Oxygen Isotope Systematics Research Articles

C–O Stable Isotope Geochemistry of Carbonate Minerals in the Nonsulfide Zinc Deposits of the Middle East: A Review

Zinc nonsulfides are well represented in the Middle East, with occurrences in Turkey, Iran, and Yemen. Their genesis can be constrained by using carbon and oxygen isotope systematics applied to carbonate minerals. The δ13C ratios of smithsonite and hydrozincite in Iran and Turkey are comprised in the typical interval of supergene Zn carbonates (−0.4 and −7.1‰ V-PDB). The oxygen isotope geochemistry is more complex. Oxygen isotope compositions of smithsonite of the Hakkari deposit (Turkey) (δ18O from 24.2 to 25.6‰ V-SMOW) point to precipitation temperatures between ~4 and ~18 °C, corresponding to a normal weathering environment at these latitudes, whereas δ18O of smithsonite from other Middle East deposits (Angouran in Iran, Jabali in Yemen) point to the precipitation from low- to medium-temperature hydrothermal fluids. The C–O isotopic compositions of hydrozincite from the Mehdi Abad, Irankuh, and Chah-Talkh deposits can be only partially compared with those of smithsonite, because the oxygen isotopes fractionation equation for hydrozincite-water is not known. A comparison between the geochemical characteristics of all Zn-nonsulfide ores in the Middle East indicates that, even though several mineral deposits are derived from supergene weathering processes, other ones have been deposited from fluids associated with magmatic activity (Angouran, Iran) or with hydrothermal systems (Jabali, Yemen). This suggests that it is not possible to apply a common interpretative model to the genesis of all nonsulfide deposits in the Middle East.

Dating the Paleoproterozoic snowball Earth glaciations using contemporaneous subglacial hydrothermal systems

The presence of Paleoproterozoic glacial diamictites deposited at low latitudes on different continents indicates that three or four worldwide glaciations occurred between 2.45 and 2.22 Ga. During that time period, the first atmospheric oxygen rise, known as the Great Oxidation Event (GOE), occurred, implying a potential connection between these events. Herein we combine triple oxygen isotope systematics and in situ and high-precision U-Pb zircon ages of mafic intrusions to date two episodes of snowball Earth glaciations. Subglacial hydrothermal alteration was induced by intrusions of high- Mg and high-Fe gabbros during the early Paleoproterozoic rifting on the Baltic Shield, which at the time was located at low latitudes. The low d18O values of hydrothermally altered rocks associated with these intrusions are attributed to high-temperature isotopic exchange between hot rock and glacial meltwater, indicating the presence of glacial ice globally. The triple oxygen isotope approach is used here to show that the d18O of glacial meltwaters during the dated episodes of snowball Earth glaciation was approximately –40 permil VSMOW (Vienna standard mean ocean water). High-Mg gabbro intrusions and associated low-d18O hydrothermally altered rocks formed during the earliest episode of snowball Earth glaciation between 2.43 and 2.41 Ga. High-Fe gabbro from the Khitoostrov locality (Karelia, Russia) hosts a d18O value of −27.3 permil and is dated here at 2291 ± 8 Ma. This age is interpreted to reflect the interaction between the intrusion and glacial meltwaters during the third Paleoproterozoic glaciation, which occurred after the GOE.

Triple oxygen isotope systematics of structurally bonded water in gypsum

The triple oxygen isotopic composition of gypsum mother water (gmw) is recorded in structurally bonded water in gypsum (gsbw). Respective fractionation factors have been determined experimentally for 18O/16O and 17O/16O. By taking previous experiments into account we suggest using 18αgsbw-gmw=1.0037; 17αgsbw-gmw=1.00195 and θgsbw-gmw=0.5285 as fractionation factors in triple oxygen isotope space.Recent gypsum was sampled from a series of 10 ponds located in the Salar de Llamara in the Atacama Desert, Chile. Total dissolved solids (TDS) in these ponds show a gradual increase from 23g/l to 182g/l that is accompanied by an increase in pond water 18O/16O. Gsbw falls on a parallel curve to the ambient water from the saline ponds. The offset is mainly due to the equilibrium fractionation between gsbw and gmw. However, gsbw represents a time integrated signal biased towards times of strong evaporation, hence the estimated gmw comprises elevated 18O/16O compositions when compared to pond water samples taken on site. Gypsum precipitation is associated with algae mats in the ponds with lower salinity. No evidence for respective vital effects on the triple oxygen isotopic composition of gypsum hydration water is observed, nor are such effects expected. In principle, the array of δ18Ogsbw vs. 17Oexcess can be used to: (1) provide information on the degree of evaporation during gypsum formation; (2) estimate pristine meteoric water compositions; and (3) estimate local relative humidity which is the controlling parameter of the slope of the array for simple hydrological situations. In our case study, local mining activities may have decreased deep groundwater recharge, causing a recent change of the local hydrology.

