Range Of Isotope Ratios Research Articles

Land use and mobility during the Neolithic in Wales explored using isotope analysis of tooth enamel.

ObjectivesThe nature of land use and mobility during the transition to agriculture has often been debated. Here, we use isotope analysis of tooth enamel from human populations buried in two different Neolithic burial monuments, Penywyrlod and Ty Isaf, in south‐east Wales, to examine patterns of land use and to evaluate where individuals obtained their childhood diet.Materials and MethodsWe employ strontium (87Sr/86Sr) and oxygen (δ18O) and carbon (δ13C) isotope analysis of enamel from adjacent molars. We compare strontium isotope values measured in enamel to locally bioavailable 87Sr/86Sr values. We combine discussion of these results with evaluation of new radiocarbon dates obtained from both sites.ResultsThe majority of enamel samples from Penywyrlod have strontium isotope ratios above 0.7140. In contrast, the majority of those from Ty Isaf have 87Sr/86Sr values below 0.7140. At Penywyrlod oxygen isotope ratios range between 25.9 and 28.2 ‰ (mean 26.7 ± 0.6 ‰, 1σ, n = 15) and enamel δ13Ccarbonate values range between −18.0 and −15.0 ‰ (mean −16.0 ± 0.8 ‰, 1σ, n = 15). At Ty Isaf oxygen isotope ratios exhibited by Neolithic individuals range between 25.4 and 27.7 ‰ (mean 26.7 ± 0.6 ‰, 1σ, n = 15) and enamel δ13Ccarbonate values range between −16.9 and −14.9 ‰ (mean −16.0 ± 0.6 ‰, 1σ, n = 15).DiscussionThe strontium isotope results suggest that the majority of individuals buried at Penywyrlod did not source their childhood diet locally. One individual in this group has strontium isotope ratios that exceed all current known biosphere values within England and Wales. This individual is radiocarbon dated to the first few centuries of the 4th millennium BC, consistent with the period in which agriculture was initiated in Wales: the results therefore provide evidence for migration during the transition to farming in Wales. In contrast, all individuals sampled from Ty Isaf post‐date the period in which agriculture is considered to have been initiated and could have sourced their childhood diet from the local region in which they were buried.

Lead isotopic ratios in source apportionment of heavy metals in the street dust of Kolkata, India

Street dust from 29 locations, in some of the busiest parts of north and south Kolkata, was analysed for heavy metal composition. The decreasing order of average metal concentrations (mg kg−1) found was Mn (390) > Pb (380) > Zn (300) > As (96) > Cu (61) > Cr (40) > Co (13) > Ag (2.1). The heavy metal composition of the Kolkata dust was compared with reported data for other cities. Enrichment factors of Pb and As were high. Multivariate statistical analysis of the heavy metals and analysis of lead isotopic ratios of the dust revealed a predominant anthropogenic influence in the contamination. The range of lead isotopic ratios found in the dust was between 0.8789 and 0.8998 with a mean Pb concentration of 383 mg kg−1. The three Pb isotope plots of street dust, diesel and rainwater clustered linearly, while coal did not fit into this trend. The highest 207/206 lead isotopic ratio obtained was from diesel with a mean value of 0.9015, followed by the rainwater sample. The application of the binary mixing model showed that about 66.86% of lead contamination in the street dust was sourced from the atmosphere. The two components extracted by the principal component analysis explained 64.34% of the total variance. Vehicular and industrial emissions appeared to be an important contributor to the accumulation of heavy metals in the dust. The health risk assessment study of the dust indicated carcinogenic risk associated with As and Cr.

