Isotopic Space Research Articles

The role of low-temperature 18O exchange in the isotopic evolution of deep subsurface fluids

Ca-Na-Cl fluids with high concentrations of dissolved reduced gases reside within fractures in crystalline Precambrian rocks around the world, and have been most intensively studied within South Africa, Fennoscandia and the Canadian Shield. In contrast to surface waters, shallow groundwaters, sedimentary basin brines and metamorphic fluids, the δ18O and δ2H values for these Ca-Na-Cl fluids typically plot to the left/above of the Global Meteoric Water Line (GMWL). To date, most interpretive frameworks for these fracture fluids have focused on their production via water-rock alteration reactions that affect both δ18O and δ2H values, resulting in co-variation of water isotope values above the GMWL. Such alteration processes include silicate hydration coupled with formation of secondary minerals, radiolytic H2 formation, isotopic exchange with a H2-rich gas, or isotope exchange with O and/or H-bearing minerals. This study presents the first compiled global isotopic dataset for these types of fluids, integrating a large amount of unpublished data with the previously published literature in order to investigate these fracture fluid systems on a global scale. Importantly this global perspective allows differentiation between fluids impacted by late-stage mixing with meteoric waters, from fluids that reflect the most saline end-members stored in the host rocks in hydrogeologic isolation from the surface hydrologic cycle. The most saline fluids are shown to occupy a more restricted range of δ18O-δ2H space than previously recognised, with end-member fluids from all of the Precambrian rock settings investigated occupying a range of δ2H δ18O isotope space within which there is no co-variation in these values. These findings suggest a set of common processes may define the isotopic signatures of these most saline end-member fluids in Precambrian settings around the world – creating common signatures identifiable in these fluids, despite differences in geologic setting. This study identifies the important role of oxygen isotopic exchange between primary fluids (associated with hydrothermal/metamorphic activity) and the host rocks, taking place under low temperature, low-volume, water-rock ratios over long (Ma) geologic timescales. This process results in progressive 18O depletion in the fluids over time, while δ2H values remain less affected. For each site the specific isotopic signature of the fracture fluid end-member depends on initial hydrothermal/metamorphic fluid composition, rates of isotopic exchange, water-to-rock ratios, and in-situ residence times. We suggest the often-observed co-variation in both δ18O-δ2H above the GMWL primarily results from late stage mixing of the fracture fluid end-members with (paleo)-meteoric water, resulting in isotopic regression back towards the GMWL, with end-points defined by the local meteoric-climatic conditions for each site.

The breeding seabird community reveals that recent sea ice loss in the Pacific Arctic does not benefit piscivores and is detrimental to planktivores

Recent dramatic reductions of winter sea ice in the northern Bering Sea have raised the possibility that a rapid ecological transformation is underway. It has been hypothesized that with sufficient sea ice loss the cold pool thermal barrier separating the northern and southern Bering Sea would be breached, potentially benefiting piscivorous seabirds with an influx of abundant forage fish, but maybe causing food limitations for planktivorous seabirds that rely on cold-water associated species of zooplankton. During 2016–2019, we examined responses of the seabird community breeding on St. Lawrence Island to variable winter sea ice extent (low in 2018–2019). The seabird community includes piscivorous black-legged kittiwakes, thick-billed and common murres, and planktivorous crested and least auklets. We used a combination of stable isotope analysis and nutritional stress status analyses to examine if the region's ecosystem has shifted to a pelagic prey base, and whether either foraging guild benefitted from such a shift. To interpret bird responses, we used stable isotope analyses of prey and trawl survey-derived abundance of forage size fish. Stable isotope values in blood tissues revealed no change in the prey base as reflected in the isotopic space used by the seabird community across the four study years. Sea-ice loss was, however, associated with increased nutritional stress in all seabird species and diverging foraging niches within guilds. Benthic forage-sized fish remained a key food source, with their abundance stable since 2010, and benthic-foraging thick-billed murres experienced relatively low nutritional stress across the study period. A significant increase in common murre nutritional stress levels between 2016 and 2017, and a shift in their isotopic niche indicative of higher reliance on benthic prey coincided with a decline in the abundance of pelagic forage fish in the region. Surface-foraging black-legged kittiwakes experienced steady increases in nutritional stress as the abundance of pelagic forage fish declined. The spring sea-ice loss was detrimental to planktivorous least and crested auklets that rely on zooplankton advected from the Bering Sea basin. During the study period both auklets have experienced severe nutritional stress (2018) and colony-wide reproductive failures (2018 and 2019). In conclusion, we found that increasingly warm conditions during 2016–2019 have challenged the adaptive ability of seabirds relying on pelagic fish and zooplankton advected into the region each summer, but have not yet resulted in a reorganization of the northern Bering Sea's benthic-dominated ecosystem.

