Iron and magnesium isotopic compositions of peridotite xenoliths from Eastern China

We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ⁵⁶Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ²⁶Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ⁵⁶Fe; ±0.10‰ for δ²⁶Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ⁵⁶Fe_ol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ⁵⁶Fe_ol-opx = −0.05 ± 0.11‰; Δ²⁶Mg_ol-opx = −0.09 ± 0.12‰; Δ²⁶Mg_ol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ⁵⁶Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ²⁶Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ⁵⁶Fe and δ²⁶Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ⁵⁶Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ⁵⁶Fe, the larger deviations from average δ⁵⁶Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ⁵⁶Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ⁵⁶Fe and slightly heavy δ²⁶Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.     

Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

Komatiites from Alexo, Canada, are well preserved and represent high-degree partial mantle melts (∼50%). They are thus well suited for investigating the Mg and Fe isotopic compositions of the Archean mantle and the conditions of magmatic differentiation in komatiitic lavas. High precision Mg and Fe isotopic analyses of 22 samples taken along a 15-m depth profile in a komatiite flow are reported. The δ²⁵Mg and δ²⁶Mg values of the bulk flow are −0.138 ± 0.021‰ and −0.275 ± 0.042‰, respectively. These values are indistinguishable from those measured in mantle peridotites and chondrites, and represent the best estimate of the composition of the silicate Earth from analysis of volcanic rocks. Excluding the samples affected by secondary Fe mobilization, the δ⁵⁶Fe and δ⁵⁷Fe values of the bulk flow are +0.044 ± 0.030‰, and +0.059 ± 0.044‰, respectively. These values are consistent with a near-chondritic Fe isotopic composition of the silicate Earth and minor fractionation during komatiite magma genesis. In order to explain the early crystallization of pigeonite relative to augite in slowly cooled spinifex lavas, it was suggested that magmas trapped in the crystal mush during spinifex growth differentiated by Soret effect, which should be associated with large and coupled variations in the isotopic compositions of Mg and Fe. The lack of variations in Mg and Fe isotopic ratios either rules out the Soret effect in the komatiite flow or the effect is effaced as the solidification front migrates downward through the flow crust. Olivine separated from a cumulate sample has light δ⁵⁶Fe and slightly heavy δ²⁶Mg values relative to the bulk flow, which modeling shows can be explained by kinetic isotope fractionation associated with Fe-Mg inter-diffusion in olivine. Such variations can be used to identify diffusive processes involved in the formation of zoned minerals.     

The effect of early diagenesis on the Fe isotope compositions of porewaters and authigenic minerals in continental margin sediments

Iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the California continental margin, where isotope compositions range from δ⁵⁶Fe = −3.0‰ to +0.4‰. At one site near Monterey Canyon off central California, organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction (DIR) and bacterial sulfate reduction, whereas at our other site in the Santa Barbara basin DIR appears to be comparatively small, and production of sulfides (FeS and pyrite) was extensive. The largest range in Fe isotope compositions is observed for Fe(II)_aq in porewaters, which generally have the lowest δ⁵⁶Fe values (minimum: −3.0‰) near the sediment surface, and increase with burial depth. δ⁵⁶Fe values for FeS inferred from HCl extractions vary between ∼−0.4‰ and +0.4‰, but pyrite is similar at both stations, where an average δ⁵⁶Fe value of −0.8 ± 0.2‰ was measured. We interpret variations in dissolved Fe isotope compositions to be best explained by open-system behavior that involves extensive recycling of Feflux. This study is the first to examine Fe isotope variations in modern marine sediments, and the results show that Fe isotopes in the various reactive Fe pools undergo isotopic fractionation during early diagenesis. Importantly, processes dominated by sulfide formation produce high-δ⁵⁶Fe values for porewaters, whereas the opposite occurs when Fe(III)-oxides are present and DIR is a major pathway of organic carbon respiration. Because shelf pore fluids may carry a negative δ⁵⁶Fe signature it is possible that the Fe isotope composition of ocean water reflects a significant contribution of shelf-derived iron to the open ocean. Such a signature would be an important means for tracing iron sources to the ocean and water mass circulation.     