Occurrence of selected trace metals and their oral bioaccessibility in urban soils of kindergartens and parks in Bratislava (Slovak Republic) as evaluated by simple in vitro digestion procedure

A total of eighty surface soil samples were collected from public kindergartens and urban parks in the city of Bratislava, and the <150µm soil fraction was evaluated for total concentrations of five metals, Cd, Cu, Hg, Pb and Zn, their oral bioaccessibilities, non-carcinogenic and carcinogenic health risks to children, and lead isotopic composition. The mean metal concentrations in urban soils (0.29, 36.1, 0.13, 30.9 and 113mg/kg for Cd, Cu, Hg, Pb and Zn, respectively) were about two times higher compared with background soil concentrations. The order of bioaccessible metal fractions determined by Simple Bioaccessibility Extraction Test was: Pb (59.9%) > Cu (43.8%) > Cd (40.8%) > Zn (33.6%) > Hg (12.8%). Variations in the bioaccessible metal fractions were mainly related to the total metal concentrations in urban soils. A relatively wide range of lead isotopic ratios in urban soils (1.1598–1.2088 for 206Pb/207Pb isotopic ratio) indicated a combination of anthropogenic and geogenic sources of metals in the soils. Lower values of 206Pb/207Pb isotopic ratio in the city centre and similar spatial distribution of total metal concentrations, together with their increasing total concentrations in soils towards the city centre, showed that traffic and coal combustion in former times were likely the major sources of soil contamination. The non-carcinogenic and carcinogenic health risks to children due to exposure to metals in kindergarten and urban park soils were low, with hazard index and cancer risk values below the threshold values at all studied sites.

Lead isotope distribution and enrichment factors in soil profiles around an abandoned Pb-smelter plant

Todos os Santos (all Saints) Bay area on Brazil’s east coast is known for one of the most significant cases of lead contamination in the country owing to the past activities of a Pb-smelter plant. This work was carried out to assess the concentration and sources of Pb based on Pb isotopes and enrichment factor of soil profiles surrounding Todos os Santos Bay in order to understand the expansion of contamination and to help the establishment of Pb regulatory standards for the region. Forty-four samples were collected from soil genetic horizons of six pedons that represent the range of dominant soil properties and geologic materials in the region. Concentrations of Pb and the isotopes 204Pb, 206Pb, 207Pb, and 208Pb were determined on an inductively coupled plasma (quadrupole) mass spectrometry. The soil enrichment factor was calculated using Al and Fe as conservative index elements. Average Pb concentration (15.87 mg kg−1) in uppermost horizons (from all six pedons) is slightly higher than soil background concentrations commonly reported in Brazil. Samples feature a wide range of Pb isotope ratios, ranging from 36.71 to 47.38 for 208Pb/204Pb, 15.00 to 15.65 for 207Pb/204Pb, 16.86 to 20.59 for 206Pb/204Pb, and 1.10 to 1.31 for 206Pb/207Pb. For the enrichment factor calculations, only Fe demonstrated a good agreement with Pb isotopic ratios. Both Pb isotopic composition and enrichment factor were useful tools to distinguishing natural and anthropogenic influence on the Pb soil concentrations.

Permian copper–polymetallic mineralization in the southern Great Xing’an Range, Northeast China: the Daolundaba copper–polymetallic deposit example

ABSTRACTThe Daolundaba Cu–polymetallic deposit is a newly discovered Cu–W–Sn deposit on the western slopes of the southern Great Xing’an Range, and its mineralization was related to an early Permian coarse-grained biotite granite. However, there is little information on the age of formation of the deposit. In this article, we present the results of our investigation into the age of the Daolundaba Cu–polymetallic deposit, which involved the selection of chalcopyrite and pyrrhotite samples for Rb–Sr isochron dating. A Rb–Sr isochron defined by the chalcopyrite samples yielded a Rb–Sr isochron age of 290.0 ± 11 Ma (MSWD = 1.2) with an initial Sr isotopic composition (ISr) of 0.71446. The pyrrhotite samples yielded a Rb–Sr isochron age of 283.0 ± 2.6 Ma (MSWD = 1.16) with an initial Sr isotopic composition (ISr) of 0.71447. The Rb–Sr isochron age determined from the chalcopyrite and pyrrhotite is 282.7 ± 1.7 Ma (MSWD = 1.13). These results indicate that the Daolundaba Cu–polymetallic deposit formed during the early Permian (282.7–290.0 Ma). The Rb and Sr contents of the chalcopyrite and pyrrhotite range from ~0.1325 to ~3.6810 ppm and from ~0.1219 to ~9.5740 ppm, respectively, and the initial Sr isotope ratios (ISr) range from 0.71047 to 0.71869, with an average of 0.714723. These isotopic characteristics indicate the ore-forming minerals of the Daolundaba Cu–polymetallic deposit originated mainly from the crust, but with small amounts of mantle material involved. The copper was derived from the associated magma whereas the W and Sn was derived from the surrounding strata. The Permian mineralization of the Xing’an–Mongolia region occurred in an active continental margin setting during subduction of the Palaeo-Asian oceanic plate beneath the Siberian Plate.