Use of stable isotopes in the evaluation of fish trophic guilds from a tropical hypersaline lagoon

AbstractEnvironmental factors, size-related isotopic changes of the most abundant species and isotopic niche overlap were investigated using stable isotopes in order to evaluate spatial changes of fish trophic guilds in the Araruama Lagoon. Based on 440 muscle samples, 17 fish species were grouped into five trophic guilds. Mean salinity was above 40 at both sites sampled and a significant spatial difference was observed. The highest δ13C mean value was observed for an omnivorous species, whereas the lowest carbon signatures were found for the three fish species belonging to the planktivorous guild. Analysis of the carbon signature of fish species in lower trophic levels showed influence of salinity variation, whilst size appeared to play a role for others. A narrow δ15N difference was observed, but the piscivorous fish species showed the highest δ15N values. The Standard Ellipses Analysis (SEA) detected spatial differences and varying degrees of isotopic niche overlap among trophic guilds, but the percentages of most overlaps (<60%) suggest that, to some extent, the guilds had a unique isotopic niche space. These results are in agreement with data previously reported for the Araruama Lagoon, that found the same prey items with varying relative importance among the most abundant species. Further studies are necessary to understand how the interaction between salinity and other factors, such as migration patterns, changes in prey availability, changes in contribution of primary sources and changes in baseline isotopic signatures could affect the stable isotope signatures shown here.

Intra-tooth stable isotope profiles in warthog canines and third molars: Implications for paleoenvironmental reconstructions

Intra-tooth stable isotope variations have been used to interpret seasonality and aridity in paleoenvironmental reconstructions of paleontological and archeological sites. However, most intra-tooth datasets only permit qualitative interpretations of seasonality, because the measured signal is attenuated due to the duration of enamel mineralization process and sampling geometry. The common warthog (Phacochoerus africanus) is an ideal organism to investigate stable isotope variation in enamel. Their canines grow continuously through the life of the individual and are therefore excellent candidates for mathematical modeling of seasonal signals and of signal attenuation; further, their isotope profiles (a series of isotope measurements) can be compared to isotope profiles of third molars (M3) to provide insights into environmental reconstructions. We first obtained paired intra-tooth enamel samples from ever-growing canines and hypsodont M3s of two extant common warthog specimens from Laikipia, Kenya. Second, from a different set of specimens, we collected data on enamel growth patterns and geometry using histological thin sections and transmitted light microscopy, and enamel mineralization parameters using micro-CT scans in each tooth type. Third, we reconstructed the timeline of unattenuated seasonal δ18O signal from canine enamel using growth rate estimates and the inverse model of Passey et al. (2005). Our results demonstrate that canines, which capture ~1.5 years of time, exhibit near-constant growth rates and simple enamel maturation geometry, whereas M3s, which also represent ~1.5 years of time, exhibit linearly decreasing growth rates and more complex maturation patterns. We compare the timelines of unattenuated seasonal δ18O signal and measured M3 profiles and find an average signal reduction of ~50% in the M3s, providing interpretations of the duration of seasonal cycles that are consistent 75% of the time. We conclude that warthog canines are well suited for the inverse model approach, and we established the model parameters for the forward and inverse methods. Timeline reconstructions based on M3 histology are promising for investigating the pattern of rainfall seasonality in the past. Finally, we found an unexpected carbon isotopic spacing of ~2‰ between canine and M3 enamel, which suggests caution in interpreting δ13C results from suid canine or molar enamel alone.

Fingerprinting subducted oceanic crust and Hainan Plume in the melt sources of Cenozoic Basalts from the South China Sea Region

AbstractGeochemical data compilation of Cenozoic basalts recovered from the South China Sea tectonic domain shows westward weakening of the influence of a focal zone‐like component in Nd–Hf, Nd–Pb and Sr–Pb, but not in Pb–Pb isotope spaces because the Pb isotopes are dominantly controlled by the high U/Pb component derived from the subducted Pacific oceanic slab. Low Th/U melt generated by recycling of marine carbonates, rather than the subduction‐related enriched mantle (EM2), signals the emplacement of the Hainan Plume at ~25 Ma. Radiogenic Hf in the pre‐existing ancient sub‐continental lithospheric mantle beneath the Cathaysia Block was greatly depleted by early‐stage magmatism influenced by the high U/Pb component. Hence, late Cenozoic basalts associated with the carbonatitic melts display contrasting Nd–Hf isotope covariations, with the Red River–Zhongnan Fault System as a dividing line, implying that the East and Southwest sub‐basins have been developed on the Cathaysia and Indochina Blocks respectively.

