Range Of Isotopic Compositions Research Articles

The zirconium stable isotope compositions of 22 geological reference materials, 4 zircons and 3 standard solutions

We report stable Zr isotope compositions of four common natural zircon grains: Penglai, 91500, GJ-1 and Mud Tank, and 22 whole rock reference materials which include fifteen magmatic rocks, one metamorphic rock, six sediments or sedimentary rocks. In addition, the isotopic calibration of the three Zr standard solutions used in different publications (IPGP-Zr, SRM3169, NIST) has been determined. The data are reported as δ94/90ZrIPGP-Zr, the permille deviation of the 94Zr/90Zr ratio relative to the IPGP-Zr standard solution. The isotopic offsets among the three Zr standard solutions are small (less than 0.033 ± 0.033‰). The isotopic composition of the zircons agrees well with previous in-situ measurements, validating the in-situ measurements. On the other hand, all the samples analyzed in this study show a large range of Zr isotope compositions of 0.602‰. This confirms the fairly large natural Zr isotopic variations. The general variation of δ94/90ZrIPGP-Zr with magmatic evolution of the various igneous samples is in agreement with previous studies, with the most magmatically evolved samples being the isotopically heavier. The two magmatic zircons are isotopically lighter compared to the magmatic rocks, which is in agreement with the enrichment of heavy isotope in the evolved samples. This is ultimately controlled by the crystallization of isotopically light zircons.

Constraints on the hydrogeochemistry and origin of the CO2-rich mineral waters from the Eastern Carpathians – Transylvanian Basin boundary (Romania)

The Eastern Carpathians, along the Neogene to Quaternary volcanic chain, host important natural resources of mineral waters. Most of these are in the form of CO2-rich natural springs of different types, associated with CO2 gas emissions. In order to understand the origin of these waters, the natural processes responsible for the observed physical–chemical features and to check the exposure and vulnerability of the hydrogeological system to potential contamination and environmental changes, 121 mineral water springs and wells were investigated. The investigations performed, offered an insight on the processes affecting the waters after their infiltration that led to a wide range of chemical and isotopic compositions. The diversity in the geological structure of the area, alteration processes and interaction with CO2, all explain the variety of the hydrogeochemical facies of the investigated waters. Three major hydrogeochemical types that are the result of the geological variety of the study area and water–rock interaction were identified: Ca–Na–Mg–HCO3, Ca–Na–Mg/Na–Ca ± Mg–HCO3–Cl ± SO4, and Na–Cl waters. Further changes in the geochemical features are attributable to mixing between different water types during their ascent, enhanced by the tectonic setting of the area. Considering the isotopic composition (δ18O, δD) of the waters, most of them proved to be of meteoric origin. Extreme isotopic shifts towards enriched δ18O and δD values of +8.59 and −12.2‰ (V-SMOW) respectively were observed in the case of 11 springs. These shifts were attributed to the interaction of water with clay minerals and diagenesis. As most of these waters are of meteoric origin, their discharge may be influenced by meteorological factors. Moreover, the mixing of aquifers with very shallow waters most likely facilitates their exposure to external contamination.

Ruby Mineralization in Murzinka–Adui Metamorphic Complex, Central Urals

The Lipovka and Alabashka ruby occurrences in the Central Urals hosted in marble of the Murzinka–Adui Anticlinorium are considered. The field works were carried out; the chemical compositions of rocks and minerals and oxygen isotopic composition of corundum and silicates were determined; and micas were dated by the K–Ar method. These occurrences are similar in geological environment and mineral assemblages to the ruby occurrences in the South Urals and deposits in Southeast Asia. At the Alabashka occurrence, ruby was also found from brecciated mineralized granite and amphibolite. The mineralized breccia and ruby-bearing marble are enriched in K- and Cr-bearing minerals. Ruby characterized by a narrow range of the oxygen isotopic composition (δ18О = 14.2–17.8‰) corresponding to that of metamorphic and metasomatic corundum. The hydrogen isotopic composition of fuchsite ranges from –78.5 to –89.6‰, while the δD value calculated for fluid equilibrated with fuchsite varies from –48.8 to –79.7‰ that is consistent with the high-temperature equilibrium with magmatic rocks. Aluminum and Cr were mobile elements and could be inflowed by fluids during ruby formation. Corundum mineralization at both occurrenceswas dated at 280–260 Ma, which is consistent with the beginning of post-collisional relaxation. The Murzinka–Adui Anticlinorium is the most prospective area in the Urals for the high-quality ruby deposits.