A reappraisal of the evolution of the palaeo-Pacific margin of Gondwana from the Pb and Os isotope systematics of igneous rocks from the southern Adelaide fold belt, South Australia

We have undertaken Pb and Os isotopic analyses of igneous rocks from the southern Adelaide fold belt to further understanding of the Neoproterozoic – early Palaeozoic evolution of the palaeo-Pacific margin of Gondwana. The new Pb isotope data and a reappraisal of existing geochronological data suggest that layered mafic intrusions at Black Hill and Cambrai may have intruded much earlier than previously thought. Intrusion may even have occurred prior to deposition of the Normanville and Kanmantoo Group sediments and the onset of what is traditionally regarded as the Delamerian Orogeny. We tentatively term this earlier event the Hallett Cove orogeny. However, equivalent ages are common in the formerly adjacent Ross Orogen. When the Pb isotope signatures of the suites of gabbros, and I-, S-, and A-type granites are compared at 487Ma they all span a very large range (206Pb/204Pbi=15.4–18.7, 207Pb/204Pbi=15.4–15.7, 208Pb/204Pbi=33.0–38.8) compared with other formerly adjacent terranes or the subsequent Lachlan fold belt. Similarly, 187Os/188Osi ratios span 0.122–1.17 but include one gabbro and one rhyolite that have subchondritic 187Os/188Osi ratios. When taken together, the combined data do not support models in which the observed range in initial isotope ratios reflects major and variable mid to upper crustal assimilation of magmas derived from the contemporary asthenospheric mantle. Rather, it appears that magmas parental to the gabbros and A-type rocks were derived from subduction-modified lithospheric mantle that has a median metasomatic age of 1.4Ga. Large-scale Pb isotopic homogenization, as proposed for the Lachlan fold belt did not occur in the Adelaide fold belt.

Reconstructing diet in Napoleon's Grand Army using stable carbon and nitrogen isotope analysis.

Historical evidence has provided information regarding disease and mortality in Napoleon Bonaparte's Grand Army, but dietary information beyond individual soldier accounts remains scarce. The purpose of this research is to reconstruct the diets of Napoleon's multiethnic army who were associated with the Russian Campaign of 1812. We conducted stable carbon and nitrogen isotope ratio analysis on femoral bone collagen of 78 individuals recovered from a salvage excavation at the mass gravesite of Šiaurės miestelis in Vilnius, Lithuania. These individuals were later discovered to be Napoleonic soldiers and camp followers who participated in the 1812 Russian Campaign. Stable carbon isotope ratios range from -19.2‰ to -11.8‰, with a mean of -17.8‰ ± 1.5‰ (1 σ). Stable nitrogen isotope ratios range from 7.1‰ to 13.6‰, with a mean of 10.5‰ ± 1.4‰ (1 σ). Both δ13 C and δ15 N values show a wide range of variation. Stable isotope data indicate considerable dietary variation in this population associated with a multiethnic and socially stratified military population. Diets ranged from predominantly C3 -based to predominantly C4 -based, with varying inputs of terrestrial, freshwater, and marine animal protein. Comparison with other European populations further denotes the exceptional range of dietary variation of soldiers and camp followers in Napoleon's army.

Stable isotopes, carbon-14 and hydrochemical composition from a basaltic aquifer in São Paulo State, Brazil