Isotopic composition of lead white pigments on qeros: implications for the chronology and production of Andean ritual drinking vessels during the colonial era

For millennia, qeros have been a primary component of ceremonially and politically important toasting rituals in the Andes and retain their cultural significance to this day. These wooden drinking vessels underwent a stylistic and technical revolution in the colonial period (1532–1821 AD). Among an array of features that distinguish colonial qeros from their Inka predecessors is the presence of lead white, a pigment that was introduced to the Andes by the Spanish. Here, we present lead (Pb) isotope measurements of lead white pigments from 20 colonial qeros from the collections of the National Museum of the American Indian, the American Museum of Natural History, the Brooklyn Museum of Art, the Metropolitan Museum of Art, and in a private collection. Although most of the vessels are not known to be associated with one another, their lead white pigments fall into three distinct and internally consistent groupings in Pb isotope space. We interpret the isotopic signatures of two of the groups to indicate that the lead white was imported from Europe. We suggest that the largest grouping (comprising pigments sampled from 12 qeros) is decorated with lead white of Andean origin. These isotopic signatures may have a chronological component, and strongly suggest some form of centralization in pigment acquisition, manufacture, and/or distribution in the colonial period.

Stable Isotope Ecology of Invasive Lionfish (Pterois volitans and P. miles) in Bermuda

Over the past three decades, lionfish (Pterois volitans and P. miles) from the Indo-Pacific have invaded the northwest Atlantic Ocean, Caribbean Sea and the Gulf of Mexico. As generalist predators with a broad diet, they could pose a major threat to economically and ecologically important species and, therefore, the overall health of coral reef ecosystems and the communities that rely on them. In Bermuda, the lionfish invasion appears to be progressing at a slower rate than elsewhere in the invaded range, providing an opportunity to study their ecological impact at an early stage. This study used stable isotope analysis of lionfish, their major prey, and their competitors to investigate the feeding ecology of lionfish in Bermuda and provide a more complete evaluation of their potential impacts on community structure and trophic interactions. Results suggest that lionfish in Bermuda primarily derive resources from the plankton-based food web, with only a small contribution from food chains supported by macroalgae. Further, it appears that lionfish resource use overlaps substantially with other similarly-sized sympatric mesopredators, in particular the coney grouper (Cephalopholis fulva), a species commonly targeted by local commercial fisheries. This visualization of Bermuda’s demersal ecosystem in two-dimensional isotope space, the first of its kind, will help track the ecological impact of lionfish over time, predict potential changes in community structure, and better inform developing control strategies for this invasive species.

Rapid and sensitive determination of antifouling organotin compounds in sediments using Gas chromatography with Tandem MS (GC – MSMS / GC – QTOF)

ABSTRACT Organotin compounds are causing deterioration effect on non-targeted aquatic species in the environment, such as shell anomalies in oysters and imposex in gastropods, even at concentrations as low nanograms per litre. Specific analytical methods have been developed for different environmental matrices. Analysis of organotin compounds in sediment because of hydrophobic properties and tend to adsorb onto particulate matter is valuable for assessing the environmental distribution and fate. Therefore, in this study, a rapid, reliable, sensitive and effective method developed for quantification and identification of antifouling organotin butyl- and phenyltin compounds in sediment samples by using Gas chromatography with Tandem MS detector (GC-MSMS and GC-QTOF). The method is based on the single extraction step followed by GC-MSMS and GC-QTOF detection. The sample extraction steps were decreased for low labour laboratory costs and time-consuming. Not only the extraction procedure but instrumental method parameters were carefully optimised in order to increase sensitivity and precision. The QTOF system with the help of mass accuracy and mass resolution showed a good sensitivity and precision to analyse sediment samples. The isotopic disturbance and utilisation isotopic space for confirmation of organotin compounds have been scrutinised in GC-QTOF. GC Triple Quadrupole (GC-MSMS) system also utilised as a secondary confirmation method for organotin compound analysis. Additionally, the sensitivity of both MS detectors under the matrix effect was investigated. The recoveries of extraction were in the range of 82–106%. The acquired results have shown that the concentration range in 10–250 ppb (ng/ml) level was denoted good linearity (r2 = 0.9725–0.9983). Moreover, limits of detection (LOD) for both butyl and phenyl fractions of organotin compounds were ranged between 0.23 and 22.11 pg/ml. Consequently, the developed method successfully was applied to certificated reference material (BCR – 462) and the targeted organotin compounds in a real marine/harbour sediment sample.

