Quantification of trace element contents in frozen fluid inclusions by UV-fs-LA-ICP-MS analysis

We have developed a new analytical setup for the determination of trace element concentrations in fluid inclusions by UV-fs-LA-ICP-MS. Laser ablation was performed at a low temperature of -40 degrees C by using a modified heating-freezing stage as the ablation cell. With this method it was possible to successfully analyse 53 of 55 frozen synthetic NaCl-H2O fluid inclusions in quartz, covering a size range between 8 mu m and 25 mu m down to a depth of 50 mu m. The high success rate could be achieved as the 194 nm UV-fs-laser allows excellent control over the opening procedure of frozen fluid inclusions. Trace element analyses were performed with a fast scanning magnetic sector field ICP-MS. The lower limits of detection for fluid inclusion analysis vary from 0.1 mu g g(-1) (for Bi-209) to 10 mu g g(-1) (for K-39). The typical analytical uncertainty, depending on the element and respective concentration level, ranges between 10% and 30% (1RSD), based on the reproducibility of experimentally synthesized fluid inclusions. All elements from a stock solution, which behaved inert during the HP/HT experiments (B, K, Cd, Te, Tl, Pb and Bi), could be recovered in the synthetic inclusions at concentrations that correspond within their specific analytical uncertainties to their original concentration of 53 mu g g(-1). The method represents a highly efficient tool for the determination of accurate trace element data on low concentration levels in small fluid inclusions with a high success rate of >90%. The latter is particularly advantageous considering the commonly time consuming characterization of fluid inclusions.NTH Graduate School GeoFluxe

Iron Exports From Catchments Are Constrained by Redox Status and Topography

Fe(III) hydroxides stabilize organic carbon (OC) and P in soils. Observations of rising stream Fe concentrations are controversially posited to result from a flushing of iron-rich deeper soil layers or a decrease of competing electron acceptors inhibiting Fe reduction (NO3- and SO42-). Here, we argue that catchment topography constrains the release of Fe, OC, and P to streams. We therefore incubated organic topsoil and mineral subsoil and modified the availability of NO3-. We found that Fe leaching was highest in topsoil. Fe, OC, and P released at quantities proportional to their ratios in the source soil. Supply of NO3- reduced Fe leaching to 18% and increased pore water OC:Fe and P:Fe ratios. Subsoil, however, was an insignificant Fe source (<0.5%). Here, the leached quantities of Fe, OC and P were highly disproportionate to the soil source with an excess of released OC and P. We tested if experimental findings scale up using data from 88 German catchments representing gradients in NO3- concentration and topography. Average stream Fe concentrations increased with decreasing NO3- and were high in catchments with shallow topography where high groundwater levels support reductive processes and topsoils are hydrologically connected to streams; but Fe concentrations were low in catchments with steep topography where flow occurs primarily through subsoils. OC:Fe and P:Fe ratios in the streams similarly varied by NO3- and topography. This corroborates the findings from the laboratory experiment and suggests that catchment topography and competing electron acceptors constrain the formation of Fe-reducing conditions and control the release of Fe, OC, and P to streams. © 2022. The Authors

(Isotopen-)Geochemie von Fe, Cu und Zn in einem Süßwasserwatt der Elbe

Die Untersuchung von stabilen Metallisotopen ermöglicht neue Einblicke in das Verhalten von Metallen in der Umwelt. Wir nutzten diese Methode, um Herkunft, Verbleib und Transport von Eisen, Kupfer und Zink in einem belasteten Süsswasserwatt zu untersuchen. Dazu wurde ein Kern aus einem Süsswasserwatt der Elbe analysiert. Der Kern zeigt eine starke Zonierung in einem oberen oxischen/suboxischen und einen unteren permanent anoxischen Teil. Die einzige signifikante Abweichung der d56Fe Werte trat direkt oberhalb des permanent anoxischen Teils auf und ist wahrscheinlich auf dissimilatorische Eisenreduktion zurückzuführen. Die d65Cu- Werte zeigen signifikante Variationen, die negativ mit den Eisen- und den C- und N-Gehalten im Sediment korrelieren. Diese Korrelationen sind wahrscheinlich auf die Mobilisierung von reduzierten Cu- Spezies aus dem Sediment zurückzuführen. Die d66Zn-Werte deuten auf Kläranlagen und städtischen Abfluss als Zn-Quellen hin. Der gleichbleibende d66Zn-Wert über die Tiefe ist außerdem ein Indiz für gleichbleibende Zn-Quellen während des Sedimentationszeitraumes