The Cretaceous Serra Geral Aquifer (SGA) is contained within one of the largest continental flood basalts in the world, reaching a thickness up to 1700 m in the center of the Parana Basin. The SGA is one of the most important groundwater reservoirs in northeastern Sao Paulo State (Brazil), responsible for water supply to cities and agriculture. In order to evaluate the geochemical and isotopic evolution of SGA, as well as to determine the mean residence time, a groundwater sampling campaign was carried out over the SGA in Sao Paulo State (Brazil) from January to April 2013. Two main hydrochemical facies were recognized: Ca–Mg–HCO3 related to water–rock interaction reactions in basaltic outcrop, such as mineral dissolution due to atmospheric CO2 uptake, and alkaline Na–HCO3 groundwater, evolved from mixing with groundwater from the underlying Guarani Aquifer System. Stable isotope (δ18O and δ2H) ratios range from −8.87 to −5.32‰ VSMOW and −61.31 to −31.64‰ VSMOW, respectively, closely following the GMWL. Spatial and temporal variations in isotope ratios are associated with the South Atlantic convergence zone activities and the type of rain responsible for recharge. Values for δ13C vary from −21.53 to −7.11‰ VPDB, while 14C activities vary from 1.2 pcm to more than 100 pcm, presenting a trend to enrichment and decrease in 14C activities westward, concordant with the regional groundwater flow direction. Most recent groundwaters have δ13C ratio contents mostly consistent with C3 plants.

Oceanic mafic magmatism in the Siletz terrane, NW North America: Fragments of an Eocene oceanic plateau?

The Siletz terrane, a predominantly mafic accreted oceanic terrane, is located in the Cascadia forearc region of Oregon, Washington and Vancouver Island. The terrane represents a late Palaeocene–Eocene large igneous province that consists of pillow lavas, massive flows and intrusive sheets. Previously it has been proposed that the Siletz terrane represents either an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data, has been used to assess the validity of the proposed tectonomagmatic models for the Siletz terrane. The trace element data show little evidence of crustal contamination, or an arc signature, and the samples have rare earth element (REE) patterns that are flat to light REE enriched. These features are similar to other oceanic plateaus such as the Ontong Java and the Caribbean. Initial isotope ratios range from 206Pb/204 Pb: 18.751 to 19.668, 207Pb/204Pb: 15.507 to 15.661, 208Pb/204Pb: 38.294 to 39.2128, 176Hf/177Hf: 0.28300 to 0.28316 (εHf: 9.0 to 14.5), 143Nd/144Nd: 0.51282 to 0.51299 (εNd: 5.0 to 8.1) and 87Sr/86Sr: 0.70302 to 0.70380. These data are consistent with a mantle source of the Siletz terrane that appears to have been heterogeneous and slightly enriched. The enriched signature has characteristics of both EM2 and HIMU components and this, combined with a calculated mantle potential temperature well above ambient mantle, indicates derivation of the Siletz magmatism from a mantle plume, possibly the Yellowstone Hotspot. We therefore conclude that the Siletz terrane represents an accreted oceanic plateau.

Stable isotopes measurements reveal dual carbon pools contributing to organic matter enrichment in marine aerosol.

Stable carbon isotope ratios in marine aerosol collected over the Southern Indian Ocean revealed δ13C values ranging from −20.0‰ to −28.2‰. The isotope ratios exhibited a strong correlation with the fractional organic matter (OM) enrichment in sea spray aerosol. The base-level isotope ratio of −20.0‰ is characteristic of an aged Dissolved Organic Matter (DOM) pool contributing a relatively homogeneous background level of DOM to oceanic waters. The range of isotope ratios, extending down to −28.2‰, is characteristic of more variable, stronger, and fresher Particulate Organic Matter (POM) pool driven by trophic level interactions. We present a conceptual dual-pool POM-DOM model which comprises a ‘young’ and variable POM pool which dominates enrichment in sea-spray and an ‘aged’ but invariant DOM pool which is, ultimately, an aged end-product of processed ‘fresh’ POM. This model is harmonious with the preferential enrichment of fresh colloidal and nano-gel lipid-like particulate matter in sea spray particles and the observed depleted δ13C ratio resulting from isotope equilibrium fractionation coupled with enhanced plankton photosynthesis in cold water (−2 °C to +8 °C). These results re-assert the hypothesis that OM enrichment in sea-spray is directly linked to primary production and, consequently, can have implications for climate-aerosol-cloud feedback systems.

