Similar Isotopic Compositions Research Articles

The effect of supercritical fluids on Nb-Ta fractionation in subduction zones: Geochemical insights from a coesite-bearing eclogite-vein system

Supercritical fluids are ideal media for mass transfer from the subducting slab into the mantle wedge. However, little is known about the role of natural supercritical fluids in subduction zones. A combined study of petrology, geochemistry and zirconology was carried out for a coesite-bearing eclogitic vein and its surrounding eclogites from the Dabie orogen, one of the typical continental subduction zones on Earth. The results are used to reveal the composition and source of supercritical fluids, the P-T conditions and timing of their formation and their geochemical effects in subduction zones. The eclogitic vein is composed of garnet, omphacite, quartz, amphibole, rutile and apatite, with minor amounts of epidote, plagioclase and zircon. Coesite was identified in omphacite in the vein, demonstrating its formation under ultrahigh-pressure (UHP) metamorphic conditions that correspond to subarc depths in oceanic subduction zones. Zircons from the vein give concordant U-Pb ages of 225 ± 5 Ma and flat HREE patterns for newly grown rims, consistent with their crystallization at the UHP eclogite facies. The vein shows similar Hf-O isotope compositions to the host UHP eclogites, indicating that the UHP vein-forming fluid is internally buffered within the UHP eclogites. Minerals in the vein contain not only higher contents of Cr, Ni, Sr, REE and HFSE, but also multiphase crystal inclusions (such as omphacite, quartz, epidote, apatite, amphibole, plagioclase, mica, rutile, calcite, and anhydrite) as well as liquid and gas phases of H2O. The calculated fluid composition of major elements is 31 wt% SiO2, 20 wt% CaO, 10 wt% (SO4)2−, 8 wt% (CO3)2−, 8 wt% FeO, 7 wt% Al2O3, and 9 wt% H2O, with traces of Na2O, K2O and TiO2. This composition is responsible for the UHP veining from supercritical silicate-rich fluids in equilibrium with peak UHP minerals. The P-T-t path of the UHP eclogite-vein system indicates that the second critical endpoint of the basalt-H2O system is located close to 3.4 GPa and 770°C. The high contents of sulfate and carbonate in the vein suggest that supercritical fluids were oxidized and likely a predominant agent to transfer sulfur and carbon at subarc depths. High Nb/Ta ratios of rutile crystals in the vein and quantitative modelling indicate that dehydration of subducting eclogites at subarc depths can produce supercritical fluids with suprachondritic Nb/Ta ratios. The UHP eclogites and mantle wedge peridotites metasomatized by supercritical fluids can acquire suprachondritic Nb/Ta ratios and thus provide a complementary reservoir to balance the subchondritic reservoirs on Earth. Based on a statistics of Nb-Ta data, two indices are proposed to identify the existence of supercritical fluids in fossil subduction zones: (1) rutile crystallized from supercritical fluids shows lower Nb contents than that from aqueous solutions and hydrous melts; (2) UHP eclogites associated with supercritical fluids exhibit a distance >0.1 in their Nb-Ta compositions to the line defined by basalts in the plot of log[Nb] vs log[Ta]. Therefore, the existence of supercritical fluids during UHP metamorphism at subarc depths can be recognized from their effect on the mobility of fluid-immobile trace elements in UHP metamorphic rocks.

Compound specific isotope evidence points to use of freshwater resources as weaning food in Middle Neolithic Paris Basin

ObjectivesA clear understanding of past weaning practices can provide invaluable insights into social issues such as infant care, fertility rate, and demographic patterns in past societies. This study presents the first archeological research employing compound specific isotope analysis (CSIA) for the reconstruction of past weaning practices.MethodsWeaning practices of two Middle Neolithic communities in the Paris Basin region: Balloy (BLR) and Vignely (VPB), are evaluated by combining previously published bone collagen stable carbon, nitrogen, and sulfur (n = 66) isotope analysis with new compound specific carbon and nitrogen isotope compositions of bone collagen (n = 10).ResultsOur results demonstrate that the diets of individuals from BLR and VPB likely incorporated freshwater resources. The signals of freshwater resources consumption are even stronger among subadults, suggesting that freshwater resources were used as weaning food at these sites.ConclusionsThe implications of our result are threefold. Currently many CSIA studies in archeology only involve either carbon or nitrogen. Our data shows that it is important to conduct CSIA on both carbon and nitrogen for a more integrated picture. Secondly, our data demonstrates that the use of a protein‐based weaning food—instead of a starch‐based weaning food (such as cereal gruel)—was likely more prevalent among the Middle Neolithic communities in the Paris Basin Region than previously thought. The finding thus prompts a rethinking of the role of protein‐based weaning food in other archeological contexts. Lastly, the common assumption that weaning foods and adult diets share similar isotopic compositions can be problematic, as the use of protein‐based, high trophic‐level weaning foods can skew the δ 15N weaning curve and produce an erroneously late estimation for weaning ages.

