Search

Abstract: Iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at ∼20 °C. Applying a Rayleigh distillation model to three experiments in which siderite (FeCO3) was continuously precipitated from solutions over the course of tens of hours, the derived fractionation factor is 103lnαFe(II)–Siderite=+0.48±0.22‰ (2σ). This experimentally determined fractionation factor is significantly smaller (by ∼1.5‰) than that calculated from vibrational frequencies and Mössbauer shifts [Geochim. Cosmochim. Acta 64 (2000) 849–865; Geochim. Cosmochim. Acta 65 (2001) 2487–2497], and lies on the high end of that inferred from natural mineral assemblages, where the Fe(II)aq–siderite fractionation factor was inferred to lie between −1.7‰ and +0.3‰ [Contrib. Mineral. Petrol. 144 (2003) 523–547]. The measured Fe(II)aq–siderite fractionation from this study suggests that siderite from Archean and Proterozoic rocks, which have moderately negative δ56Fe values (−1.5‰ to −0.5‰), may reflect precipitation from Fe(II)aq whose origin was hydrothermal. In contrast, natural siderites that have higher δ56Fe values apparently require non-hydrothermal sources of Fe and/or different pathways of formation, or may reflect the effects of cation substitution on the Fe(II)aq–carbonate fractionation factor. [Copyright &y& Elsevier]

Nanophase Fe metal grains (np-Fe°) are a product of space weathering, formed by processes related to meteorite impacts, and solar-wind sputtering on airless planetary bodies, such as the Moon. Iron isotopes of lunar soils are fractionated during these processes, and the np-Fe° in the finest (<10 μm), mature, size fractions of the soil become enriched in heavier isotopes by ∼0.3‰ in 56Fe/54Fe in comparison to the bulk rocks (0.03±0.05‰), from which the soil was formed. A positive correlation of δ56Fe values with the soil maturity index, IS/FeO, suggests that the high δ56Fe values reflect production of nanophase Fe metal that is produced by space weathering that occurs on airless planetary bodies. Furthermore, the enrichment of δ56Fe in the smallest size fraction of lunar soils supports a model for creation of np-Fe° through vapor deposition induced by micrometeorites, as well as that by solar-wind sputtering. [Copyright &y& Elsevier]

The Fe isotope composition of Proterozoic to modern clastic sedimentary rocks and aerosols defines a range in δ[sup 56]Fe values that is only slightly more variable than the range of Fe isotope compositions measured in terrestrial igneous rocks, indicating that chemical weathering, sedimentary transport, and diagenesis play only a minor role in producing Fe isotope variations in environments where Fe redox conditions have been controlled by current levels of atmospheric oxygen. In contrast, the Fe isotope compositions of hot fluids (>300 °C) from mid-ocean-ridge (MOR) spreading centers define a narrow range that is shifted to lower δ[sup 56]Fe values by 0.2‰-0.5‰ as compared to igneous rocks. These new data allow a conceptual model for the Fe isotope composition of the oxic oceans that predicts large ranges in Fe isotope composition under conditions of changing aerosol and MOR Fe fluxes, such as during periods of major worldwide glaciation. [ABSTRACT FROM AUTHOR]

Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in δ56Fe values (per mil deviations in 56Fe/54Fe ratios) in nature from −2.50‰ to +1.5‰. Re-assessment of the range of δ56Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to ±0.05‰, which represents an unparalleled baseline with which to interpret Fe isotope variations in nature. All of the isotopic variability in nature lies in fluids, rocks, and minerals that formed at low temperature. Equilibrium (“abiotic”) isotopic fractionations at low temperatures may explain the range in δ56Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) (ΔFe(III)–Fe(II)=2.75‰). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in δ56Fe values are also consistent with the experimentally measured isotopic fractionations produced by Fe-reducing bacteria.In this work, we touch on a number of aspects of Fe isotope geochemistry that bear on its application to geochemical problems in general, and biological cycling of metals in particular. We report on new state-of-the-art Fe isotope analytical procedures, which allow precisions of ±0.05‰ (56Fe/54Fe) on samples <300 ng in size. In addition, we discuss the implications of experimental work on Fe isotope fractionations during metabolic processing of Fe by bacteria and the need to take a “mechanistic” approach to understanding the pathways in which Fe isotopes may be uniquely fractionated by biology. Additionally, we discuss experimental methods, such as the use of enriched isotope tracers that are necessary to evaluate if experimental isotope exchange reactions are transient kinetic fractionations, equilibrium isotopic exchange reactions, or a combination of both, which can be caused by the complexities of multiple isotope exchange reactions taking place in an experimental system. [Copyright &y& Elsevier]

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from –2.5 to +1.0‰ in 56Fe/54Fe ratios relative to the bulk Earth. With a current state-of-the-art precision of ±0.05‰ for the 56Fe/54Fe ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BIFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have δ56Fe=0.00±0.05‰, as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BIFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of δ56Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)–Fe(II)–hematite–magnetite–Fe carbonate. δ56Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high δ56Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive δ56Fe values; these seem best explained through production of high 56Fe/54Fe reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low δ56Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-δ56Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00410-002-0418-x. [ABSTRACT FROM AUTHOR]

During the last interglaciation (LIG; ∼129–116 ka), global mean sea level (GMSL) was 6–9 m above present. However, the source, or sources, of the higher-than-present sea level are only partially confirmed with far fewer geologic constraints than GMSL itself. Because of modest LIG Greenland Ice-Sheet retreat that raised sea level by < 2.5 m, Antarctic ice sheets are hypothesized to have contributed significantly to the LIG GMSL highstand, but direct evidence is limited. Here we infer ice-sheet presence or absence on West Antarctica and the Antarctic Peninsula using sediment geochemistry in the Bellingshausen Sea at Ocean Drilling Program (ODP) Site 1096. In particular, a combination of Sr-Nd-Pb isotopes and trace-element ratios allows differentiation between silt sourced from West Antarctica and the Antarctic Peninsula. From the Holocene (<11.7 ka) back through Marine Isotope Stage (MIS) 5d (∼116 ka), we find in ∼50% of our samples glacially eroded silt sourced from under the West Antarctic Ice Sheet that was transported to the Bellingshausen Sea in the Antarctic Circumpolar Current, in addition to silt sourced from under the proximal Antarctic Peninsula Ice Sheet. However, West Antarctic-sourced silt is absent during the LIG (MIS 5e) when only Antarctic Peninsula-sourced silt was deposited at ODP Site 1096. This lack of West Antarctic-sourced silt is consistent with the two ice-core constraints on West Antarctic Ice Sheet LIG size, which combined with our record point towards the absence of the West Antarctic Ice Sheet during the LIG. We therefore provide the first marine-based sedimentary evidence that supports West Antarctic Ice-Sheet absence during the LIG. • The Bellingshausen Sea contains West Antarctic silt during the last glacial cycle. • The Bellingshausen Sea lacks West Antarctic silt during the last interglaciation. • West Antarctic last-interglacial deglaciation would explain its silt absence. [ABSTRACT FROM AUTHOR]

Provides information on iron isotope biosignatures. How the fractionation of light stable isotopes is controlled by inorganic processes related to temperature changes and phase transitions and by biological processes; Biological fractionation of transition metal elements; Experiments performed to determine the magnitude of Fe-isotopic fractionation; Results of the experiments; Implications.

Presents information on a study which investigated a series of high-K mafic lavas from Deep Springs Valley, California. Geologic setting; Petrography; Analytical methods; Conclusions.

Reports that basalts derived from upwelling asthenosphere in the region of the Rio Grande rift, southwestern United States, have Nd and Hf isotope ratios that lie significantly off the ocean-island Nd-Hf array. Interpretation of the low 176Hf/177Hf ratios in these basalts as reflecting derivation from ancient asthenospheric mantle that melted at shallow levels beneath the oceans; Isotope and chemical data for Cenozoic lavas from the Rio Grande rift; More.

The article presents authors' response to a comment on their previous study on Yakutian eclogite xenoliths. They explain several criticisms made on the previous study which include the chemical study of eclogite xenoliths, kimberlite pipes and the issue of eclogite genesis. They also discuss the continuation of a study on the eclogites from various pipes throughout Yakutia, Russia, as well as kyanite eclogites.