Sr–Pb isotopes signature of Lascar volcano (Chile): Insight into contamination of arc magmas ascending through a thick continental crust

In the Central Volcanic Zone of the Andean cordillera (CVZ), thickening of the continental crust during the last 30 Ma led to the production of increasingly differentiated magmas. Better constrain on the interactions between ascending magmas and the thick continental crust are needed to understand the contamination processes and their implications on the geochemical diversity of arc magmas.Lascar volcano figures among the most active Quaternary volcano of the CVZ and yet geochemical studies on its volcanic products remain scarce. We lead major-, trace elements and Pb–Sr isotopes measurements on 10 new Lascar samples representative of the different petrologic types (andesitic to dacitic).206Pb/204Pb (18.770–18.823), 207Pb/204Pb (15.636–15.655), 208Pb/204Pb (38.711–38.809) and 87Sr/86Sr isotope ratios (0.705731–0.706564) show small variations within the previously defined isotopic field of Lascar. In Pb–Sr or Pb–Pb isotope space, the data points show a triangular distribution that requires the contribution of at least three end-member components at the source of Lascar magmas. We built up a two-stages three-components assimilation fractional crystallization model (AFC) that reproduces the compositional range of Lascar magmas. It allows us to constrain the composition and nature of the three end-member components: (BA) a common mantellic source with low 87Sr/86Sr (<0.70575), low 206Pb/204Pb (<18.768), 207Pb/204Pb (<15.637) and 208Pb/204Pb (<38.711), (LC) a lower crust and/or slab component radiogenic in Sr (87Sr/86Sr > 0.70656) and in Pb (206Pb/204Pb > 18.82; 207Pb/204Pb > 15.647; 208Pb/204Pb > 38.79), (UC) a felsic upper continental crust (>65.1 wt% of SiO2) with similar Sr–Pb isotope composition as LC (87Sr/86Sr > 0.70643; 206Pb/204Pb > 18.823; 207Pb/204Pb > 15.655; 208Pb/204Pb > 38.809). Both BA and LC components occur in lavas with <57 wt% SiO2.This model predicts that Lascar magmas assimilated ≤30% of lower crust and ≤29% of upper crust. Our conclusions are in good agreement with the CVZ literature that reports the contribution of three ubiquitous end-member components at the source of CVZ magmas, with similar natures and origins.

The role of Bells in the continuous accretion between the CM and CR chondrite reservoirs.

CM meteorites are dominant members of carbonaceous chondrites (CCs), which evidently accreted in a region separated from the terrestrial planets. These chondrites are key in determining the accretion regions of solar system materials, since in Mg and Cr isotope space, they intersect between what are identified as inner and outer solar system reservoirs. In this model, the outer reservoir is represented by metal‐rich carbonaceous chondrites (MRCCs), including CR chondrites. An important question remains whether the barrier between MRCCs and CCs was a temporal or spatial one. CM chondrites and chondrules are used here to identify the nature of the barrier as well as the timescale of chondrite parent body accretion. We find based on high precision Mg and Cr isotope data of seven CM chondrites and 12 chondrules, that accretion in the CM chondrite reservoir was continuous lasting <3 Myr and showing late accretion of MRCC‐like material reflected by the anomalous CM chondrite Bells. We further argue that although MRCCs likely accreted later than CM chondrites, CR chondrules must have initially formed from a reservoir spatially separated from CM chondrules. Finally, we hypothesize on the nature of the spatial barrier separating these reservoirs.