Petrogenesis of early cretaceous intermediate to felsic rocks in Shanghai, South China: Magmatic response to Paleo-Pacific plate subduction

Voluminous magmatism and extensional deformation are the prominent late Mesozoic tectonic features in East Asia, which recorded the evolutionary history of the continental marginal lithosphere. Due to the thick covering of Quaternary and Pliocene sediments in the region, the Early Cretaceous magmatism in northeastern South China is not well constrained. In this study, we collected samples from the boreholes in Shanghai, East China and provided geochronological, elemental and isotopic analyses of andesite and granodiorite. Zircon UPb age results indicate that andesites and granodiorites were formed at ca. 135 to 102 Ma. The andesites exhibit moderate MgO contents, slightly radiogenic Sr (87Sr/86Sr(t) = 0.706149 to 0.706170) and variable unradiogenic Nd and Hf isotopic compositions (εNd(t) = −5.32 to −5.12; εHf(t) = −10.8 to −0.6). The granodiorites exhibit arc-like trace-element patterns with high Sr/Y ratios, as well as slightly radiogenic initial Sr (87Sr/86Sr(t) = 0.706754 to 0.706790) and moderately unradiogenic Nd and Hf isotopic compositions (εNd(t) = −6.54 to −6.27; εHf(t) = − 4.0 to −6.7). Both andesites and granodiorites exhibit similar initial Pb isotopic compositions (andesites: 206Pb/204Pb = 18.232–18.259, 207Pb/204Pb = 15.547–15.548, and 208Pb/204Pb = 38.338–38.350; granodiorites: 206Pb/204Pb = 17.930–17.946, 207Pb/204Pb = 15.542–15.543, and 208Pb/204Pb (38.256–38.293). The results recommend that the ca.130 Ma andesites were produced by the partial melting of an enriched lithospheric mantle, which was previously metasomatized with sediments in the source region in a continental arc setting of the Paleo-Pacific subduction. The ca.102 Ma adakitic granodiorites probably originated from partial melting of the lower crust with the addition of mantle-derived magmas. With the consolidation of previous studies, we argue that the flat slab subduction of the paleo-Pacific Plate led to the formation of these intermediate to felsic magmatism.

Strontium, Hydrogen and Oxygen Behavior in Vent Fluids and Plumes from the Kueishantao Hydrothermal Field Offshore Northeast Taiwan: Constrained by Fluid Processes

Strontium (Sr), hydrogen (H) and oxygen (O) in vent fluids are important for understanding the water–rock interaction and hydrothermal flux in hydrothermal systems. We have analyzed the Sr, H and O isotopic compositions of seawater, vent fluid and hydrothermal plume samples in the Kueishantao hydrothermal field, as well as their calcium (Ca), total sulfur (S), Sr, arsenic (As), stibium (Sb), chlorine (Cl) and manganese (Mn) concentrations for understanding the origin and processes of fluids. The results suggest that most As, Sb and Mn are leached from andesitic rocks into the fluids, and most Ca and Cl remained in the deep reaction zone during the fluid–andesitic rock interaction. The ranges of 87Sr/86Sr, δDV-SMOW and δ18OV-SMOW values in the yellow spring, white spring and plumes are small. The 87Sr/86Sr, δDV-SMOW and δ18OV-SMOW values of fluids and plumes are like those of ambient seawater, indicating that the Sr, H and O of vent fluids and hydrothermal plumes are derived primarily from seawater. This suggests that the interaction of andesite and subseafloor fluid is of short duration and results in the majority of As, Sb and Mn being released into fluids, while most Ca and Cl remained in the deep reaction zone. In addition, there was no significant variation of Sr, H and O isotopic compositions in the upwelling fluid, keeping the similar isotopic compositions of seawater. There are obvious correlations among the pH values, As and Sb concentrations, and H isotopic compositions of the vent fluids and hydrothermal plumes, implying that the As and Sb concentrations and H isotopic compositions can trace the dispersion of plumes in the ambient seawater. According to the Sr concentrations and 87Sr/86Sr values, the water/rock ratios are 3076~8124, which is consistent with the idea that the interaction between fluid and andesite at the subseafloor is of short duration. The hydrothermal flux of Sr discharged from the yellow spring into the seawater is between 2.06 × 104 and 2.26 × 104 mol/yr, and the white spring discharges 1.18 × 104~1.26 × 104 mol/yr Sr if just andesites appear in the reaction zone.