Owing to the association with diamonds, eclogite xenoliths have received disproportionate attention given their low abundance in kimberlites. Several hypotheses have been advanced for the origin of eclogite xenoliths, from the subduction and high-pressure melting of oceanic crust, to cumulates and liquids derived from the upper mantle. We have amassed a comprehensive data set, including major- and trace-element mineral chemistry, carbon isotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os, and oxygen isotopes in ultrapure mineral and whole-rock splits from eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia. Furthermore, eclogites from two other Yakutian kimberlite pipes, Mir and Obnazhennaya, have been studied in detail and offer contrasting images of eclogite protoliths. Relative to eclogites from southern Africa and other Yakutian localities, Udachnaya eclogites are notable in the absence of chemical zoning in mineral grains, as well as the degree of light rare earth element (LREE) depletion and unradiogenic Sr; lack of significant oxygen, sulfur, and carbon isotopic variation relative to the mantle; and intermineral radiogenic isotopic equilibration. Several of these eclogites could be derived from ancient, recycled, oceanic crust, but many others exhibit no evidence for an oceanic crustal protolith. The apparent lack of stable-isotope variation in the Udachnaya eclogites could be due to the antiquity of the samples and consequent lack of deep oceanic and biogenically diverse environments at that time. Those eclogites that are interpreted to be non-recycled have compositions characteristic of Group A eclogites from other localities that also have been interpreted as being directly from the mantle. At least two separate and diverse isotopic reservoirs are suggested by Nd isotopic whole-rock reconstructions. Most samples were derived from typical depleted mantle. However, two groups of three samples each indicate both enriched mantle and possible ultra-depleted mantle present beneath Yakutia during the late Archean and early Proterozoic. The vast majority of eclogites studied from the Obnazhennaya pipe also exhibit characteristics of Group A eclogites and are probably derived directly from the mantle. However, the eclogites from the Mir kimberlite are more typical of other eclogites world-wide and show convincing evidence of a recycled, oceanic crustal affinity. We concur with the late Ted Ringwood that eclogites can be formed in a variety of ways, both within the mantle and from oceanic crustal residues. [ABSTRACT FROM AUTHOR]

Reports on the petrography and geochemistry of eclogites from Siberia, Russia. Groups of eclogite suites; Characterizations of high-calcium group samples; Elements of the low-calcium group; Types of zoning in garnet grains; Distribution coefficients of trace elements.

The performance of multi-collector secondary ion mass spectrometry (MC-SIMS) for Mg isotope ratio analysis was evaluated using 17 olivine and 5 pyroxene reference materials (RMs). The Mg isotope composition of these RMs was accurately and precisely determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), and these measured isotope ratios were used to evaluate SIMS instrumental mass bias as a function of the forsterite (Fo) content of olivine. The magnitude of the Mg isotope matrix effects were ~3‰ in δ25Mg, and are a complex function of olivine Fo content, that ranged from Fo 59.3 to Fo 100. In addition to these Mg isotope matrix effects, Si+ ion yields and Mg+/Si+ ion ratios varied as a complex function of the Fo content of the olivine RMs. For example, Si+ ion yields varied by ~33%. Based on the observations, we propose instrumental bias correction procedures for SIMS Mg isotope analysis of olivine using a combination of Mg+/Si+ ratios and Fo content of olivine. Using this correction method, the accuracy of δ25Mg analyses is 0.3‰, except for analysis of olivine with Fo 86 – 88 where instrumental biases and Mg+/Si+ ratios change dramatically with Fo content, making it more difficult to assess the accuracy of Mg isotope ratio measurements by SIMS over this narrow range of Fo content. Five pyroxene RMs (3 orthopyroxenes and 2 clinopyroxenes) show smaller ranges of instrumental bias (~1.4‰ in δ25Mg) as compared to the olivine RMs. The instrumental bias for the 3 orthopyroxene RMs do not define a linear relationship with respect to enstatite (En) content, that ranged from En 8 5. 5 – 96.3. The clinopyroxene RMs have similar En and wollastonite (Wo) contents but have δ25Mg values that differ by 0.5‰ relative to their δ25Mg values determined by MC-ICP-MS. These results indicate that additional factors (e.g., minor element abundances) likely contribute to SIMS instrumental mass fractionation. In order to better correct for these SIMS matrix effects, additional pyroxene RMs with various chemical compositions and known Mg isotope ratios are needed. • We investigated matrix effects of SIMS Mg isotope analyses of olivine and pyroxene. • Instrumental bias of olivine is estimated by using numbers of olivine standards. • Matrix effect depends on both forsterite content and Mg+/Si+ secondary ion yields. • Precision and accuracy of Mg isotope analysis of olivine is improved to be ≤0.3‰. • Radio-frequency plasma ion source has an advantage on precise Mg isotope analyses. [ABSTRACT FROM AUTHOR]

The article discusses biogeochemical cycling of iron isotopes. A remarkably large portion of the iron inventory on Earth is isotopically homogeneous, including igneous rocks and sedimentary rocks that have undergone minimal chemical change after deposition. Although iron-isotope variations within the mantle could arise as a result of high-pressure mineral fractionation and/or chemical changes, these variations are apparently homogenized during magma generation, producing oceanic and continental crust with 56Fe/54Fe ratios that are indistinguishable from those of bulk Earth. The relatively large range in iron isotope compositions measured for Precambrian sedimentary rocks and minerals spans the entire range yet observed in nature, and if such compositions were produced when the oceans were iron rich, extremely large quantities of iron must have been cycled. Reductive dissolution of ferric hydroxide/oxide minerals during anoxic reactions may produce aqueous iron-bearing minerals such as siderite and magnetite through microbial dissimilatory iron reduction, where bacteria pump electrons to the Fe3+ phase to create reduced iron externally.

Shales are a major sink for K into seawater delivered from continental weathering, and are potential recorders of K cycling. High precision K isotope analyses reveal a > 0.6 ‰ variation in δ41K values (41K/39K relative to NIST SRM 3141a) from a set of well characterized post-Archean Australian shale (PAAS) samples. By contrast, loess samples have relatively homogenous δ41K values (− 0.5 ± 0.1 ‰), which may represent the average K composition of upper continental crust. Most of the shales analyzed in this study have experienced K enrichment relative to average continental crust, and the majority of them define a trend of decreasing δ41K value (from − 0.5 to − 0.7 ‰) with increasing K content and K/Na ratio, indicating cation exchange in clays minerals is accompanied by K isotope fractionation. Several shale samples do not follow the trend and have elevated δ41K values up to − 0.1 ‰, and these samples are characterized by variable Fe isotope compositions, which reflect post-depositional processes. The K isotope variability observed in shales, in combination with recent findings about K isotope fractionation during continental weathering, indicates that K isotopes fractionate during cycling of K between different reservoirs, and K isotopes in sediments may be used to trace geological cycling of K. [ABSTRACT FROM AUTHOR]

Abstract Hematite is the most thermodynamically stable Fe oxide at the Earth's surface and its isotopic composition may record past biogeochemical Fe cycling and paleo-environmental conditions. Proper interpretation of its isotopic values requires an understanding of the equilibrium Fe isotope fractionation factor between hematite and other Fe-bearing minerals and aqueous species. Here, we use the three-isotope method (54Fe-56Fe-57Fe, with 56Fe/54Fe and 57Fe/56Fe ratios expressed in delta notation relative to the IRMM-014 isotope standard) to simultaneously determine Fe isotope exchange, via tracking an enriched tracer-isotope (i.e. 57Fe), and measure mass-dependent isotope fractionation between aqueous Fe(II) (Fe(II) aq) with structural-Fe(III) in hematite, as determined from variations in 56Fe/54Fe ratios. Specifically, we used a reversal-approach to equilibrium by reacting three hematite samples of varying particle size and reactivity with two Fe(II) aq solutions that had initial 56Fe/54Fe ratios above and below the presumed equilibrium value. We confirm that Fe(II) aq readily exchanges with hematite at low temperature, and that the extent of exchange depends on hematite particle size. In three-isotope plots, δ56Fe values of Fe(II) aq exhibit unique trajectories depending on the initial hematite particle size. Rapid exchange occurring between Fe(II) aq and small hematite particles are affected by an apparent mixing reaction, which results in the δ56Fe value of Fe(II) aq to approach the isotopic composition of hematite. These trajectories exhibit inflections to more negative δ56Fe values during continued exchange. The location of these inflections depend on particle size and are interpreted to represent a change in the recrystallization mechanism from rapid "heterogeneous" recrystallization, which is limited to surface exchange, to "homogeneous" recrystallization involving the bulk hematite. Slow isotopic exchange occurring at longer reaction times appears to approximate equilibrium for all particle sizes with measured Fe(II) aq -hematite Fe isotope fractionation factors at 22 °C ranging from −2.61 ‰ (±0.26‰, 2σ) to −3.14 ‰ (±0.34‰, 2σ) in δ56Fe, and an average value of −2.8‰. This value is consistent with estimated fractionation factors measured during microbial Fe reduction experiments and with calculated and spectroscopically derived reduced partition function ratios. The three-isotope method provides a robust measure of equilibrium fractionation if proper constrains are utilized to eliminate extrapolation from partial exchange. Furthermore, it can provide insight into the mechanism of mineral-fluid exchange which is necessary for accurate quantification of the extent of mineral recrystallization when using isotopic tracers. [ABSTRACT FROM AUTHOR]