The δ53Cr isotope composition of komatiite flows and implications for the composition of the bulk silicate Earth

Data on the chromium stable isotope composition of planetary reservoirs have the potential to provide information about core formation, partial melting and conditions of the Moon formation. In order to detect the small Cr isotopic differences between various reservoirs in the solar system, their compositions need to be precisely constrained. The current BSE value of δ53Cr = −0.11 ± 0.06‰ (Sossi et al., 2018) cannot resolve differences between achondrites, (Vesta δ53Cr = −0.17 ± 0.05‰) and chondrites (carbonaceous δ53Cr = −0.12 ± 0.05‰; ordinary δ53Cr = −0.11 ± 0.04‰). The composition of the bulk silicate Earth (BSE) is often used as a reference point for comparisons to other planetary reservoirs. However, past attempts to estimate the Cr isotopic composition of the BSE have been unable to provide a well-constrained BSE value. Traditional methods, using mantle peridotites, are affected by the susceptibility of Cr isotopes to fractionation during metasomatism. More recently, the Cr isotope composition of the BSE has been calculated using komatiites, in addition to mantle peridotites, to produce a more precise value (Sossi et al., 2018). In order to constrain the BSE composition to a higher precision, the δ53Cr of remarkably fresh komatiite lava flows from three localities, ranging in age from 2.7 Ga to 89 Ma, have been investigated in detail. These included the Tony's Flow in the Belingwe Greenstone Belt, Zimbabwe, the Victoria's Lava Lake in Fennoscandia, and komatiites from Gorgona Island in Colombia.In the komatiites studied, a range in Cr isotopic compositions was found, from δ53Cr = −0.16 ± 0.02 to −0.01 ± 0.02‰. We show that the high degrees of partial melting that produced the komatiites, did not result in Cr isotopic fractionation between the komatiitic melt and mantle residue. However, limited Cr isotopic fractionation is found to be a consequence of komatiite lava differentiation. For the lava flows with high Mg content and high Cr2+/ƩCrTOT (the molar ratio of Cr2+/(Cr2+ + Cr3+)), such as Tony's Flow and Gorgona, δ53Cr increases in the evolved portion of the magma during olivine fractionation due to the preferential inclusion of light Cr into olivine. Other flows with lower MgO content do not show this behaviour because a smaller fraction of the Cr is contained in olivine. The effects of fractional crystallisation must, therefore, be taken into account when calculating the Cr isotopic composition of the source of komatiite lavas.The weighted average of δ53Cr for the komatiite lavas analysed is −0.12 ± 0.04‰ (n = 5) and represents our best estimate for the Cr isotopic composition of the BSE. It agrees with the previous estimates, while providing an improvement to the uncertainty. There is no resolvable difference between this value and that of chondritic meteorites. Our data also indicate that the δ53Cr of the mantle has been constant since at least the Archean.

A Global Assessment of Copper, Zinc, and Lead Isotopes in Mineral Dust Sources and Aerosols

The stable isotope compositions of Cu and Zn in major geochemical reservoirs are increasingly studied with the aim to develop these isotope systems as tools to investigate the global biogeochemical cycles of these trace metals. The objectives of the present study were (i) to expand the range of Cu, Zn, and Pb isotope compositions of mineral dust by analyzing samples from major mineral dust sources in Asia and Africa (Chinese Loess Plateau, Chinese deserts, Thar desert, Sahel region) and (ii) to assess the potential impact of human activities on the isotope composition of aerosols by synthesizing published Cu and Zn isotope compositions in aerosols and natural and anthropogenic sources. For the newly analyzed mineral dust areas in Asia and Africa, δ^{65}Cu_{NIST-976} values range from −0.54 to +0.52‰, δ^{66}Zn_{JMC-Lyon} values from −0.07 to +0.57‰, and {206}^Pb/{204}^Pb values from 18.522 to 19.696. We find a significant geographic control with samples from the Thar Desert having the heaviest isotopic compositions (δ^{65}Cu_{NIST-976} = +0.48 ± 0.06‰, δ^{66}ZN_{JMC-Lyon} = +0.49 ± 0.11‰) and samples from the Sahel and the Badain Jaran desert having the lightest Zn isotope composition (δ^{66}ZN_{JMC-Lyon} = +0.19 ± 0.15‰ and +0.07 ± 0.07‰, respectively). We find important variations in the isotope signatures between particle size fractions with heavier isotopic compositions in the smallest and largest particle size fractions and lighter isotopic compositions in the mid particle size fractions. Associations with the mineralogical composition are less clear. Newly analyzed aerosol samples for Beijing and Xi'an show δ^{65}Cu_{NIST-976} values of +0.29 ± 0.19‰ and +0.16 ± 0.04‰, δ^{66}Zn_{JMC-Lyon} values of −0.36 ± 0.04‰ and +0.02 ± 0.06‰, and {206}^PB/{204}Pb values of 18.129 ± 0.003 and 18.031 ± 0.003, respectively. Based on a synthesis of published and novel data, we suggest improved ranges and mean values for the isotopic composition of mineral dust from selected locations in Asia and Africa and of anthropogenic sources such as non-exhaust traffic emissions, combustion, electroplating and galvanization. This should serve as a valuable reference for future studies using these isotope systems. This paper demonstrates univocally that human activity introduces a wide range of Zn isotope compositions into the atmospheric environment and, thus, impacts the biogeochemical cycle of Zn.

