Enriched Isotope Ratios Research Articles

Characterizing Peridotite Xenoliths From Southern Vietnam: Insight Into the Underlying Lithospheric Mantle

AbstractWe present data for lithospheric mantle xenoliths sampled from two alkali basalts in south‐central Vietnam, Pleiku and Xuan Loc, including fertile spinel peridotites. To better determine the origins of the Indochinese subcontinental lithospheric mantle (SCLM), including impacts of posited tectonic extrusion, we present major and trace elements, and 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in xenolith mineral separates. Most peridotites from Pleiku and Xuan Loc have fertile major element compositions, “depleted” and “spoon‐shaped” rare earth element (REE) patterns, interpreted to record prior melt depletion followed by melt metasomatism, and variable but generally depleted isotopic signatures (e.g., 87Sr/86Sr = 0.70238–0.70337 and 143Nd/144Nd = 0.512921–0.514190). A small group of refractory peridotites have “enriched” REE patterns suggesting more extensive metasomatism and enriched isotope ratios (87Sr/86Sr = 0.70405 and 143Nd/144Nd = 0.512755–0.512800). The presence of both fertile and refractory xenoliths records a heterogeneous SCLM beneath Vietnam. Based on geothermobarometry calculations, fertile xenoliths have equilibrium temperatures of 923–1,034°C and pressures of 11.7–15.8 kbar, while refractory xenoliths have comparable temperatures of 923–1,006°C, but lower pressures of 7.1–10.0 kbar, suggesting refractory rocks are dominantly present at shallower depths. We suggest that the lithospheric mantle has experienced variable melt extraction around 1.0–1.3 Ga, producing heterogeneous major element compositions. While we cannot rule out partial removal and replacement of the lithosphere, large‐scale delamination is not necessary to explain observed characteristics. The entire SCLM was more recently metasomatized by melts resembling Cenozoic basalts, suggesting recent asthenospheric melting has modified the SCLM by melt infiltration.

Backward Trajectory Analysis Based on Specific Humidity Correction for the Influence of Moisture Sources on Precipitation Isotopes in the Western Loess Plateau, China

Based on the precipitation samples collected in Minxian in the western Loess Plateau from 2019 to 2021, this study analyzes the influence of moisture sources on precipitation isotopes. Through the specific humidity correction of the backward trajectory, it is found that Xinjiang and the middle and lower reaches of the Yangtze River may be the main moisture sources. Through cluster analysis of backward trajectories, it is found that in the summer half-year, the water vapor from the east and northeast leads to the precipitation with a depleted isotope ratio, while the water vapor from the south and northwest leads to the precipitation with an enriched isotope ratio; in the winter half, water vapor from the east leads to precipitation with enriched isotope ratios, while water vapor from the northwest and northeast leads to precipitation with depleted isotope ratios. In addition, the precipitation isotope values showed a tendency to deplete with the duration of water vapor transport in the summer half-year, but this tendency was not obvious in the winter half-year. The relationship between precipitation isotopes and water vapor transport height showed a positive correlation in both the winter half-year and summer half-year. Using the Potential Source Contribution Factor analysis methods and the backward trajectory after the specific humidity revision, it was found that the potential evaporation source areas in the summer half-year are larger in extent, mainly distributed in the eastern and southern regions of the sampling site, and the contribution of local recycled vapor to precipitation is 32.17%; while the potential evaporation source areas in the winter half-year are smaller in extent, they are only distributed in the southern region of the sampling site, and the contribution of local recycled vapor to precipitation is 24.66%.

Syn-collisional magmatic record of Indian steep subduction by 50 Ma

Abstract Subduction of Indian continental lithosphere during the Asia-India collision played an important role in the formation and evolution of the Himalaya-Tibetan orogen. However, the geometry of early Indian continental subduction remains debated. Given that the Indian continent is characterized by enriched isotope ratios (87Sr/86Sr > 0.730, εNd(t) < −10), relative to those in subducted oceanic materials (87Sr/86Sr < 0.704, εNd(t) ≈ +8), changes in the composition of magmatic rocks with time, in particular their radiogenic isotope ratios, is used to constrain the timing and nature of continental subduction. This study reports the field relations, zircon U-Pb ages and geochemical composition of a syn-collisional batholith that crosscuts the central Indus-Yarlung Zangbu suture in the Saga area of southern Tibet. Zircon U/Pb ages for the batholith mainly range from 50 to 46 Ma. Samples from the Lopu Range batholith have enriched zircon Hf (εHf(t) = −0.4 to −8.6) and whole rock 87Sr/86Sri = 0.7094–0.7121 and εNd(t) = −7.3 to −9.8, suggesting that they were derived from a mixture of juvenile Gangdese and isotopically enriched Indian crustal materials. This result indicates that subduction of Indian crustal rocks occurred before 50 Ma in the central Himalaya. The geochemical composition and distribution of high volume ca. 51 Ma magmatism in the Gangdese belt, combined with thermal models of the subduction zone, suggests a steepening of the subducted Indian continental lithosphere occurred between the onset of India-Asia collision (59 Ma) and 46 Ma in the central-eastern Himalaya.

