ion lithophile elements (LILE), particularly Sr (3125–7018 ppm ppm), Ba (393–1300 ppm), U (10–787 ppm) and light REEs (LREEs) (1058–1569 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, Ta and Hf. Nb and V show, however, much higher concentrations ranging from 140 ppm to >1000 ppm, and 603 to 1008 ppm, respectively. Mineralogically, the antiskarn lithotypes consist predominantly of diopsidic (Di66-82 Hd18-34) to aegirine-augite clinopyroxene (Di38-55 Ae21-35) and andradite (And86-94 Gro1-9) with subordinate amounts of micas (biotite/phlogopite) and titanite. The paragenetically later mineral assemblages involve amphibole, chlorite, epidotes, baryte, fluorite, kaolinite, carbonates and quartz. The main Nb-P-Fe ± LREE ± U-Th ore minerals consist of Fe oxides (titanomagnetite principally), pyrochlore group minerals, phosphates (apatite, monazite-(Ce)), and REE-fluorcarbonates (bastnäsite-(Ce)) with subordinate sulfides (chalcopyrite, sphalerite, and galena), fluorite and baryte. Development of the antiskarn calc-silicate paragenesis and related strategic metal-bearing mineralization is consistent with derivation from a carbonatite melt through assimilation of silicate country rocks including the Silurian-Devonian gneissic granitoid of the Laknouk suite. From an economic perspective, the widespread occurrence of high-Ti (0.7–16. 9 wt% Ti) and V- (0.2–0.7 wt% V) magnetite opens new opportunities in regard of exploration for V; an emerging strategic commodity whose presence has to be reassessed in the Twihinate prospect. The currently described antiskarn paragenesis may be therefore used as reliable exploration metallotect to target prospective occurrences in the search for critical metals including V as by product.

Rare earth elements as new biogeochemical proxies in deep-sea mussels

We report on the abundances of REE in a comprehensive suite of shells of deep-sea chemosynthetic mussels from hydrothermal vents and cold seeps. Except for mussel shells from oceanic hydrothermal sites that often show extremely pronounced Eu anomalies ((Eu/Sm)sn = 2–200), and abundances for this element that can occasionally exceed 200 ng/g, REE concentrations are usually low and typically between 10−4 and 10−3 times the shale reference.In addition to exhibiting commonly high Eu anomalies, mussel shells from hydrothermal vents are depleted in light-REE and heavy-REE compared to reference shales (e.g., Pr/Sm)sn < 1, (Tb/Yb)sn = 1–6). These features are inherited from hydrothermal fluid. Mussel shells from cold seeps have very different REE concentrations, which also reflect the compositions of the waters they filter: their (Eu/Sm)sn ratios are much lower (<2) and are much less heavy-REE fractionated ((Tb/Yb)sn = 0.5–2.7). Furthermore, the REE distributions show a clear dichotomy between thiotrophic and methanotrophic mussels. The latter show marked enrichments in light-REE and even sometimes La enrichments much larger than those classically observed in deep-sea waters, leading to huge positive La anomalies (La/La* > 10). These light-REE enrichments are likely related to REE-dependent methanol dehydrogenase enzymes used by the symbionts that these mussels host in their gills. These data show that REE chemistry is a promising tool to study chemosynthetic faunas living near hydrothermal vents or cold seeps. Furthermore, REE chemistry, coupled with stable isotopes, should reveal the footprint of aerobic methanotrophy in carbonates formed in cold seeps, but also potentially in ancient sediments.

Distribution, source and behavior of rare earth elements in surface water and sediments in a subtropical freshwater lake influenced by human activities

As tracers, rare earth elements (REEs) can reflect the influence of human activities on the environmental changes in aquatic systems. To reveal the geochemical behavior of REEs in a water–sediment system influenced by human activities, the contents of REEs in the surface water and sediment in the Chaohu Lake Basin were measured by inductively coupled plasma mass spectrometry (ICP–MS). The results show that the ΣREE contents in the surface water are 0.10–0.850 μg L−1, the ΣREE contents in the sediments are 71.14–210.01 μg g−1, and the average contents are 0.24 μg L−1 and 126.72 μg g−1, respectively. Almost all water and sediment samples have obvious light REE (LREE) enrichment, which is the result of the input of LREE-rich substances released by natural processes and human activities (industrial and agricultural production). Under the alkaline water quality conditions of Chaohu Lake, REEs (especially LREEs) are easily removed from water by adsorption/coprecipitation reactions with suspended colloidal particles, which leads to the enrichment of LREEs in sediments. The Ce anomaly of the water–sediment system is related to the oxidation environment, while the Eu anomaly is related to the plagioclase crystallization. Significant Gd anomalies was observed in the downstream of rivers flowing through urban areas, which was related to the anthropogenic Gd wastewater discharged by hospitals. The ∑REE–δEu and provenance index (PI) discrimination results are consistent, indicating that the sediments in Chaohu Lake mainly come from rivers flowing through the southwest farmland. Furthermore, the spatial distribution of REEs shows that these tributaries are significantly affected by agricultural activities. The distribution and accumulation of REEs in Chaohu Lake are the result of the interaction of natural and human processes. The results can provide a scientific reference for the distribution and environmental behavior of REEs in aquatic environments disturbed by human beings.