The geochemical behavior and isotopic composition of Hg in a mid-Pleistocene western Mediterranean sapropel

The concentrations of mercury (Hg) and other trace metals (Ni, Cu, Zn, Mo, Ba, Re, U) and the Hg isotopic composition were examined across a dramatic redox and productivity transition in a mid-Pleistocene Mediterranean Sea sapropel sequence. Characteristic trace metal enrichment in organic-rich layers was observed, with organic-rich sapropel layers ranging in Hg concentration from 314 to 488 ng/g (avg = 385), with an average enrichment in Hg by a factor of 5.9 compared to organic-poor background sediments, which range from 39 to 94 ng/g Hg (avg = 66). Comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers Hg to the seafloor. Near complete scavenging of Hg from the water column renders the sapropel Hg isotopic composition representative of mid-Pleistocene Mediterranean seawater. Sapropels have an average δ²⁰²Hg value of −0.91‰ ± 0.15‰ (n = 5, 1 SD) and Δ¹⁹⁹Hg value of 0.11‰ ± 0.03‰ (n = 5, 1 SD). Background sediments have an average δ²⁰²Hg of −0.76‰ ± 0.16‰ (n = 5, 1 SD) and Δ¹⁹⁹Hg of 0.05‰ ± 0.01‰ (n = 5, 1 SD), which is indistinguishable from the sapropel values. We suggest that the sapropel isotopic composition is most representative of the mid-Pleistocene Tyrrhenian Sea.     

Effects of aridity and vegetation on plant-wax δD in modern lake sediments

We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (ε_a) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of ²H and ¹⁸O associated with evaporation of lake waters. The most negative ε_a values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum ε_a values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ¹⁸O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.     

Iron isotope systematics in estuaries: The case of North River, Massachusetts (USA)

Recent studies have suggested that rivers may present an isotopically light Fe source to the oceans. Since the input of dissolved iron from river water is generally controlled by flocculation processes that occur during estuarine mixing, it is important to investigate potential fractionation of Fe-isotopes during this process. In this study, we investigate the influence of the flocculation of Fe-rich colloids on the iron isotope composition of pristine estuarine waters and suspended particles. The samples were collected along a salinity gradient from the fresh water to the ocean in the North River estuary (MA, USA). Estuarine samples were filtered at 0.22 μm and the iron isotope composition of the two fractions (dissolved and particles) were analyzed using high-resolution MC-ICP-MS after chemical purification. Dissolved iron results show positive δ⁵⁶Fe values (with an average of 0.43 ± 0.04‰) relative to the IRMM-14 standard and do not display any relationships with salinity or with percentage of colloid flocculation. The iron isotopic composition of the particles suspended in fresh water is characterized by more negative δ⁵⁶Fe values than for dissolved Fe and correlate with the percentage of Fe flocculation. Particulate δ⁵⁶Fe values vary from −0.09‰ at no flocculation to ∼0.1‰ at the flocculation maximum, which reflect mixing effects between river-borne particles, lithogenic particles derived from coastal seawaters and newly precipitated colloids. Since the process of flocculation produces minimal Fe-isotope fractionation in the dissolved Fe pool, we suggest that the pristine iron isotope composition of fresh water is preserved during estuarine mixing and that the value of the global riverine source into the ocean can be identified from the fresh water values. However, this study also suggests that δ⁵⁶Fe composition of rivers can also be characterized by more positive δ⁵⁶Fe values (up to 0.3‰) relative to the crust than previously reported. In order to improve our current understanding of the oceanic iron isotope cycling, further work is now required to determine the processes controlling the fractionation of Fe-isotopes during continental run-off.     

The origin and history of ordinary chondrites: A study by iron isotope measurements of metal grains from ordinary chondrites

Chondrules and chondrites provide unique insights into early solar system origin and history, and iron plays a critical role in defining the properties of these objects. In order to understand the processes that formed chondrules and chondrites, and introduced isotopic fractionation of iron isotopes, we measured stable iron isotope ratios ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe in metal grains separated from 18 ordinary chondrites, of classes H, L and LL, ranging from petrographic types 3-6 using multi-collector inductively coupled plasma mass spectrometry. The δ⁵⁶Fe values range from −0.06 ± 0.01 to +0.30 ± 0.04‰ and δ⁵⁷Fe values are −0.09 ± 0.02 to +0.55 ± 0.05‰ (relative to IRMM-014 iron isotope standard). Where comparisons are possible, these data are in good agreement with published data. We found no systematic difference between falls and finds, suggesting that terrestrial weathering effects are not important in controlling the isotopic fractionations in our samples. We did find a trend in the ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe isotopic ratios along the series H, L and LL, with LL being isotopically heavier than H chondrites by ∼0.3‰ suggesting that redox processes are fractionating the isotopes. The ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios also increase with increasing petrologic type, which again could reflect redox changes during metamorphism and also a temperature dependant fractionation as meteorites cooled. Metal separated from chondrites is isotopically heavier by ∼0.31‰ in δ⁵⁶Fe than chondrules from the same class, while bulk and matrix samples plot between chondrules and metal. Thus, as with so many chondrite properties, the bulk values appear to reflect the proportion of chondrules (more precisely the proportion of certain types of chondrule) to metal, whereas chondrule properties are largely determined by the redox conditions during chondrule formation. The chondrite assemblages we now observe were, therefore, formed as a closed system.     