Molybdenum isotopic constraint from Java on slab inputs to subduction zone magmatism

Molybdenum isotope is a diagnostic tracer for crustal and mantle components in arc magmatism. However, the mechanism of Mo isotopic variation in arc magmas is still debated, e.g., input of different subduction components into the mantle wedge versus isotopic fractionation during dehydration of subducted slab. Here we present whole-rock Mo-Sr-Nd-Hf-Pb isotopic data for the Continental Arc Basalt (CAB) and Back Arc Basalt (BAB) from Java, Indonesia, to investigate the role of slab inputs in Mo isotopic variation of the Sunda arc magmatism. The CAB samples have variable K2O contents (0.44–2.49 wt%) and are mainly classified as calc-alkaline series, while the BAB samples are shoshonitic with markedly high K2O contents (2.12–6.90 wt%) relative to the CABs. The Java CABs and BABs have similar Mo isotopic compositions (δ98/95Mo = −0.65 to −0.07‰ and −0.66 to −0.07‰, respectively, relative to NIST SRM3134), suggesting that such a significant Mo isotopic variation should not be caused solely by the isotopic fractionation during the subduction. Instead, δ98/95Mo values of the Java basalts positively correlate with Pb isotopic ratios. This implies that the Mo isotopic variations in the Java arc rocks should result from the metasomatism in the mantle wedge by hybrid agents, including varying proportions of melts from subducted sediments (with heavy Mo isotope) and melts from the subducted altered upper oceanic crust (SAOC) (with light Mo isotope). The light Mo isotope of the Java arc rocks, compared with the Mariana arc basalts, suggests that melts from the SAOC have much lighter Mo isotopic compositions than the components from the lower oceanic crust. Thus, Mo isotope has great potential to distinguish the components from the subducted upper and lower oceanic crust. The Java CABs show along-arc variations in Mo-Sr-Nd-Hf-Pb isotopes, which is related closely with the thermal status of the subducted slab. Upwelling of the asthenosphere due to the slab tearing beneath the Java arc might have enhanced the partial melting of subducted sediments nearby the slab window. The complicated subduction system in the Sunda arc has strongly controlled the geochemical composition of arc magmas, which changes with input of different subduction components into the mantle wedge along arc.

Zinc isotope constraints on the formation of sedimentary exhalative (SEDEX) ore deposits: New evidence from the Franklin, NJ mining district

We report δ66Zn for the high temperature metamorphic zinc oxide and silicate minerals franklinite (Fr) (Zn2+Fe3+2O4), zincite (Zc) (ZnO) and willemite (Wlm) (Zn2SiO4) from the Franklin, NJ, historic mining district. With reference to the JMC-Lyon standard, δ66Zn franklinite ranges from −0.10 to 0.48‰ with an average of 0.20 ± 0.17‰ (n = 22). δ66Zn willemite ranges from 0.23 to 0.48‰ with an average of 0.37 ± 0.09‰ (n = 7). δ66Zn zincite ranges from 0.29 to 0.60‰ with an average of 0.47 ± 0.12‰ (n = 9). These data suggest that the analyzed phases fractionate heavy zinc in the order Fr < Wlm < Zc. Taken as a group, these minerals have an average δ66Zn of 0.30 ± 0.19‰. This is0.16‰ heavier than an estimated global mean δ66Zn for sphalerite (ZnS) of 0.12 ± 0.16‰ for seafloor zinc deposits. Our results are consistent with fractionation factors that predict that Zn oxides and silicates (protoliths of these ores) should be isotopically heavier than sphalerite when precipitated from fluids of the same temperature with similar zinc isotope compositions. Our samples from Sterling Hill are taken from two short transects across the orebody. Calcite (Cal) (CaCO3) from the same samples has δ13C from −0.54 to 1.46‰ with an average of 0.79 ± 0.51‰ (VPDB; n = 30), while δ18O ranges from 9.72 to 15.12‰ with an average of 12.42 ± 1.35‰ (VSMOW; n = 30). Results for these two isotope systems are consistent with earlier studies. δ13C decreases smoothly with distance towards the west along both transects; δ18O in contrast stays very close to its mean on the longer, southern transect but increases with distance to the west along the shorter, northern transect. There is no apparent covariation between δ66Zn and either of the other isotopic ratios measured.

Molecular and carbon isotope composition of hydrocarbons from ambers of the Eocene Shenbei coalfield (Liaoning Province, NE China)

The molecular and (bulk and compound-specific) stable carbon isotope composition of ambers from the Shenbei coalfield, northeastern China, were analysed to determine their origin and palaeobotanical affinitions. The applicability of the δ13C data of saturated diterpenoid hydrocarbons for palaeoenvironmental reconstruction was addressed by comparison with published carbon isotope ratios of diterpenoids from adjacent areas. The detected sesquiterpenoids are common biomarkers for the gymnosperm families of Pinaceae and Cupressaceae. The diterpenoids dominate the saturated hydrocarbon fraction with high abundances of kaurane-type compounds. Abietane-type, cadalene-type and alkylated compounds are the main biomarkers in the aromatic hydrocarbons in the ambers. Proposed typical biomarkers for amber (e.g., amberene and homoamberene) have also been detected and probably derived from the diagenesis of the abietane-type terpenoids in the studied ambers. The biomarker composition suggest that the ambers probably originated from gymnosperms such as Taxodium and Pinus. The δ13C values of bulk amber and individual compounds fall within the range of ancient ambers, but the δ13C values of bulk amber are isotopically more enriched than modern ambers. The δ13C values of compounds in hydrocarbon fractions vary in broad ranges. In the saturated hydrocarbon fraction, the δ13C value of sesquiterpenoids (–28.0‰ to –24.5‰) fluctuates more than that of diterpenoids (–22.5‰ to –21.8‰), and the average δ13C value of the latter is approximately 4‰ higher than the former. Aromatic hydrocarbons are on average enriched in 13C by around 2‰ compared to the δ13C of non-aromatic compounds, the result is influenced by isotopically light sesquiterpenoids in the saturated fraction. The similar carbon isotope compositions of aromatic abietane-type compounds and abietane confirm that progressive aromatization does not have a significant impact on the δ13C value of compounds. The obtained higher δ13C values of saturated diterpenoids in contrast to that of lower Eocene lignite from the nearby regions provide evidence for growth of the precursor plants of the amber in dryer habitats or differences in precipitation distribution between different locations and ages.