The origins of iron formations remain somewhat enigmatic despite much progress over the past decades. This is due in part to continuing research demonstrating that these deposits do not have a single origin but rather can be formed under various and variable conditions. This study describes a formation pathway for an Algoma-type banded iron formation (BIF) from the 2.75 billion-year-old Weld Range of Western Australia, based on petrographic and Fe isotope analyses of drill core samples. The BIF is composed of alternating layers of jaspilitic and Fe-poor chert, magnetite layers of varying thickness and abundance, and rare pyrite. Petrographic analysis indicates the minerals formed in the order hematite, magnetite, and then pyrite, with the latter two clearly replacive of “primary” bedded hematite. Hematite δ 56 Fe values from all portions of the drill core are positive and essentially identical (δ 56 Fe = 0.61 ± 0.05‰). Magnetite δ 56 Fe values are positive, as well (δ 56 Fe = 0.51 ± 0.07‰) but vary systematically with total magnetite content of the core samples. Pyrite δ 56 Fe values are positive and higher than both hematite and magnetite (δ 56 Fe = 1.03 ± 0.05‰). Combined, these analyses reveal a multistage formational model for the Weld Range BIF. Initial deposition occurred by the partial oxidation of hydrothermally-sourced aqueous Fe(II) to form a Fe(III)-Si co-precipitate, probably by anoxygenic photosynthetic iron oxidizers. This material would have been converted to hematite under equilibrium conditions with excess aqueous Fe(II). Magnetite formed by a later intrusion of reducing fluids, mostly along bedding planes, but also as cross-cutting veins. This produced large quantities of magnetite layers parallel, and sub-parallel, to the original bedding, but that do not represent a primary depositional feature. Pyrite was produced in some portions of the unit, likely under equilibrium conditions, by later infiltration of reducing fluids containing sulfide. The entirely positive δ 56 Fe values for all Fe phases in the Weld Range BIF stands in stark contrast to the more massive Superior-type BIFs of the Paleoproterozoic Hamersley and Transvaal basins of Western Australia and South Africa, respectively, which have δ 56 Fe values that average close to 0‰ but vary over nearly the entire range of δ 56 Fe values measured in nature (δ 56 Fe = −2.5 to +2.6‰). The Weld Range BIF δ 56 Fe values are similar to those of older Algoma-type BIFs such as the Paleoarchean Isua BIF of Greenland but are less positive on average. This overall trend of decreasing average δ 56 Fe values through time likely records an overall increasing trend of oxygenation of the Archean surface ocean from 3.8 to 2.45 billion years ago. [ABSTRACT FROM AUTHOR]

Controls on Archean seawater chemistry remain controversial. Many studies have suggested that it was largely controlled by oceanic hydrothermal fluid circulation. Recent work, however, from clastic sequences, Hf–O isotope data from detrital zircons, and models for the Rb/Sr evolution of the continental crust suggest that intense continental weathering and low-temperature surface alteration were more important than previously thought during the early Archean. This is consistent with biogeochemical studies that suggest the Archean had a diverse microbial ecology, which would, in part, need to be sustained by nutrients (e.g., phosphorus) that were derived from continental weathering. To further quantify continental weathering during the early Archean, we analyzed 3.26 Ga barite from the Fig Tree Group, South Africa for strontium, oxygen, and sulfur isotope compositions. We propose that the seawater component of the barite is characterized by 87 Sr/ 86 Sr ratios >0.701, which is significantly more radiogenic than contemporaneous mantle (∼0.7007–0.7008). The radiogenic nature of seawater at this time suggests that the continental weathering flux at 3.26 Ga had a large impact on ocean chemistry 400 million years earlier than previously suggested. [ABSTRACT FROM AUTHOR]

230 Th-excess is rare in most arc lavas, but common in the Cascades, yet the origin of such excesses remains unclear. At Mt. Shasta, age-corrected ( 230 Th/ 232 Th) and ( 238 U/ 232 Th) activity ratios range from 1.108 to 1.290 and from 0.987 to 1.309 (27.3% 230 Th-excess to 6.1% 238 U-excess), respectively. Although small degrees of zircon crystallization (<0.3%) may yield high ( 230 Th/ 238 U) 0 in derivative magmas, high Zr contents, the lack of zircon as a liquidus phase, and low Th/U ratios in Mt. Shasta lavas argue against zircon fractionation. Instead, melting models suggest 230 Th-excesses are imparted on lavas through mixing mantle-derived magmas with partial melts of a mafic amphibolite lower crust where garnet was produced in the residuum through amphibole and plagioclase destabilization. The hot nature of Cascade magmas suggests that high intrusion temperatures promoted dehydration melting in the deep crust. At Mt. Shasta, the destruction of the ancestral cone (Sand Flat) was followed by four cone-building stages, three of which lie in the age range of U-series geochronology. Lavas within individual eruptive stages have relatively constant ( 230 Th/ 232 Th) 0 ratios that are interpreted to reflect specific mixtures of mantle ( m ) and lower crustal ( lc ) melts that are characteristic of a specific stage ( M m : l c ). High ( 230 Th/ 232 Th) 0 ratios identify higher proportions of lower crust in the Misery Hill stage ( M m : l c = ∼ 85 : 15 ), whereas low ( 230 Th/ 232 Th) 0 ratios reflect the more mantle-like composition of the Shastina lavas ( M m : l c = ∼ 95 : 5 ); in the case of Shastina lavas, very low 87 Sr/ 86 Sr ratios, down to 0.7029, support a substantial mantle contribution. Changes in ( 230 Th/ 232 Th) 0 ratios correlate with eruptive volume, where the most voluminous stage (Misery Hill) is inferred to have the largest proportion of crustal melt and highest ( 230 Th/ 232 Th) 0 ratios. Variable ( 230 Th/ 238 U) 0 ratios within, and between, eruptive groups likely reflect a combination of residence time in the lower crust and differential assimilation of bulk, non-garnet-bearing crust that had ( 230 Th/ 238 U) = 1. The volume-( 230 Th/ 232 Th) 0 relations are accompanied by correlations with 87 Sr/ 86 Sr ratios, where the most radiogenic Sr is associated with the largest eruptive volumes, indicating that the largest magmatic episodes produced the largest amount of lower crustal interaction. The new U–Th and Sr isotope measurements of this study, along with U-series data for other Cascade centers suggest that interaction with the lower crust exerts greater control on Cascade magma chemistry than previously recognized. Indeed, the relatively dry nature of the Cascades may offer a unique opportunity to better understand the influence of the deep crust in young continental arcs, as larger subduction components in other settings may overprint any lower crustal signature and produce lavas with large 238 U-excesses. [ABSTRACT FROM AUTHOR]

Enriched Fe isotope tracer studies demonstrate that aqueous Fe(II) undergoes electron transfer and atom exchange with goethite. Such processes influence contaminant fate and trace-element mobility, and result in stable Fe isotope fractionation in both biological and abiological processes. To date, the majority of experimental studies of aqueous Fe(II) and Fe oxide interactions have been done at circumneutral pH. The effect of pH variations on the rate and extent of Fe isotope exchange between aqueous Fe(II) and iron oxide minerals, as well as the natural mass-dependent fractionation between these species, has not been adequately explored. Here, the three-isotope method ( 57 Fe– 56 Fe– 54 Fe), using an enriched 57 Fe tracer, was used to investigate the effect of pH (between 2.5 and 7.5) on the rate and extent of isotopic exchange. 56 Fe/ 54 Fe ratios were used to determine the natural, mass-dependent stable isotope fractionation, between aqueous Fe(II) and goethite. Three Fe(II) solutions differing in their initial 56 Fe/ 54 Fe ratios were used to approach isotopic equilibrium from multiple directions. The 57 Fe-enriched tracer data indicate that the extent of isotopic exchange between Fe(II) aq and goethite was positively correlated with pH, where the least amount of exchange occurred at the lowest pH. Similarly, initial kinetic isotope fractionations were influenced by pH; at low pH, minimal kinetic isotope effects were observed relative to large effects at high pH, suggesting a relation between the extent of sorbed Fe(II) and kinetic isotope effects. Continued exchange over time at high pH, however, erases the initial kinetic isotope effects, and the system fundamentally reached isotopic equilibrium by the end of the experiment. Our results show that the interplay between kinetic and equilibrium effects may prevent confident extrapolation to infer equilibrium fractionation factors when only small amounts of Fe exchange occur. [ABSTRACT FROM AUTHOR]