Lithium isotope compositions of U.S. coals and source rocks: Potential tracer of hydrocarbons

Kerogen in organic-rich rocks contains trace amounts of lithium (Li) that has been overlooked as a contributor to the global Li geochemical cycle. This study examined a variety of coals where kerogen is concentrated (>50% organic carbon) and hydrocarbon source rocks of different ages, depositional environments and thermal maturity to determine their range of Li isotopic compositions (δ7Li‰) and factors that influence their compositions.Using Secondary Ion Mass Spectrometry (SIMS), we analyzed 22 coals and 4 hydrocarbon source rocks (Types I, II, III), to determine the δ7Li of kerogen in situ, without chemical isolation of phases that can alter their original isotopic compositions. The δ7Li values of the coals surveyed are distinctly isotopically light (<0‰) compared to most natural minerals and fluids. In immature coals, with a vitrinite reflectance in oil (VRo) of ≤0.5%, kerogen δ7Li values average –23.4 ± 1.1‰ and become heavier with increasing thermal grade to temperatures of gas generation (VRo ~1.3%). The linear correlation between δ7Li and VRo suggests that 6Li may be preferentially released to pore fluid from kerogen during thermal maturation. Notably, authigenic clays forming at diagenetic temperatures substitute Li from pore fluids into silicate layers, therefore, the Li isotopic composition of the clays may record fluid isotopic compositions influenced by organic-Li sources.NanoSIMS isotopic maps of the Lower Bakken Shale, and SIMS measurements of the Green River Shale show isotopically light Li associated with C-dominated areas, and heavier δ7Li with Si-dominated areas of the hydrocarbon source rocks. We conclude that kerogen is a source of isotopically light Li that contributes to fluids during thermal maturation and hydrocarbon generation. Kerogen may be a significant contributor of Li to pore fluids and its distinctly light Li isotopic composition relative to other terrestrial waters and minerals could provide a tracer of organic inputs to the global geochemical cycle.

In Situ Elemental and Sr Isotope Characteristics of Magmatic to Hydrothermal Minerals from the Black Mountain Porphyry Deposit, Baguio District, Philippines

AbstractAt the Black Mountain porphyry Cu-Au deposit in the Baguio district, Northern Luzon (Philippines), pre- and synmineralized rocks preserve magmatic and hydrothermal minerals (e.g., plagioclase, amphibole, titanite, and epidote) spanning the complete paragenesis of the deposit. Strontium isotope values in early crystallized plagioclase phenocrysts from all felsic porphyries can be divided into two types. The type-I plagioclase crystals show relatively homogeneous Sr isotope values (0.7035–0.7038, 1σ &amp;lt;0.0001), indicating crystallization from a relatively stable and long-lived felsic magma chamber. The type-II plagioclase grains have a wider range of Sr isotope compositions (0.7032–0.7039, 1σ &amp;lt;0.0001), indicating mafic and/or felsic magma recharge. In magmatic titanite, Nb/Ta values are higher than those in the whole rock, while Zr/Hf and Y/Ho values are lower. In hydrothermal titanite and epidote, the ratios are similar to those in the whole rock. These patterns reflect crystallization effects imposed during the magmatic stage but an absence of differentiation during the hydrothermal stage. The consistent gradual decrease in total rare earth element, Y, Zr, and U contents in both hydrothermal titanite and epidote from early to late stages indicates the effect of hydrothermal fluid evolution with decreasing temperature. The variation of 87Sr/86Sr values in magmatic amphibole, plagioclase, and hydrothermal epidote in felsic and mafic rocks indicates the addition of mafic magma-derived fluid into the felsic magma-derived fluid. One extra source of fluid (probably derived from wall-rock limestone) was required to generate the highly radiogenic 87Sr/86Sr values of some epidote (0.7038–0.7053). Thus, in situ elemental and Sr isotope variation in minerals from different paragenetic stages can be used to interpret formation process and source for both magmas and hydrothermal fluids.