Sub-Bottom Profiler and Side Scan Sonar investigations, with the assistance of hydrochemical and isotopic analysis of Sawa Lake, Al-Muthana Governorate, Southern Iraq

A marine geophysical survey are including Sub Bottom Profilers and Side Scan Sonar integrating with under-water imagery, as well as hydrochemical and isotopic investigations have been carried out for Sawa Lake, one of the unique, shallow and closed water body in Iraq. The results showed that the depth of the Lake is shallow, ranged between 1 m in areas near the edges of the Lake and 2 m in the Lake center. The Recharge site has been detected exactly (this is a first recording (Nov. 2012)), as a hole. It has a longitudinal an axis of 50 m with NW-SE tren, which resembles the trending of the longitudinal axis of the Lake and Abu-Jir fault trending. The maximum width of this hole ranging from 23.0-29.5 m. The hole depth about 20 m, but may be greater then, because the emergence of a high proportion of noise in the geophysical sections. The Lake bottom is homogeneity and does not has any diacritical features. Also, the results showed discontinuous layers on either side of the hole due to the existance a number of caves under the bottom of the Lake extends into several meters with different dimensions and forms. Hydrochemical analysis of the water samples collected from different depths inside the hole by scientific diving with specefic equibments and the result showed that the water is NaCl type, with domination of Mg and Cl over Ca and SO4. The Stiff diagram, detected that very high TDS arising mainly from sodium and chloride, it is mostly typical marine water. Values of δ18O and δ2H in water samples of Sawa Lake, when plotted along the global meteoric water line shows that the three collected water samples from the lake, which have a similar isotopic composition, indicating that all of them are receiving water from the same flow zones, with diffuse recharge type. Also, the isotopic parameter showed that the Sawa lake water suffering from strong evaporation.

Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth's Core Formation and Differentiation of the Ureilite Parent Body

AbstractWe report the first Ni and Cr stable isotope data for ureilite meteorites that are the mantle residue of a carbon‐rich differentiated planet. Ureilites have similar Ni stable isotope compositions as chondrites, suggesting that the core‐mantle differentiation of ureilite parent body (UPB) did not fractionate Ni isotopes. Since the size of Earth is potentially larger than that of UPB; with diameter &gt;690 km), resulting in higher temperatures at the core‐mantle boundary of Earth, it can be predicted that the terrestrial core formation may not directly cause Ni stable isotope fractionation. On the other hand, we also report high‐precision Cr stable isotope composition of ureilites, including one ureilitic trachyandesite (ALM‐A) that is enriched in lighter Cr stable isotopes relative to the main‐group ureilites, which suggests that the partial melting occurred on UPB. The globally heavy Cr in the UPB compared to chondrites can be caused by sulfur‐rich core formation processes.

Thallium isotope compositions of subduction-zone fluids: Insights from ultra-high pressure eclogites and veins in the Dabie terrane, eastern China

Dehydration of the subducted slab is a crucial process in the generation of arc magmas. However, the mineralogical controls of slab dehydration remain uncertain. Thallium (Tl) isotopes have emerged as tracers of subducted slab components, but the Tl isotope characteristics of slab fluids are not known. High-pressure metamorphic veins and host eclogites in the Dabie terrane (China) provide important information on the composition and evolution of subduction zone fluids. In this study, we present the first Tl concentration and Tl isotope data for two high-pressure metamorphic epidote-rich veins formed through breakdown of lawsonite, as well as host eclogites and phengite separates, from the Ganghe and Hualiangting areas. The high Tl concentrations in phengite separates and identical Tl isotope compositions between phengites and their host bulk eclogites demonstrate that phengite controls the overall inventory of Tl in the host eclogites. Generally, the vein samples from both areas have much lower Tl concentrations than their host eclogites, suggesting that the fluids from which the veins precipitated were derived from phengite-bearing eclogites that retained Tl in the residue. The Ganghe eclogites display similar Tl isotope compositions compared with Ganghe omphacite-epidote vein. This similarity indicates that the vein-forming fluid was likely derived from the host eclogites, and that Tl isotopes did not fractionate during fluid-release. Three generations of veins in the Hualiangting area record variable Tl isotope values and were likely derived from multi-stage dehydration of heterogeneous host eclogites, some of which may have contained a sediment component. We argue that fluids derived from eclogite dehydration are likely characterized by higher Cs/Tl ratios compared to their host eclogites. Arc magmas that display high Cs/Tl (i.e. >15) that cannot be caused by variations in slab source composition, coupled with relatively low Ba/Th ratios, are likely to directly record the presence of residual phengite at subarc depths.