Varying levels of boreal summer insolation and associated Earth system feedbacks led to differing climate and ice-sheet states during late-Quaternary interglaciations. In particular, Marine Isotope Stage (MIS) 11 was an exceptionally long interglaciation and potentially had a global mean sea level 6 to 13 metres above the present level around 410,000 to 400,000 years ago, implying substantial mass loss from the Greenland ice sheet (GIS). There are, however, no model simulations and only limited proxy data to constrain the magnitude of the GIS response to climate change during this 'super interglacial', thus confounding efforts to assess climate/ice-sheet threshold behaviour and associated sea-level rise. Here we show that the south GIS was drastically smaller during MIS 11 than it is now, with only a small residual ice dome over southernmost Greenland. We use the strontium-neodymium-lead isotopic composition of proglacial sediment discharged from south Greenland to constrain the provenance of terrigenous silt deposited on the Eirik Drift, a sedimentary deposit off the south Greenland margin. We identify a major reduction in sediment input derived from south Greenland's Precambrian bedrock terranes, probably reflecting the cessation of subglacial erosion and sediment transport as a result of near-complete deglaciation of south Greenland. Comparison with ice-sheet configurations from numerical models suggests that the GIS lost about 4.5 to 6 metres of sea-level-equivalent volume during MIS 11. This is evidence for late-Quaternary GIS collapse after it crossed a climate/ice-sheet stability threshold that may have been no more than several degrees above pre-industrial temperatures. [ABSTRACT FROM AUTHOR]

Abstract: Dissimilatory microbial iron oxide reduction (DIR) has been hypothesized to be an important respiratory pathway on early Earth, potentially generating significant quantities of Fe(II) that have been preserved in Proterozoic and Archean sedimentary rocks. In particular, DIR has been implicated in the formation of magnetite in Precambrian marine sediments. To date, however, only one modern sedimentary environment exists where in situ magnetite formation has been linked to DIR: the Bay of Vidy in Lake Geneva, Switzerland. Previous work at this locality has characterized a magnetic susceptibility anomaly that reflects the presence of fine-grained magnetite produced via microbial reduction of amorphous Fe(III) oxides that enter the Bay of Vidy from a nearby sewage treatment plant. In this study, we report on the Fe isotope composition of aqueous and solid-phase Fe in the Bay of Vidy sediments. Extensive Fe(III) reduction has occurred, resulting in the conversion of nearly all reactive (non-silicate) Fe(III) to a variety of Fe(II)-bearing phases, with mixed Fe valence magnetite being a minor but easily detectable component (0.5–8wt.%). Very little Fe isotope variation was observed in any solid phase Fe components, including magnetite, although significant fractionation was observed between aqueous and solid-phase Fe(II). Because Fe mass-balance was dominated by the solid phase, little net change in δ56Fe values for Fe(II)-bearing components was produced despite clear evidence for DIR. This study provides a basis for interpreting instances in the rock record where DIR was the driving force for Fe(II) production and magnetite formation, yet no significant deviations in δ56Fe values were preserved. A key implication of the results is that Fe isotope homogeneity is not sufficient to rule out a biological mechanism for magnetite formation, and this should be taken into account when examining the Precambrian rock record. [Copyright &y& Elsevier]

A strongly layered garnet pyroxenite, consisting of cm-scale layers of garnetite, garnet orthopyroxenite, garnet clinopyroxenite, and websterite, occurs in the Biskupice garnet peridotite, which is located in migmatitic gneiss of the Gföhl Assemblage in the Vysočina District, western Moravia. Major- and trace-element compositions and REE patterns of minerals and layers in the pyroxenite are consistent with origin of the layers by HT-HP crystallization and accumulation of variable proportions of garnet, orthopyroxene, and clinopyroxene. The present compositions of the minerals (pyroperich garnet, low-A1, low-Ca enstatite and low-A1, high-Ca diopside) are the result of extensive subsolidus re-equilibration at ~900 °C and ~37 kbar, during the Variscan (332 Ma) Orogeny. Neodymium and strontium isotopic values (ε,Nd335 +1.8 to +2.1; (87Sr/86Sr)335, 0.7049) and negative HFSE anomalies indicate a crustal component in the melt from which the layers crystallized. Most garnet pyroxenites and eclogites in Gföhl garnet peridotites are thought to have formed by HT-HP crystallization from silicate melts containing crustal components, and the Biskupice garnet pyroxenite is an excellent example of such a petrogenetic process. [ABSTRACT FROM AUTHOR]

To ascertain the response of the southern Greenland Ice Sheet (GIS) to a boreal summer climate warmer than at present, we explored whether southern Greenland was deglaciated during the Last Interglacial (LIG), using the Sr-Nd-Pb isotope ratios of silt-sized sediment discharged from southern Greenland. Our isotope data indicate that no single southern Greenland geologic terrane was completely deglaciated during the LIG, similar to the Holocene. Differences in sediment sources during the LIG relative to the early Holocene denote, however, greater southern GIS retreat during the LIG. These results allow the evaluation of a suite of GIS models and are consistent with a GIS contribution of 1.6 to 2.2 meters to the ≥4-meter LIG sea-level highstand, requiring a significant sea-level contribution from the Antarctic Ice Sheet. [ABSTRACT FROM AUTHOR]

Abstract: The largest excursion in kerogen δ 13C and bulk/mineral δ 56Fe values yet measured in the ancient rock record occurs in rocks of ∼2.7 to 2.5Ga age. New Fe isotope data integrated with previously collected C isotope data on the same samples document the metabolic diversity of microbial communities in the Neoarchean Hamersley Province of the Pilbara Craton in Western Australia. Samples of shales, carbonates, and mixed carbonate/shale lithologies were collected from three drill cores; two cores from the depocenter of the province and one from the margin. Shallow-water clastic/carbonate rocks deposited in the center of the province (Tumbiana Formation) record kerogen δ 13C values that indicate C cycling by various anaerobic or aerobic methane pathways, but the restricted range in δ 56Fe values indicates little or no Fe redox cycling. Deep-water sediments deposited contemporaneously in both parts of the Hamersley Province (Jeerinah Formation) record slightly positive δ 56Fe values in the relatively shallower and suboxic margin, but strongly negative δ 56Fe values in the deeper euxinic depocenter of the province, a pattern consistent with Fe cycling via a basin Fe shuttle, driven by bacterial dissimilatory iron reduction (DIR). Kerogen δ 13C values from these units indicate coupling of microbial Fe cycling to aerobic methanotrophy or anaerobic oxidation of methane. Younger black shales, intercalated with iron formation (Marra Mamba Iron Formation) in the depocenter, record a shift to near-zero δ 56Fe values reflecting an Fe budget dominated by hydrothermal and clastic sources. However, time-equivalent, Fe-rich carbonate/shale lithologies deposited on the margin of the province (Carawine Dolomite) have δ 56Fe values that steadily decrease from near zero to strongly negative values. These relatively Fe-rich carbonates may reflect a carbonate trap of a DIR-driven Fe shuttle, similar to the sulfidic trap in the euxinic portion of the Jeerinah Formation. In contrast, younger shallow-water carbonates deposited by turbidity currents in relatively deep water in the center of the province (Wittenoom Formation), have δ 56Fe values that correlate with Fe concentrations, a pattern that indicates Fe cycling by Rayleigh fractionation through precipitation of iron oxides from aqueous ferrous iron. [Copyright &y& Elsevier]