The Allende multicompound chondrule (ACC)—Chondrule formation in a local super‐dense region of the early solar system

AbstractIn Allende, a very complex compound chondrule (Allende compound chondrule; ACC) was found consisting of at least 16 subchondrules (14 siblings and 2 independents). Its overall texture can roughly be described as a barred olivine object (BO). The BO texture is similar in all siblings, but does not exist in the two independents, which appear as relatively compact olivine‐rich units. Because of secondary alteration of pristine Allende components and the ACC in particular, only limited predictions can be made concerning the original compositions of the colliding melt droplets. Based on textural and mineralogical characteristics, the siblings must have been formed on a very short time scale in a dense, local environment. This is also supported by oxygen isotope systematics showing similar compositions for all 16 subchondrules. Furthermore, the ACC subchondrules are isotopically distinct from typical Allende chondrules, indicating formation in or reaction with a more 16O‐poor reservoir. We modeled constraints on the particle density required at the ACC formation location, using textural, mineral‐chemical, and isotopic observations on this multicompound chondrule to define melt droplet collision conditions. In this context, we discuss the possible relationship between the formation of complex chondrules and the formation of macrochondrules and cluster chondrites. While macrochondrules may have formed under similar or related conditions as complex chondrules, cluster chondrites certainly require different formation conditions. Cluster chondrites represent a mixture of viscously deformed, seemingly young chondrules of different chemical and textural types and a population of older chondrules. Concerning the formation of ACC calculations suggest the existence of very local, kilometer‐sized, and super‐dense chondrule‐forming regions with extremely high solid‐to‐gas mass ratios of 1000 or more.

Lead, Sulfur, and Oxygen Isotope Systematics in Hydrothermal Precipitates from the 14°S Hydrothermal Field, South Mid‐Atlantic Ridge

AbstractAt the inside corner between the South Mid‐Atlantic Ridge (SMAR) and the Cardno fracture zone, the 14°S hydrothermal field, with its abundant silica‐rich sulfides, was identified as a volcanic massif, which provided the first opportunity to study inside corner‐related hydrothermal mineralization. S, Pb, and O isotopes were measured in hydrothermal sulfide samples from the two sites (volcanic top and slope sites) in the 14°S field. The homogeneous Pb isotope values (207Pb/204Pb = 15.466 − 15.472; 206Pb/204Pb = 18.242 − 18.252) and the δ34S values (top site, +6.3 to +6.85‰; slope site, +2.37 to +3.36‰) suggest that the source of metals and most of the sulfur is the upper crust, whereas some sulfur is sourced from downward‐penetrating seawater, especially in the top site. The calculated oxygen isotope equilibration temperatures from quartz and seawater are between 94 and 144°C for the top site and between 179 and 196°C for the slope site. These isotopic proxies, together with geological background survey data, have an important implication: Discrepancies between the sites may result from variations in crust permeability, which also affect the mixing level between downward‐penetrating seawater and upward hydrothermal end‐member fluids and determine the various mineralization types.

Examining Archean methanotrophy

The carbon isotope ratios preserved in sedimentary rocks can be used to fingerprint ancient metabolisms. Organic carbon in Late Archean samples stands out from that of other intervals with unusually low δ^(13)C values (∼−45 to −60‰). It was hypothesized that these light compositions record ecosystem-wide methane cycling and methanotrophy, either of the aerobic or anaerobic variety. To test this idea, we studied the petrography and carbon and oxygen isotope systematics of well-known and spectacular occurrences of shallow water stromatolites from the 2.72 Ga Tumbiana Formation of Western Australia. We examined the carbonate cements and kerogen produced within the stromatolites, because methanotrophy is expected to leave an isotopic fingerprint in these carbon reservoirs. Mathematical modeling of Archean carbonate chemistry further reveals that methanotrophy should still have a discernible signature preserved in the isotopic record, somewhat diminished from those observed in Phanerozoic sedimentary basins due to higher dissolved inorganic carbon concentrations. These stromatolites contain kerogen with δ^(13)C_(org) values of ∼−50‰. By microsampling different regions and textures within the stromatolites, we determined that the isotopic compositions of the authigenic calcite cements show a low degree of variation and are nearly identical to values estimated for seawater at this time; the lack of low and variable δ^(13)C_(carb) values implies that methanotrophy does not explain the low δ^(13)C_(org) seen in the coeval kerogen. These observations do not support a methanotrophy hypothesis, but instead hint that the Late Archean may constitute an interval wherein autotrophs employed markedly different biochemical processes of energy conservation and carbon fixation.

Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

AbstractSedimentological insights underpin many of the important recent advances in understanding of Earth system behaviour in the Neoproterozoic Era. This article reviews three main areas: (i) chemical proxies and their preservation, with emphasis on carbonate facies; (ii) glacial and post‐glacial facies, including their age constraints; (iii) sedimentary evidence for biotic innovations and responses. Chemostratigraphy plays an important role in ordering Neoproterozoic events and defining disturbances to the carbon cycle. There is increasing attention being paid to assessing the role of diagenetic origination or modification of chemostratigraphic signals. Alongside this, new criteria for identifying primary dolomite and precursor metastable phases such as ikaite have been developed. In respect of oxygenation, geochemical proxies substantiate the concept of a Neoproterozoic Oxygenation Event as a very gradual transition, the ocean being at any one time a heterogeneous assemblage of ferruginous, sulphidic and oxic conditions, with some evidence of increasing deep‐sea oxidation through the Ediacaran Period. Techniques such as Fe‐speciation need to be supplemented with proxies sensitive to suboxic conditions. More generally, it is predicted that petrographically constrained microanalytical studies will also become more important in reconstructing palaeoenvironmental conditions. The global distribution of Neoproterozoic glacial deposits combined with palaeomagnetic evidence supports the concept of panglaciations in which ice sheets reached sea‐level in the tropics. Advances in radiometric dating have demonstrated the synchronous onset of global (Sturtian) glaciation at 717 Ma and the demise of a second (Marinoan) glaciation at 635 Ma, and plausibly indicate long durations for each (55 Myr for Sturtian and 5 to 15 Myr for Marinoan). However, a compilation of radiometric dates indicates ambiguities indicating the need for further improvements to the radiometric and Sr‐isotope database to understand events within the Sturtian time frame, the timing of onset of Marinoan glaciations, and the age and synchroneity of individual negative δ13C anomalies. Sturtian deposits are typically thick, rift‐related successions containing a range of environments influenced or dominated by dynamic glaciers, as well as ice‐free marine intervals. Marinoan glacial deposits, by contrast, tend to be thin and continental. During the latter interval, oxygen isotope systematics of sulphate demonstrate that atmospheric CO2was high, as predicted by Snowball Earth theory, and that sedimentation was influenced by orbital forcing. The Sturtian record, by comparison, needs to be searched for evidence of cold‐climate hiatuses on the one hand and orbital forcing on the other. Cap carbonate formation appears to have coincided with rising sea‐levels following panglaciations. Snowball theory considers that they formed rapidly in the postglacial greenhouse, but an alternative model of slower formation with clastic sediment starvation during transgression may prove to be consistent with new data and models showing extensive glacier terminations on land. For all facies, but especially caps, the use of microanalytical techniques and holistic studies of petrogenesis are future priorities. There are important discrepancies between molecular clock predictions of early metazoan origination and the hard evidence from sedi‐mentary records which largely depend on local exceptional preservation by early diagenesis. A variety of life survived panglaciation, and there is little evidence that glaciations directly caused oxygenation or stimulated evolution.

O, Sr and Nd isotopic constraints on Cenozoic granitoids of Northwestern Anatolia, Turkey: Enrichment by subduction zone fluids

The oxygen and strontium isotope compositions of Cenozoic granitoids cropping out in the İzmir-Ankara-Erzincan suture zone help constrain the petrological evolution of magmatism in northwest Anatolia. The magmatism was mostly widespread between late Eocene (∼37 Ma) and the middle Miocene (∼14–15 Ma), and is represented by volcanic and plutonic rocks of orogenic affinity, of which Ezine, Eğrigöz, Çataldağ and Kozak are the largest Tertiary granitic plutons exposed in northwest Anatolia. They vary from granite to granodiorite, and are subalkaline, belonging to the high-K calc-alkaline I-type granite series. All these characteristics, combined with major, trace element geochemical data as well as mineralogical and textural evidence, reveal that the Oligocene-Miocene granitoids of NW Anatolia are comparable with volcanic arc granites, formed in a transitional oceanic to continental collisional tectonic setting, from a hybrid source, having crustal and mantle components that underwent further interaction with the upper crust. These plutons have initial 87Sr/86Sr ratios of 0.7072–0.7094, and εNd(t) values ranging from −3.48 to −1.20. These characteristics also indicate that a crustal component played an important role in the petrogenesis of NW Anatolian Oligocene-Miocene granitoids. The moderately evolved Ezine, Eğrigöz, Çataldağ and Kozak granitoids, have δ18O values that are consistent with those of normal I-type granites (6–10‰), but the δ18O relationships among minerals of samples collected from the intrusive contacts which are closest to mineralized zones, indicate a major influence of hydrothermal processes under subsolidus conditions. The oxygen isotope systematics of the samples from these plutons result from the activity of high-δ18O fluids (magmatic water), with major involvement of low-δ18O fluids (meteoric water) evident, near the edge zone of these plutons. This is most evident in δ18O quartz-feldspar pairs from these granitoids, which commonly have values characteristic of open-system hydrothermal conditions, and is consistent with the presence of large scale base-metal mineralization around the NW Anatolian granitoids.