Molybdenum isotopic behavior during intense weathering of basalt on Hainan Island, South China

The behavior of molybdenum (Mo) isotopes during chemical weathering was investigated through analysis of saprolite and soil samples from two weathering profiles developed on Neogene basalts of Hainan Island in southern China. The Penglai and Nanyang profiles exhibit strong (mean ∼60 wt.% kaolinite) and extreme (mean ∼85 wt.% kaolinite) degrees of weathering, respectively. Our results show that light Mo isotopes are selectively retained in residual minerals, including kaolinite and hematite, whereas heavy Mo isotopes are preferentially leached out. Relative to the Mo-isotopic composition of the underlying unaltered parent basalt –0.38 ± 0.05 ‰ (2σ); vs. NIST 3134), the δ98/95Mo values of the overlying saprolite and surface soil at Penglai are modestly lower (–0.81 to –0.39‰) and higher (–0.35 to –0.12‰), respectively. In this profile, both δ98/95Mo and total organic carbon (TOC) decrease with depth, and their positive correlation in the soil layer (r = +0.99, p(α) < 0.01, n = 6) implies that heavier Mo isotopes are fractionated into organic matter. In the more intensely weathered Nanyang profile, the δ98/95Mo of the underlying unaltered parent basalt is lower –0.56 ± 0.05‰ (2σ) than that at Penglai, and the weathered samples show a larger range of relatively lower values (–1.39 to –0.33‰), which may reflect adsorption of Mo by Fe oxides. Throughout the weathering profile, Fe2O3 correlates positively with Mo concentration (r = +0.86, p(α) < 0.01, n = 32) but negatively with δ98/95Mo (r = –0.69, p(α) < 0.01, n = 32), implying that adsorption onto Fe-oxides played a key role in Mo mobility and isotopic fractionation. Extremely negative δ98/95Mo values (to –1.39‰) in the middle saprolite were likely linked to hydrothermal activity, as indicated by unusual enrichments of trace metals (Ni, Cu, Mo), low Hf/Y ratios, and a positive Eu/Eu* anomaly. Furthermore, the overlying soil layers of both profiles are characterized by enrichment of heavier Mo isotopes and higher Mo concentrations relative to their parent rocks. This may have been caused by additional inputs of Mo from atmospheric precipitation (average δ98/95Mo = +0.94 ± 0.14‰, 1σ, n = 3). Based on mass balance modeling of Mo isotopes and concentrations through the integrated profile, we infer Mo inputs from volcanic ash in the basalt-derived soils of northern Hainan Island. This study highlights the range of potential factors controlling Mo retention and isotopic fractionation as a function of variable weathering intensity, which may help to explain the large range of Mo isotopic compositions observed in river catchments at a global scale.

Assessing mercury distribution using isotopic fractionation of mercury processes and sources adjacent and downstream of a legacy mine district in Tuscany, Italy

Mercury (Hg) concentrations and isotopic compositions in a range of sample types collected from the legacy Abbadia San Salvatore Mine (ASSM) area were used to evaluate the distribution of Hg in the region. The district generated more than 100,000 metric tons of Hg releasing extensive amounts of gaseous Hg emissions and producing large amounts of mine waste calcine from which Hg can be mobilized into the local and regional environments. Direct and indirect impact from cinnabar ore mining and processing resulted in elevated Hg concentrations in the calcine, soils, lake and stream sediments, and fish samples collected at and downstream of ASSM. The contribution of Hg from the different sources and processes resulted in a wide range of δ202Hg and Δ199Hg isotopic compositions (−2.25 to 0.96‰, −0.12 to 0.72‰, respectively). Fish samples resulted in negative values for δ202Hg (−0.53 to −1.21‰) and positive Δ199Hg (0.12–0.73‰). Primary cinnabar ore and present-day geothermal water and precipitate showed distinctly negative δ202Hg (−0.96 to −2.25‰), whereas waste calcines were enriched in 202Hg (δ202Hg from about −1 to +1‰); soils and sediments show intermediate compositions, reflecting different extents of contributions from the various Hg sources and processes. These sources and processes of Hg include weathering cinnabar, gaseous Hg emissions from ore processing, and geothermal activity to a lesser extent. Hg speciation in concert with Hg isotope ratios can better ascertain source attribution and assist in identifying Hg pathways into the environment to distinguish the more bioavailable forms of Hg entering the ecosystem.

Molecular indicators of methane metabolisms at cold seeps along the United States Atlantic Margin