Sulfide enrichment at an oceanic crust-mantle transition zone : Kane Megamullion (23°N, MAR)

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 230 (2018): 155-189, doi:10.1016/j.gca.2018.03.027.The Kane Megamullion oceanic core complex located along the Mid-Atlantic Ridge (23°30′N, 45°20′W) exposes lower crust and upper mantle directly on the ocean floor. We studied chalcophile elements and sulfides in the ultramafic and mafic rocks of the crust-mantle transition and the mantle underneath. We determined mineralogical and elemental composition and the Cu isotope composition of the respective sulfides along with the mineralogical and elemental composition of the respective serpentines. The rocks of the crust-mantle transition zone (i.e., plagioclase harzburgite, peridotite-gabbro contacts, and dunite) overlaid by troctolites are by one order of magnitude enriched in several chalcophile elements with respect to the spinel harzburgites of the mantle beneath. Whereas the range of Cu concentrations in spinel harzburgites is 7–69 ppm, the Cu concentrations are highly elevated in plagioclase harzburgites with a range of 90–209 ppm. The zones of the peridotite-gabbro contacts are even more enriched, exhibiting up to 305 ppm Cu and highly elevated concentrations of As, Zn, Ga, Sb and Tl. High Cu concentrations show pronounced correlation with bulk S concentrations at the crust-mantle transition zone implying an enrichment process in this horizon of the oceanic lithosphere. We interpret this enrichment as related to melt-mantle reaction, which is extensive in crust-mantle transition zones. In spite of the ubiquitous serpentinization of primary rocks, we found magmatic chalcopyrites [CuFeS2] as inclusions in plagioclase as well as associated with pentlandite [(Fe,Ni)9S8] and pyrrhotite [Fe1−xS] in polysulfide grains. These chalcopyrites show a primary magmatic δ65Cu signature ranging from −0.04 to +0.29 ‰. Other chalcopyrites have been dissolved during serpentinization. Due to the low temperature (<300 °C) of circulating fluids chalcophile metals from primary sulfides have not been mobilized and transported away but have been trapped in smaller secondary sulfides and hydroxides. Combined with the Cu deposits documented in the crust-mantle transition zones of various ophiolite complexes, our results indicate that the metal enrichment, increased sulfide modes, and potentially formation of small sulfide deposits could be expected globally along the petrological Moho.This research was funded by a Diamond Grant of the Polish Ministry of Science and Higher Education (DI2012 2057 42 to J. Ciazela), and partly supported by grants of the U.S. National Science Foundation (OCE1434452 and OCE1637130 to H.J.B. Dick), and the German Science Foundation (Bo2941/4-1 to R. Botcharnikov)

Subduction zone dynamics in the SW Pacific plate boundary region constrained from high-precision Pb isotope data