An isotopic generation: four decades of stable isotope analysis in African archaeology

ABSTRACTThe application of stable isotope analysis in African archaeology has a long and distinguished pedigree. Many new developments were inspired by questions in the archaeology of the continent, helped considerably by the availability of African landscapes as natural laboratories for providing the essential underpinning of isotope ecology systematics. Stable isotopes are now regularly built into multi-disciplinary archaeological projects spanning the very long temporal and geographical span of human history on the African continent. On the fiftieth anniversary of Azania: Archaeological Research in Africa we evaluate these endeavours — the achievements, problems, constraints and potential of stable isotopic research in African archaeology. We begin with a discussion of the continent's role in developing understandings of stable carbon, nitrogen, oxygen and strontium isotope systematics, as used to interpret change in the archaeological record. We discuss the application of these principles within several areas of archaeological research, drawing on case studies from among late Pleistocene/Holocene hunter-gatherers, early food producers, complex societies of the African Iron Age and the individual life histories of the African slave trade. There is clearly much potential for further application in African archaeology.

Semi-arid zone caves: Evaporation and hydrological controls on δ18O drip water composition and implications for speleothem paleoclimate reconstructions

Oxygen isotope ratios in speleothems may be affected by external processes that are independent of climate, such as karst hydrology and kinetic fractionation. Consequently, there has been a shift towards characterising and understanding these processes through cave monitoring studies, particularly focussing on temperate zones where precipitation exceeds evapotranspiration. Here, we investigate oxygen isotope systematics at Wellington Caves in semi-arid, SE Australia, where evapotranspiration exceeds precipitation. We use a novel D2O isotopic tracer in a series of artificial irrigations, supplemented by pre-irrigation data comprised four years of drip monitoring and three years of stable isotope analysis of both drip waters and rainfall. This study reveals that: (1) evaporative processes in the unsaturated zone dominate the isotopic composition of drip waters; (2) significant soil zone ‘wetting up’ is required to overcome soil moisture deficits in order to achieve infiltration, which is highly dependent on antecedent hydro-climatic conditions; (3) lateral flow, preferential flow and sorption in the soil zone are important in redistributing subsurface zone water; (4) isotopic breakthrough curves suggest clear evidence of piston-flow at some drip sites where an older front of water discharged prior to artificial irrigation water; and (5) water residence times in a shallow vadose zone (<2 m) are highly variable and can exceed six months. Oxygen isotope speleothem records from semi-arid regions are therefore more likely to contain archives of alternating paleo-aridity and paleo-recharge, rather than paleo-rainfall e.g. the amount effect or mean annual. Speleothem-forming drip waters will be dominated by evaporative enrichment, up to ∼3‰ in the context of this study, relative to precipitation-weighted mean annual rainfall. The oxygen isotope variability of such coeval records may further be influenced by flow path and storage in the unsaturated zone that is not only drip specific but also influenced by internal cave climatic conditions, which may vary spatially in the cave.

Formation of the Neoarchean Bad Vermilion Lake Anorthosite Complex and spatially associated granitic rocks at a convergent plate margin, Superior Province, Western Ontario, Canada

The Bad Vermilion Lake Anorthosite Complex (henceforth, the BVLA Complex) in western Ontario is one of the well-exposed, anorthosite-bearing, Archean layered intrusions in the Superior Province, Canada. This study presents new whole-rock major and trace element data for the various units of the Complex, oxygen isotope data for the anorthosite, and major and trace element data for the spatially associated granitic rocks intruding the BVLA Complex to constrain their petrogenetic and geodynamic origin. Zircons from granitic rocks have yielded a 207Pb/206Pb age of 2716±18Ma, constraining the minimum intrusion age of the Complex.Despite deformation and greenschist facies metamorphism, primary igneous textures are locally well preserved in the BVLA Complex. Its whole-rock major and trace elemental compositions and the oxygen isotopic systematics appear not to have been substantially modified by deformation and metamorphism. Mantle-like oxygen isotope signatures and major and trace element compositions are inconsistent with significant crustal contamination of the BVLA Complex during its emplacement. The existence of primary calcic igneous plagioclase, coherent negative Nb anomalies (Nb/Nb*=0.08–0.88), and geochemical similarities between gabbros from the BVLA Complex and gabbros from Cenozoic arcs collectively suggest an intra-oceanic subduction zone geodynamic setting for the Complex. Near-flat REE patterns in the various units of the BVLA Complex suggest that they were derived from melting of a shallow source beneath a subarc mantle wedge. Trends in immobile major (e.g., MgO) and trace (e.g., Zr) element data indicate that the mineralogical composition of the Complex can be explained by fractional crystallization and accumulation of olivine, orthopyroxene, clinopyroxene, plagioclase and possibly amphibole.Compositionally, the bordering granitic rocks are A2-type and strongly enriched in Th and REE (>100 times chondrite) and depleted of Ba, Sr, Eu and Ti. We suggest that they formed in a post-collisional, extensional, tectonic regime following emplacement of the BVLA Complex in an oceanic arc.

