Influence of high marine Ca/SO4 ratio on alteration of submarine basalts at 2.41 Ga documented by triple O and Sr isotopes of epidote

Over the course of Earth’s history, marine sulfate concentrations have been increasing in response to long-term atmospheric oxygenation. In contrast to modern oceans, where abundant sulfate precipitates in hot oceanic crust as anhydrite, Precambrian oceans contained much less (~0–10 mM) sulfate, so that submarine hydrothermal systems were comparatively poor in anhydrite. As a step towards exploring the role of chemical evolution of seawater solutes, we investigate the reaction between basalt and seawater that took place at the ca. 2.43–2.41 Ga Vetreny Belt (Karelia craton, NW Russia) using fluid inclusion and multi-isotope measurements complemented by reactive transport and static aqueous-mineral equilibrium calculations. Using fluid inclusion measurements by LA-ICP-MS, we constrain the Sr concentration in the least modified seawater-derived fluids and address the effect of phase separation. Then, we complement the previous δ18O – Δ′17O datasets with new 87Sr/86Sr measurements performed on 2.41 Ga epidote from the Vetreny Belt, and recent (0–6 Ma) oceanic epidote from Reykjanes, Iceland and the drilling site 504B in the eastern Pacific Ocean. The 2.41 Ga epidote with 87Sr/86Srinitial of 0.7029–0.7042 and Δ′17O of –0.06 to 0.00‰ is best explained by a relatively high fraction (~90%) of marine Sr that was delivered from contemporaneous seawater with 87Sr/86Sr ≈ 0.7045, and without significant removal by early anhydrite. Using Monte-Carlo simulation of a dual-porosity model, we constrain the range of possible exchange trajectories based on the variability of physical parameters (porosity, fluid flow velocity, fracture spacing, recrystallization rates). Further, we use a series of static equilibrium seawater-basalt reaction calculations with emphasis on the possible range of marine Ca/SO4 values at 2.41 Ga. Our calculations demonstrate that co-existing quartz and epidote in absence of feldspars represent equilibrium with less-evolved hydrothermal fluids. Consequently, equilibrium assemblage of quartz and epidote provide an insightful archive for marine Sr. Based on our modeling and assumptions about marine 87Sr/86Sr and Sr/Ca ratios, the 2.41 Ga epidotes document a seawater-basalt reaction where the initial fluid contained between 30 and 40 mM of Ca and 0–10 mM SO4, representing a high marine input and the possible effect of phase separation. Based on our data, we suggest that high Ca/SO4 ratio of seawater (≫1) and low concentration of anhydrite in submarine systems of the contemporaneous oceans promote a higher fraction of seawater Sr to be permanently stored in silicates of altered oceanic crust. In contrast, modern altered oceanic crust is depleted in radiogenic Sr due to partitioning into anhydrite, which partly returns into the ocean upon cooling

Uranium 238U/235U isotope ratios as indicators of reduction: Results from an in situ biostimulation experiment at Rifle, Colorado, USA

The attenuation of groundwater contamination via chemical reaction is traditionally evaluated by monitoring contaminant concentration through time. However, this method can be confounded by common transport processes (e.g. dilution, sorption). Isotopic techniques bypass the limits of concentration methods, and so may provide improved accuracy in determining the extent of reaction. We apply measurements of {sup 238}U/{sup 235}U to a U bioremediation field experiment at the Rifle Integrated Field Research Challenge Site in Rifle, Colorado (USA). An array of monitoring and injection wells was installed on a 100 m{sup 2} plot where U(VI) contamination was present in the groundwater. Acetate-amended groundwater was injected along an up-gradient gallery to encourage the growth of dissimilatory metal reducing bacteria (e.g. Geobacter species). During amendment, U concentration dropped by an order of magnitude in the experiment plot. We measured {sup 238}U/{sup 235}U in samples from one monitoring well by MC-ICP-MS using a double isotope tracer method. A significant {approx}1.00{per_thousand} decrease in {sup 238}U/{sup 235}U occurred in the groundwater as U(VI) concentration decreased. The relationship between {sup 238}U/{sup 235}U and concentration corresponds approximately to a Rayleigh distillation curve with an effective fractionation factor ({alpha}) of 1.00046. We attribute the observed U isotope fractionation to a nuclear field shift effect during enzymatic reduction of U(VI){sub (aq)} to U(IV){sub (s)}