This study presents high-precision Pb isotope data obtained by MC-ICP-MS for lavas from the Solomon island arc that covers ca. 1000 km of the SW Pacific plate boundary. Following a reversal of subduction polarity, the presence of the old subducted Pacific oceanic crust, and the presently subducting Indian-Australian plate beneath the entire island arc is confirmed by geophysical and geochemical evidence. New high-precision Pb isotope data now permit to distinguish more clearly between Indian and Pacific-type subduction components in this complex plate tectonic constellation. The values range from Pb-206/Pb-204 = 18.345 to 18.845, Pb-207/Pb-204 = 15498 to 15.545, and Pb-208/Pb-204 = 38.120 to 38.372, indicating the absence of pelagic sediments in the sub-arc mantle. These data reveal a predominance of subduction components derived from ancient subduction of the Pacific plate. In contrast, lavas with Indian-type Pb isotope signatures are confined to the central New Georgia Group of the Solomon Islands, where the young Woodlark Ridge spreading centre is currently subducting. All other Solomon arc segments with a lower geothermal gradient have apparently not yet been overprinted by younger subduction components over the past 6 Myrs. Moreover, trace element and Pb isotope data for Woodlark Ridge basalts show that subduction components from the fossil Pacific slab have locally modified the source of Woodlark Basin lavas. Altogether, our study highlights the potential of high-resolution Pb isotope measurements to distinguish between Indian and Pacific-type subduction components along the SW Pacific plate boundary. (C) 2011 Elsevier B.V. All rights reserved

Iron isotope composition of aqueous phases of a lowland environment

Environmental context Iron (Fe) isotope analysis is a powerful tool to understand the transport of Fe within and from soils to rivers. We determined Fe isotopes and Fe concentrations of soil solutions at different depths and found that the Fe isotope compositions are modified owing to adsorption onto Fe oxides, especially in the subsoil. Hence Fe-rich capillary rising groundwater or seeping Fe-rich surface water are depleted in Fe and potentially other metals in Fe oxide-rich soil horizons. Abstract The mobility of iron (Fe) in soils is strongly affected by redox conditions, which also affect Fe input into groundwater and rivers. Stable Fe isotope analyses allow further investigation of Fe translocation processes within, into and out of soils. Soil solutions taken from a Gleysol in a lowland area (NW Germany) at different depths revealed that Fe concentration and isotope ratios strongly varied with abundance of solid Fe oxides. Low Fe-56 values of -1.7 parts per thousand and minimum Fe concentrations of similar to 0.2mgL(-1) were recorded in soil solutions of Fe-rich horizons. Soil solutions of a Fe-poor horizon, however, yielded higher Fe-56 values (-0.39 parts per thousand) and Fe concentrations of up to 68mgL(-1). The water of an adjacent drainage ditch featured Fe-56 values of -1.1 parts per thousand, in strong contrast to +0.60 parts per thousand of short-range ordered Fe oxide deposits in the ditch bed. We attribute the coupled low Fe-56 values and Fe concentrations to combined adsorption and atom exchange between dissolved Fe and Fe oxides. Consequently Fe oxide-poor horizons had higher Fe-56 values and dissolved Fe concentrations. Outflow of Fe-rich groundwater and surface water during rainfall into rivers is responsible for high Fe-56 for Fe-oxide precipitates and low riverine Fe-56 values

Redox-dependent fractionation of iron isotopes in suspensions of a groundwater-influenced soil