Devolatilization or melting of carbonates at Meteor Crater, AZ?

AbstractWe have investigated the carbonates in the impact melts and in a monolithic clast of highly shocked Coconino sandstone of Meteor Crater, AZ to evaluate whether melting or devolatilization is the dominant response of carbonates during high‐speed meteorite impact. Both melt‐ and clast‐carbonates are calcites that have identical crystal habits and that contain anomalously high SiO2 and Al2O3. Also, both calcite occurrences lack any meteoritic contamination, such as Fe or Ni, which is otherwise abundantly observed in all other impact melts and their crystallization products at Meteor Crater. The carbon and oxygen isotope systematics for both calcite deposits suggest a low temperature environment (&lt;100 °C) for their precipitation from an aqueous solution, consistent with caliche. We furthermore subjected bulk melt beads to thermogravimetric analysis and monitored the evolving volatiles with a quadrupole mass spectrometer. CO2 yields were &lt;5 wt%, with typical values in the 2 wt% range; also total CO2 loss is positively correlated with H2O loss, an indication that most of these volatiles derive from the secondary calcite. Also, transparent glasses, considered the most pristine impact melts, yield 100 wt% element totals by EMPA, suggesting complete loss of CO2. The target dolomite decomposed into MgO, CaO, and CO2; the CO2 escaped and the CaO and MgO combined with SiO2 from coexisting quartz and FeO from the impactor to produce the dominant impact melt at Meteor Crater. Although confined to Meteor Crater, these findings are in stark contrast to Osinski et al. (2008) who proposed that melting of carbonates, rather than devolatilization, is the dominant process during hypervelocity impact into carbonate‐bearing targets, including Meteor Crater.

Carbon and oxygen isotope systematics of a Paleoproterozoic cap-carbonate sequence from the Sausar Group, Central India

Well-preserved sedimentological features of the Paleoproterozoic age Sausar Group of central India indicate glaciogenic origin for the diamictite in the lower part of the unit. Deglaciation was responsible for a marine transgression and the deposition of proximal glaciomarine facies and a cap-carbonate, followed by fine clastics with manganese ore. Analyses of δ13C and δ18O contents in the carbonate unit overlying the diamictite, together with Sr and Ba contents and REE data, indicate the preservation of primary geochemical signatures. The average δ13Ccarb content ranges between −3.1‰ and +0.1‰ V-PDB, having peak negative δ13C excursions of up to −7.4‰ V-PDB, similar to Paleoproterozoic and Neoproterozoic cap-carbonates elsewhere. High Sr and Ba contents indicate the presence of aragonite and barite precursors, similar to well-studied Neoproterozoic cap-carbonates. A single continuous carbonate unit shows δ13C excursions from −4.4 to +2.6‰ V-PDB. The lithological association and chemostratigraphy are comparable with the carbonate–tillite association of the Huronian Supergroup of Canada, the Snowy Pass Supergroup of the USA, the Transvaal Supergroup of South Africa, and the Turee Creek Group of Australia.

Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs

Abstract Oxygen isotopes are useful tracers of silicate melt generation processes because of the relatively constant abundance of oxygen in silicate reservoirs and the large isotopic fractionation that can occur between 18O and 16O during low ( 350 °C) temperature alteration processes at Earth's surface. Studies of oceanic island basalts (OIB) have demonstrated the important role of assimilation of hydrothermal altered crust on 18O/16O ratios, as well as evidence that some OIB mantle sources contain recycled oceanic or continental crust and lithosphere based on correlations between oxygen and radiogenic isotopes. To further investigate how oxygen isotope signatures may be used as tracers in intraplate volcanic rocks, we report olivine compositions from South African olivine melilitites. Olivine melilitites are considered to be related to Group 1 kimberlites and form from asthenospheric melting beneath mature oceanic islands or under off-craton continental lithosphere. South African olivine melilitites also exhibit radiogenic isotopic signatures similar to high-μ (HIMU; high-238U/204Pb) OIB, suggesting sources containing subducted oceanic lithosphere. Olivine from South African melilitites has trace element compositions that are consistent with a magmatic origin from a HIMU-type mantle melt and have a remarkably restricted range in primary 18O/16O ratios (δ18O = 4.99–5.26‰; Average = 5.14 ± 0.17‰, 2σ) that are within the mantle olivine range (δ18O = 5.2 ± 0.3‰). These compositions indicate that South African olivine melilitites require a HIMU mantle source with the oxygen isotope characteristics of ambient peridotite mantle and can be explain through either: (1) intra-mantle differentiation processes that fractionate U(and Th) from Pb, but not 18O/16O ratios, or (2) a dominantly peridotitic source with HIMU-like trace-element and radiogenic isotope characteristics inherited from equilibration and remixing of ancient recycled oceanic lithosphere. In contrast, some HIMU ocean island basalts require mantle sources with low-δ18O, indicating that they originate from distinct recycled mantle lithologies (e.g., pyroxenite/eclogite).