Functional and trophic diversity of fishes in the Mekong-3S river system: comparison of morphological and isotopic patterns

The Lower Mekong River and its three major tributaries, the Sekong, Sesan, and Srepok rivers, are systems with high aquatic biodiversity that now face impacts from new hydroelectric dams. Despite the ecological, economic, and cultural importance of the freshwater biota, knowledge about aquatic ecology in this region remains poor. We used morphological and stable isotope data to explore how fish functional and trophic diversity vary between the four rivers that comprise the Mekong-3S river system. During our field surveys, the Sesan had experienced greatest flow alteration from dams and had lowest taxonomic and functional diversity, with species less packed and less evenly dispersed within morphological space compared to the other rivers. The Sekong had greatest functional diversity, with species less packed in morphological space. Species in the Mekong and Srepok were more evenly distributed in morphological space and had intermediate levels of functional diversity. Isotopic niche diversity in the Sesan did not appear to be significantly different from the Srepok and Sekong rivers. Conversely, the more species-rich Mekong fish assemblage encompassed a greater isotopic space with species less packed and evenly distributed. Greater trophic redundancy was observed amongst fishes of the 3S rivers than the Mekong. Species functional redundancy could buffer river food webs against species loss caused by dams; however, additional traits and niche dimensions should be evaluated to test this hypothesis. Overall, morphological and isotopic evidence from the Mekong and 3S rivers indicate that river impoundment and flow regulation function as an environmental filter that reduces fish functional diversity.

Assessing trophic flexibility of a predator assemblage across a large estuarine seascape using blood plasma stable isotope analysis

The northcentral Gulf of Mexico (nGoM) encompasses a range of environments that support a speciose predatory assemblage. Large predatory species are often viewed as trophically-analogous, sharing similar isotopic niches. To investigate the regional and seasonal isotopic variability of a predator assemblage across the coastal waters of the nGoM (Louisiana, Mississippi, and Alabama), we analyzed the stable isotopes of carbon, nitrogen, and sulfur using a fast turnover tissue, blood plasma, for seven dominant predator species. Carbon, nitrogen, and sulfur isotope values varied regionally, with a significant interaction between region and season for nitrogen and sulfur. Species' isotopic niches varied regionally, as well as seasonally, leading to varying levels of isotopic niche overlap among species. Blacktip sharks Carcharhinus limbatus and Atlantic sharpnose sharks Rhizoprionodon terraenovae in particular demonstrated flexibility in their isotopic relationships, whereas blacknose sharks C. acronotus maintained a more consistent isotopic niche regardless of region or season. No biologically significant overlap was observed for blacknose sharks, bull sharks C. leucas, or spinner sharks C. brevipinna, suggesting that these species occupy distinct isotopic space. Overlap calculations using two isotopes (carbon and nitrogen) produced markedly higher overlap versus three isotopes (carbon, nitrogen, and sulfur), which demonstrates that predatory roles may be oversimplified when using only carbon and nitrogen. These findings highlight the value of examining seasonal variation in trophic roles using fast turnover tissues and provide the first triple-isotope characterization of a common predatory assemblage in the northcentral Gulf of Mexico.

Triple oxygen isotope systematics as a tracer of fluids in the crust: A study from modern geothermal systems of Iceland

Triple oxygen isotope systematics of ancient hydrothermally altered rocks has been previously used to constrain environmental conditions of the Precambrian. To validate those studies, we report high-precision triple oxygen isotope measurements (expressed as Δ′17O with reference slope 0.528) of quartz, epidote and well fluids from modern geothermal areas of Iceland, Krafla and Reykjanes, as well as measurements from the extinct 6 Ma Geitafell hydrothermal system. At these systems, basalts reacted with distinct fluid sources at temperatures ranging from 250 to 400°C. Resolvable difference between isotope compositions of surface waters and rocks enables novel insights into boiling, isotope exchange at variable water-rock ratios, and remelting of altered rock. Our measurements of δD, Δ′17O, and δ′18O in well fluids show that the reactions proceeded at water-rock ratios of 0.1 to 2, and reveal the addition of meteoric water in the Reykjanes system, and near-surface boiling and steam-liquid separation at Krafla. The δ′18O and Δ′17O values of fluids shift due to exchange with rocks at high temperature following the slope 0.51 in the triple isotope space. Near-surface boiling and steam-liquid separation cause shifts in δD and δ′18O values of the fluids. Their Δ′17O values do not change significantly owning to the shallow slope of liquid-vapor equilibrium fractionation relative to the reference line slope. Epidote δ′18O and Δ′17O values in all three localities closely resemble the isotope composition of local fluid sources. The measured slope of triple oxygen isotope fractionation between quartz and epidote at 250–400°C is 0.526±0.001. We suggest that triple oxygen isotope composition of epidote can be used as a direct first-order proxy for the equilibrium fluid δ′18O and Δ′17O values. The calibrated quartz-water equilibrium fractionation for triple oxygen isotopes yields general agreement with the local fluid sources, within ±1.5‰ of their δ18O values, while the Δ′17O agree within ±0.02‰. We present in situ δ18O measurements in a quartz crystal from Krafla that show several ‰ heterogeneities which may affect the reconstructed equilibrium fluid values. We tested the effect of shallow crustal contamination on the Δ′17O values of rhyolitic glasses from Krafla, including those quenched and extracted by drilling, that likely formed by assimilation of low-δ18O hydrothermally altered crust. Our Δ′17O measurements constrain the degree of crustal assimilation to 10–20 %. Our study shows that the Δ′17O values measured in geothermal fluids, secondary minerals and low δ18O contaminated magmas can provide key information on the conditions of water-rock reaction and magma genesis, and contain additional details that were not accessible through conventional analyses of δD and δ18O.