O/O and D/H in goethite from a North American Oxisol of the Early Eocene climatic optimum

An Early Eocene Oxisol in the Ione Formation of California formed in a coastal continental weathering environment at a paleolatitude of ∼38°N. The dominant minerals in the Oxisol are goethite, quartz, and kaolinite. Material balance calculations were applied to new measurements of chemical composition, D/H, and ¹⁸O/¹⁶O ratios of Oxisol samples to determine the δD (−150 ± 3‰) and δ¹⁸O (−2.4 ± 0.3‰) values of the goethite (α-FeOOH). These data, in combination with the global meteoric water line (MWL), yielded an isotopic temperature of 21(±4) °C. The nominal value of 21 °C contrasts with the modern mean annual temperature (MAT) of 16 °C in that area. The warmer temperature is consistent with formation of the goethite during the Early Eocene climatic optimum. The isotopic composition of the goethite and a temperature of 21 °C imply ancient water with a δD value of −61(±4)‰ and a δ¹⁸O value of −8.9(±0.5)‰. This Early Eocene δ¹⁸O (or δD) value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a MAT of 21 °C.At times of warm global climates, the location of a near-surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions. A simple Rayleigh-type condensation model indicates that, if one “follows the isotherm”, the associated scatter in δD and δ¹⁸O of precipitation in very warm global climates should shift (for a given isotherm) to more negative values that may be detectable in proxy records. The isotopic results from the goethite of the Early Eocene Oxisol appear to add to evidence in support of this idea.     

Natural fractionation of U/U

The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the ²³⁸U/²³⁵U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed ²³⁶U-²³³U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ²³⁸U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ²³⁸U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ²³⁸U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ²³⁸U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ²³⁸U (−0.41‰ to −0.16‰) compared to seawater, which has δ²³⁸U of −0.41 ± 0.03‰. Granites define a range of δ²³⁸U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ²³⁸U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble U^VI to insoluble U^IV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that ²³⁸U/²³⁵U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in ²⁰⁷Pb/²⁰⁶Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old.     

Environmental memory and a possible seasonal bias in the stable isotope composition of (U-Th)/He-dated goethite from the Canadian Arctic

Goethite (Ax-2) from Axel Heiberg Island (∼80°N) on the margin of the Arctic Ocean is the dominant mineral in a sample of “petrified” Eocene wood, but U, Th, and He measurements suggest that the goethite (α-FeOOH) crystallized in the latest Miocene/Pliocene (ca. 5.5 to 2.8 Ma). Measured δD and δ¹⁸O values of Ax-2 are −221 (±6)‰ and −9.6 (±0.5)‰, respectively. The inferred δD and δ¹⁸O values of the ancient water were about −139‰ and −18.6‰, respectively, with a calculated temperature of crystallization of 3 (±5)°C, which compares with the modern summer (J-J-A) temperature of 3 °C and contrasts with a modern MAT of −19 °C. Published results from various biological proxies on nearby Ellesmere Island indicate a Pliocene (∼4 Ma) MAT of either −6 or −0.4 °C and corresponding seasonal amplitudes of about 18 or 13 °C. A conductive heat flow model suggests that a temperature of 3 °C could represent goethite crystallization at depths of ∼100-200 cm (for MAT = −6 °C) or ∼250-450 cm (for MAT = −0.4 °C) over seasonally restricted intervals of time.The δ¹⁸O value of the Ax-2 water (−18.6‰) is more positive than the modern J-J-A precipitation (−22‰). In combination, the paleotemperatures and δ¹⁸O values of ancient waters (from Ax-2 and published results from three Eocene or Pliocene proxy sites on Axel Heiberg and Ellesmere Islands) are consistent with a warm season bias in those isotopic proxies. The results are also consistent with higher proportions of J-J-A precipitation in the annual total. If so, this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates, and suggests that the Arctic hydrologic cycle, as expressed in the seasonal distribution and isotopic composition of precipitation (perhaps modified by a warmer Arctic Ocean), differed from modern.The δ¹³C value of the Fe(CO₃)OH component in the Ax-2 goethite is +6.6‰, which is much more positive than expected if crystallizing goethite incorporated CO₂ derived primarily from oxidation of relict Eocene wood with δ¹³C values of about −24‰. This apparent paradox may be resolved if the goethite is a product of oxidation of ¹³C-rich siderite, which had previously replaced wood in an Eocene methanogenic burial environment. Thus, the goethite retains a carbon isotope “memory” of a diagenetic Eocene event, but a δD and δ¹⁸O record of the latest Miocene/Pliocene Arctic climate.     