Anaerobic oxidation of methane (AOM) and the environmental conditions supporting AOM on continental margins is an essential component to global methane budgets. Diagnostic lipid biomarkers and their compound specific isotope analysis preserved in authigenic carbonates at cold seeps can serve as “fingerprints” to archaeal−bacterial consortia involved in AOM. However, despite the discovery of several hundreds of seeps along the United States Atlantic Margin (USAM), there are relatively few biomarker investigations of cold seep carbonates along this passive margin. A lipid biomarker, carbon isotope, and DNA marker gene study was therefore undertaken to determine the microbial origins of authigenic carbonates from two USAM seeps, Norfolk and the Baltimore Canyon seep fields. Results from this study capture a distinct archaeal lipid signature from putative methanotrophic archaea, including archaeol (I), sn-2-hydroxyarchaeol, 2,6,10,15,19-pentamethylicosane (PMI), and crocetane. The 13C-depleted AOM-related archaeal lipid samples (i.e., archaeol: −91.6‰, sn-2-hydroxyarchaeol: −129.2‰, PMI −92.8‰, and crocetane: −70.9‰) confirm the dominance of methane assimilation and isotope fractionation during AOM. These results are consistent with the detection of archaeal anaerobic methanotrophs (ANMEs) based on 16S rRNA gene sequencing. The Norfolk authigenic carbonate contained ANME-1a, -1b, 2a-2b, and 2c whereas only the ANME-2 clade was detected at Baltimore and present as the subclusters 2a-2b and -2b. The ANME-2d clade may also be present, particularly at the Baltimore seep site, given the high abundance of Candidatus Methanoperedens nitroreducens detected in the mcrA gene sequencing. The presence of terminally branched fatty acids, antesio- and iso-C15:0 components, as well as C16:1ω7 with δ13C values as low as −107.6‰, are indicative of sulfate-reducing bacteria (SRB) at the Norfolk seep site and supports syntrophy of SRB with methane-oxidizing archaea. In contrast, nitrate-driven AOM in syntrophy M. nitroreducens at the Baltimore seep site is consistent with elevated fatty acid δ13C values and lack of Deltaproteobacteria at the Baltimore seep site. Taken together, the range in lipid composition, distribution, and carbon isotopic composition observed at the Norfolk and Baltimore seep sites suggests AOM is performed by multiple archaea instead of a single species and may be paired with either or both nitrate- and sulfate-reduction. Given the heterogeneous nature of cold seep ecosystems, this study fills a critical spatial gap in our knowledge of AOM activity at two seep sites along a passive margin.

In Situ Fe and S isotope analyses in pyrite from the 3.2Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction.

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7–2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28‐ to 3.26‐Gyr‐old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ56Fe values of −1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass‐independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ33S values > +3‰, while the other groups display more variable and closer to 0‰ Δ33S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ33S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass‐independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro‐organisms closely related to the last common ancestor had the ability to reduce Fe(III).

Molybdenum Mass Fractions and Stable Isotope Compositions of Sedimentary Carbonate and Silicate Reference Materials

A double‐spike method in combination with MC‐ICP‐MS was applied to obtain molybdenum (Mo) mass fractions and stable isotope compositions in a suite of sedimentary silicate (marine, lake, stream, estuarine, organic‐rich sediment, shales, slate, chert) and carbonate reference materials (coral, dolomite, limestones, carbonatites), and a manganese nodule reference material, poorly characterised for stable Mo isotope compositions. The Mo contents vary between 0.076 and 364 μg g−1, with low‐Mo mass fractions (&lt; 0.29 μg g−1) found almost exclusively in carbonates. Intermediate Mo contents (0.73–2.70 μg g−1) are reported for silicate sediments, with the exception of chert JCh‐1 (0.24 μg g−1), organic‐rich shale SGR‐1b (36.6 μg g−1) and manganese nodule NOD‐A‐1 (364 μg g−1). The Mo isotope compositions (reported as δ98Mo relative to NIST SRM 3134) range from −1.77 to 1.03‰, with the intermediate precision varying between ± 0.01 and ± 0.12‰ (2s) for most materials. Low‐temperature carbonates show δ98Mo values ranging from 0.21 to 1.03‰ whereas δ98Mo values of −1.77 and −0.17‰ were obtained for carbonatites CMP‐1 and COQ‐1, respectively. Silicate materials have δ98Mo values varying from −1.56 to 0.73‰. The range of δ98Mo values in reference materials may thus reflect the increasingly important relevance of Mo isotope investigations in the fields of palaeoceanography, weathering, sedimentation and provenance, as well as the magmatic realm.

Magnetic mineral constraint on lead isotope variations of coal fly ash and its implications for source discrimination

Coal fly ash in the atmosphere affects air quality and potentially influences the global climate by promoting oceanic productivity. Although accurately tracing the sources of fly ashes is vital for emission control, it remains a challenging task. Stable lead (Pb) isotope analysis is a useful tool for tracing atmospheric pollution but it fails to accurately address coal combustion emissions due to the broad range of Pb isotopic composition of coal. Environmental magnetic parameters can be used as a rapid and economical proxy for tracing atmospheric pollutants (including coal fly ashes) and have the potential for discriminating emission sources. In this study, we combined magnetic parameters with Pb isotopic signatures in order to better discriminate the sources of coal fly ash. Both magnetic particles and Pb are highly concentrated in the fly ashes compared with the feed coals. Most of the fly ashes exhibit higher 206Pb/207Pb and lower 208Pb/206Pb ratios than those of the feed coals. Furthermore, the Pb isotopic compositions of the fly ashes are highly correlated (p < 0.01) with the concentrations of magnetic particles (especially hematite), suggesting that the variation of Pb isotopes in the fly ashes is controlled by the adsorption of Pb on magnetic minerals. Based on the established relationship between magnetic minerals and Pb isotopes within coal fly ashes, we re-analyzed previously reported magnetic and Pb isotopic data from atmospheric dust and demonstrated the effectiveness of the combined method in discriminating coal fly ash in the atmosphere.