Re-assessment of the effect of fractional crystallization on Mo isotopes: Constraints from I-type granitoids and their enclosed mafic magmatic enclaves

In recent years, considerable progress has been made on the study of Mo isotopes in high-temperature geological processes. However, it is still controversial whether Mo isotope fractionation occurs during magmatic differentiation. Here we reassess the effect of magma differentiation on Mo isotope fractionation using published data in conjunction with our new analysis of Mo isotopes in well-characterized I-type granitoids and their enclosed mafic magmatic enclaves (MMEs) from two plutons in the North Qilian Orogen. The MMEs in each pluton, representing earlier cumulates with greater modal proportions of amphibole and biotite, show similar Mo elemental and isotopic compositions to their host granitoids. The absence of covariation of Mo isotopes with Dy/Dy*, Fe2O3 and K/Rb in both plutons further indicates that fractional crystallization of amphibole, biotite and Fe3+-rich minerals does not fractionate Mo isotopes in these granitoids. On the basis of re-assessment of published and our newly obtained Mo isotope data, we find that there is no clear evidence for Mo isotope fractionation during fractional crystallization in igneous rock. Instead, Mo isotopes of our samples correlate positively with Sr isotopes and Rb/La ratios, and negatively with Nd isotopes and Ce/Pb ratios, indicating clearly that magma source compositional variation controls the variation of Mo isotope compositions of these granitoids.

Trace element evolution of magnetite in iron oxide-apatite deposits: Case study of Daling deposit, Eastern China

In recent years, magnetite trace elements have increasingly been applied in studies into ore deposits. The magnetite trace element characteristics of different deposit types have been summarized, but to date no satisfactory explanation has been obtained as to why magnetite samples from some iron oxide-apatite (IOA) deposits exhibit similar trace element characteristics to those in iron-oxide copper gold (IOCG) deposits, or even skarn deposits. The Daling IOA deposit is located in the Luzong volcanic basin, Eastern China. Multi-stage magnetite have developed within the deposit; which provides a good opportunity to study the evolution process of magnetite trace elements in an IOA deposit. Here, sensitive high-resolution ion microprobe (SHRIMP) in situ sulfur isotope analysis was carried out for pyrites at different depths in the deposit. Pyrites at different locations exhibited similar sulfur isotope compositions, falling within the magmatic sulfur range (0.1‰–5.2‰), indicating that the ore-forming fluid of the Daling deposit was not affected by the Triassic evaporite, and that the trace element characteristics of its magnetite were mainly controlled by its magmatic-hydrothermal evolution process. Magnetite within the deposit can be divided into three sub-stages. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis revealed that magnetite (Mag-a) in vertical veinlets exhibits the highest Ti, V, Mn, and Zn contents; magnetite (Mag-b), which is associated with diopside and tremolite in coarse veinlets, exhibits the highest Mg, Al, Co, and Ni contents and brecciated magnetite (Mag-c) exhibits the lowest V content, the highest Mn and Zn contents, and similar Mg, Al, Ti, and Co contents to those of Mag-b. Comparing the trace elements of the three types of magnetite revealed that hydrothermal fluid can assimilate and absorb albite-diopside altered rock that forms in the early stage, resulting in hydrothermal fluid that is rich in Mg, Al and Si. Thus, magnetite samples in IOA deposits can exhibit similar features to those in skarn deposits through multi-stage, long-distance fluid evolution. Therefore, before using magnetite trace elements to discuss the characteristics of hydrothermal fluid, it is important to carefully investigate the basic geological characteristics of a given deposit and the mineralogical characteristics of its magnetite.

Distribution of Pb isotopes in different chemical fractions in bed sediments from lower reaches of the Xiangjiang River, Hunan province of China