Petrogenesis of ilmenite-bearing mafic intrusions: A case study of Abu Ghalaga area, South Eastern Desert, Egypt

Neoproterozoic Abu Ghalaga ilmenite-bearing mafic rocks were subjected to geological, petrographical and geochemical studies integrated with mineral chemistry of ilmenite, pyrite and magnetite in order to deduce their petrogenesis. Abu Ghalaga mafic intrusions hosting the largest ilmenite reserve in the Egyptian part of the Arabian-Nubian Shield. Field observations revealed that the intrusions impound ilmenite ore, which occurs as interlayer of massive bands or disseminated within gabbroic rocks. Petrographically, Abu Ghalaga mafic intrusions include different kinds of gabbro; olivine pyroxene gabbro, pyroxene gabbro, pyroxene hornblende gabbro, altered gabbro and Fe-Ti-rich gabbro. They have calc-alkaline to tholeiitic magmas, which are depleted in high field strength elements (e.g., Ta, Nb and Th) relative to low field strength elements (e.g., Ba, Sr and Rb), and exhibit light rare earth elements enrichment relative to heavy rare earth elements with positive Eu anomaly (Eu/Eu*= 0.8-2.4). Variable mineralogical and wide variation of bulk rock geochemistry are ascribed to fractional crystallization and hydrothermal overprinting. Abu Ghalaga gabbroic rocks are derived from fractional crystallization of depleted mantle magmas that were modified by ascending fluids from subducted slab in volcanic arc setting. Fe-Ti oxides required high oxygen fugacity (ƒO2) and water contents by crystal settling of Ti-rich mantle which is applicable with subduction setting. It can be inferred that Abu Ghalaga mafic rocks are neither related to ophiolite nor to Alaskan type but to one of the layered intrusions hosting Fe-Ti oxides.

Petrogenesis of Jurassic granitic plutons in Liaodong Peninsula, NE China: Insights into the subduction of Paleo − Pacific plate

• Jurassic granites in Liaodong Peninsula formed at 184–179 and 162–151 Ma. • Granite geochemistry reveals the thinning of continental crust from Early to Late Jurassic . • The granites constrain the subduction history of the Paleo–Pacific plate. There is a debate about the subduction of Paleo − Pacific plate in the Jurassic tectonic evolution of the northeastern North China Craton. Here we present new geochronological and geochemical investigations for Early and Late Jurassic granites on Liaodong Peninsula. The Early Jurassic granites (184–179 Ma) show high SiO 2 and low MgO contents, and are classified as I − type granites, with enrichment in large − ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depletion in high − field − strength elements (HFSEs) and heavy REEs (HREEs), and enriched Sr − Nd − Hf − O isotopic compositions. Combined with their typical adakitic properties as high Sr, low Y and Yb contents, and high Sr/Y ratios, we conclude that they originated from thickened lower crust. The spatio − temporal distribution of Early Jurassic intrusive and volcanic rocks and large scale of compressive deformation suggest the onset of flat subduction of Paleo − Pacific plate. In contrast, the Late Jurassic granites (162–151 Ma) are classified as I − type granites, with enrichment in LILEs and LREEs and depletion in HFSEs and HREEs and more enriched Sr − Nd − Hf − O isotopic compositions. Based on Sr and Y contents and Sr/Y ratios, we divided them into two groups. Group 1 show high SiO 2 and low MgO contents and typical adakitic properties, indicating an origin of thickened lower crust. In contrast, group 2 show relatively higher SiO 2 and lower MgO contents, with low Sr and high Y contents and high Nb/U ratios, suggesting an origin of lower crust in normal thickness. The spatio − temporal distribution of Late Jurassic bimodal igneous rocks and continuous volcanic and sedimentary deposition as well as the shifted accretionary complexes, indicate the oblique subduction of Paleo − Pacific plate.