Using stable isotopes to identify major flow pathways in a permafrost influenced alpine meadow hillslope during summer rainfall period

AbstractGlobal warming has leaded to permafrost degradation, with potential impacts on the runoff generation processes of permafrost influenced alpine meadow hillslope. Stable isotopes have the potential to trace the complex runoff generation processes. In this study, precipitation, hillslope surface and subsurface runoff, stream water, and mobile soil water (MSW) at different hillslope positions and depths were collected during the summer rainfall period to analyse the major flow pathway based on stable isotopic signatures. The results indicated that (a) compared with precipitation, the δ2H values of MSW showed little temporal variation but strong heterogeneity with enriched isotopic ratios at lower hillslope positions and in deeper soil layers. (b) The δ2H values of middle‐slope surface runoff and shallow subsurface flow were similar to those of precipitation and MSW of the same soil layer, respectively. (c) Middle‐slope shallow subsurface flow was the major flow pathway of the permafrost influenced alpine meadow hillslope, which turned into surface runoff at the riparian zone before contributing to the streamflow. (d) The slight variation of δ2H values in stream water was shown to be related to mixing processes of new water (precipitation, 2%) and old water (middle‐slope shallow subsurface flow, 98%) in the highly transmissive shallow thawed soil layers. It was inferred that supra‐permafrost water levels would be lowered to a less conductive, deeper soil layer under further warming and thawing permafrost, which would result in a declined streamflow and delayed runoff peak. This study explained the “rapid mobilization of old water” paradox in permafrost influenced alpine meadow hillslope and improved our understanding of permafrost hillslope hydrology in alpine regions.

Elucidating the climate and topographic controls on stable isotope composition of meteoric waters in Morocco, using station-based and spatially-interpolated data

Understanding the main controls on stable isotope variations in precipitation is fundamental for the interpretation of the hydrological cycle. However, spatio-temporal variations in δ18Op are poorly known in Morocco. Herein, we explore the relative influence of meteorological variables, spatial and orographic (altitudinal) effects, atmospheric circulation and moisture sources on precipitation stable isotopes in Morocco. Precipitation events and two-years-long monthly records from 17 rain-gauge stations in Morocco are investigated and compared in this study to global gridded records of monthly and annual stable isotopes in precipitation. We highlight that the main spatial controls on precipitation stable isotopes are the topography and the distance from marine source. The most depleted mean annual isotopes are located in the High Atlas Mountains (δ18Op=−9.56‰ and δ2Hp=−59.3‰), while the most enriched isotope ratios exist in southwestern Morocco (δ18Op=−2.35‰ and δ2Hp=−7.47‰). The well-constrained relationship between δ18Op and altitude describes a gradient of 0.11–0.18‰ per 100m. The seasonal variation is expressed by a general enrichment that reaches −4.8‰ during the dry season, related to the recycled vapor contained within the summer precipitation. Notwithstanding the scarcity of temperature and precipitation measurements, the amount effect is observed in multiple stations during several rain events and precipitation seems to have more influence on δ18Op than temperature. Backward moisture trajectories indicate a distinct depletion in δ18Op in extreme events originating from the Atlantic Ocean. The presence of a rain shadow effect is also revealed on the lee side of High Atlas Mountains, southeastern Morocco.

Insights from stable isotopes and hydrochemistry to the Quaternary groundwater system, south of the Ismailia canal, Egypt