Constraints on the sources of ore metals in Mississippi Valley-type deposits in central and east Tennessee, USA, using Pb isotopes

The Mississippi Valley-Type (MVT) Central and East Tennessee (TN) districts contain lead-zinc deposits that occur in the Early Ordovician carbonates of the Knox Group. Although both districts share similar host rock, have similar temperatures of formation, and typically fill open spaces of collapse breccias or replace their host carbonates, previous studies may suggest that these ores did not form from the same mineralizing fluids and may have different Pb sources. Nu Plasma MC-ICP-MS Pb isotopic analyses on sulfides from Central and East TN were conducted and the metal sources evaluated. Pb isotope ratios from the Young Mine in the East TN District plot within ranges defined by other East TN District samples from previous studies. Generally, Pb isotope ratios or ores from the Elmwood Mine in the Central TN District are more radiogenic and plot between samples from the southern Appalachians and Illinois-Kentucky districts. It appears that the thick, black, metal-rich shales of the Appalachian Basin have been the source of the Pb-Zn and ore fluids of MVT deposits that formed the East TN deposits. When East TN Pb data from this study are compared to Central TN Pb data, Central TN Pb is commonly more radiogenic, suggestive of a mineralizing fluid with different compositions. Lead isotopic signature of Central TN ores may imply potential mixing between end-member sources originating from the Southern Appalachians and Illinois basins.Lead isotopes in MVT and carbonate-hosted base metal deposits in US, South America, and Europe display two basic types of Pb-isotopic signatures. The classic MVT deposits in the mid-continent US (SE and Central Missouri, Illinois-Kentucky, and Upper Mississippi Valley districts) have highly radiogenic Pb-isotopic compositions and they generally display linear Pb-isotopic arrays. In contrast, deposits from Europe (Irish-type, Silesian-type, Alpine-type, and Cévennes region), the Appalachians, and Andean-type deposits have lower Pb-isotopic values. The European deposits (except for the Irish-type) have a restricted range of isotopic ratios, which suggest a single mineralizing event of relatively short duration. It appears that basement rocks or sedimentary rocks derived from the basement contributed Pb to many MVT and carbonate-hosted base-metal deposits worldwide. Hydrothermal leaching of radiogenic Pb from the crystalline basement rocks may have occurred during formation of these ore deposits. High 238U/204Pb values for several districts also suggest a supracrustal origin of Pb.

Evaluating the suitability of different environmental samples for tracing atmospheric pollution in industrial areas

Samples of lichens, snow and particulate matter (PM10, 24 h) are used for the source identification of air pollution in the heavily industrialized region of Ostrava, Upper Silesia, Czech Republic. An integrated approach that uses different environmental samples for metal concentration and Pb isotope analyses was applied. The broad range of isotope ratios in the samples indicates a combination of different pollution sources, the strongest among them being the metallurgical industry, bituminous coal combustion and traffic. Snow samples are proven as the most relevant indicator for tracing metal(loid)s and recent local contamination in the atmosphere. Lichens can be successfully used as tracers of the long-term activity of local and remote sources of contamination. The combination of PM10 with snow can provide very useful information for evaluation of current pollution sources.

Ancient impactor components preserved and reworked in martian regolith breccia Northwest Africa 7034

Northwest Africa (NWA) 7034 and paired stones represent unique samples of martian polymict regolith breccia. Multiple breccia subsamples characterized in this work confirm highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) contents that are consistently elevated (e.g., Os ∼9.3–18.4ppb) above indigenous martian igneous rocks (mostly <5ppb Os), equivalent to ∼3wt% of admixed CI-type carbonaceous chondritic material, and occur in broadly chondrite-relative proportions. However, a protracted history of impactor component (metal and sulfide) breakdown and redistribution of the associated HSE has masked the original nature of the admixed meteorite signatures. The present-day 187Os/188Os ratios of 0.119–0.136 record a wider variation than observed for all major chondrite types. Combined with the measured 187Re/188Os ratios of 0.154–0.994, the range in Os isotope ratios indicates redistribution of Re and Os from originally chondritic components early in the history of the regolith commencing at ∼4.4Ga. Superimposed recent Re mobility reflects exposure and weathering at or near the martian and terrestrial surfaces. Elevated Os concentrations (38.0 and 92.6ppb Os), superchondritic Os/HSE ratios, and 187Os/188Os of 0.1171 and 0.1197 measured for two subsamples of the breccia suggest the redistribution of impactor material at ∼1.5–1.9Ga, possibly overlapping with a (partial) resetting event at ∼1.4Ga recorded by U–Pb isotope systematics in the breccia. Martian alteration of the originally chondritic HSE host phases, to form Os-Ir-rich nuggets and Ni-rich pyrite, implies the influence of potentially impact-driven hydrothermal systems. Multiple generations of impactor component admixture, redistribution, and alteration mark the formation and evolution of the martian regolith clasts and matrix of NWA 7034 and paired meteorites, from the pre-Noachian until impact ejection to Earth.