This paper reports Pb isotopes in different fractions following the three step BCR and 1 M HCl extractions on river sediments from lower reaches of the Xiangjiang river in China, and highlights the importance of Pb isotopes in heavy metal contamination assessment. Lead concentrations and Pb isotopes in bulk sediments and sediment fractions (leachates and residues) from the river were analysed using ICP-MS techniques. Results showed that sediments were highly enriched with Pb with enrichment factors >5.5, while Pb in sediments was dominated by reducible and residual Pb fractions, residing mainly in Fe-oxide and silicate minerals. Pb isotopes in sediments was characterized by radiogenic Pb produced from the decay of uranium and thorium with 206Pb/207Pb ratios of 1.1744 for less radiogenic Pb and 1.1816 for more radiogenic Pb. The leachates and residues from BCR extraction generally had similar Pb isotope compositions, of which the 206Pb/207Pb ratios were 1.1798 ± 0.002 and 1.1844 ± 0.008 respectively. Differentiation of Pb isotopes between BCR leachates and residues was insignificant. However, differentiation between leachates and residues using 1 M HCl extraction was significant, as shown by average 206Pb/207Pb ratios of 1.1746 ± 0.005 and 1.1858 ± 0.008 for leachates and residues respectively. Pb isotopic tracing suggests that Pb in sediments from Zhuzhou section arose from the mixing of anthropogenic Pb from coal combustion (39%) and mining-smelting for Pb-Zn ores (58%); while Pb in sediments from Xiangtan, Changsha and Xiangyin sections arose from the mixing of anthropogenic Pb from mining-smelting for Pb-Zn ores (54%), and lithologically inherited Pb from granite weathering (35%) with a small amount of contribution from coal combustion (10%). The present study suggests that the BCR extraction scheme was not appropriate for ecological risk assessment of heavy metal contamination in mining-impacted (ore-Pb dominated) river sediments.

Geochemical composition of dykes along the Cameroon Line (CL): Petrogenesis and similarities with the Central Atlantic Magmatic Province

We report whole-rock geochemistry and Sr–Nd–Pb isotopic compositions of mafic dykes intruded in the Precambrian granito-gneissic basement complex, exposed at Nyos, Batibo, Dschang and Foumban on the Cameroon Line. The dykes are alkaline (Batibo), transitional (Foumban), and subalkaline (Nyos, Batibo and Dschang) with SiO2 of 45–54 wt% and MgO of 2–9 wt%, similar to dykes reported in other areas of the Cameroon Line (CL) and the Central Atlantic Magmatic Province (CAMP). The abundances of rare earth elements (REE) and the Primitive Mantle normalised patterns for the Nyos, Batibo and Dschang dykes are similar to those of MORB, indicating that the dykes formed at shallower depths by a higher degree of partial melting relative to the Foumban dykes and the alkaline lavas of the CL. The transitional basaltic dykes with steeper REE patterns have their sources at deeper levels in the lithospheric mantle, possibly the garnet-spinel transition zone and were generated by a lower degree partial melting of the lithospheric and plume components. The Nyos and Batibo subalkaline dykes show similar isotopic compositions with a spectrum extending from depleted (DMM-like) to enriched (EM1-like) mantle, indicating the similarity in their source components. The Dschang dykes show distinct isotopic characteristics with relatively unradiogenic Nd-Pb isotope compositions compared to the Batibo and Nyos dykes. The Foumban transitional dykes with characteristic wide ranges in Sr-Nd-Pb isotopic compositions reveal varying contributions from enriched mantle components (EM1 and EM2) in addition to its plume signature similar to those of CL lavas. The Nyos and Batibo dykes alongside other dykes on the CL have low TiO2 abundances (<2 wt%), negative PM-normalised Nb-anomalies, and moderately to strongly enriched REE patterns, and isotopic composition that overlaps with those of CAMP, suggesting a similar lithospheric origin.

Bioclast-controlled patchy barite cementation – Origin and impact on reservoir properties in deeply buried Upper Jurassic sandstones, North Sea

Reservoir non-destructive patchy barite cement occurs in volumes up to 16 vol% in deeply (>5150 m) buried shoreface sandstones in the Hejre field, Danish Central Graben, North Sea. Barite typically occurs as an accessory cement in the sandstones of the Upper Jurassic Heno Formation; however in the Hejre field, barite appears more abundant in the crestal-most well than in other wells. The purpose of this study is to describe the characteristics and the origin of patchy barite cement besides implications for reservoir properties. Petrographic investigations by optical and scanning electron microscopy supplemented by isotopic analysis by SIMS (secondary ion mass spectroscopy) were applied to reveal the barite cementation process. Porosity and permeability of the sandstones were measured by conventional core analysis. Barite cement occurs in the Hejre field with three morphologies: crystals arranged in rosettes, crystal fillings of bioclasts, and multiple-oriented intergrown small crystals in veins. SIMS showed similar sulphur and oxygen isotopic compositions for these barite morphologies, and this composition resembles evaporite-derived barite cement. A hydrothermal origin of the fluid is supported by the associated precipitation of pyrite, chalcopyrite, sphalerite and galena, and a high Sr/Ba ratio in barite. Conventional core analysis revealed major reductions in porosity and permeability when barite cement occurs as fracture filling or in combination with calcite cement in shell lags. However, these occurrences are rare and have a minor influence on the overall reservoir properties. Minor reduction of porosity and permeability characterizes sandstones with barite cement in rosettes of radiating crystals. This growth pattern was probably promoted by slow seep of sulphate-rich fluids which, by mixing with barium-bearing formation water became super-saturated with respect to barite as they moved towards lower temperature and pressure zones. Faults acted as pathways for transport of sulphate-rich fluids possibly from evaporite deposits, whereas barium may have originated from compactional fluids leached out of underlying volcaniclastic rocks. Thermochemical sulphate reduction led to precipitation of sulphide minerals, minor sulphur fractionation and fluid acidification, whereby calcareous bioclasts dissolved. Barite precipitated when the fluids entered these large- or over-sized pores with small obstacles causing prolonged fluid-rock interaction. The bioclast-replacing and patchy barite cement caused only limited reduction of reservoir porosity and permeability compared with barite-free sandstones from the Heno Formation found at a similar burial depth.