Geochemistry of volcanic rocks from Oldoinyo Lengai, Tanzania: Implications for mantle source lithology

Oldoinyo Lengai is the only active carbonatite volcano within the East African Rift Valley in northern Tanzania. The volcano is dominated by peralkaline silicate rocks with natrocarbonatites. This study presents new mineralogical and geochemical data, including Sr–Nd–Pb–Hf–Mg isotopic compositions, for volcanic rocks at Oldoinyo Lengai and lavas from the nearby Gregory Rift Valley. The samples analyzed in this study include olivine melilitite, melanephelinite, wollastonite nephelinite, and phonolite. The olivine melilitites and melanephelinites have highly fractionated rare-earth-element (REE) patterns with (La/Yb)N values of 26.4–64.9, suggesting that they formed from magmas generated by low-degree (up to ∼7%) of partial melting within the garnet stability field. The wollastonite nephelinites have much higher (La/Sm)N values but lower (Sm/Yb)N values relative to typical oceanic island basalts (OIB), with flat heavy REE patterns [(La/Yb)N = ∼22]. The phonolites have elevated REE abundances but with patterns intermediate between the other two sample groups [(La/Yb)N = ∼41]. All samples have primitive-mantle-normalized incompatible element patterns that are characterized by negative K and Rb anomalies but no significant Eu anomalies. They also have elevated Yb contents relative to the compositions of modeled garnet peridotite-derived melts, suggesting that they were derived from a sublithospheric source containing enriched HIMU-like recycled oceanic crustal material. However, the wollastonite nephelinites have significantly positive Ba, U, Sr, and Pb anomalies similar to those found within the Oldoinyo Lengai natrocarbonatites. The wollastonite nephelinites might have been sourced from a region of sub-continental lithospheric mantle (SCLM) that was previously metasomatized by interaction with carbonatite melts. The phonolites in the study area have also weakly positive Pb and Sr anomalies indicative of some interaction with the SCLM. All samples have δ26Mg values (−0.39‰ ± 0.07‰) lighter than the composition of normal mantle material (−0.25‰ ± 0.04‰). In addition, a negative correlation between δ26Mg values and MgO concentrations suggests derivation from a source region containing recycled carbonate. The samples from the study area define a mixing array between HIMU- and EM1-type OIB in Sr–Nd and Pb–Pb isotopic correlation diagrams, and have pronounced Nd–Hf isotopic decoupling, plotting below the mantle regression line in Nd–Hf isotopic space. The negative deviation from the Nd–Hf isotopic mantle array and the presence of an EM1-type mantle component in the Sr–Nd isotopic compositions of the Oldoinyo Lengai volcanic rocks can be generated by recycling of E-MORB-type oceanic crustal material with an age of 1.5–1.0 Ga.

Heterogeneous mantle melting and magmatic processes at the East Pacific Rise (2.6–3.1°S): Evidence from mid-ocean ridge basalt geochemistry and Sr–Nd–Pb isotopes