Paleoproterozoic TTG gneisses and granites of the Huai’an Complex: Geochemical and zircon U-Pb-Hf data provide insights into subduction history and crustal growth

Tonalite-trondhjemite-granodiorite (TTG) suites and granitoids are major components of the Precambrian continental crust. Here we investigate the Paleoproterozoic TTG gneisses and granites from the Huai’an Complex based on new and published geochemical and geochronological data with a view to trace the tectonic evolution of the North China Craton (NCC). The TTG gneisses show high Mg#, Sr and Sr/Y, low Yb, positive Eu anomalies, as well as enriched light REE (LREE) and depleted heavy REE (HREE) patterns. The magmatic zircons of TTG gneisses show εHf(t) of 9.1 to −2.8 and 176Lu/177Hf of 0.046. Geochemical characteristics suggest that the parental magmas of TTG gneisses were sourced from the partial melting of subducted hydrated oceanic crust and mantle wedge, with fractional crystallization of plagioclase and amphibole. The granites are magnesian, medium-K to high-K calc-alkaline, metaluminous to weakly peraluminous, and have decreasing P2O5, Al2O3, TiO2, MgO, CaO, and Sr with increasing SiO2, increasing Pb with increasing SiO2, decreasing Ta/Nb with increasing TiO2, positive Eu anomalies and low HREE. The magmatic zircons of granites show εHf(t) of 2.72 to −2.6 and 176Lu/177Hf of 0.033. Geochemical characteristics classify the granites into I-type granites and indicate that magma derived through partial melting of infra-crustal igneous rocks and subcontinental lithospheric mantle with fractional crystallization of apatite, K-feldspar, plagioclase and mica. Geochemical characteristics and zircon U-Pb data indicate that the Huai’an Complex was a long-lived magmatic arc and in crustal growth stage during 2.51–2.39 Ga. The crystallization ages of TTG gneisses and granites in the Huai’an Complex show a general younging-trend from north to south along NE-SW direction, likely suggesting a scissor-like subduction model with an oceanic lithosphere subduction earlier beneath the northern part of the Huai’an Complex than the southern part.

Characteristics and Provenance Implications of Rare Earth Elements and Nd Isotope in PM2.5 in a Coastal City of Southeastern China

The source apportionment of fine particulate matters, especially PM2.5, has drawn great attention worldwide. Since rare earth elements (REEs) and Nd isotopes can serve as source tracers, in this study, the characteristics and provenance implications of REEs and Nd isotopes in PM2.5 of four seasons in Xiamen city, China, were investigated. The range of the ratios of ΣREE to PM2.5 was 1.04 × 10−5 to 8.06 × 10−4, and the mean concentration of REEs in PM2.5 were in the order of spring &gt; autumn &gt; winter &gt; summer. According to the geoaccumulation index (Igeo), spring was the season in which anthropogenic sources had the greatest impact on the REEs in PM2.5. The chondrite-normalized REE distribution patterns exhibited light rare earth elements (LREEs, including La, Ce, Pr, Nd, Pm, Sm and Eu) enrichment and a flat heavy rare earth elements (HREEs, including Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) pattern. Significant negative Eu anomalies and no significant Ce anomalies were observed in the PM2.5. The results of La-Ce-Sm ternary plots indicated that the REEs in the PM2.5 might be related to both natural and anthropogenic sources. Combined with the Nd isotope, the 143Nd/144Nd versus Ce/Ce* plot further illustrated that the REEs in the PM2.5 seemed to mostly originate from multiple potential sources, in which vehicle exhaust emissions, coal burning and cement dust made a great contribution to REEs in PM2.5.

Geochemical characteristics of mafic intrusive rocks from the Naga Hills Ophiolite, north‐east India: Constraints on petrogenesis and tectonic setting

The Naga Hills Ophiolite (NHO) of Late Jurassic to Eocene age is one of the well‐exposed domains of Tethyan ophiolites, located in the northern margin of the Indo‐Myanmar Orogenic Belt, north‐east India. It is composed of suites of tectonite peridotite and cumulative ultramafic, mafic intrusive‐extrusive rocks, felsic intrusive, radiolarian, and marine pelagic sediments. The mafic intrusives of the NHO exhibit rhythmic to cryptic layering and isotropic in nature. Based on their mineralogy, the mafic intrusives are identified as gabbro, gabbronorite, hornblende gabbro, and olivine gabbro. The gabbros are adcumulate and mesocumulate and essentially composed of plagioclase feldspar, clinopyroxene, orthopyroxene, hornblende, with minor olivine, serpentine, chlorite, and magnetite. These mafic intrusive rocks show affinity to tholeiitic parental magma and fractionated trends in the multi‐elemental diagram, with negative anomalies at Zr, P, Ce and positive anomalies at Sr, Nb, and Ti suggesting that the magmatism was influenced by subduction zone components. The majority of the samples represent light rare earth elements (LREE) depletion (LaN/SmN = 0.28–0.86), except two samples that display LREE enrichment (LaN/SmN = 2.11–3.03), with negligible negative Eu anomaly. Petrogenetic modelling suggests 5%–20% partial melting of a depleted mid‐ocean ridge basalt‐type mantle source, within the spinel lherzolite stability field, as well as low degree partial melting (&lt;10%) of an enriched mantle source probably metasomatized by sediment‐melt from a subducted slab. Therefore, based on mineralogical and geochemical data, we propose that the mafic intrusive of the NHO were generated in the intra‐oceanic arc system within the Neotethys Ocean with the active participation of possible wet basaltic melt from spinel‐lherzolite source‐mantle, subsequently modified by melts derived from sediments in a subduction zone.