Oxygen isotope systematics in the aragonite–CO2–H2O–NaCl system up to 0.7 mol/kg ionic strength at 25 °C

To investigate the oxygen isotope systematics in the aragonite–CO2–H2O–NaCl system, witherite (BaCO3) was precipitated quasi-instantaneously and quantitatively from Na–Cl–Ba–CO2 solutions of seawater-like ionic strength (I=0.7mol/kg) at two pH values (∼7.9 and ∼10.6) at 25°C. The oxygen isotope composition of the witherite and the dissolved inorganic carbon speciation in the starting solution were used to estimate the oxygen isotope fractionations between HCO3− and H2O as well as between CO32− and H2O. Given the analytical error on the oxygen isotope composition of the witherite and uncertainties of the parent solution pH and speciation, oxygen isotope fractionation between NaHCO3° and HCO3−, as well as between NaCO3− and CO32−, is negligible under the experimental conditions investigated.The influence of dissolved NaCl concentration on the oxygen isotope fractionation in the aragonite–CO2–H2O–NaCl system was also investigated at 25°C. Aragonite was precipitated from Na–Cl–Ca–Mg–(B)–CO2 solutions of seawater-like ionic strength using passive CO2 degassing or constant addition methods. Based upon our new experimental observations and published experimental data from lower ionic strength solutions by Kim et al. (2007b), the equilibrium aragonite–water oxygen isotope fractionation factor is independent of the ionic strength of the parent solution up to 0.7mol/kg. Hence, our study also suggests that the aragonite precipitation mechanism is not affected by the presence of sodium and chloride ions in the parent solution over the range of concentrations investigated.

Komsomolskaya diamondiferous eclogites: evidence for oceanic crustal protoliths

The Komsomolskaya kimberlite is one of numerous (>1,000) kimberlite pipes that host eclogite xenoliths on the Siberian craton. Eclogite xenoliths from the adjacent Udachnaya kimberlite pipe have previously been geochemically well characterized; however, data from surrounding diamond-bearing kimberlite pipes from the center of the craton are relatively sparse. Here, we report major- and trace-element data, as well as oxygen isotope systematics, for mineral separates of diamondiferous eclogite xenoliths from the Komsomolskaya kimberlite, suggesting two distinct subgroups of a metamorphosed, subducted oceanic crustal protolith. Using almandine contents, this suite can be divided into two subgroups: group B1, with a high almandine component (>20 mol%) and group B2, with a low almandine component (<20 mol%). Reconstructed REE profiles for B1 eclogites overlap with typical oceanic basalts and lack distinct Eu anomalies. In addition, elevated oxygen isotope values, which are interpreted to reflect isotopic exchange with seawater at low temperatures (<350 °C), are consistent with an upper-oceanic crustal protolith. Reconstructed REE profiles for B2 eclogites are consistent with oceanic gabbros and display distinct Eu anomalies, suggesting a plagioclase-rich cumulate protolith. In contrast to B1, B2 eclogites do not display elevated oxygen isotope values, suggesting an origin deep within the crustal pile, where little-to-no interaction with hydrothermal fluids has occurred. Major-element systematics were reconstructed based on mineral modes; group B1 eclogites have higher MgO wt% and lower SiO2 wt%, with respect to typical oceanic basalts, reflecting a partial melting event during slab subduction. Calculated residues from batch partial melt modeling of a range of Precambrian basalts overlap with group B1 trace-element chemistry. When taken together with the respective partial melt trajectories, these melting events are clearly linked to the formation of Tonalite–Trondhjemite–Granodiorite (TTG) complexes. As a result, we propose that many, if not all, diamondiferous eclogite xenoliths from Komsomolskaya represent mantle ‘restites’ that preserve chemical signatures of Precambrian oceanic crust.