Carbonatites define the largest range in Fe isotope compositions yet measured for igneous rocks, recording significant isotopic fractionations between carbonate, oxide, and silicate minerals during generation in the mantle and subsequent differentiation. In contrast to the relatively restricted range in δ56Fe values for mantle-derived basaltic magmas (δ56Fe = 0.0 ± 0.1‰), calcite from carbonatites have δ56Fe values between −1.0 and +0.8‰, similar to the range defined by whole-rock samples of carbonatites. Based on expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative δ56Fe values of ~ −0.3‰ or lower can be explained by equilibrium with a silicate mantle. More negative δ56Fe values were probably produced by differentiation processes, including crystal fractionation and liquid immiscibility. Positive δ56Fe values for carbonatites are, however, unexpected, and such values seem to likely reflect interaction between low-Fe carbonates and Fe3+-rich fluids at igneous or near-igneous temperatures; the expected δ56Fe values for Fe2+-bearing fluids are too low to produced the observed positive δ56Fe values of some carbonatites, indicating that Fe isotopes may be a valuable tracer of redox conditions in carbonatite complexes. Further evidence for fluid-rock or fluid-magma interactions comes from the common occurrence of Fe isotope disequilibrium among carbonate, oxide, silicate, and sulfide minerals in the majority of the carbonatites studied. The common occurrence of Fe isotope disequilibrium among minerals in carbonatites may also indicate mixing of phenocyrsts from distinct magmas. Expulsion of Fe3+-rich brines into metasomatic aureols that surround carbonatite complexes are expected to produce high-δ56Fe fenites, but this has yet to be tested. [ABSTRACT FROM AUTHOR]

Abstract: The Sandvik ultramafic body, Island of Gurskøy, Western Gneiss Region, Norway, is a mantle fragment that contains polymetamorphic mineral assemblages and affords a unique view into the response of subcontinental lithospheric mantle to repeated orogenic/magmatic events. The Sandvik peridotite body and nearby outcrops record four paragenetic stages: 1) pre-exsolution porphyroclasts of ol+grt+opx (high-Ca )+cpx (low-Ca), which equilibrated at 1100–1200 °C and 6.5–7.0 GPa; 2) kelyphite containing ol+grt+spl+opx (low-Ca)+am (high-Al), as well as exsolved pyroxene containing opx+cpx+spl in equilibrium with matrix olivine, at 725 °C and 1.5 GPa; 3) granoblastic matrix of ol+spl+opx (low-Ca)+am (high-Al), at 700 °C and 1.0 GPa. A nearby outcrop contains a fourth assemblage consisting of ol+chl+opx+am. Lu–Hf and Re–Os model ages of garnet peridotite indicate melt depletion at 3.3 Ga [Beyer, E.E., Brueckner, H.K., Griffin, W.L., O''Reilly, S.Y., Graham, S., 2004. Archean mantle fragments in Proterozoic crust, Western Gneiss Region, Norway. Geology 32, 609–612.; Lapen, T.J., Medaris, L.G. Jr., Johnson, C.M., and Beard, B.L., 2005. Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm–Nd and Lu–Hf isotope analyses of the Sandvik ultramafic body, Norway. Contributions to Mineralogy and Petrology 150, 131–145.], marking the time of separation from the convecting mantle. Lu–Hf whole rock and mineral isochron ages of constituent garnet peridotite and garnet pyroxenite layers in the Sandvik body reflect cooling and emplacement at ~1.25 Ga and ~1.18 Ga, respectively, whereas Sm–Nd whole rock and mineral ages of the garnet pyroxenite layers and the garnet peridotite are consistent with metasomatic alteration at ~1.15 Ga [Lapen, T.J., Medaris, L.G. Jr., Johnson, C.M., and Beard, B.L., 2005. Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm–Nd and Lu–Hf isotope analyses of the Sandvik ultramafic body, Norway. Contributions to Mineralogy and Petrology 150, 131–145.]. The isochron ages likely record lithospheric modification associated with the 1.25–1.00 Ga Sveconorwegian orogeny and represent the youngest age of the Stage 1 mineral assemblage equilibration. A 606±39 Ma Sm–Nd isochron age of the Stage 2 kelyphite assemblage is consistent with partial re-equilibration of the porphyroclastic assemblage during continental rifting associated with opening of the Iapetus Ocean between Baltica and Laurentia at ~600 Ma, or extension between Baltica and Siberia that may have been associated with opening of the Ægir Sea. The age of kelyphite, therefore, places the Sandvik peridotite in the uppermost mantle prior to Silurian shortening between the Baltic and Laurentian continents. [Copyright &y& Elsevier]

Abstract: Basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the Puyehue–Cordón Caulle volcanic complex in the Andean Southern Volcanic Zone (SVZ) were analyzed for major and trace element, Sr isotope, and U–Th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis. Internal U–Th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages, indicating a short period (<1000 yr) of crystal residence in the magma prior to eruption. The (230Th/232Th) ratios define a narrow range (0.80–0.83) compared to that of all SVZ lavas (0.72–0.97), suggesting that Puyehue basalt was derived from a relatively uniform mantle source. Dacites and rhyolites have the largest U excesses and likely evolved via fractional crystallization of a plagioclase-dominated mineral assemblage. In contrast, basalts have 1 to 6% 230Th excesses, a characteristic not previously observed in frontal arc stratovolcanoes of the Andean SVZ. The 230Th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust. Lower crustal processes, therefore, have dampened the 238U excesses that were generated during fluid addition to the mantle wedge. Although prior 238U–230Th–226Ra studies of lavas from other southern SVZ stratovolcanoes (36 to 41° S) have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc, our results suggest that MASH processes envisioned by Hildreth and Moorbath [W. Hildreth, S. Moorbath, Crustal contributions to arc magmatism in the Andes of central Chile, Contrib. Mineral. Petrol. 98 (1988) 455-489] in the northern SVZ also occur in the southern SVZ, where the crust is relatively thin. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Iron isotope compositions of low-metamorphic grade samples of Archean–Paleoproterozoic sedimentary rocks obtained from fresh drill core from the Kaapvaal Craton in South Africa and from the Pilbara Craton in Australia vary by ~3‰ in 56Fe/54Fe ratios, reflecting a variety of weathering and diagenetic processes. Depositional ages for the 120 samples studied range from 3.3 to 2.2 Ga, and Fe, C, and S contents define several compositional groups, including samples rich in Fe, organic carbon, carbonate, and sulfide. The δ 56Fe values for low-Corg, low-Ccarb, and low-S sedimentary rocks are close to 0‰, the average of igneous rocks. This range is essentially the same as that of Corg-poor late Cenozoic loess, aerosol, river loads, and marine sediments and those of Corg-poor Phanerozoic–Proterozoic shales. That these δ 56Fe values are the same as those of igneous rocks suggests that Fe has behaved conservatively in bulk sediments during sedimentary transport, diagenesis, and lithification since the Archean. These observations indicate that, if atmospheric O2 contents rose dramatically between 2.4 and 2.2 Ga, as proposed by many workers, such a rise did not produce a significant change in the bulk Fe budget of the terrestrial sedimentary system. If the Archean atmosphere was anoxic and Fe was lost from bedrock during soil formation, any isotopic fractionation between aqueous ferrous Fe (Feaq 2+) and Fe-bearing minerals must have been negligible. In contrast, if the Archean atmosphere was oxic, Fe would have been retained as Fe3+ hydroxides during weathering as it is today, which would produce minimal net isotopic fractionation in bulk detrital sediments. Siderite-rich samples have δ 56Fe values of −0.5±0.5‰, and experimentally determined Feaq 2+-siderite fractionation factors suggest that these rocks formed from Feaq 2+ that had similar or slightly higher δ 56Fe values. The δ 56Fe values calculated for Feaq 2+ overlaps those of modern submarine hydrothermal fluids, but it is also possible that Feaq 2+ had δ 56Fe values higher than those of modern hydrothermal fluids, depending upon the Feaq 2+–Fe carbonate fractionation factor that is used. In contrast, Corg-rich samples and magnetite-rich samples have strongly negative δ 56Fe values, generally between −2.3‰ and −1.0‰, and available fluid–mineral fractionation factors suggest that the Fe-bearing minerals siderite and magnetite in these rocks formed in the presence of Feaq 2+ that had very low δ 56Fe values, between −3‰ and −1‰. Reduction of Fe3+ hydroxide by sulfide, precipitation of sulfide minerals, or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low-δ 56Fe Feaq 2+. We interpret microbial dissimilatory Fe3+ reduction (DIR) as the best explanation for producing such low δ 56Fe values for Feaq 2+, and our results suggest that DIR was a significant form of respiration since at least 2.9 Ga. [Copyright &y& Elsevier]