ABSTRACT We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6–3.1°S. These samples are low-K tholeiites and show significant variation in their major element compositions (e.g. 4.60–8.18 wt% MgO, 8.34–12.12 wt% CaO, 9.78–14.25 wt% Fe2O3, and 0.06–0.34 K2O wt%). Trace element abundances of the 2.6–3.1°S MORB are variably depleted (e.g. (La/Sm), N = 0.51–0.78, Zr/Y = 2.35–3.42, Th/La = 0.035–0.056, and Ce/Yb = 2.38–3.96) but closely resemble the average N-MORB. In the compatible elements (Ni and Cr) against incompatible element Zr plots, the 2.6–3.1°S MORB show well-defined negative correlations, together with a liquid line of descent (LLD) modelling and petrographic observations, implying a significant role of olivine, plagioclase and clinopyroxene fractionation during magma evolution. When compared to global MORB and peridotites, the 2.6–3.1°S MORB and most of the other axial lavas from the South EPR show similar Zn/Fe, Zn/Mn, and Fe/Mn ratios, attesting to a peridotite-dominated mantle lithology. However, the relationships between incompatible trace element ratios, such as Zr/Rb and Nb/Sm, and the negative correlation between Zr/Nb and 87Sr/86Sr indicate a geochemically heterogeneous mantle source. The mantle beneath the South EPR likely consists of two components, with the enriched component residing as physically distinct domains (e.g. veins or dikes) in the depleted peridotite matrix. In the Sr–Nd–Pb isotope space, the South EPR MORB lie along the mixing lines between the depleted MORB mantle (DMM) and the ‘C’-like Pukapuka endmember. We infer that low-F melts derived from these enriched materials may cause localized mantle heterogeneity (veins or dikes) via an infiltration process. Subsequent melting of the refertilized mantle may impart an isotopically distinct characteristic to South EPR MORB.

Isotopic niche is not equal to trophic niche.

The work of Sheppard et al. (Ecol. Lett., 21, 2018, 1395) relies on the strong assumption that isotopic niche is equal to trophic niche. Here I raise three main concerns showing that classic hypotheses built upon trophic niche cannot be directly interpreted in isotopic space. Future studies should always keep isotopic and trophic niches distinct.

Nucleosynthetic Sr–Nd Isotope Correlations in Chondrites: Evidence for Nebular Thermal Processing and Dust Transportation in the Early Solar System

Abstract We conducted high-precision Sr and Nd isotopic measurements in bulk chondrites using a complete sample digestion technique. Our new data indicate that enstatite and ordinary chondrites possess uniform and small, but resolvable, Sr and Nd isotopic deviations from terrestrial rocks. In contrast, the Sr isotope ratios varied across different classes of carbonaceous chondrites (CM, CO, and CV). The deviation of data from the s-process mixing line in Sr–Nd isotopic space likely resulted from the incorporation of calcium-aluminum-rich inclusions (CAIs) into carbonaceous chondrite parent bodies. Planetary-scale Sr and Nd isotopic heterogeneities among terrestrial rocks, enstatite, ordinary chondrites, and CAI-subtracted carbonaceous chondrites suggest a heterogeneous distribution of s-process-enriched materials in the early solar system, probably caused by nebular thermal processing. The observed Sr and Nd isotopic variation across the CAI-subtracted carbonaceous chondrites cannot be explained solely by nebular thermal processing, but is likely attributable to s-process-depleted silicate grains that repeatedly circulated among the early solar system. These grains were transferred and incorporated at varying degrees into the formation region of the parent bodies of individual carbonaceous chondrites.

Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands

Mantle plume models have been widely applied to explain the formation of ocean island basalts (OIB), with high-3He/4He in their lavas being explained by sampling of a primitive deep mantle source. The Juan Fernandez Islands have 3He/4He (7.8–18 RA) similar to or higher than in mid-ocean ridge basalts (MORB; 8 ± 1 RA) and have been used to both support and refute the mantle plume hypothesis. Ambiguity regarding the origin of the Juan Fernandez Islands primarily originates from interpretation of mantle source signatures between the lava series from the two main islands, Robinson Crusoe and Alexander Selkirk. To examine this issue, we report new whole-rock and olivine separate 187Os/188Os ratios and major-, trace-, and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances. The HSE and trace element abundances in Juan Fernandez main shield lavas can be explained by up to 30% olivine removal, together with spinel crystallization at 1–5 kbar, whereas Robinson Crusoe rejuvenated lavas can be reproduced by higher-pressure fractional crystallization (up to 10 kbar). An assemblage of 30 modal % olivine and 1–5 modal % spinel, combined with the additional contribution of primary melt trapped in olivine inclusions reproduces the range of HSE compositions observed in Juan Fernandez Archipelago olivine grains. Ratios of 187Os/188Os for Juan Fernandez lavas are generally less radiogenic than global OIB and show no correlation with indices of fractionation, indicating that they reflect mantle source compositions. Younger basanite lavas from Robinson Crusoe represent rejuvenated volcanism dominantly from a depleted lithospheric mantle source (187Os/188Os < 0.13) mixed with a high-3He/4He component from the main shield building stage. These lavas are similar in origin and composition to other Pacific rejuvenated lavas (e.g., Samoa, Hawaii). Robinson Crusoe main shield lavas are from a high-3He/4He (>18 RA) and enriched mantle source (187Os/188Os = 0.1312) similar to the ‘C’ or ‘FOZO’ component, whereas Alexander Selkirk lavas are consistent with a dominant contribution from a depleted, low-3He/4He (<10 RA) mantle component. The mantle sources of the shield lavas yield subtle variations in Sr-Nd-Os-Pb isotope space and have no clear variations with relative or absolute abundances of the HSE or trace elements. These results are consistent with a heterogeneous mantle plume model, with initial eruption of lavas from a primitive high-3He/4He mantle source ∼4 million years ago (Ma) to form Robinson Crusoe Island, which also led to 3He enrichment of the oceanic lithosphere, followed by eruption of Alexander Selkirk lavas from a more depleted mantle source at ∼2 Ma. Juan Fernandez lavas show that mantle heterogeneity preserved in OIB can occur over short-timescales (<2 Ma) and can impact lithospheric compositions, leading to eruption of rejuvenated lavas with unusual isotopic characteristics.