Radiocarbon Dating, Mineralogy, and Isotopic Composition of Hackberry Endocarps from the Neolithic Site of Aşikli Höyük, Central Turkey

Carbonate is abundant in many Neolithic tells and is a potentially useful archive for dating and climate reconstruction. In this paper, we focus on the mineralogy, radiocarbon dating, and stable isotope systematics of carbonate in hackberry endocarps. Hackberry fruits and seeds are edible in fresh and stored forms, and they were consumed in large quantities in many Neolithic sites in the Near East, including the site of our study, Aşıkli Höyük in central Anatolia, an Aceramic Neolithic tell occupied from about 9.4 to &gt; 10.3 BP (7.4 to &gt; 8.3 BCE). Detailed 14C age control provided by archaeological charcoal permits a test of the fidelity in 14C dating of hackberry endocarps. Modern endocarps and leaves yield fraction modern 14C values of 1.050–1.066, consistent with levels present in the atmosphere when sampled in 2009. On the other hand, archaeological endocarps yield consistently younger ages than associated charcoal by ca. 130 14C years (ca. 220 calendar years) for samples about 10,000 years old. We speculate this is caused by the slight addition of calcite or recrystallization to calcite in the endocarp, as detected by scanning electron microscopy. Subtle addition or replacement of calcite by primary aragonite is not widely recognized in the 14C community, even though similar effects are reported from other natural carbonates such as shell carbonate. This small (but consistent) level of contamination supports the usefulness of endocarps in dating where other materials like charcoal are lacking. Before dating, however, hackberries should be carefully screened for mineralogical preservation and context. We examined the carbon and oxygen isotopic systematics of the fossil endocarps to try to establish potential source areas for harvesting. Most of the hackberries are enriched in 18O compared to local water sources, indicating that they were drawing on highly evaporated soil water, rather than the local (perched and regional) water table sampled in our study. Isotopic evidence therefore suggests that most but not all of the hackberries were harvested from nearby mesas well above the local streams and seeps fed by the water table.

Radiocarbon Dating, Mineralogy, and Isotopic Composition of Hackberry Endocarps from the Neolithic Site of Aşikli Höyük, Central Turkey

Carbonate is abundant in many Neolithic tells and is a potentially useful archive for dating and climate reconstruction. In this paper, we focus on the mineralogy, radiocarbon dating, and stable isotope systematics of carbonate in hackberry endocarps. Hackberry fruits and seeds are edible in fresh and stored forms, and they were consumed in large quantities in many Neolithic sites in the Near East, including the site of our study, Aşıkli Höyük in central Anatolia, an Aceramic Neolithic tell occupied from about 9.4 to &amp;gt; 10.3 BP (7.4 to &amp;gt; 8.3 BCE). Detailed14C age control provided by archaeological charcoal permits a test of the fidelity in14C dating of hackberry endocarps. Modern endocarps and leaves yield fraction modern14C values of 1.050–1.066, consistent with levels present in the atmosphere when sampled in 2009. On the other hand, archaeological endocarps yield consistently younger ages than associated charcoal by ca. 13014C years (ca. 220 calendar years) for samples about 10,000 years old. We speculate this is caused by the slight addition of calcite or recrystallization to calcite in the endocarp, as detected by scanning electron microscopy. Subtle addition or replacement of calcite by primary aragonite is not widely recognized in the14C community, even though similar effects are reported from other natural carbonates such as shell carbonate. This small (but consistent) level of contamination supports the usefulness of endocarps in dating where other materials like charcoal are lacking. Before dating, however, hackberries should be carefully screened for mineralogical preservation and context. We examined the carbon and oxygen isotopic systematics of the fossil endocarps to try to establish potential source areas for harvesting. Most of the hackberries are enriched in18O compared to local water sources, indicating that they were drawing on highly evaporated soil water, rather than the local (perched and regional) water table sampled in our study. Isotopic evidence therefore suggests that most but not all of the hackberries were harvested from nearby mesas well above the local streams and seeps fed by the water table.

Oxygen three-isotope ratios of silicate particles returned from asteroid Itokawa by the Hayabusa spacecraft: A strong link with equilibrated LL chondrites

Oxygen three-isotope ratios of four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in seven silicate particles from asteroid Itokawa were measured to investigate oxygen isotope systematics of asteroidal materials. In order to obtain highly precise and accurate oxygen isotope ratios (∼±0.3‰ in δ18O), we used a new indium mounting method that minimizes potential instrumental mass fractionation due to surface topography. The oxygen isotope data of the seven Itokawa particles are distributed above the terrestrial fractionation line with indistinguishable ΔO17(=δO17−0.52×δO18) values of +1.34±0.36‰ (2SD; n=22), which is similar to those of Itokawa particles analyzed by Yurimoto et al. (2011) and Nakamura et al. (2012). However, δ18O values of the seven Itokawa particles are tightly clustered (+3.9‰ to +5.9‰ VSMOW) compared to previous analyses of the Itokawa particles (+1.4‰ to +8.8‰).The average Δ17O values and 2SD of the two equilibrated ordinary chondrites (EOCs), which were analyzed for comparison, are +1.31±0.49‰ for St. Séverin (LL6) and +0.83±0.64‰ for Guareña (H6). The Δ17O values of the seven Itokawa particles are similar to those of St. Séverin. The average bulk δ18O value of the seven Itokawa particles is estimated as +4.8‰, which is within the δ18O range of bulk LL5 and LL6 chondrites. It is suggested that Itokawa particles are similar to those of equilibrated LL chondrites, which strengthens a link between asteroid Itokawa and equilibrated LL chondrites that fell to Earth.The seven Itokawa particles and two EOCs have δ18O values of high-Ca pyroxene that are lower than those of olivine, indicating that oxygen isotopes between the two mineral phases are not in equilibrium. Apparent oxygen isotope equilibrium temperatures estimated from a pair of low-Ca pyroxene and olivine are excessively higher than those from a pair of plagioclase and high-Ca pyroxene (∼800 °C) and peak metamorphic temperatures. Oxygen isotopes in olivine and low-Ca pyroxene do not represent equilibrium. On the other hand, oxygen isotope ratios in high-Ca pyroxene and plagioclase were mass-dependently fractionated accompanied by recrystallization (and possibly by diffusion) during thermal metamorphism. Values of δ18O in high-Ca pyroxene were lowered by exchange, which may have resulted in the reverse δ18O fractionation between olivine and high-Ca pyroxene.