Pressure-temperature evolution of the Qingshuiquan mafic granulite: Implications for Proto-Tethys subduction in the East Kunlun orogenic belt, northern Tibetan Plateau

AbstractGranulite is in general a key metamorphic rock that can be used to understand the tectonic architecture and evolutionary history of an orogenic belt. The Qingshuiquan mafic granulite in the East Kunlun orogenic belt, northern Tibetan Plateau, occurs as tectonic boudins together with lower-grade ophiolitic mélange assemblages within an amphibolite-facies crystalline basement. In this study, we investigated the geochemistry, geochronology, mineralogy, and phase modeling of the Qingshuiquan mafic granulite. Based on mineralogical observations and microstructures, three mineral assemblage generations were distinguished: an assemblage found as inclusions within garnet and amphibole comprising clinopyroxene + plagioclase + amphibole + quartz + ilmenite + rutile (M1); an inferred peak assemblage of garnet + clinopyroxene + plagioclase + amphibole + quartz + ilmenite ± orthopyroxene (M2) in the matrix; and a retrograde assemblage of amphibole and biotite coronae (M3) around clinopyroxene or orthopyroxene. Thermobarometric calculations and phase equilibrium modeling constrained a clockwise pressure-temperature (P-T) path for the Qingshuiquan mafic granulite with peak T conditions of 830–860 °C at 8.0–9.5 kbar. Prior to the peak T conditions, a pressure maximum of ~11 kbar at ~800 °C was recorded by rutile, ilmenite, and clinopyroxene inclusions in garnet and amphibole. The retrograde path was defined by a decompression segment followed by final cooling. The whole-rock geochemical results indicated that the protolith of the Qingshuiquan mafic granulite was similar to present-day enriched mid-ocean-ridge basalt (E-MORB) displaying low total rare earth element (REE) concentrations and a slight enrichment of light REEs, as well as flat high field strength element patterns in the primitive mantle–normalized trace-element diagram. Geochronologic results revealed that the protolith crystallization age of the mafic granulite is 507 ± 3 Ma, and the timing of granulite-facies metamorphic overprint is 457–455 Ma. This evidence, taken together with results from previous studies, indicates that the protolith of the Qingshuiquan mafic granulite can be interpreted as basaltic rocks of Proto-Tethys oceanic crust that experienced a first high-pressure granulite-facies imprint followed by subsequent decompression and granulite-facies overprint at slightly lower P and slightly higher T. This granulitefacies metamorphism can be attributed to the subduction of Proto-Tethys oceanic crust, which also generated numerous contemporaneous subduction-related magmatic rocks in the East Kunlun orogenic belt.

Petrogenesis and Tectonic Implication of the Hongtaiping High-Mg Diorite in the Wangqing Area, NE China: Constraints from Geochronology, Geochemistry and Hf Isotopes

This study presents new data from zircon U–Pb dating and Hf isotope analysis, as well as whole-rock major- and trace-element compositions of the Hongtaiping high-Mg diorite in the Wangqing area of Yanbian, NE China. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb dating gives an eruption age of ca. 267 Ma for the high-Mg diorite. These samples have MgO contents of 13.30% to 16.58% and high transition metal element concentrations, classified as sanukite. Their rare earth element (REE) contents range from 45.2 to 68.4 ppm and are characterized by slightly positive Eu anomalies (Eu/Eu* = 1.08–1.17). They show enrichment in light REEs (LREEs) and depletion in heavy REEs (HREEs), with LREE/HREE ratios = 6.54–6.97 and (La/Yb)N values = 7.24–8.08. The Hongtaiping high-Mg diorite is enriched in Rb, U, K, and Sr, but depleted in Th, Nb, and Ta. High MgO contents, Mg# values, and transition metal element concentrations imply that the magma experienced insignificant crystallization fractionation and crustal contamination. Relatively homogenous positive Hf isotopic values indicate that the original magma was generated by the partial melting of a depleted mantle wedge that was metasomatized by subducting slab fluids. The magma was generated by the moderate degree partial melting (20%–30%) of a garnet lherzolite source. Combined with previous studies, this shows that the high-Mg diorite was formed by the northward subduction of the Paleo-Asian oceanic plate during the Middle Permian.