Reconciling petrological and isotopic mixing mechanisms in the Pitcairn mantle plume using stable Fe isotopes

Ocean-island basalts (OIB) are the products of intra-plate mantle melting above mantle plumes. These hot, buoyant upwellings rise from the deep mantle, possibly the core-mantle boundary, to the Earth's surface. Radiogenic isotope signatures of trace elements in OIB are consistent with a variety of crustally-derived recycled components in their source that are suggested to reside in the plume as eclogite or their pyroxenitic melt-reaction products. The global OIB compositional spectrum exhibits a continuum of radiogenic isotope characteristics (Sr-Nd-Pb-Hf) that indicate mixing of these plume components. However, the petrological mechanisms underpinning this mixing remain elusive. Here, we report stable Fe isotope data for plume-derived lavas of Pitcairn Island from the southwest Pacific. Iron isotope compositions, corrected for olivine fractionation, show a strong co-variation with radiogenic Nd-Sr-Pb isotopes. At Pitcairn, two isotopically distinct components have been identified, one being an enriched-mantle 1 (EM-1) component and the other being a more primitive component, which plots in multiple radiogenic isotope space in the so-called focal zone (FOZO), tentatively identified here as the plume matrix. We suggest a two-stage scenario in which Fe isotopes are first fractionated towards heavier values during formation of a reaction-zone pyroxenite. Partial silicate melt derived from a recycled crustal eclogite, assumed here to be of a subducted sediment origin, reacts with ambient peridotite to form an isotopically heavier Fe isotope pyroxenite with memory of its precursor in radiogenic isotopes. Subsequently, both, isolated and enriched reaction zone pyroxenites and ambient plume matrix reach their solidus and mixing of their partial melts to variable proportions create the spread in radiogenic and stable isotope systematics. This mixing of components is coherent with the widely observed disparity in trajectories between plume matrix and enriched components in long-lived (Sr-Nd-Pb-Hf) and extinct (W) radiogenic and stable (Fe-Mg) isotope systematics and marks a fundamental process for chemical diversity in OIB. It supports a model of isolated plume components in a primitive mantle matrix. Enriched components in the plume that contribute to this mixing appear to be solely pyroxenitic with no or very little direct contribution from the original eclogite. Timing and loci of pyroxenite formation remain to be elucidated.

An environmental baseline for food webs at deep-sea hydrothermal vents in Manus Basin (Papua New Guinea)

Food webs and trophic interactions provide a measure of ecosystem function and have been highlighted as an important element for environmental baselines and environmental impact assessments. Stable isotopes have long been used in the deep sea as a means to explore and summarize food webs, especially in chemosynthetic ecosystems, where there may be multiple sources of primary production with distinctive isotopic compositions. Hydrothermal vents in Manus Basin have been targeted for mining and, if mining proceeds, vent food webs will be altered. Here we describe a pre-mining baseline for the food-web state for 3 active vent fields. Manus vents support at least 9 feeding guilds, and both reverse TriCarboxylic Acid (rTCA) and Calvin-Benson-Bassham (CBB) carbon fixation pathways are inferred for symbionts of holobiont taxa. There are site differences in the C-N-S isotopic trait spaces (convex hulls) of Manus Basin vent food webs that likely reflect environmental variability. Isotopic diversity metrics (divergence, dispersion, evenness) are similar in Solwara 1, South Su, and PACMANUS vent food webs, but South Su has a higher degree of isolation in isotopic space (uniqueness). Because convex hull and isotopic diversity metrics are easy to calculate from multidimensional isotopic data, they may be useful food-web indicators for environmental management.