Processes controlling the composition of seafloor hydrothermal fluids in silicic back-arc or neararc crustal settings remain poorly constrained despite growing evidence for extensive magmatichydrothermal activity in such environments. We conducted a survey of vent fluid compositions from two contrasting sites in the Manus back-arc basin, Papua New Guinea, to examine the influence of variations in host rock composition and magmatic inputs (both a function of arc proximity) on hydrothermal fluid chemistry. Fluid samples were collected from felsic-hosted hydrothermal vent fields located on Pual Ridge (PACMANUS and Northeast (NE) Pual) near the active New Britain Arc and a basalt-hosted vent field (Vienna Woods) located farther from the arc on the Manus Spreading Center. Vienna Woods fluids were characterized by relatively uniform endmember temperatures (273-285°C) and major element compositions, low dissolved CO 2 concentrations (4.4mmol/kg) and high measured pH (4.2-4.9 at 25°C).Temperatures and compositions were highly variable at PACMANUS/NE Pual and a large, newly discovered vent area (Fenway) was observed to be vigorously venting boiling (358°C) fluid. All 2

High-precision Fe isotopic data for 104 samples, including modern and ancient (≥ 3.7 Ga) subduction-related magmas and mantle peridotites, are presented. These data demonstrate that mid-ocean ridge and oceanicisland basalts (MORBs and OIBs) have on average small, but distinctly (~+ 0.06‰) higher 56 Fe/ 54 Fe ratios than both modern and Eoarchean boninites and many island arc basalts (IABs) that are interpreted to form by large degrees of flux melting of depleted mantle sources. Additionally boninites and many IABs have iron isotopic compositions similar to chondrites, fertile mantle peridotites, Eoarchean mantle peridotites, and basalts from Mars and Vesta. The observed variations are best explained by the bulk silicate Earth having a near-chondritic iron isotopic composition, with~+ 0.3‰ equilibrium isotope fractionation between Fe 3+ and Fe 2+ and preferential extraction of isotopically heavier, incompatible Fe 3+ during mantle melting to form oceanic crust (as represented by MORBs and OIBs). A quantitative model that relates the iron isotopic composition of basaltic magmas to the degree of partial melting, Fe 3+ /Fe 2+ ratio, and buffering capacity of the mantle is presented. The concept that redox conditions may influence iron isotopic fractionation during melting provides a new approach for understanding the redox conditions of magma genesis on early Earth and Mars. Experimental and theoretical work is required to establish iron isotopic fractionation as an oxybarometer of mantle melting.

isotope compositions are best determined using purified (matrix-clean) sulfur standards and sample solutions using the chemical purification protocol we present. For in situ analysis, where the complex matrix cannot be removed prior to analysis, appropriately matrix-matching standards and samples removes matrix artifacts and yields sulfur isotope ratios consistent with conventional techniques using matrix-clean analytes. Our method enables solid samples to be calibrated against aqueous standards; a consideration that is important when certified, isotopically-homogeneous and appropriately matrix-matched solid standards do not exist.Further, bulk and in situ analyses can be performed interchangeably in a single analytical session because the instrumental setup is identical for both. We validated the robustness of our analytical method through multiple isotope analyses of a range of reference materials and have compared these with isotope ratios determined using independent techniques. Long-term reproducibility of S isotope compositions is typically 0.20 ‰ and 0.45 ‰ (2σ) for solution and laser analysis, respectively. Our method affords the opportunity to make accurate and relatively precise S isotope measurement for a wide range of sulfur-bearing materials, and is particularly appropriate for geologic samples with complex matrix and for which high-resolution in situ analysis is critical.

High‐precision iron isotopic compositions for Fe‐bearing geological reference materials and chondrites with a wide range of matrices (e.g., silicates, oxides, organic‐bearing materials) are reported. This comprehensive data set should serve as a reference for iron isotopic studies across a range of geological and biological disciplines for both quality assurance and inter‐laboratory calibration. Where comparison is available, the iron isotopic compositions of most geological reference materials measured in this study were in agreement with previously published data within quoted uncertainties. Recommendations for the reporting of future iron isotopic data and associated uncertainties are also presented. Long‐term repeat analyses of all samples indicate that highly reproducible iron isotopic measurements are now obtainable (± 0.03‰ and ± 0.05‰ for δ56Fe and δ57Fe, respectively).