Storage Changes Stable Isotope Composition of Cucumbers

Vegetable food stuff produced under controlled and identical conditions from one farm of identical “age” (batch) has a similar isotopic composition. This fact can be used to control the origin of vegetables. This question is of special relevance when food-contaminations have to be traced back to the producer, or certain production claims have to be controlled. However, as vegetables are harvested, brought to whole-sale merchants and to retail shops, where they remain until being bought by the consumer, one has to consider possible changes in isotopic composition during this transfer period, when comparing vegetables of questioned origin with reference samples taken directly from the field/producer. We investigated changes in the isotope composition of vegetables during storage by studying as an example cucumbers from one batch. We stored the cucumbers in a vegetable storage under controlled conditions and removed one sample every day and analyzed its isotopic composition. We found changes in the δ15N and δ18O isotope values over the investigated period of 21 days, with both parameters showing positive linear correlations, and maximum enrichments with time of more than 1.5‰ for δ15N and more than 2‰ for δ18O. However, within the interval the samples remained in a saleable condition the isotope variations remained more or less within the variability of the sample batch. Our study demonstrates that changes in the isotopic signature in vegetables might occur after harvest during storage and have to be taken into account when (commercial) samples collected in a market are investigated.

The gallium isotopic composition of the Moon

In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The δ71Ga values of these samples range from -0.10 to +0.66‰ (where δ71Ga is the relative difference between the 71Ga/69Ga ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than δ71Ga values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk δ71Ga value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have δ71Ga values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher δ71Ga values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution.Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Δ71Gaplagioclase-melt of -0.3‰, (where Δ71Gaplagioclase-melt is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have δ71Ga values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP.We estimate that the δ71Ga value of the LMO was ∼0.14‰ prior to the onset of plagioclase crystallization and extraction. Whether this δ71Ga value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 ± 0.06‰, Kato et al., 2017) and volatile loss drove the LMO to higher δ71Ga values prior to formation of the lunar crust.

δ53Cr values of catchment runoff exhibit seasonality regardless of bedrock Cr concentrations: Role of periodically changing chromium export fluxes

AbstractOne pre‐requisite for the construction of a global chromium isotope mass balance is detailed understanding of Cr isotope systematics in the critical zone where redox‐processes can modify the isotope signature of geogenic Cr input into the hydrosphere. A Cr isotope inventory of bedrock, soil, and runoff was performed in a Central European headwater catchment underlain by amphibolite, situated in the vicinity of two previously studied catchments underlain by different bedrock types (serpentinite and leucogranite). Fresh bedrock in the amphibolite catchment NAZ contained ~300 mg/kg Cr, serpentinite at PLB contained ~800 mg/kg Cr, and leucogranite at LYS contained ~2 mg/kg Cr. Monthly hydrochemical monitoring at all three sites revealed higher Cr(VI) export fluxes in winter than in summer. NAZ was characterized by a distinct seasonality in the δ53Cr values, with minima during winter/spring snowmelts (−0.35‰) and maxima during dry summers (0.40‰). Similar seasonality in δ53Cr values had been reported from PLB and LYS. Bedrock at all three sites had similar Cr isotope composition close to −0.10‰, a value indistinguishable from the δ53Cr value of bulk silicate Earth (BSE). Positive mean δ53Cr value of NAZ runoff indicated Cr‐isotope fractionations during weathering of geogenic Cr(III), combined with adsorption of the resulting Cr(VI) on soil particles during pedogenesis. However, the mass‐weighted mean δ53Cr of NAZ runoff was lower (−0.08‰), indistinguishable from the Cr isotope signature of bedrock. The same pattern of lower mass‐weighted mean δ53Cr values of runoff, compared to arithmetic mean δ53Cr values of runoff, were observed also at PLB and LYS. We suggest that elevated Cr runoff fluxes in winter remove some of the residual isotopically light Cr that accumulated in the soil during summer. Seasonality in runoff δ53Cr values appears to be a relatively widespread phenomenon, de‐coupled from Cr availability for chemical weathering.