Stable oxygen and carbon isotope systematics of exquisitely preserved Turonian foraminifera from Tanzania — Understanding isotopic signatures in fossils

The interpretation of δ18O and δ13C analyses of extinct foraminifera is hampered by insufficient knowledge on the contribution of environmental versus biological factors on the measured values. We present single-species stable isotope data for 18 benthic and 11 planktic foraminiferal species and one calcareous dinoflagellate cyst species from a Turonian equatorial record measured on samples collected in SE Tanzania. All microfossils analyzed were excellently preserved and results are compared to isotopic measurements from bulk sediment of the same material. Remarkably low intra-specific variability (standard deviations < 0.2‰) across the studied interval and consistently large benthic/planktic offsets in δ18O (~ 2.8‰) and δ13C (~ 1.5‰) indicate absence of major long-term environmental fluctuations and presence of a well stratified water column. Stratigraphic isotopic trends show small shifts that are parallel in all studied benthic species (with the exception of the δ13C record of Lenticulina species), but within-sample inter-specific offsets reflect a combination of habitat differences and species-specific isotopic disequilibrium fractionation. Among planktic taxa closely related taxa tend to have similar isotopic values. Biserial taxa have the lowest δ18O values (~− 4.7‰) but also have relatively low δ13C values (~ 1.6‰) indicating that, for the assumption of a surface water habitat, additional metabolic effects need to be involved, which supports the concept of their opportunistic lifestyle. Among trochospiral planktics the keeled taxa with compressed chambers have slightly higher δ18O (~− 4.2‰) and lower δ13C (~ 2.3‰) values than taxa with globular chambers (~− 4.4‰ and ~ 2.4‰, respectively), suggesting differences in habitat or in peak seasonal abundance. Benthic taxa are subdivided into three isotopically distinct groups with differences reflecting mineralogy, life strategy and habitat; Group I: aragonitic taxa (Epistomina, Colomia africana), Group II: calcitic trochospiral taxa with isotopic values seemingly close to equilibrium (Berthelina berthelini, Oridorsalis umbonatus, Lingulogavelinella globosa, Lingulogavelinella convexa) and Group III: calcitic taxa with low δ18O values and large scatter in δ13C values (Lenticulina). Four other benthic species show values between Groups II and III. The wealth and consistency of data, especially for benthic taxa, permit refined conclusions. The five studied species of Epistomina have a similar isotopic signature and are inferred to form their aragonitic shells with close to equilibrium values. Small isotopic offsets are seen among species that differ in orientation of the septal foramen, but no offsets are seen that seem to co-vary with test shape or ornamentation. The conical species C. africana has δ18O values ~ 0.2‰ lower and δ13C values ~ 0.7‰ lower than species of Epistomina. The latter offset probably reflects an infaunal habitat for C. africana. Epistomina and Group II species are recommended for paleoenviromental studies. Mono-specific Lenticulina δ18O values may be used if increased by correction factors of 0.4‰ to 0.9‰, but this taxon's low and variable δ13C values are interpreted to reflect incorporation of respiratory CO2, possibly combined with fast growth rates related to opportunism, rather than equilibrium with benthic dissolved inorganic carbon. The observed stability of inter-specific isotopic offsets suggests disequilibrium isotopic effects are relatively consistent for many taxa in this section, and correction factors are proposed that may help to generate more robust Upper Cretaceous foraminiferal isotopic records. However, δ18O/δ13C covariance in some benthic inter-specific comparisons suggests offsets might vary with changes in temperature, food supply or other factors so the appropriateness of the correction factors should be confirmed before being applied at different sites. Similarities between the observed single-species isotopic patterns from this and other studies are used to present a summarizing scheme of influences on the stable isotopic composition in foraminiferal tests. By applying the new insights from our study to data from other Cretaceous studies we demonstrate that the selection of species for isotopic analysis and understanding these signals are critical to the outcome of paleoceanographic interpretations.