Factors Influencing the Radiocarbon Dating of Human Skeletal Remains from the Dnieper River System: Archaeological and Stable Isotope Evidence of Diet from the Epipaleolithic to Eneolithic Periods

AbstractRecent research has identified the existence of a freshwater reservoir effect influencing the radiocarbon dating of human skeletal remains from the Dnieper region of Ukraine (Lillie et al. 2009). The current study outlines the evidence for freshwater resource exploitation throughout the period ~10,200–3700 cal BC, and presents the available evidence for the existence of dietary offsets in the14C dates obtained. We have obtained human skeletal material from 54 Epipaleolithic to Mesolithic period individuals and 267 Neolithic to Eneolithic individuals, from 13 cemeteries, since our research in Ukraine began in 1992. Here, we present the initial results of stable isotope analysis of Eneolithic individuals from the Igren VIII cemetery alongside the Epipaleolithic to Eneolithic samples that have previously been analyzed. When contrasted against the evidence from the prehistoric fauna and fish remains studied, and modern fish species from the Dnieper region, we continue to see variability in diets at the population level, both internally and across cemeteries. We also observed temporal variability in human diets across these chronological periods. The fish samples (both archaeological and modern) show a wide range of isotope ratios for both δ13C and δ15N, which could prove significant when interpreting the dietary sources being exploited. This information directly informs the14C dating program as an inherent degree of complexity is introduced into the dating of individuals whose diets combine freshwater and terrestrial sources in differing quantities and at differing temporal and/or spatial scales (e.g. Bronk Ramsey et al. 2014).

Metal enrichment and lead isotope analysis for source apportionment in the urban dust and rural surface soil

To understand the metal accumulation in the environment and identify its sources, 29 different metal contents and lead (Pb) isotope ratios were determined for 40 urban dust samples, 36 surface soil samples, and one river sediment sample collected in the municipality of Beijing, China. Results showed that cadmium, copper (Cu), mercury, Pb, antimony (Sb), and zinc demonstrated to be the typical urban contaminants and mostly influenced by the adjacent human activities with higher content to background ratios and SD values. Among the 29 metal elements investigated, Cu and Sb were found to be the most distinct elements that were highly affected by the developing level and congestion status of the cities with much higher contents in dust in more developed and congested cities. There was a relatively wider range of Pb isotope ratios of country surface soil than those of urban dust. The results of source identification based on Pb isotope ratios showed that coal combustion was the first largest Pb source and vehicle exhaust was the second largest source. The sum of them accounted for 74.6% mass proportion of overall Pb pollution on average. The surface soil sample collected at an iron mine had the highest 204Pb/206Pb, 207Pb/206Pb, and 208Pb/206Pb ratios indicating ore had much higher ratios than other sources. The fine particle subsamples had higher 204Pb/206Pb, 207Pb/206Pb, and 208Pb/206Pb ratios than the coarse particle subsamples indicating more anthropogenic sources of coal combustion and vehicle exhaust for fine particles and more background influence for coarse particles. These results help with pinpointing the major Pb sources and applying suitable measures for the target sources.