Photoautotrophic bacteria that oxidize ferrous iron (Fe[II]) under anaerobic conditions are thought to be ancient in origin, and the ferric (hydr)oxide mineral products of their metabolism are likely to be preserved in ancient rocks. Here, two enrichment cultures of Fe(II)-oxidizing photoautotrophs and a culture of the genus Thiodictyon were studied with respect to their ability to fractionate Fe isotopes. Fe isotope fractionations produced by both the enrichment cultures and the Thiodictyon culture were relatively constant at early stages of the reaction progress, where the 56Fe/54Fe ratios of poorly crystalline hydrous ferric oxide (HFO) metabolic products were enriched in the heavier isotope relative to aqueous ferrous iron (Fe[II]aq) by ∼1.5 ± 0.2‰. This fractionation appears to be independent of the rate of photoautotrophic Fe(II)-oxidation, and is comparable to that observed for Fe isotope fractionation by dissimilatory Fe(III)-reducing bacteria. Although there remain a number of uncertainties regarding how the overall measured isotopic fractionation is produced, the most likely mechanisms include (1) an equilibrium effect produced by biological ligands, or (2) a kinetic effect produced by precipitation of HFO overlaid upon equilibrium exchange between Fe(II) and Fe(III) species. The fractionation we observe is similar in direction to that measured for abiotic oxidation of Fe(II)aq by molecular oxygen. This suggests that the use of Fe isotopes to identify phototrophic Fe(II)-oxidation in the rock record may only be possible during time periods in Earth’s history when independent evidence exists for low ambient oxygen contents. [Copyright &y& Elsevier]

Large equilibrium isotope fractionation occurs between Fe(III) and Fe(II) in very dilute (≤22 mM Cl−) aqueous solutions, reflecting significant differences in bonding environments. Separation of Fe(III) and Fe(II) is attained by rapid and complete precipitation of Fe(III) through carbonate addition, followed by separation of supernatant and ferric precipitate; experiments reported here produce an equilibrium ΔFe(III)–Fe(II)=+2.75±0.15‰ for 56Fe/54Fe at room temperature (22±2°C). The timescales required for attainment of isotopic equilibrium have been determined by parallel isotope tracer experiments using 57Fe-enriched iron, which are best fitted by a second-order rate law, with K=0.18±0.03 s−1. Based on this rate constant, ∼15–20% isotopic exchange is estimated to have occurred during Fe(III)–Fe(II) separation, which contributes <0.10‰ uncertainty to the equilibrium ΔFe(III)–Fe(II). Under the experimental conditions used in this study, >97% Fe(II) exists as [FeII(H2O)6]2+, and >82% Fe(III) exists as [FeIII(H2O)6]3+ and [FeIII(H2O)6−n(OH)n]3−n; assuming these are the dominant species, the measured Fe isotope fractionation is approximately half that predicted by Schauble et al. [Geochim. Cosmochim. Acta 65 (2001) 2487–2497] at 20–25°C. Although this discrepancy may be due in part to the experimentally unknown isotopic effects of chloride interacting with Fe-hexaquo or Fe-hydroxide complexes, or directly bonded to Fe, there still appears to be at this stage a >1‰ difference between prediction and experiment. [Copyright &y& Elsevier]

Modern and ancient lacustrine carbonate build‐ups provide uniquely sensitive sedimentary and geochemical records for understanding the interaction between tectonics, past climates, and local and regional scale basin hydrology. Large (metre to decametre), well‐developed carbonate mounds in the Green River Formation have long been recognized along the margins of an Eocene lake, known as Lake Gosiute. However, their mode of origin and significance with respect to palaeohydrology remain controversial. Here, new sedimentological, Sr isotope data and structural evidence show that significant spring discharge led to the formation of a decametre size complex of shoreline carbonate mounds in the upper Wilkins Peak Member of the Green River Formation at Little Mesa and adjacent areas in the Bridger Basin, Wyoming, USA. Sedimentological evidence indicates that spring discharge was predominantly subaqueous but was, at times, also subaerial, which produced tufa cascades and micro‐rimstone dam structures. The 87Sr/86Sr ratios measured from these subaerial spring deposits are less radiogenic (87Sr/86Sr = 0.71040 to 0.71101) than contemporaneous palaeolake carbonates (87Sr/86Sr = 0.71195 to 0.71561) because their parent groundwaters likely interacted with less‐radiogenic Palaeozoic carbonate. Calcite‐cemented sandstone cones and spires (87Sr/86Sr = 0.71037 to 0.71057) in the Wasatch Formation directly below the spring deposits suggest that groundwaters derived from Palaeozoic carbonates preferentially flowed along thrust faults. These results imply that high spring discharge coincided with lake high stands of the upper Wilkins Peak Member, suggesting that recharge at the north‐west margin of the Bridger Basin contributed to Lake Gosiute's water budget and lowered the salinity of an underfilled, evaporative lake basin. The findings of this study provide criteria for the recognition of groundwater discharge in palaeolake systems which may lead to the discovery of palaeospring systems in other ancient lake deposits. [ABSTRACT FROM AUTHOR]

Dissimilatory iron reduction (DIR) plays an essential role in biogeochemical Fe cycling in anoxic environments. At near-neutral pH, in both biotic and abiotic systems, aqueous Fe(II) (Fe(II) aq) interacts with reactive ferric (hydr)oxides via electron transfer and atom exchange that is catalyzed by large amounts of sorbed Fe(II). This may result in substantial Fe isotope exchange, which, at equilibrium, produces up to a ∼4‰ 56Fe/54Fe fractionation between coexisting oxide/hydroxide and Fe(II) aq , depending on mineralogy. The role of biology in such systems has been interpreted to lie in the production of Fe(II) rather than a specific "vital" effect, such as enzymatic and kinetic processes. Under acidic abiotic conditions, however, the lack of sorbed Fe(II) generates little Fe isotope exchange, and, by extension, it has been expected that little exchange would occur during DIR at low pH if sorbed Fe(II) is the key component for catalyzing isotopic exchange. In this study, we explored the extent and mechanism of Fe isotope exchange between Fe(II) aq and ferric hydroxides (ferrihydrite and goethite), including determination of the 56Fe/54Fe fractionations during DIR by Acidianus strain DS80 at pH ∼ 3.0 and 80 °C, over 19 days of incubation. Significant Fe(III) reduction occurred for both minerals along with large changes in Fe isotope compositions for Fe(II) aq , indicating Fe isotope exchange. Solid-phase extractions using HCl confirmed a lack of sorbed Fe(II), which suggests that a mechanism other than sorption is required to catalyze Fe isotope exchange during DIR at low pH. Reactive Fe(III) (Fe(III) reac) extracted from the mineral surface allowed for the calculation of the Fe pools that underwent isotopic exchange. A total of ∼20% of goethite and ∼60% of ferrihydrite underwent isotopic exchange over 19 days. For goethite from biotic experiments, we calculate a Fe(III) reac -Fe(II) aq fractionation factor of 1.57 ± 0.52‰, which is larger than the abiotic equilibrium fractionation factor (∼0.73‰ at 80 °C). This result is consistent with previous work on DIR of goethite at neutral pH, where a fractionation factor larger than equilibrium was interpreted to reflect an isotopically distinct "distorted surface layer" of goethite produced during exchange with Fe(II) aq. In contrast to goethite, the difference between the Fe(III) reac -Fe(II) aq fractionation factor for ferrihydrite from our biotic reactors (2.91 ± 0.40‰) and the abiotic equilibrium fractionation factor (∼2.28‰ at 80 °C, under silica-free conditions) is smaller. Ultimately, the contrast in the extent of Fe isotope exchange between biotic and abiotic experiments emphasizes the importance of biology in promoting Fe isotope exchange in acidic systems. We speculate that the unique role of biology at low pH in catalyzing Fe isotope exchange, not seen in equivalent abiotic systems, must lie in the transport of electrons to the ferric hydroxide surface that produces Fe(II) atoms in situ. This suggests that isotopic exchange occurs on an atom-by-atom basis as Fe(III) is reduced to Fe(II), followed by the release of Fe(II) into solution. This study demonstrates that significant variations in Fe isotope compositions may be uniquely produced in acidic environments where microbial Fe cycling occurs via DIR, compared to minor isotopic variations observed previously in acidic abiotic systems. [ABSTRACT FROM AUTHOR]