Redox conditions control the release of iron (Fe) into soil pore waters. A fluctuating groundwater table in soils results in significant changes in redox conditions with both time and depth. The effects of short-term differences in redox conditions on the stable isotope inventory of dissolved Fe in such soils have not yet been studied. Bulk Fe isotope compositions of a Gleysol yielded delta Fe-57 values from + 0.3 parts per thousand (humic topsoil, Ah horizon) to -0.2 parts per thousand (Fe-enriched subsoil, CrBg horizon). In microcosm experiments, soil suspensions of the Ah and CrBg horizons were subjected to controlled redox conditions ranging from high redox potential (E-H) (>430 mV, pH 5.1 to 5.6), moderate E-H (similar to 330 mV, pH 4.9 to 5.9), to low E-H (<= 170 mV, pH 5.2 to 6.7). Membrane-filtered (0.45 mu m) solutions taken from the suspensions were analysed for their Fe concentrations (Fe2+, and total Fe: Fe-tot) and isotopic compositions. The microcosm experiments demonstrated that the ferrihydrite-and organic-rich Ah horizon is a highly dynamic and rapidly responding reservoir with respect to Fe mobilization and isotopic fractionation at low E-H. Iron concentrations and isotope ratios of the solutions from the Ah horizon varied depending on E-H with negative delta Fe-57 values (-0.4 parts per thousand) and Fe-tot (-1.6 mg L-1) at moderate E-H, and even lower delta Fe-57 values (-1.1 parts per thousand) but high Fe-tot (similar to 7.8 mg L-1) at low E-H. At high E-H, delta Fe-57 values slowly increased from + 0.3 parts per thousand to + 1.0 parts per thousand and Fetot decreased to similar to 0.2 mg L-1 within six weeks. The goethite-rich CrBg horizon constitutes a stable redox-insensitive pool with very low amounts of mobilized Fe and a small degree of isotopic fractionation, even after exposure to low E-H over several weeks. In a natural open system, removal of Fe from the dynamic Ah horizon will result in progressively higher soil delta Fe-57 values due to preferential release of Fe-54. Vertical movement of a low-delta Fe-57 solution from the topsoil may result with time in the formation of a subsoil with delta Fe-57 values that are lower than the topsoil after repeated low and high E-H cycles. At high E-H, Fe mobility in the Ah horizon is much lowered, but release of Fe with high delta Fe-57 values is in agreement with earlier studies for the formation of Fe pools (Fe oxides, colloids, organic complexes) with high delta Fe-57 signatures. (C) 2014 Elsevier B.V. All rights reserved

Geochemical evolution of the Rabaul volcanic complex, Papua New Guinea - Insights from HFSE, Sr-Nd-Hf, and Fe isotopes

The Rabaul volcanic complex, Papua New Guinea, is among the few calderas worldwide with ongoing volcanic activity. Its volcanic lithologies vary from mafic lavas from outer caldera cones to differentiated lavas within. In this study, representative lavas have been analysed for their major and trace element concentrations and radiogenic Sr-Nd-Hf isotope compositions to study the geochemical evolution of the magmatic system. Stable Fe isotopes and concentrations of high field strength elements (HFSE) complement the analyses as novel tools to assess the effect of high-temperature fractional crystallisation during ongoing differentiation. Major element systematics reveal a typical fractional crystallisation sequence of olivine, pyroxene, and plagioclase as the critical process controlling the magmatic evolution. A distinct increase of Zr/Hf from the basaltic (older) outer caldera lavas (similar to 39) to the dacitic (younger) inner caldera lavas (similar to 41-44) can be explained by fractionation of clinopyroxene and amphibole. Ratios of Nb/Ta tend to decrease with an increasing degree of differentiation, consistent with fractional crystallisation of amphibole but not clinopyroxene. Additional fractionation of Ti-magnetite and rather oxidising conditions are further supported by the Fe isotope compositions in the inner caldera lavas (delta Fe-57 from +0.03 to +0.22 parts per thousand, +/- 0.04 parts per thousand, 2 SD). The high Nb/Ta in more primitive outer caldera samples are coupled with increasing rare earth elements (REE) abundances, and slab melt-like subduction components can explain high Sr/Y and Gd-N/Yb-N in the magma sources. Complementary enrichments in fluid-mobile trace elements indicate that slab dehydration controlled the sub-arc enrichment of the inner caldera volcanism. Coupled Hf-Nd isotope compositions reveal the presence of the Indian-Australian mantle domain beneath Rabaul and a temporal trend towards sediment melt components overprinting inner caldera lavas. In conclusion, geochemical features show a temporal evolution controlled by (i) variable influence of partial slab melts vs slab fluids and (ii) a change in fractional crystallisation patterns from solely olivine and pyroxene-controlled to increasingly titano-magnetite and amphibole-controlled fractionation