Petrographic and geochemical characteristics of nodular carbonate-bearing fluorapatite in the lacustrine shale of the Shahejie Formation, Dongying Depression, Bohai Bay Basin

A varying abundance of carbonate-bearing fluorapatite (CFA) nodules in Eocene lacustrine shale in the Bohai Bay Basin was observed; however, mineralogical and geochemical awareness is lacking, which limits the specific understanding of CFA formation mechanisms and paleoenvironmental proxies. We specified the petrology, trace elements (TE), rare earth elements and yttrium (REY) of the CFA nodules by using an integrated approach of cathodoluminescence, micro-Fourier transform infrared spectroscopy, electron microprobe analysis and laser ablation–inductively coupled plasma–mass spectrometry. The results suggest that CaO (25.85 to 39.95 wt%), P2O5 (17.19 to 29.35 wt%) and F (0.71 to 2.99 wt%) are the dominant components of the CFA nodules, with low Mn/Fe ratios and few incorporations of CO32−. They show selective concentrations of Sr (avg. 14,963.3 ppm), Ba (avg. 1831.6 ppm), U (avg. 314.2 ppm), Zr (avg. 263.5 ppm), and Th (avg. 185.4 ppm) and apparent negative Y anomalies with low Y/Ho ratios (18.40 to 28.90). These CFA nodules were classified into three types according to REY patterns normalized to the Post-Archean Australian Shale (PAAS). Type A nodules enclosed by micritic calcite laminae are middle rare earth elements (MREE) enriched which yield a typical “bell-shaped” REY pattern, nonanomalies of Ce and a good correlation of ΣREY with Ce/Ce* (R2 = 0.90) and Ba (R2 = 0.95). Type B nodules in the clay and organic-rich laminae were better phosphatized and display more heavy rare earth elements (HREE) depletion, minimal Ce anomalies and a good correlation between CaO and P2O5 (R2 = 0.84) than type A nodules. Type C nodules are rare but show light rare earth elements (LREE) enrichment relative to type A and B nodules. The adsorption of P, TE and REY by Fe-(oxyhydr)oxide and organic matter is significant for CFA formation through the “dissolution-recapture-reprecipitation” process in the ambient pore water of the Fe reduction zone during early diagenesis. The retention of HREE as HREE(CO3)2− in more reducing bottom water is critical to the HREE depletion for type B and C nodules, while the release of LREE from bacterial-mediated degradation of organic matter into ambient pore water enhances the concentration of LREE on type C nodules. Deviation of the REY patterns is probably related to the differential suboxic redoxclines for the deposition of calcite-rich and clay- and organic-rich laminae of shale, which are considered the depositional environment proxies of lacustrine shale.

Late Permian rift-related volcanic rocks from the Zhongba Terrane, Southern Tibet: implications for the opening of the Neotethys

ABSTRACT The questions of how and when the Neotethys Ocean opened are crucial for understanding the evolution of the Tethys Realm and the break-up of Gondwana. However, the evolution of the Neotethys remains controversial. Herein, we report the zircon U–Pb ages, whole-rock major as well as trace elements, and Sr–Nd isotopic data for the basaltic volcanic rocks from the Zhongba Terrane in the western Yalung–Zangbo Suture Zone. The results demonstrate that the Saga basalts were extruded during the Late Permian (257–258 Ma). All samples are characterized based on the low SiO2 content (44.03–50.33 wt%) and belong to the high-K calc-alkaline series. Furthermore, they exhibit light rare earth element enrichment ((La/Yb)N = 12.7–26.7); slightly positive Eu anomaly(Eu/Eu* = 1.01–1.14); prominent Nb and Ta positive anomalies; slightly negative Zr, Hf, and Ti anomalies and exhibit high ratios of Ta/Hf (0.93–1.21), Zr/Y (8.68–10.63), and Nb/Zr (0.39–0.45). The (87Sr/86Sr)i ratio varied from 0.7050 to 0.7063, with relatively constant and depleted εNd(t) values ranging from 0.52 to 1.08. The rocks were probably sourced from a low-degree partial melting of the asthenospheric mantle in a rift environment and had undergone the process of fractional crystallization. These characteristics indicate that the Saga basalts differ from the rift-related magmatic rocks produced by the Panjal Traps. Consequently, combined with the recently reported detailed fieldwork, the Zhongba Terrane disintegrated from the northern margin of Gondwana during the Late Permian, promoting the opening of the Neotethys Ocean, which is most likely caused by the northward subduction of the Paleotethys Ocean.