Summary Stable isotopic ratios of water (δ 18 O and δ 2 H) and major dissolved ions were analyzed from samples collected from the area south of the Ismailia canal, Egypt. This region has recently experienced rapid growth that has increased both surface and groundwater use. The data from the samples were used to identify recharge sources, and mixing and salinization processes in the groundwater system in this region. On the basis of the isotopic data and geochemical data, four end-members were defined representing, groundwater in a Quaternary aquifer, groundwater in a Miocene aquifer, water in the Ismailia canal, and wastewater from the 10th of Ramadan city. Several different mixing trends were recognized in the study area. As a consequence of mixing with groundwater in the Miocene aquifer, groundwater in the Quaternary aquifer was found to have depleted isotopic signatures and increased total dissolved ions (TDI) toward the south of the study area, near subsurface Miocene structural highs. Another mixing trend, consisting of, enriched isotopic ratios and lower TDI values toward the north in the Quaternary aquifer, indicated mixing with surface water, i.e., the Ismailia canal and freshwater ponds. A third trend of locally high TDI values together with depleted stable isotopic data, i.e., the Ramsis area and the vicinity of the well field, were interpreted to be due to excessive pumping for irrigation and reclamation activities, which in turn resulted in upconing of the deeper saline groundwater from the Miocene aquifer. Lastly, leakage from the wastewater ponds into the recharge process was suggested by considering chemical and isotopic signature of the Quaternary groundwater sample No. 31. These results strongly indicate the urgent need for monitoring, protection and remediation of the Quaternary aquifer in order to guarantee the sustainability of water resources in the study area. This work also illustrates the efficiency of using stable isotopes and major ion chemistry to improve our understanding of a flow system.

Abrupt millennial-scale changes in intensity of Southern Hemisphere westerly winds during marine isotope stages 2–4

Research Article| May 01, 2011 Abrupt millennial-scale changes in intensity of Southern Hemisphere westerly winds during marine isotope stages 2–4 Thomas E. Whittaker; Thomas E. Whittaker * 1Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton, New Zealand *Current address: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA; E-mail: twhittak@unm.edu. Search for other works by this author on: GSW Google Scholar Chris H. Hendy; Chris H. Hendy 1Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton, New Zealand Search for other works by this author on: GSW Google Scholar John C. Hellstrom John C. Hellstrom 2School of Earth Sciences, University of Melbourne, VIC 3010, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Thomas E. Whittaker * 1Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton, New Zealand Chris H. Hendy 1Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton, New Zealand John C. Hellstrom 2School of Earth Sciences, University of Melbourne, VIC 3010, Australia *Current address: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA; E-mail: twhittak@unm.edu. Publisher: Geological Society of America Received: 14 Oct 2010 Revision Received: 12 Dec 2010 Accepted: 14 Dec 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2011 Geological Society of America Geology (2011) 39 (5): 455–458. https://doi.org/10.1130/G31827.1 Article history Received: 14 Oct 2010 Revision Received: 12 Dec 2010 Accepted: 14 Dec 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Thomas E. Whittaker, Chris H. Hendy, John C. Hellstrom; Abrupt millennial-scale changes in intensity of Southern Hemisphere westerly winds during marine isotope stages 2–4. Geology 2011;; 39 (5): 455–458. doi: https://doi.org/10.1130/G31827.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The strength of mid-southern latitude westerly atmospheric circulation plays an important role in global climate. Due to a lack of long, continuous, high-resolution paleoclimate archives from mid-southern latitudes, it remains unclear what factors control changes in its intensity and how past changes affected the climates of landmasses in their path. Here we show growth rate and stable isotope (δ18O, δ13C) profiles from a South Island, New Zealand, stalagmite (HW3) that permit centennial-scale investigation of Southern Hemisphere westerly paleointensity between 73 and 11 ka. Correlation between HW3 growth rate and isotope profiles suggests sensitivity to changes in annual precipitation, a factor controlled by westerly intensity. Low growth rates and relatively enriched isotope ratios define long-term trends in HW3, supporting existing evidence that weaker westerlies predominated during the last glacial period. Abrupt millennial-scale events occur frequently, such that the HW3 record resembles Greenland ice core stable isotope profiles. Furthermore, nearly synchronous timing of nine prominent wet and cool intervals with Heinrich events supports studies showing that increased westerly intensity is closely linked to North Atlantic cooling. As well as Heinrich events, the HW3 profiles also show an Antarctic Cold Reversal–like event during deglaciation, advocating for a bipolar seesaw of global climate at that time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seasonality of stable carbon and nitrogen isotopes within the pelagic food web of Taihu Lake

Particulate organic matter (POM), dissolved organic matter (DOM), bacteria and cladoceran were sampled seasonally at Zhihugang Estuary and Lake Center in Taihu Lake. The δ 13 C of the four organic matter fractions showed consistent temporal variation, with heaviest values in summer and lower at other times of the year. The cladoceran δ 13 C showed a significant correlation with that of POM, reflecting a heavy dietary dependence on POM during the study period. The bacteria became enriched in 13 C compared with that of DOM throughout the sampling dates, although no significant relationship was found between the two fractions. δ 13 C values of POM, cladoceran and bacteria were all negative significantly correlated with oxidation and reduction potential (ORP), and specific conductivity (SpCond). As for δ 15 N, the seasonal pattern of food web components was variable. The POM δ 15 N signature exhibited the most enriched isotope ratios during the summer months when dissolved inorganic nitrogen (DIN) nutrients were at their lowest concentrations. The consumption of DIN in summer can explain in part the progressive accumulation of heavy nitrogen isotopes during this period. Spatially, δ 13 C and δ 15 N of the food web components were all slightly depleted at Estuary than that at Lake Center during the study period, possibly due to large allochthonous inputs at Zhihugang Estuary. Relatively wide ranges of stable isotopic values from both sites suggest that seasonality should be considered when attempting to establish food web structures in a eutrophic lake.