Mantle heterogeneity beneath the South China Sea: Chemical and isotopic evidence for contamination of ambient asthenospheric mantle

Large quantities of continental lithosphere are known to enter the mantle during continental rifting, but their possible contribution in the generation of melts under a nascent ocean remains enigmatic. Here we report new in situ chemical and SrPb isotopic data for volcanic glass and feldspar in volcanic breccia in the South China Sea (SCS) formed after cessation of SCS spreading. Similar to volcanic breccia (whole rocks), the volcanic glasses and feldspars in breccia can also be grouped into early (11–8 Ma, e.g. 25R-17R) and late (<8 Ma, e.g. 10R-7R) stages. Early stage glasses and minerals in breccia display limited chemical and isotopic variability, whereas late stage glasses and feldspars show a significantly wide range in chemical and SrPb isotopic compositions. Geochemical and radiogenic isotope variations in glasses from the SCS breccia have delineated two distinct mantle sources: a depleted mantle component (DM), and an enriched mantle component (EM-2). The influence of EM-2 component is less diluted in late-stage magmatism compared to early stage magmatism. The temporal variation in compositions of volcanic glasses from the SCS are consistent with dynamic melting of a heterogeneous mantle. Due to progressive thickening of the lithosphere, low-degree melting would preferentially sample the more fertile EM-2 component during the late stage of volcanism. The new results, in combination with literature data of post-spreading lavas, provide evidence for an origin of the EM-2 (DUPAL) signature from recent introduction of continental lithosphere via delamination. We conclude that shallow recycling of continental lithosphere introduced into the upper mantle during continental rifting can be an important mechanism contributing to mantle heterogeneity.

Mantle degassing along strike-slip faults in the Southeastern Korean Peninsula

On September 12, 2016, a ML 5.8 earthquake hit Gyeongju in the southeastern part of the Korean Peninsula (SeKP), although the area is known to be far from the boundary of the active plate. A number of strike-slip faults are observed in heavily populated city areas (e.g., Busan, Ulsan, Pohang, and Gyeongju). However, dissolved gases related to the active faults have rarely been studied despite many groundwater wells and hot springs in the area. Here we report new results of gas compositions and isotope values of helium and carbon dioxide (CO2) in fault-related fluids in the region. Based on gas geochemistry, the majority of gas samples are abundant in CO2 (up to 99.91 vol.%). Measured 3He/4He ratios range from 0.07 to 5.66 Ra, showing that the mantle contribution is up to 71%. The range of carbon isotope compositions (δ13C) of CO2 is from −8.25 to −24.92‰, showing mantle-derived CO2 is observed coherently where high 3He/4He ratios appear. The weakening of faults seems to be related to enhanced pressures of fluids containing mantle-derived helium and CO2 despite the ductile lower crust underneath the region. Thus, we suggest that the SeKP strike-slip faults penetrate into the mantle through ductile shearing.

Thallium isotopic composition of phlogopite in kimberlite-hosted MARID and PIC mantle xenoliths