The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland

Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth’s oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H₂S ∼ 200 μM in the bottom waters (or about 300 μM total sulfide: ΣS²⁻ = H₂S + HS⁻ + S²⁻). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a δ⁹⁸Mo_oxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ⁹⁸Mo_river = 0.9 ± 0.1‰. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1‰, but the dissolved Mo is even heavier at δ⁹⁸Mo_sulfidic = 1.8‰. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with δ⁹⁸Mo values significantly fractionated at −0.8‰ to −1.2‰ both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by ∼0.3-1.5‰. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive δ⁹⁸Mo trend with depth, from δ⁹⁸Mo = 1.2‰ to 1.4‰ while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of δ⁹⁸Mo = 0.9-1.0‰.These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H₂S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems.     

Controls on iron-isotope fractionation in organic-rich sediments (Kimmeridge Clay, Upper Jurassic, Southern England)

This study explores the fractionation of iron isotopes (⁵⁷Fe/⁵⁴Fe) in an organic-rich mudstone succession, focusing on core and outcrop material sampled from the Upper Jurassic Kimmeridge Clay Formation type locality in south Dorset, UK. The organic-rich environments recorded by the succession provide an excellent setting for an investigation of the mechanisms by which iron isotopes are partitioned among mineral phases during biogeochemical sedimentary processes.Two main types of iron-bearing assemblage are defined in the core material: mudstones with calcite ± pyrite ± siderite mineralogy, and ferroan dolomite (dolostone) bands. A cyclic data distribution is apparent, which reflects variations in isotopic composition from a lower range of δ⁵⁷Fe values associated with the pyrite/siderite mudstone samples to the generally higher values of the adjacent dolostone samples. Most pyrite/siderite mudstones vary between −0.4 and 0.1‰ while dolostones range between −0.1 and 0.5‰, although in very organic-rich shale samples below 360 m core depth higher δ⁵⁷Fe values are noted. Pyrite nodules and pyritized ammonites from the type exposure yield δ⁵⁷Fe values of −0.3 to −0.45‰. A fractionation model consistent with the δ⁵⁷Fe variations relates the lower δ⁵⁷Fe pyrite and siderite ± pyrite mudstones values to the production of isotopically depleted Fe(II) during biogenic reduction of the isotopically heavier lithogenic Fe(III) oxides. A consequence of this reductive dissolution is that a ⁵⁷Fe-enriched iron species must be produced that potentially becomes available for the formation of the higher δ⁵⁷Fe dolostones. An isotopic profile across a dolostone band reveals distinct zonal variations in δ⁵⁷Fe, characterized by two peaks, respectively located above and below the central part of the band, and decoupling of the isotopic composition from the iron content. This form of isotopic zoning is shown to be consistent with a one-dimensional model of diffusional-chromatographic Fe-isotope exchange between dolomite and isotopically enriched pore water. An alternative mechanism envisages the infiltration of dissolved ferrous iron from variable (high and low) δ⁵⁷Fe sources during coprecipitation of Fe(II) ion with dolomite. The study provides clear evidence that iron isotopes are cycled during the formation and diagenesis of organic carbon-rich sediments.     