Diffuse degassing at Longonot volcano, Kenya: Implications for CO2 flux in continental rifts

Magma movement, fault structures and hydrothermal systems influence volatile emissions at rift volcanoes. Longonot is a Quaternary caldera volcano located in the southern Kenyan Rift, where regional extension controls recent shallow magma ascent. Here we report the results of a soil carbon dioxide (CO2) survey in the vicinity of Longonot volcano, as well as fumarolic gas compositions and carbon isotope data. The total non-biogenic CO2 degassing is estimated at <300kgd−1, and is largely controlled by crater faults and fractures close to the summit. Thus, recent volcanic structures, rather than regional tectonics, control fluid pathways and degassing. Fumarolic gases are characterised by a narrow range in carbon isotope ratios (δ13C), from −4.7‰ to −6.4‰ (vs. PDB) suggesting a magmatic origin with minor contributions from biogenic CO2. Comparison with other degassing measurements in the East African Rift shows that records of historical eruptions or unrest do not correspond directly to the magnitude of CO2 flux from volcanic centres, which may instead reflect the current size and characteristics of the subsurface magma reservoir. Interestingly, the integrated CO2 flux from faulted rift basins is reported to be an order of magnitude higher than that from any of the volcanic centres for which CO2 surveys have so far been reported.

Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc

We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (238U/232Th, 230Th/232Th and 226Ra/230Th) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of 238U to 230Th, this results in elevated (230Th/232Th) (where brackets indicate activity ratios) over time-scales of ∼350ka. This process leads to the high (230Th/232Th), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (230Th/232Th)=1.2–2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments.

Recycling of crustal material by the Iceland mantle plume: New evidence from nitrogen elemental and isotope systematics of subglacial basalts

We report new nitrogen (N2) abundance and isotope (δ15N) data for 43 subglacial basaltic glasses from the neovolcanic zones of Iceland, a key locality in studies of mantle plume geochemistry and crust–mantle processes. New helium and argon abundance and isotope data are also reported to supplement previous studies (Füri et al., 2010; Barry et al., 2014), allowing elemental ratios (e.g., N2/40Ar∗ where 40Ar∗=radiogenic 40Ar) to be calculated. Subglacial basaltic glasses with N2>2μcm3STP/g show a wide range in δ15N values, from −2.91 to +11.96‰ (vs. Air), with values >6‰ only observed at one locality in the Eastern Rift Zone. Elemental ratios involving N2, i.e., N2/3He, and N2/40Ar∗, span several orders of magnitude from 2.5×105 to 9.0×107, and 32.8 to 1.46×106, respectively. In contrast, argon isotope ratios (40Ar/36Ar) are limited, ranging from air-like (∼298.6) values up to 1330. Glasses exhibit a wide range in helium isotope ratios (8–26 RA), with clear distinctions between individual rift segments.A number of processes have extensively modified original mantle source N isotope and relative abundance compositions – most significantly air interaction, crustal contamination in some instances, and possibly degassing-induced fractionation. Under the assumption that the starting 4He/40Ar∗ production ratio of Iceland mantle is identical to the depleted MORB mantle (DMM), a filtering protocol for the entire N dataset, based upon 40Ar/36Ar and 4He/40Ar∗ ratios, was adopted to identify samples with unmodified δ15N values. Consequently, we identify 22 samples that define the Icelandic mantle N-isotope distribution (δ15N=−2.29 to +5.71‰). Using the filtered dataset, we investigate simple binary mixing scenarios involving N2/3He–N2/40Ar∗–δ15N variations to identify mantle end-member compositions. Mixing scenarios are consistent with a recycled component in the Iceland mantle source, defined by a high and heterogeneous δ15N end-member. Moreover, this end-member is coupled to the high 3He/4He signature, and is characterized by He depletion and/or the presence of excess N2. These features strongly suggest the presence of recycled crustal N-component(s) integrated into and/or entrained by the Iceland plume source.These new results reveal the highly heterogeneous nature of nitrogen in the hybrid Iceland plume source, consistent with models based on trace elements and radiogenic isotopes that advocate for significant heterogeneity of recycled crustal component(s) sampled by the Iceland plume. A relatively young age of the recycled crustal material (possibly Phanerozoic) is consistent with the association of positive δ15N values and high N2/40Ar∗ ratios with constraints from radiogenic isotopes (e.g., Pb), thus indicating a relatively short time-interval (∼108years) between subduction of crustal material and entrainment by the Iceland mantle plume.