Two‐Stage Mafic‐Felsic Magma Interactions and Related Magma Chamber Processes in the Arc Setting: An Example From the Enclave‐Bearing Calc‐Alkaline Plutons, Chinese Altai

AbstractA systematic data set of petrography, mineralogy, geochronology, and geochemistry is reported for typical arc granitoids and associated mafic microgranular enclaves (MME) from the Chinese Altai, Central Asian Orogenic Belt. Zircon U‐Pb dating indicates that they crystallized coevally at ∼395 Ma. Magma hybridization modeling suggests that the granitoid parental magma formed by mixing between a mafic and a felsic endmember. The sharp decrease of An values from cores to rims (e.g., from ca. 80 to 40) of plagioclase crystals points to polybaric crystallization accompanied by degassing. Petrographic evidence and plagioclase in situ Sr isotopic compositions ((87Sr/86Sr)i = 0.7053–0.7071) confirm the interaction of at least two isotopically different magmas during the mineral crystallization. A model for the formation of the enclave‐bearing calc‐alkaline plutons in arc settings is presented: At depth, mantle and crustal melting and efficient magma mixing controlled the principal chemical compositions of the granitoid intrusions. Then in the higher level chamber, decompression‐dominated crystallization and the mingling and limited mixing between blobs of mafic magma and the enclosing granitoid magma ultimately determined the texture, mineral composition, and enclave morphology. The MME have (87Sr/86Sr)i of 0.7048–0.7062, εNd(t) of −0.5 to +2.6, and zircon εHf(t) of +2.3 to +5.4, while the host granitoids have similar Sr isotopic compositions ((87Sr/86Sr)i = 0.7054–0.7064), but generally lower whole‐rock εNd(t) and zircon εHf(t) values (−2.2 to +0.4 and +0.6 to +4.6, respectively). The petrogenetic analysis and isotopic characters suggest that granitoid emplacement was a means of crustal growth in the Chinese Altai orogen.

Unraveling the petrogenesis of the Miocene La Peña alkaline intrusive complex, Mendoza, Argentina: Insights from the study of the disregarded late dykes

The La Peña Complex (LPC) is a silica-undersaturated alkaline potassic intrusive system, with a subduction-related signature, linked to the early Miocene retroarc magmatism of the Southern Central Andes, in the flat slab segment. The LPC is composed of several intrusions, predominantly plutonic (clinopyroxenite, malignite and syenite), cross cut by a voluminous swarm of radial and annular dikes with mostly volcanic-subvolcanic textures and variable compositions (foid-bearing alkali feldspar trachyte, trachyte, benmoreite, ledmorite, syenite, tephrite, tephriphonolite and alkaline lamprophyre). In the TAS classification these rocks plot in the alkaline series covering a wide spectrum of compositions following two different trends: 1) alkaline (potassic) strongly silica-undersaturated series, from tephrite, phonotephrite to tephra-phonolite, and 2) mid-alkaline, less silica-undersaturated series, ranging from basaltic trachyandesite to trachyandesite (benmoreite), and trachyte. Dikes from the alkaline series show higher K2O/Na2O ratios and Sr, La, Ce, contents compared to those from the mid-alkaline series. Rocks of the alkaline series are richer in K-feldspar, sodalite, leucite (pseudoleucite), biotite, potassic-ferro-pargasite and garnet than the less silica-undersaturated (trachytic) rocks, reflecting a stronger alkaline potassic affinity. A review of geochemical, isotopic and mineralogical data, and a new geochemical modeling performed on the LPC dikes, suggests that both trends represent separated magmatic series that evolved from two different parental magmas lodged ∼30 km deep in the crust. Our results suggest that the compositional variations observed in LPC dikes, cannot be explained by a simple magmatic evolution via fractional crystallization from a unique parental magma, and that an assimilation and fractional crystallization (AFC) process is required to explain some compositional differences. Our results suggest an upper crustal contaminant (evolved rocks) with a Grenvillian isotope signature. On the other hand, analyses of feldspar crystals from the tephriphonolitic dikes indicate local mixing effects, between an evolved tephriphonolitic melt and a less evolved and hotter mafic magma.The origin of both parental magmas could be explained by different melting degrees of the same mantle source, a phlogopite-bearing spinel lherzolite metasomatized by subduction derived fluids. We consider as a possible explanation that alkaline and coeval calc-alkaline magmatism in this part of the Andes, is due to local heterogeneities in the mantle source, and different degrees of partial melting Similar isotopic compositions of the LPC dikes, with those from other Miocene magmatic occurrences with arc-signature and similar age (e.g., Paramillos de Uspallata, Las Máquinas basalt, Abanico Fm and Farellones Fm) suggest an analogous mantle source for these rocks, from arc and retroarc in the Pampean flat slab regions. However, our results suggest that the isotopic trend contamination of LCP is different from that of Paramillos de Uspallata and other arc rocks of the Southern Volcanic Zone. The crustal contaminant of LPC possibly has another composition that those of Precordillera and Principal Cordillera Miocene rocks. The age of LPC rocks (∼19 Ma) and their arc-related signature agree with the eastward broadening of the arc magmatism between 17 and 19 Ma in this part of the flat slab. According to our interpretations, the LPC is a singular occurrence of two alkaline magmatic series on destructive plate margins, associated with calc-alkaline magmatism, occurring closely in time and space.