Advanced characterization of hydrothermal flows within recharge and discharge areas using rare earth elements, proved through a case study of two-phase reservoir geothermal field, in Southern Bandung, West Java, Indonesia

Rare earth element (REE) analysis is effective for tracing water–rock interactions and solute transport in geothermal and groundwater systems. This study aimed to clarify hydrothermal flows within recharge and discharge areas of a geothermal system. We selected a well-known high-temperature geothermal field with two-phase reservoir in Southern Bandung, West Java (Indonesia) as a case study target, and collected 31 river/spring samples and 8 well rock samples. The measured total REE series concentrations ranged widely in the water samples, i.e., rivers: 3.4–35 ppt, cold springs: 0.3–284 ppt, and hot springs: 1.4 ppt to 102 ppm. Enrichment of light REEs (LREEs) in the water samples was characterized more clearly by Post Archean Australian Shale normalization than by chondrite normalization. Speciation analysis derived the following interpretations: dissolved REEs in the river water originated from free ions or Ln3+ that are typically contained in near-surface groundwater, and the dominant components were carbonate (LnCO3+) and Ln3+ complexes in the cold springs and LnCO3+ complexation in the hot springs. Steam and gas condensation into less-oxygenated groundwater can be indicated by rich LnCO3+. Only one hot spring was interpreted as being directly connected with the reservoir by strong H2S condensation and LREE enrichment. The water of the other cold and hot springs probably underwent dilution, evaporation, and magmatic gas condensation through shallow, deep, and deep-perched aquifers, permeable zones, and a major fault within the recharge and discharge areas. REE results also suggest that the main constituents of the aquifers are carbonates and silicate minerals and hydrothermally altered lahars, andesitic lava, and pyroclastic flow deposits. REEs facilitate detailed interpretation of geochemistry facies in shallow–deep waters of a hydrothermal system. Finally, integration of the analysis results of REEs, major anions and cations, water isotopes, and strontium isotopes improved a conceptual model of groundwater flows and recharge–discharge interactions in aquifers feeding a geothermal reservoir.

Sheared and Granular Peridotites from the Udachnaya–East Kimberlite (Yakutia): Mineralogy, Chemistry, and PGE Patterns

Abstract —Mantle xenoliths of fresh sheared and granular peridotites from the Udachnaya kimberlite pipe (Yakutian kimberlite province, Siberia, Russia) are studied in terms of mineralogy, chemistry, and behavior of platinum-group elements (PGE), with a focus on difference between the two types of peridotites and on the role of PGEs in their origin. The analyzed samples of sheared and granular peridotites differ in mineral-liquid equilibrium temperatures and pressures: 1230–1350 °C, 56–67 kbar inferred for the former and 750–1300 °C, 30–67 kbar for the latter. The contents of major oxides, trace elements, and PGEs differ both between sheared and granular peridotites and within each group. Some sheared and all granular peridotites are enriched in incompatible elements, have low percentages of clinopyroxene and contain garnet rich in middle rare-earth elements (MREE). These features result from interaction of the peridotites with melts and related fluids of presumably carbonate compositions containing incompatible elements in high abundances. Sheared peridotites were more strongly enriched and gained more Ca, Al, and Ti than the granular variety. On the other hand, granular peridotites have higher light REE (LREE) and lower heavy REE (HREE) enrichment, which may record compositional difference of metasomatic agents. Metasomatism acted in two stages: first silicate agents caused LREE enrichment and HREE depletion of melts and then the fractionated melts percolated through the mantle and affected the compositions of granular peridotites and, partly, sheared peridotites. The two types of peridotites differ also in PGE patterns, with largely variable Os–Pt concentrations in granular peridotite. This difference may be due to uneven distribution of the Os–Ir and Pt–Os–Ir intermetallic compounds, which formed as sulfide melting residue in the lithospheric mantle. Sheared peridotites show almost uniform Os–Pt distribution and are enriched in Pd, Re, and partly Pt. The enrichment possibly occurred during metasomatism when the percentages of garnet and clinopyroxene increased while sulfide phases precipitated into interstitials.