Improvements in mass spectrometry have made it possible to identify naturally occurring K isotope ( 39 K/ 41 K) variability in terrestrial samples that can be used in a variety of geological and biological applications that involve cycling of K such as clay or evaporite formation. However, our ability to interpret K isotope variability is limited by a poor understanding of how K isotopes are fractionated at low temperatures. In this study, we conducted recrystallization experiments of eight K-salts in order to measure the K isotope fractionation factor between the salt and the saturated K solution (Δ 41 K min-sol ). Measured Δ 41 K min-sol are +0.50‰ for K 2 CO 3 ·1.5H 2 O, +0.32‰ for K 2 SO 4 , +0.23‰ for KHCO 3 , +0.06‰ for K 2 C 2 O 4 ·H 2 O, +0.02‰ for KCl, −0.03‰ for K 2 CrO 4 , −0.15‰ for KBr, and −0.52‰ for KI. Overall the Δ 41 K min-sol decreases with increasing r for K in crystals, where r is the average distance between a K atom and its neighboring atoms of negative charge. Salts with monovalent anions and salts with divalent anion complexes define different linear trends with distinct slopes on a plot of Δ 41 K min-sol - r . We applied ab initio lattice dynamics and empirical crystal-chemistry models to calculation of K isotope fractionation factors between K salts; both methods showed that the calculated inter-mineral K isotope fractionation factors (Δ 41 K min-KCl ) are highly consistent with experimentally derived Δ 41 K min-KCl under the assumption of consistent β factors for different saturated K solutions. Formulations for the crystal-chemistry model further indicate that both anion charge and bond length r are the principle controlling factors for K isotope fractionation, and the K isotope fractionation factors correlate with r following a 1/ r 3 relationship. Our experiment and theoretical study confirms the existence of significant equilibrium K isotope fractionation at ambient conditions, and the K isotope fractionation factors for halides and sulfate obtained in this study provide a basis for future K isotope studies on evaporites. [ABSTRACT FROM AUTHOR]

The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36°S, is among the most active latest Pleistocene-Holocene rhyolitic centers globally and has been inflating at a rate of >20 cm a-1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, ~20 km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic precursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied frontal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope compositions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcanism during the early post-glacial (25-19 ka) and Holocene (c. 8-2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element compositions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus silicic cumulates. A long-term rate of mass addition--extrapolated from surface deformation accumulated over the past decade--is comparable with those that have produced moderate- to large-volume caldera-forming eruptions elsewhere. [ABSTRACT FROM AUTHOR]

The past behavior of the Greenland ice sheet can provide important insight into climatic thresholds that may initiate and drive major ice-sheet retreat. Particle-size-specific magnetic and Sr–Nd–Pb isotope records from Eirik Ridge sediments south of Greenland track southern Greenland ice sheet (sGIS) erosional signatures over the past ∼430 ka by discriminating changes in sediment source and transport over the Eirik Ridge. Ground-truthed magnetic and isotopic compositions of subglacial silt from south Greenland's Precambrian bedrock terranes constrain independent magnetic and isotopic estimates of Eirik Ridge silt provenance, which in turn indicate that the southern Greenland ice sheet (sGIS) retreated within its present margin during three of the four previous interglaciations over the past ∼430 ka. Retreat of the sGIS was extensive during the Marine Isotope Stage (MIS) 5e, 9, and 11 interglaciations, continuing unabated despite declining insolation during MIS 5e and 9, with near complete deglaciation in MIS 11. Retreat of the sGIS during MIS 7 was minimal, notwithstanding strong insolation forcing, while Holocene retreat slowed shortly after peak insolation. The reconstruction of sGIS retreat during the last five deglacial and interglacial periods suggests that a threshold for extensive sGIS retreat exists between the insolation and CO 2 states of the Holocene and the MIS 5e and 9 interglaciations, with CO 2 exerting a stronger control on sGIS retreat than insolation. Our results also suggest that the extent and stability of the sGIS in the Holocene is anomalous in the context of late-Quaternary interglaciations. [ABSTRACT FROM AUTHOR]

The Green River Formation (GRF) provides one of the premier paleoclimate archives of the Early Eocene Climatic Optimum (∼50 Ma), representing the apex of the early Cenozoic greenhouse climate. Rhythmic lake-level variability expressed in the GRF has inspired numerous hypotheses for the behavior of the Eocene hydrologic cycle, including its linkage to astronomical forcing, solar variability, and the El Niño Southern Oscillation (ENSO). However, the lack of sufficient proxy data to document atmospheric water-mass transport and the geographic pattern of evaporation/precipitation/runoff has made it difficult to discriminate between different models for astronomical forcing. Variable 87 Sr/ 86 Sr ratios of bedrock that encompass the GRF provide an opportunity to reconstruct the spatial expression of the Eocene hydrologic cycle and its linkage to lake level. Here Sr isotope data from the Wilkins Peak Member, a rhythmic succession that has been demonstrated to record Milankovitch forcing of lake levels, indicate that high lake levels reflect an increased proportion of runoff from less radiogenic rocks west of the basin, eliminating a number of the existing astronomical-forcing hypotheses. The 87 Sr/ 86 Sr variability is consistent with a change in mean ENSO state, which is predicted by climate models to be linked to orbital-insolation. Thus, the 87 Sr/ 86 Sr data reveal a coupling of high frequency (ENSO) and low frequency (astronomical) climate variability, and also predict the existence of sizable astronomically-forced alpine snowpack during the last greenhouse climate. More broadly, this study demonstrates the utility of 87 Sr/ 86 Sr as a powerful tool for reconstructing the deep-time hydrologic cycle. [ABSTRACT FROM AUTHOR]

Before the Great Oxidation Event (GOE) 2.4–2.2 billion years ago it has been traditionally thought that oceanic water columns were uniformly anoxic due to a lack of oxygen-producing microorganisms. Recently, however, it has been proposed that transient oxygenation of shallow seawater occurred between 2.8 and 3.0 billion years ago. Here, we present a novel combination of stable Fe and radiogenic U–Th–Pb isotope data that demonstrate significant oxygen contents in the shallow oceans at 3.2 Ga, based on analysis of the Manzimnyama Banded Iron Formation (BIF), Fig Tree Group, South Africa. This unit is exceptional in that proximal, shallow-water and distal, deep-water facies are preserved. When compared to the distal, deep-water facies, the proximal samples show elevated U concentrations and moderately positive δ 56 Fe values, indicating vertical stratification in dissolved oxygen contents. Confirmation of oxidizing conditions using U abundances is robustly constrained using samples that have been closed to U and Pb mobility using U–Th–Pb geochronology. Although redox-sensitive elements have been commonly used in ancient rocks to infer redox conditions, post-depositional element mobility has been rarely tested, and U–Th–Pb geochronology can constrain open- or closed-system behavior. The U abundances and δ 56 Fe values of the Manzimnyama BIF suggest the proximal, shallow-water samples record precipitation under stronger oxidizing conditions compared to the distal deeper-water facies, which in turn indicates the existence of a discrete redox boundary between deep and shallow ocean waters at this time; this work, therefore, documents the oldest known preserved marine redox gradient in the rock record. The relative enrichment of O 2 in the upper water column is likely due to the existence of oxygen-producing microorganisms such as cyanobacteria. These results provide a new approach for identifying free oxygen in Earth's ancient oceans, including confirming the age of redox proxies, and indicate that cyanobacteria evolved prior to 3.2 Ga. [ABSTRACT FROM AUTHOR]

Strontium isotopes from lacustrine carbonates record detailed weathering histories of exposed bedrocks, and thus are potentially useful for understanding interactions between tectonics and climate that may be driven by local or regional factors in basin-scale hydrologic systems. We combine extensive 87 Sr/ 86 Sr and δ 18 O datasets from the Green River Formation in order to identify environmental factors driving the evolution of Sr concentration and isotopic composition in lacustrine systems through the use of Sr mass balance modeling. Two models are developed. The first tests the effect of drainage capture that drove a constant-volume lake from balance-filled to overfilled conditions, and is applied to the Laney Member of the Green River Formation. The second model focuses on lacustrine evolution in response to changing the concentration, composition, and mass of water influx in a variable-size Sr reservoir (lake). This model is applied to the Wilkins Peak Member of the Green River Formation. 87 Sr/ 86 Sr ratios are correlated with δ 18 O values only during the Laney Member, when both are readily explained by a common forcing mechanism (drainage capture). Modeling of Sr isotope compositions and concentrations indicates a short residence time for Sr (~ 10 3 –10 4 years or less) in both balance-overfilled and underfilled phases of the Green River Formation lacustrine system. This in turn suggests that paleo-lacustrine sediments in most lakes can preserve Sr isotope records with high-resolution (~ 10 2 years) timescales. Rates of Sr sequestration in carbonate are shown to have a strong influence on lacustrine Sr concentration, and high Sr concentrations of lacustrine carbonate consistent with high salinity are observed in the underfilled Wilkins Peak Member and the balance-filled Tipton and lower Laney Members. In the Laney Member, 87 Sr/ 86 Sr mass-balance modeling results provide additional support for previous interpretations that the introduction of a large drainage system produced an isotopic shift across the lower LaClede/upper LaClede boundary. Major drainage reorganization is not required to drive high variability in 87 Sr/ 86 Sr ratios, however. Modeling shows that variability in 87 Sr/ 86 Sr ratios of ~ 0.004 observed in the Wilkins Peak Member can be explained by change in the characteristics of intrabasinal water sources during highstand vs. lowstand conditions. [ABSTRACT FROM AUTHOR]