Iron oxide – Apatite, iron oxide – copper – gold deposits and magmas: A bubbly connection

Iron oxide-apatite (IOA) and iron oxide-copper-gold deposits (IOCG) are often spatially and temporally related with one another and with coeval magmatism. However, a genetic model that accounts for observations of natural systems remains elusive, with few observational data able to distinguish among working hypotheses that invoke meteoric fluid, magmatic-hydrothermal fluid, and immiscible melts. Here, we use high-resolution trace element concentrations in magnetite, hematite and pyrite, high-precision Fe and O stable isotope data of magnetite and hematite grains, δD of magnetite and actinolite, and Re and Os in magnetite and pyrite from the Los Colorados IOA and Candelaria and Mantoverde IOCG deposits in the Chilean Iron Belt to elucidate the origin of IOA and IOCG systems. At Los Colorados, Ti, V, Al, and Mn are enriched in magnetite cores and decrease systematically from core to rim, a trend consistent with magmatic and/or magmatic-hydrothermal magnetites. High Co/Ni ratios of pyrite from Los Colorados are also consistent with a magmatic-hydrothermal origin. δD values for magnetite and actinolite indicate a mantle source for H. Values of d56Fe and d18O for magnetite and hematite from all deposits indicate a magmatic source for Fe and O. The Re-Os systematics overlap data from Andean porphyry Cu-Mo deposits and are consistent with a magmatic-hydrothermal origin. Together, the data are consistent with a genetic model wherein 1) magnetite cores crystallize from silicate melt; 2) these magnetite crystals are nucleation sites for aqueous fluid that exsolves and scavenges Fe, P, S, Cu, Au from silicate melt; 3) the magnetite-fluid suspension is less dense that the surrounding magma, allowing ascent; 4) as the suspension ascends, magnetite grows in equilibrium with the fluid and takes on a magmatic-hydrothermal character (i.e., lower Al, Mn, Ti, V); 5) during ascent, magnetite, apatite and actinolite are deposited to form IOA deposits; 6) the further ascending fluid transports Fe, Cu, Au and S toward the surface where hematite, magnetite and sulfides precipitate to form IOCG deposits. This model is globally applicable and explains the observed temporal and spatial relationship between magmatism and formation of IOA and IOCG deposits

An inter-laboratory assessment of the thorium isotopic composition of synthetic and rock reference materials

We present a concerted international effort to cross-calibrate five synthetic Th isotope reference materials (UCSC Th "A", OU Th "U", WUN, IRMM-35 and IRMM-36), and six rock reference materials (UCSC TML, Icelandic ATHO, USGS BCR-2, USGS W-2, USGS BHVO-2, LV18) using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). We then compare our new values with a compilation of literature mass spectrometric data for these reference materials and derive recommended "consensus" 230Th/232 Th values for each. We also present isotope dilution U and Th concentration data for four rock reference materials (UCSC TML, Icelandic ATHO, USGS BCR-2, USGS W-2). © 2008 International Association of Geoanalysts

A new tool to assess Clinical Diversity In Meta‐analyses (CDIM) of interventions

Objective: To develop and validate Clinical Diversity In Meta-analyses (CDIM), a new tool for assessing clinical diversity between trials in meta-analyses of interventions. Study design and setting: The development of CDIM was based on consensus work informed by empirical literature and expertise. We drafted the CDIM tool, refined it, and validated CDIM for interrater scale reliability and agreement in three groups. Results: CDIM measures clinical diversity on a scale that includes four domains with 11 items overall: setting (time of conduct/country development status/units type); population (age; sex; patient inclusion criteria/baseline disease severity, comorbidities); interventions (intervention intensity/strength/duration of intervention; timing; control intervention; cointerventions);and outcome (definition of outcome; timing of outcome assessment). The CDIM is completed in two steps: first two authors independently assess clinical diversity in the four domains. Second, after agreeing upon scores of individual items a consensus score is achieved. Interrater scale reliability and agreement ranged from moderate to almost perfect depending on the type of raters. Conclusion: CDIM is the first tool developed for assessing clinical diversity in meta-analyses of interventions. We found CDIM to be a reliable tool for assessing clinical diversity among trials in meta-analysis