An analytical system for the measurement of stable hydrogen isotopes in ambient volatile organic compounds

Abstract. Stable isotope measurements in atmospheric volatile organic compounds (VOCs) are an excellent tool to analyse chemical and dynamical processes in the atmosphere. While up to now isotope studies of VOCs in ambient air have mainly focussed on carbon isotopes, we herein present a new measurement system to investigate hydrogen isotope ratios in atmospheric VOCs. This system, consisting of a gas chromatography pyrolysis isotope ratio mass spectrometer (GC-P-IRMS) and a pre-concentration system, was thoroughly characterised using a VOC test mixture. A precision of better than 9 ‰ (in δ 2H) is achieved for n-pentane, 2-methyl-1,3-butadiene (isoprene), n-heptane, 4-methyl-pentane-2-one (4-methyl-2-pentanone), methylbenzene (toluene), n-octane, ethylbenzene, m/p-xylene and 1,2,4-trimethylbenzene. A comparison with independent measurements via elemental analysis shows an accuracy of better than 9 ‰ for n-pentane, n-heptane, 4-methyl-2-pentanone, toluene and n-octane. Above a minimum required pre-concentrated compound mass the obtained δ 2H values are constant within the standard deviations. In addition, a remarkable influence of the pyrolysis process on the isotope ratios is found and discussed. Reliable measurements are only possible if the ceramic tube used for the pyrolysis is sufficiently conditioned, i.e. the inner surface is covered with a carbon layer. It is essential to verify this conditioning regularly and to renew it if required. Furthermore, influences of a necessary H3+ correction and the pyrolysis temperature on the isotope ratios are discussed. Finally, the applicability to measure hydrogen isotope ratios in VOCs at ambient levels is demonstrated with measurements of outside air on 5 different days in February and March 2015. The measured hydrogen isotope ratios range from −136 to −105 ‰ forn-pentane, from −86 to −63 ‰ for toluene, from −39 to −15 ‰ for ethylbenzene, from −99 to −68 ‰ for m/p-xylene and from −45 to −34 ‰ for o-xylene.

Strontium and oxygen isotopic profiles through 3 km of hydrothermally altered oceanic crust in the Reykjanes Geothermal System, Iceland

The Iceland Deep Drilling Program well RN-17 was drilled 3km into a section of hydrothermally altered basaltic crust in the Reykjanes geothermal system in Iceland. The system is located on the landward extension of the Mid-Atlantic Ridge, and the circulating hydrothermal fluid is modified seawater, making Reykjanes a useful analog for mid-oceanic ridge hydrothermal systems. We have determined whole-rock Sr and O isotope compositions, and Sr isotope compositions of epidote grains from the RN-17 cuttings and RN-17B core. Whole rock oxygen isotope ratios range from −0.13 to 3.61‰ V-SMOW, and are isotopically lighter than fresh MORB (5.8±0.2‰). The concentrations of Sr in the altered basalt range from well below to well above concentrations in fresh rock, and appear to be strongly correlated with the dominant alteration mineralogy. Whole rock Sr isotope ratios ranged from 0.70329 in the least altered crystalline basalt, to 0.70609 in the most altered hyaloclastite samples; there is no correlation with depth. Sr isotope ratios in epidote grains measured by laser ablation MC-ICP-MS ranged from 0.70360 to 0.70731. Three depth intervals, at 1000m, 1350m, and 1650m depth, have distinctive isotopic signatures, where 87Sr/86Sr ratios are elevated (mean value>0.7050) relative to background levels (mean altered basalt value ~0.7042). These areas are proximal to geothermal feed zones, and the 1350m interval directly overlies the transition from dominantly extrusive to dominantly intrusive lithologies. Oxygen isotope measurements yield integrated water/rock ratios of 0.4 to 4.3, and suggest that hydrothermal fluids must have formerly had a component of meteoric water. Strontium isotopic measurements provide a more sensitive indication of seawater interaction and require significant exchange with seawater strontium. Both isotopic systems indicate that the greenschist-altered basalts were in equilibrium with hydrothermal fluids at a relatively high mean water/rock (Wt.) ratio ranging from about 0.5 to 4. These ratios are higher than estimates from ODP Hole 504B and IODP Hole 1256D, but are consistent with values inferred from vent fluids from 21° and 13°N on the East Pacific Rise (Albarède et al., 1981; Michard et al., 1984; Alt et al., 1996; Harris et al., 2015).