Dating and geochemistry of zircon and apatite from rhyolite at the UNESCO geosite Rupnica (Mt. Papuk, northern Croatia) and the relationship to the Sava Zone

The Rupnica geosite, a key locality of the UNESCO-protected Papuk Geopark in northern Croatia, is well-known for an excellent exposure of columnar jointing in volcanic rock. This rock is defined as an albite rhyolite that comprises almost pure albite phenocrysts within a fine-grained matrix composed of microphenocrysts of albite, quartz and devitrified volcanic glass. Primary accessory minerals are clinopyroxene, apatite, zircon and magnetite. Haematite, apatite and anatase were found as inclusions in zircon. The albite rhyolite is characterized by a highly siliceous, peraluminous, oxidized (ferroan), dry, alkali-calcic to alkalic composition, with low CaO, MgO, and MnO contents and high FeOT/(FeOT+MgO) ratios. Normalized trace element contents display positive anomalies of K, Pb, and Zr as well as negative anomalies of Nb, P, Ti, Ba and Eu, together with an enrichment of light rare-earth elements (REE) relative to heavy REE. Zircon from the rhyolite of Rupnica is characterized by ratios of Th/U=1.13 and Zr/Hf=55 and contents of HfO2=1.04 wt. % typical for an early-stage igneous zircon crystallized from a dry high-temperature magma in a deep magma chamber. Apatite REE patterns show enrichment of light REE over heavy REE and a pronounced Eu anomaly, typical for apatite from granitoids formed in an oxidizing environment. The magma is of A-type and was generated at high temperatures at 800–900 °C by partial melting of lower- to mid-crustal rocks. The age of the albite rhyolite of Rupnica is Late Cretaceous at 80.8±1.8 (2σ) Ma, according to U-Pb dating of zircon, coeval with geochemically similar igneous rocks of Mt. Požeška Gora and Mt. Kozara within the Sava Zone.

The dynamic formation process of the CB chondrite Gujba

The many unique characteristics of CB chondrites have resulted in the impact hypothesis becoming the favoured model for their formation. Here, we further investigate the formation mechanisms of CB chondrites by analysing the elemental and K isotope compositions of chondrules and bulk fractions from the CBa chondrite Gujba. Similar to previous work, the refractory element ratios in the Gujba chondrules show evidence of a differentiated precursor, with the Nb/Ta, Zr/Hf, Sc/Th and Zr/Th ratios showing fractionation relative to other chondrites. In addition, the bulk fractions, and to a lesser extent the chondrules with attached matrix and metals, display significantly more refractory element fractionation and a large enrichment in light REEs. Based on EDS elemental mapping and comparisons with previous studies, the most likely source of this highly fractionated material appears to be the small amount of heterogeneously distributed interstitial fine-grained material within Gujba. These large refractory element fractionations (i.e., Nb/Ta, Zr/Hf, Sc/Th Zr/Th, and LREE/HREE) are best explained by a significant partial melting process such as crustal formation. Nevertheless, the mechanism of patrial melting cannot be conclusively determined with the data available here. The K isotopic compositions of the Gujba chondrules analyzed here range from −2.24‰ to −0.41‰ in δ41K, whereas the bulk analyses show δ41K values of −0.81‰ to −0.72‰. This range of chondrule K isotope compositions is significantly larger, and extends to much lighter compositions, compared to all other chondrites measured so far by bulk ICP-MS. In addition, the Gujba chondrules display a clear negative correlation of K isotopic composition with K concentration, with the chondrules showing the lightest K isotope compositions having the highest K concentrations. This distinctive correlation indicates that evaporation was likely the dominant process affecting the K isotopic variation observed in the Gujba chondrules. Nevertheless, the extremely light δ41K values seen in the most K-rich chondrules (which are lighter than any other early solar system material so far measured) indicate that incomplete condensation likely took place before evaporation. As such, we propose a two-stage model to explain the formation of chondrules in Gujba, with Stage 1 characterized by incomplete condensation of vaporized material with an average isotopic fractionation factor (α) of 0.9984 (when using the most K enriched chondrule to constrain the model), and Stage 2 representing partial evaporation in a vapor plume with an average α range of 0.9976 to 0.9990. Using these α values we calculate an approximate vapor saturation index value of 0.935 for condensation and between 0.903 and 0.960 for evaporation. This formation process requiring both condensation and evaporation for CB chondrules is consistent with an impact generated vapor plume and further expands our understanding of CB chondrite formation.

Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation

Alkaline rocks are generally enriched in rare metals (e.g., Nb, Ta, and Zr) and rare earth elements (REE), but the key factors controlling Nb-Ta-REE enrichment remain unclear. The Boziguoer Nb (Ta-Zr-Rb-REE) deposit in Southwest Tianshan (northern margin of Tarim Basin) is China’s largest, with reserves of 0.32 Mt Nb2O5 and 0.02 Mt Ta2O5. It is an alkaline felsic complex 4.45 km in length and 0.5–1.3 km in width, composed of alkalic granite and syenite, which can be subdivided into syenite I and syenite II. The main minerals in each lithofacies are the same (albite, K-feldspar, quartz, arfvedsonite and aegirine). The Nb in the deposit is mainly hosted in pyrochlore supergroup minerals, ubiquitous in alkalic granite and syenite of the Boziguoer deposit. The wide variation in cations (Ca, Na, REE, U, Th) in the A-site further classifies the Boziguoer pyrochlore supergroup minerals as fluornatropyrochlore, fluorcalciopyrochlore and fluorkenopyrochlore. All Boziguoer pyrochlore supergroup minerals are Nb-rich and Ta-poor at the B-site and dominated by F at the Y-site. These cation occurrence illustrate a new mechanism of substitution in the Boziguoer pyrochlore supergroup minerals (2Ca2+ +Ti4+ +4Ta5+ = REE3+ +A-V + 5Nb5+, where A-V is the A-site vacancy). This substitution mechanism is different from that in the pyrochlore supergroup minerals from other rocks such as carbonatite and nepheline syenite, which are dominated by the replacement of Ba (Rb, Sr) with Ca+ Na + A-V. In addition, the substitution of REE (mainly La, Ce) for Ca in the Boziguoer pyrochlore supergroup minerals is likely a result of either REE enrichment or a change in the REE partition coefficient during the evolution of the alkaline magma. Both the pyrochlore supergroup minerals and their host rocks display negative large ion lithophile element (LILE; K, Rb, Sr, and Ba) anomalies, positive high-field-strength element (HFSE) anomalies and light rare earth element (LREE) enrichment with negative Eu anomalies. This is consistent with the crystallization of the pyrochlore supergroup minerals from the magma rather than from hydrothermal fluids, suggesting a magmatic origin. These findings indicate that the mechanisms of pyrochlore supergroup minerals crystallization in alkaline magma may be significantly different from those in carbonatite and nepheline syenite, and that magmatic differentiation processes may have played a role in the enrichment of the Boziguoer deposit by Nb.

Petrogenesis and Tectonics of Eocene–Oligocene Phonolites of Mecejana, Ceará, NE Brazil: the Role of the Fernando de Noronha Fracture Zone, Equatorial Atlantic

Abstract Located on the western continental tip of the Fernando de Noronha transform fracture (FNTF), northeastern Brazil, the Eocene–Oligocene Mecejana Phonolitic Province (MPP) was subjected to a multi-methodological study, comprising field and microscopic characterization, mineral and whole-rock chemistry, and Sr-Nd isotopes. These rocks intrude the Precambrian crystalline basement, forming mainly plugs oriented in the NE–SW direction. They are composed of alkali basalt, phono-tephrite, tephri-phonolite, and phonolite with phenocrysts of nepheline and sanidine, and minor diopside, aegirine-augite, aegirine, kaersutite, biotite and titanite. Chemically, they are sodic (Na2O/K2O &amp;gt; 1) and display Light Rare Earth Elements (LREE) enrichment, concave upward or flat Heavy Rare Earth Elements (HREE) patterns, weak to strong Eu anomaly, and high contents of Nb, Ta and Zr. They delineate two chemical groups, the less-evolved one comprising basalt to tephri-phonolites and the more evolved constituted by phonolites. Oxide and trace element modeling points out to an evolution by fractional crystallization, leaving olivine-rich gabbro and foidolite/nepheline syenite cumulates, respectively. The analyzed samples have Sr-Nd isotopic ratios akin to ocean island basalt (OIB) rocks (positive εNd of +3.0 to +0.9 at 30 Ma) and low to high 87Sr/86Sri (at 30 Ma) of 0.70264–0.70955. The data suggest a three-stage evolution model for the MPP: (1) &amp;lt;5% partial melting of a metasomatized phlogopite-amphibole-garnet peridotite generates an alkali basalt melt, (2) polybaric differentiation of this melt at crustal level (~570–220 MPa, 1040–645°C) leads firstly to phono-tephrites and tephri-phonolites and then to (3) foidolite and nepheline syenite. Petrographic, chemical, and isotopic characteristics of the MPP and similar rocks of the Fernando de Noronha islands suggest the control by the FNTF and reactivation of NE–SW trending deep-rooted Precambrian faults allowing channeling and emplacement of the alkaline magmas.