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are rare mantle xenoliths entrained by kimberlites. Their high phlogopite modes (15 to ∼100vol.%) and consequent enrichments in alkali metals and H2O suggest a metasomatic origin. Phlogopite also has high concentrations (>0.2μg/g) of thallium (Tl) relative to mantle abundances (<3ng/g). Thallium isotope ratios have proven useful in tracing the input of Tl-rich materials, such as pelagic sediments and altered oceanic crust, to mantle sources because of their distinct isotopic compositions compared to the peridotitic mantle. This study presents the first Tl isotopic compositions of well-characterised phlogopite separates from MARID and PIC samples to further our understanding of their genesis. The PIC rocks in this study were previously interpreted as the products of kimberlite melt metasomatism, whereas the radiogenic and stable N–O isotope systematics of MARID rocks suggest a parental metasomatic agent containing a recycled component.The ε205Tl values of phlogopite in both PIC (–2.7±0.8; 2s.d., n=4) and MARID samples (–2.5±1.3; 2s.d., n=21) overlap with the estimated mantle composition (–2.0±1.0). PIC phlogopite Tl contents (∼0.4μg/g) are suggestive of equilibrium with kimberlite melts (0.1–0.6μg/g Tl), based on partitioning experiments in other silica-undersaturated melts. Kimberlite Tl-ε205Tl systematics suggest their genesis does not require a recycled contribution: however, high temperature-altered oceanic crust cannot be ruled out as a component of the Kimberley kimberlites’ source. Mantle-like ε205Tl values in MARID samples also seem to contradict previous suggestions of a recycled contribution towards their genesis. Recycled components with isotopic compositions close to mantle values (e.g., high temperature-altered oceanic crust) are still permitted. Moreover, mass balance mixing models indicate that incorporation into the primitive mantle of 1–30% of a low temperature-altered oceanic crust+continental crust recycled component or 1–50% of continental crust alone could be accommodated by the Tl–ε205Tl systematics of the MARID parental melt. This scenario is consistent with experimental evidence and existing isotopic data. One PIC phlogopite separate has an extremely light Tl isotopic composition of –9.9, interpreted to result from kinetic isotopic fractionation. Overall, phlogopite is the main host mineral for Tl in metasomatised mantle and shows a very restricted range in Tl isotopic composition, which overlaps with estimates of the mantle composition. These results strongly suggest that negligible high temperature equilibrium Tl isotopic fractionation occurs during metasomatism and reinforces previous estimates of the mantle’s Tl isotopic composition.

Tourmaline in a Mesoarchean pelagic hydrothermal system: Implications for the habitat of early life

The RNA World hypothesis requires the synthesis of RNA to allow the emergence of life on Earth. Hydrothermal systems have been proposed as potential candidates for constructing complex biomolecules. However, in order to successfully form RNA, it is necessary to stabilize ribose, a RNA carbohydrate component. Borate has been found to stabilize ribose. Therefore, boron rich hydrothermal systems are important environments concerning the origin of life on Earth.The 3.2-Ga Dixon Island Formation of the West Pilbara Superterrane, Western Australia, is a volcano-sedimentary sequence. The Formation represents a Mesoarchean pelagic hydrothermal system, which formed adjacent to an immature island arc. Fine-grained tourmaline, in addition to biogenic carbonaceous matter and spherulitic and tubular bacteriomorphs, are found in black chert. A boron-rich environment was responsible for the formation of these deposits. To explore the implications of such a boron enriched environment on microbial activity, modes of occurrence and chemical compositions of the tourmaline were examined.The tourmaline is schorl or dravite of the alkali tourmaline group and the boron isotope compositions range in δ11B from -7.3 to +2.6‰. The tourmaline occurs in microcrystalline quartz matrix of black chert veins that cross cut a volcanic unit and also in a bedded black chert, which overlays the volcanic unit. The volcanic unit contains highly altered zones with hydrothermal veins. The associated lithologic and stratigraphic features suggest that the black chert veins were the conduits for upward moving hydrothermal fluids, which reached the sea floor. Subsequently, the volcanic unit was covered by organic matter-rich cherty sediments that in part were fed, and/or altered, by the hydrothermal fluids.These results suggest that the origin of boron enrichment to form Dixon Island tourmaline is not the associated sedimentary mineral assemblage, which includes diagenetic clay, low-grade metamorphic mica, and organic matter. Instead, the tourmaline was directly precipitated from hydrothermal fluid, enriched in boron. Furthermore, the hydrothermal fluids had already concentrated the boron, in the Mesoarchean pelagic system, prior to the apex of organic matter production and microbial activity. Our findings support a hypothesis that the boron-enriched hydrothermal environment aided the survival and evolution of early life.

Kinetic and Metabolic Isotope Effects in Zooxanthellate and Non-zooxanthellate Mediterranean Corals Along a Wide Latitudinal Gradient