Early diagenetic processes during Mn-carbonate formation: evidence from the isotopic composition of authigenic Ca-rhodochrosites of the Baltic Sea

The formation of authigenic Ca-rich rhodochrosite (ACR) in sapropelic sediments of the Gotland Basin, Baltic Sea, is governed by deepwater renewal processes whereby saline water from the North Atlantic flushes the brackish anoxic Baltic Deeps. The carbon and oxygen isotopic compositions of these Mn-carbonates suggest that ACR formation takes place just below the sediment surface and that dissolved compounds from the deepwater column, such as water and bicarbonate molecules, were incorporated in ACR during authigenesis. Porewaters near the sediment surface display δ¹⁸O values of −5.4‰ (VSMOW) and are generally depleted in ¹⁸O, compared to the oxygen isotopic composition of water in equilibrium with Mn-carbonate solid solutions (ACR δ¹⁸O values are −4.6‰). This suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during Mn-carbonate formation. The reduction of sulfate having δ¹⁸O values of +8.4‰ accounts for a permanent enrichment of porewater ¹⁸O and observed δ¹⁸O values at depth equal to −4.6‰. However, this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and ACR close to the sediment surface where Mn-carbonate formation takes place. Based on isotopic mass balance calculations, we suggest that MnO₂ with δ¹⁸O values of +8.9‰ released oxygen enriched in ¹⁸O into the anoxic porewaters close below the sediment surface. This process should occur after oxygenation events during deepwater renewal when MnO₂ accumulates at the surface of anoxic sediments. Manganese carbonates formed in these waters display δ¹⁸O values of ∼1.0‰ heavier than values expected solely from the initial deepwater composition. This quantitatively explains the discrepancy between paleosalinities calculated from ACR δ¹⁸O based on Mn-carbonate/water isotopic equilibrium fractionation and direct observations for the same period. Our results emphasize the important role of microbial MnO₂ reduction during rhodochrosite authigenesis and suggest that Mn(II) activity, rather than alkalinity, is the limiting component for sedimentary Mn-carbonate formation.     

Iron isotope fractionation during leaching of granite and basalt by hydrochloric and oxalic acids

Transport of iron (Fe) within hydrothermal and soil environments involves the transferral into aqueous solutions by leaching of complex, polyminerallic rocks. Understanding the isotope fractionation mechanisms during this process is key for any application of the Fe-isotope system to biogeochemical studies. Here, we reacted biotite granite and tholeiite-basalt with 0.5 M hydrochloric acid and 5 mM oxalic acid solutions at ambient temperature. Solution aliquots were recovered over a seven-day period and analysed for major and trace element concentrations and Fe isotopic compositions. In all experiments, Fe initially released into solution was isotopically lighter, with Δ⁵⁶Fe_{solution-rock} as low as −1.80‰ in the granite-hydrochloric acid system. The oxalic acid experiments showed similar patterns but smaller fractionation. In all experiments, the Δ⁵⁶Fe_{solution-rock} reduced over time, which would be in line with the formation of a leached layer as proposed before [Brantley S. L., Liermann L. J., Guynn R. L., Anbar A., Icopini G. A., and Barling J. (2004) Fe isotopic fractionation during mineral dissolution with and without bacteria. Geochim. Cosmochim. Acta68(15), 3189-3204]. Granite and basalts reacting with hydrochloric acid reached apparent steady-state values of −0.60 ± 0.15‰ and −0.40 ± 0.20‰, respectively, whilst experimental values with oxalic acid were −1.0 ± 0.15‰ and −0.50 ± 0.15‰. During the granite experiments, alteration of biotite to chlorite, followed by dissolution of chlorite, were likely the dominant processes, whilst in the basalt experiments, dissolution of pigeonite was likely the principal source of Fe. Variations in pH during the hydrochloric acid experiments were minimal, remaining below 0.5 at all times. In oxalic acid solutions, the pH increased to over 4, leading likely to precipitation of secondary minerals and adsorption/co-precipitation of Fe onto mineral surfaces. These processes could contribute to the greater fractionation observed in the final stages of the oxalic acid experiments. Our results highlight the importance of mineralogy and fluid composition on the Fe-isotope systematics during weathering. The fractionation processes identified for granites and basalts are in line with those inferred from field observations in soils, sediments, groundwater and hydrothermal deposits and from laboratory studies of single-mineral leaching.     

Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions

Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ¹⁸O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO₂, Al₂O₃). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ⁵⁶Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ¹⁸O and δ⁵⁶Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ⁵⁶Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ¹⁸O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ¹⁸O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ¹⁸O and δ⁵⁶Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).     

Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths

We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ²⁵Mg = −0.14 ± 0.06; δ²⁶Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ⁷Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ⁷Li = 3.5 ± 0.5‰, δ²⁵Mg = −0.10 ± 0.03‰ and δ²⁶Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ⁷Li and δ²⁶Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ⁷Li and δ²⁶Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ⁷Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ⁷Li seen in some oceanic basalts.     

Stable isotope composition of Hellenic bottled waters

Bottled waters are an increasingly significant product in the human diet. In this work, we present a dataset of stable isotope ratios for bottled waters sampled in Greece. A total of 25 domestic brands of bottled still waters, collected on the Greek market in 2009, were analysed for δ¹⁸O and δ²H. The measured stable isotope ratios range from − 9.9‰ to − 6.9‰ for δ¹⁸O and from − 67.50‰ to − 46.5‰ for δ²H. Comparison of bottled water isotope ratios with natural spring water isotope ratios demonstrates that on average the isotopic composition of bottled water tends to be similar to the composition of naturally available local water sources, showing that bottled water isotope ratios preserve information about the water sources from which they were derived and suggesting that in many cases bottled water should not be considered as an isotopically distinct component of the human diet. This investigation also helped to determine the natural origin of bottled water, and to indicate differences between the natural and production processes. The production process may influence the isotopic composition of waters. No such modification was observed for sampled waters in this study. The isotopic methods applied can be used for the authentication of bottled waters and for use in the regulatory monitoring of water products.     

Grain-scale iron isotopic distribution of pyrite from Precambrian shallow marine carbonate revealed by a femtosecond laser ablation multicollector ICP-MS technique: Possible proxy for the redox state of ancient seawater

The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth’s surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS).We have determined that the grain-scale iron isotopic distribution of pyrite from coeval samples shows a bimodal (2.7 and 2.3 Ga) or unimodal pattern (2.9, 2.6, and 0.7 Ga). In particular, pyrite from the 2.7 Ga Fortescue Group shows a unique bimodal distribution with highly positive (+1.0‰ defined as Type 1) and negative δ⁵⁶Fe values (−1.8‰ defined as Type 2). Type 1 and 2 pyrites occasionally occur within different siliceous layers in the same rock specimen. Layer-scale iron isotopic heterogeneity indicates that the iron isotopic ratios of the two types of pyrite are not homogenized by diagenesis after deposition. Some cubic pyrites have a core with a positive δ⁵⁶Fe value (1‰) and a rim with a crustal δ⁵⁶Fe value (0‰). The observed isotopic zoning suggests that the positive δ⁵⁶Fe value is a primary signature at the time of stromatolite formation, while secondary pyrite precipitated during diagenesis.The positive δ⁵⁶Fe value of Type 1 and the large iron isotopic difference between Type 1 and 2 (2.8‰.) suggest partial Fe(II) oxidation in the 2.7-Ga shallow sea, i.e., pyritization of ⁵⁶Fe-enriched ferric oxyhydroxide (Type 1) and ⁵⁶Fe depleted Fe²⁺aq in seawater (Type 2). Type 2 pyrite was probably not produced by microbial iron redox cycling during diagenesis because this scenario requires a higher abundance of pyrite with δ⁵⁶Fe of 0‰ than of −1.8‰. Consequently, the degree of Fe(II) oxidation in the 2.7-Ga shallow sea can be estimated by a Fe²⁺aq steady-state model. The model calculation shows that half the Fe²⁺aq influx was oxidized in the seawater. This implies that O₂ produced by photosynthesis would have been completely consumed by oxidation of the Fe²⁺aq influx. Grain-scale iron isotopic distribution of pyrite could be a useful index for reconstructing the redox state of the Archean shallow sea.     

Boron isotopic composition of atmospheric precipitations and liquid-vapour fractionations

Boron isotope compositions (δ¹¹B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The ¹¹B/¹⁰B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in ¹⁰B compared to the ocean value (δ¹¹B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)₃ present in precipitations. A boron meteoric water line, δD = 2.6δ¹¹B − 133, is proposed which describes the relationship between δD and δ¹¹B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ¹¹B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ¹¹B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.     

High precision iron isotope measurements of meteoritic material by cold plasma ICP-MS

The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ⁵⁶Fe and ± 0.25 ‰ for δ⁵⁸Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ⁵⁶Fe_chondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.