The chemical and isotopic compositions of Precambrian Fe-rich chemical sedimentary rocks have figured prominently in discussions on the Fe biogeochemical cycle and redox conditions in the early Earth. Broad trends of decreasing δ56Fe values for Eoarchean to Paleoproterozoic iron formations (IFs) and jaspilites (hematite-chert) with decreasing age reflect a general increase in extent of oxidation of aqueous Fe(II) (Fe(II) aq) in marine environments, consistent with increasing oxygenation of surface environments, particularly since the Mesoarchean. Such trends may record a shift from anaerobic to aerobic oxidation, in part reflecting increasing nutrient abundances that follow increased emergence of continental crust. It is commonly proposed that the size of the biosphere in the early Archean was largely nutrient limited, specifically P limited. Highly positive δ56Fe values for Fe oxides in chemical sedimentary rocks deposited in open marine environments at this time are consistent with excess Fe(II) aq. Here, we present new data on Fe isotopes and trace element chemistry for 3.4–3.5 Ga jaspilites from the North Pole region of the Pilbara Craton, including the first example of a land-sea transition preserved in the lower Dresser Formation as jaspilite deposition in restricted basin and open-marine settings. A review of the possible mechanisms of Fe(II) aq oxidation in the early Archean - including O 2 generated by oxygenic photosynthesis, UV photo-oxidation, or photoferrotrophy (anoxygenic photosynthesis) - suggests that the latter is most likely, in light of evidence for low-O 2 contents and the strong role that silica plays in inhibiting abiologic Fe(II) oxidation. Y + REE contents of lower Dresser Formation jaspilites indicate a freshwater component in the restricted basin settings, in contrast to exclusively seawater components in the open-marine environments. δ56Fe values for open-marine jaspilites are highly positive (δ56Fe ~ +1 to +2.4‰), similar to other jaspilites and IFs of Eoarchean to Paleoarchean age. Assuming photoferrotrophy as the oxidative pathway, this would indicate electron donor (Fe(II) aq) excess and nutrient limitation. For the restricted-basin jaspilites of the lower Dresser Formation, δ56Fe values extend to slightly negative values (δ56Fe ~ −0.4‰), correlating with changes in Y + REE contents that indicate fluid mixing. In toto , there is a range of ~3‰ in δ56Fe values across the land-sea transition of the lower Dresser Formation, significantly exceeding the range previously measured in early Archean jaspilites. Combined with an advection-dispersion-reaction model for Fe(II) aq oxidation via photoferrotrophy, these observations suggest a range of nutrient abundance with environment, where samples from near-shore or restricted settings record excess nutrients from terrestrial input and hence electron donor limitation. Integrating the new results from the Pilbara Craton with the broader Fe isotope database for Eoarchean through Paleoarchean jaspilites and IFs, as well as a simple mass-balance model relating δ56Fe values and P contents, it becomes apparent that the relatively high nutrient abundance seen only in restricted settings at ~3.5 Ga becomes more characteristic of the open oceans by ~3.2 Ga. This expansion of nutrient availability is correlated with a marked increase in 87Sr/86Sr ratios for seawater, which can be shown to be a proxy for P delivery to the open oceans from the continents via weathering fluxes. Collectively, these findings demonstrate the importance of considering geologic context in interpreting Fe isotope and chemical compositions of jaspilites and IFs. Capture of the land-sea transition of the lower Dresser Formation provides insights into the biosphere that would have been missed in sample suites that only record open-marine conditions. [ABSTRACT FROM AUTHOR]

Abstract: The Fe isotope fractionation factors among aqueous ferrous iron (), aqueous FeS clusters (FeSaq), and nanoparticulate mackinawite under neutral and mildly acidic and alkaline pH conditions have been determined using the three-isotope method. Combined voltammetric analysis and geochemical modeling were used to determine the Fe speciation in the experimental systems. The equilibrium 56Fe/54Fe fractionation factor at 20°C and pH 7 has been determined to be −0.32±0.29 (2σ)‰ between (minor FeSaq also present in the experiment) and mackinawite. This fractionation factor was essentially constant when pH was changed to 6 or 8. When equal molarity of HS− and were added to the system, however, the isotopic fractionation at pH 7 changed to −0.64±0.36 (2σ)‰, correlating with a significant increase in the proportion of FeHS+ and FeSaq. These results highlight a more important role of aqueous Fe–S speciation in the equilibrium Fe isotope fractionation factor than recognized in previous studies. The isotopic fractionation remained constant when temperature was increased from 20°C to 35°C for fractionation factors between , and mackinawite and between dominantly FeHS+ and mackinawite. Synthesis experiments similar to those of and at pH 4 show consistent results: over time, the aqueous Fe–mackinawite fractionation decreases but even after 38days of aging the fractionation factor is far from the equilibrium value inferred using the three-isotope method. In contrast, at near-neutral pH the fractionation factor for the synthesis experiment reached the equilibrium value in 38days. These differences are best explained by noting that at low pH the FeS mackinawite particles coarsen more rapidly via particle aggregation, which limits isotopic exchange, whereas at higher pH mackinawite aggregation is limited, and Fe isotope exchange occurs more rapidly, converging on the equilibrium value. These results suggest that mackinawite formed in natural environments at near-neutral or alkaline pH are unlikely to retain kinetic isotope fractionations, but are more likely to reflect equilibrium isotope compositions. This in turn has important implications for interpreting iron isotope compositions of Fe sulfides in natural systems. [Copyright &y& Elsevier]

Prior to the ad 1902 Plinian eruption of 8 km3 of dacite and subsequent growth of the >1 km3 Santiaguito dacite dome complex, Santa María volcano grew into an 8 km3 composite cone over ∼75 kyr in four phases (at 103–72, 72, 60–46, and 35–25 ka). The 1902 eruption occurred after an ∼25 kyr period of repose in growth of the composite cone. To provide context for processes that ultimately led to the 1902 eruption, we present geochemical and isotopic (Sr, Nd, Pb, U-series) data from lavas of the composite cone for which ages are constrained by 40Ar/39Ar dating. The four cone-building phases comprise basaltic to basaltic-andesite lava (51·4–56·1% SiO2) whose major- and trace-element compositions suggest that crystallization was important in differentiation. Relative to other Central American arc volcanoes, these lavas also have large 238U excesses and high 207Pb/204Pb ratios that imply melting of a mantle wedge modified to an unusual extent by fluid from subducted crust and sediment of the Cocos plate. Major- and trace-element and isotopic variations over time imply that mafic recharge and magma mixing were prevalent during early phases of cone-building, whereas assimilation processes were more dominant during the latest stage of cone growth. Indeed, some early erupted basalts have lower 143Nd/144Nd and higher 87Sr/86Sr ratios than more SiO2-rich basaltic andesites that erupted during the final phase of cone-building. These features point to an assimilant that is not typical continental crust and instead may be more like mid-ocean ridge basalt with respect to major- and trace-element composition and Sr, Nd, Pb, and U–Th isotope ratios. Energy-constrained modeling of a parental basalt that undergoes crystal fractionation, assimilation and periodic recharge with basalt in the lower crust can reproduce lava compositions erupted during phases I–III and the early part of phase IV. Modeling further indicates that assimilation within the lower crust of partially melted garnet-amphibolite metabasalt, without basaltic recharge, may produce the youngest cone-forming lavas in phase IV. These models link the 8 km3 of cone growth over 75 kyr to the mass flux of magma into the crust. Our findings suggest an along-arc magma flux into the lower crust beneath Santa María of >20 km3 km−1 Myr−1, which is higher than anticipated in recent numerical–thermal approaches to basalt–crust interaction. Consequently, the thermal incubation period needed to produce hybrid basaltic-andesite magma may be only a few tens of thousand years. [ABSTRACT FROM PUBLISHER]