No abstract

A series of artificial maturation (anhydrous, semi-open pyrolysis) experiments on Green River oil shale have been performed to simulate the thermal maturation of type I kerogen. The goals of this program were to develop a kinetic model of petroleum generation from oil shale and to characterize the yield and composition of petroleum as a function of artificial thermal maturity. The thermal maturity level (EASY%Ro = 0.48–1.28%) is based upon the kinetic model of kerogen degradation and is equivalent to vitrinite reflectance maturity. Here, we compare the structural characteristics of kerogen and bitumen during artificial maturation of oil shale using quantitative Fourier transform infrared (IR) spectroscopy. Quantitative comparison was enabled by a novel method for the preparation of bitumen for IR spectroscopy. Bitumen can be a reaction intermediate during maturation of kerogen, and the IR data indicate that bitumen has a structure intermediate between that of kerogen and generated petroleum. Moreover, the IR data reveal that the composition of bitumen changes with maturity, with trends that are similar in some aspects to those observed previously in kerogen, but different in others. Kerogen is characterized by the early depletion of oxygenated functional groups prior to petroleum generation (EASY%Ro < 0.9%) and then a late enrichment of oxygen at higher artificial maturity (EASY%Ro > 1.2%). In contrast, bitumen shows initial enrichment of oxygenated functional groups at low artificial maturity (EASY%Ro < 0.8%) and subsequent depletion at higher maturity. Kerogen evolution follows the previously observed trend with aliphatic carbon chains that became shorter and/or more branched as kerogen is consumed during all stages of artificial maturation. Bitumen, in contrast, appears to have aliphatic carbon chains that lengthen within the same artificial maturity range as bitumen is predominantly generated from kerogen. The aliphatic carbon content of bitumen is greater than that of kerogen at all levels of artificial maturity. Both kerogen and bitumen become more aromatic in character with increasing thermal maturity, especially above artificial levels EASY%Ro > 0.9%. This similarity likely results from loss of aliphatic chains from both organic fractions during petroleum generation, suggesting that both kerogen and bitumen can be direct sources for petroleum. The loss of aliphatic carbons from aromatic centers in both kerogen and bitumen leads to protonation of the residual aromatic rings. The IR spectra of kerogen and bitumen indicate very similar degrees of protonation of those aromatic rings.

No abstract

The SuSu Knolls and DESMOS hydrothermal fields are located in the back-arc extensional transform zone of the Eastern Manus Basin. In 2006, highly acidic and RSO 4 -rich vent fluids were collected at both sites and analyzed for the chemical and isotopic composition of major and trace species. Fluids exiting the seafloor have measured temperatures from 48 to 215°C and are milky white in appearance due to precipitation of elemental S 0 . Vent fluid concentrations of Na, K, and Mg are depleted by as much as 30% relative to seawater, but have the same relative abundance. In contrast, the fluids are highly enriched in dissolved RCO 2 , Cl, SiO 2(aq) , Fe, and Al relative to seawater. Measured pH (25°C) ranged from 0.95 to 1.87 and aqueous RSO 4 ranged from 35 to 135 mmol/kg. The chemical and isotopic composition points to formation via subsurface mixing of seawater with a Na-, K-, Mg-, and Ca-free, volatile-rich magmatic fluid exsolved from subsurface magma bodies during a process analogous to subaerial fumarole discharge. Estimates of the magmatic end-member composition indicate a fluid phase where H 2 O > SO 2 > CO 2 % Cl > F. The hydrogen and oxygen isotopic composition of H 2 O and carbon isotopic composition of RCO 2 in the vent fluids strongly suggest a contribution of slab-derived H 2 O and CO 2 to melts generated in the mantle beneath the Eastern Manus volcanic zone. Abundant magmatically-derived SO 2 undergoes disproportionation during cooling in upflow zones and contributes abundant acidity, SO 4 2À , and S 0 to the venting fluids. Interaction of these highly acidic fluids with highly altered mineral assemblages in the upflow zone are responsible for extensive aqueous mobilization of SiO 2(aq), Fe, and Al. Temporal variability in the speciation and abundance of aqueous S species between 1995 and 2006 at the DESMOS vent field suggests an increase in the relative abundance of SO 2 in the magmatic end-member that has mixed with seawater in the subsurface. Results of this study constrain processes responsible for the formation of hot-spring fluids in magmatically active back-arc environments and the resulting chemical exchange between the lithosphere and water column.