Many calcifying organisms exert significant biological control over the construction and composition of biominerals which are thus generally depleted in oxygen-18 and carbon-13 relative to the isotopic ratios of abiogenic aragonite. The skeletal δ18O and δ13C values of specimens of Mediterranean zooxanthellate (Balanophyllia europaea and Cladocora caespitosa) and non-zooxanthellate corals (Leptopsammia pruvoti and Caryophyllia inornata) were assessed along an 8° latitudinal gradient along Western Italian coasts, spanning ~2 °C and ~37 W m-2 of annual average sea surface temperature and solar radiation (surface values), respectively. Seawater δ18O and δ13CDIC were surprisingly constant along the ~850 km latitudinal gradient while a ~2 and ~4 ‰ variation in skeletal δ18O and a ~4 and ~9 ‰ variation in skeletal δ13C was found in the zooxanthellate and non-zooxanthellate species, respectively. Albeit Mediterranean corals considered in this study are slow growing, only a limited number of non-zooxanthellate specimens exhibited skeletal δ18O equilibrium values while all δ13C values in the four species were depleted in comparison to the estimated isotopic equilibrium with ambient seawater, suggesting that these temperate corals cannot be used for thermometry-based seawater reconstruction. Calcification rate, linear extension rate and skeletal density were unrelated to isotopic compositions. The fact that skeletal δ18O and δ13C of zooxanthellate corals were confined to a narrower range at the most isotopically depleted end compared to non-zooxanthellate corals, suggests that the photosynthetic activity may restrict corals to a limited range of isotopic composition, away from isotopic equilibrium for both isotopes. Our data show that individual corals within the same species express the full range of isotope fractionation. These results suggest that metabolic and/or kinetic effects may act as controlling factors of isotope variability of skeleton composition along the transect, and that precipitation of coral skeletal aragonite occurs under controlling kinetic biological processes, rather than thermodynamic control, by yet unidentified mechanisms.

Synthesis of In‐House Produced Calibrated Silver Phosphate with a Large Range of Oxygen Isotope Compositions

The large range of stable oxygen isotope values of phosphate‐bearing minerals and dissolved phosphate of inorganic or organic origin requires the availability of in‐house produced calibrated silver phosphate of which isotopic ratios must closely bracket those of studied samples. We propose a simple protocol to synthesise Ag3PO4 in a wide range of oxygen isotope compositions based on the equilibrium isotopic fractionation factor and the kinetics and temperature of isotopic exchange in the phosphate–water system. Ag3PO4 crystals were obtained from KH2PO4 that was dissolved in water of known oxygen isotope composition. Isotopic exchange between dissolved phosphate and water took place at a desired and constant temperature into PYREX™ tubes that were placed in a high precision oven for defined run‐times. Samples were withdrawn at desired times, quenched in cold water and precipitated as Ag3PO4. We provide a calculation sheet that computes the δ18O of precipitated Ag3PO4 as a function of time, temperature and δ18O of both reactants KH2PO4 and H2O at t = 0. Predicted oxygen isotope compositions of synthesised silver phosphate range from −7 to +31‰ VSMOW for a temperature range comprised between 110 and 130 °C and a range of water δ18O from −20 to +15‰ VSMOW.

Rb–Sr geochronology and geochemistry of the Xiaotongchang basalt-hosted copper deposit in the Jinping area, SW China

The Xiaotongchang basalt-hosted copper deposit in Jinping, southern Yunnan Province, SW China, is located in the southern part of the Emeishan large igneous province. Here, we use Rb–Sr, Re–Os and S–Pb–C–O isotope data to constrain the age and genesis of the deposit. Petrological investigations reveal two main stages of mineralization. The early-stage is characterized by disseminated or massive sulfides in an assemblage of pyrite + chalcopyrite + quartz + albite + calcite. Rb-Sr isotopic dating of early-stage chalcopyrite yields an age of 256 ± 9.4 Ma (MSWD = 5.4), which is slightly younger than the eruption age of the Jinping basalts. The δ34SCDT values of the sulfides range from +5.8‰ to +7.7‰, suggesting that the metals of the early-stage copper mineralization originated from the Emeishan basalts and that the ore-forming fluids were contaminated by crustal rocks. These units may indicate that the early-stage orebodies represents syngenetic hydrothermal mineralization related to the eruption of Emeishan flood basalts. The late-stage mineralization is characterized by a pyrite + chalcopyrite + bornite + chalcocite + quartz + calcite assemblage that is developed in veins or breccia. The Re-Os isotopic dating of late-stage chalcopyrite samples reveals that they formed at 240–231 Ma. The Pb isotopic compositions ((206Pb/204Pb)t = 17.521–19.227, (207Pb/204Pb)t = 15.480–15.760, and (208Pb/204Pb)t = 37.778–38.731; t = 235 Ma) show that the late-stage copper orebodies have a heterogeneous source and originated mainly from the crust. The late-stage sulfides have higher δ34SCDT values (+7.9‰ to +10.7‰) than the early-stage sulfides. The narrow range of sulfur isotopic compositions of the sulfides associated with both stages and the Pb isotopic compositions of late-stage chalcopyrite indicate that some of the metals associated with late-stage mineralization originated from the Jinping basalts. The C–O isotopic compositions of calcite samples show that the magmatic hydrothermal fluids in the late stage flowed through and were contaminated by the Maokou limestone. Therefore, we suggest that the late-stage orebodies represent magmatic hydrothermal mineralization associated with the Late Triassic rhyolite porphyry and that the tectono-magmatic hydrothermal fluid remobilized copper from the basalts. The copper deposits formed at favourable interfaces within the basalt stratigraphy or in favorable structural locations.