Bioaccumulation and enantiomeric profiling of organochlorine pesticides and persistent organic pollutants in the killer whale ( Orcinus orca) from British and Irish waters

Concentrations and enantiomeric profiles for a range of organochlorine compounds are reported in blubber samples from a number of individual killer whales ( Orcinus orca) from British and Irish waters. Elevated contaminant levels and enriched isotopic ratios were determined in one individual whale sampled in the Scottish Western Isles compared to the others suggesting marine mammal based dietary influences. The potential application of isotopic ratios to model contaminant uptake, enantioselective enrichment and accumulation is demonstrated. Data are presented which provide information on enantioselective enrichment factors (EFs) for o, p′-DDT, α-HCH and toxaphene congeners CHB26 and CHB 50. This dataset further improves the current database on reported levels of a number of contaminants and provides additional background information on potential metabolic processes in killer whales from British and Irish waters.

D/H and 18O/16O ratios of water in the crater lake at Kusatsu-Shirane volcano, Japan

The D/H and 18O/16O ratios of water in the active crater lake situated on the Kusatsu-Shirane volcano, Japan are about 20 and 6‰, respectively, higher than local meteoric water. The ratios show seasonal variations superimposed on a gradual change over nine years. The isotopic ratios started to increase in early 1990 and decrease in the spring of 1995. The seasonal variation which is high in winter and low in summer correlates with the temperature difference between lake water and ambient air. The large temperature difference in winter enhances the evaporation of lake water and produces the enriched isotopic ratios relative to the ratios in summer. The accumulation of snow and the decrease in the flux of meteoric water into the lake strengthens the winter-time isotopic enrichment. The enriched isotopic ratios of the lake water over a long time result from the addition of an end member with heavy isotopic ratios contained in a thermal fluid supplied to the lake. Considering the water balance in the lake, the isotopic ratios of the thermal fluid were found to be close to the lake water itself, suggesting the circulation of the lake water seeping through lake floor. Based on the correlation between Cl−concentration and the isotopic ratios, the contribution by the heavy end member was estimated to be 25–36% relative to the enrichment by evaporation. The heavy end member could be a liquid phase evolved from a parental fluid, which is a mixture of local meteoric water and a magmatic fluid as found in high-temperature volcanic gases.

Origin and interaction of mafic and felsic magmas in an evolving late orogenic setting: the Early Paleozoic Terra Nova Intrusive Complex, Antarctica

The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively.

Relationship between deformation, equilibration temperatures, REE and radiogenic isotopes in mantle xenoliths (Ray Pic, Massif Central, France): an example of plume-lithosphere interaction?

Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental lithospheric mantle. Variations in modal mineralogy, mineral chem istry, REE patterns and radiogenic isotope data suggest that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with LREE-depleted compositions, high 143Nd/144Nd, low 87Sr/86Sr and unradiogenic Pb isotope ratios; (ii) lherzolites, harzburgites and a wehrlite with LREE-enriched patterns, low 143Nd/144Nd, high 87Sr/86Sr and radiogenic Pb isotope ratios. The former closely resemble depleted MORB-source mantle. The latter are related to enrichment by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the “low velocity component” observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 °C) undeformed regions are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 °C) regions have undergone more deformation and are more depleted both in LREE and in isotope compositions.

Analytical considerations in trace metal isotope analysis using fast atom bombardment-induced ionization

Clinical studies of mineral absorption have been limited by a lack of convenient, sensitive and accurate techniques for the measurement of isotopic enrichment. A fast atom bombardment (FAB) ion gun can be used to generate metal ions in sufficient amounts for the mass spectral determination of both natural and enriched isotope ratios. The amount of sample required for these measurements is on the order of nanomoles of metal. FAB ionization has adequate precision for clinical measurements and has often been used. However, it has been observed that FAB ionization does not give a theoretical isotope ratio for either natural or enriched abundances. It is believed that most, if not all, of the observed deviation is due to fractionation in the ionization process. The work reported in this paper forms the basis of this conclusion. A brief study of metal monohydride formation is also included.