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Water Research
15 August 2018
An upscaled rate law for mineral dissolution in heterogeneous media: The role of time and length scales
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Hang Wen, Li Li An upscaled rate law is developed for mineral dissolution in heterogeneous media under variable residence time (flow velocity) and length scale conditions, expanding the previous rate law under one flow and length condition (Wen and Li, 2017). A total of 640 Monte-Carlo numerical experiments were carried out with magnesite dissolution within quartz matrix in spatially heterogeneous media characterized by permeability variance (
15 August 2018
Constraining compositional proxies for Earths accretion and core formation through high pressure and high temperature Zn and S metal-silicate partitioning
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Brandon Mahan, Julien Siebert, Ingrid Blanchard, Stephan Borensztajn, James Badro, Fr
15 August 2018
Concentration and isotopic composition of dissolved Pb in surface waters of the modern global ocean
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Paulina Pinedo-Gonz
15 August 2018
The potential of near-entrance stalagmites as high-resolution terrestrial paleoclimate proxies: Application of isotope and trace-element geochemistry to seasonally-resolved chronology
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Peter E. Carlson, Nathaniel R. Miller, Jay L. Banner, Daniel O. Breecker, Richard C. Casteel Sub-annually resolved environmental proxies can be valuable archives of climate change, but they are rare in terrestrial settings, and it can be difficult to verify their annual nature. We suggest that speleothems that grow in well-ventilated zones of caves may preserve such high-resolution records. Near-entrance cave environments are characterized by year-round, near-atmospheric CO2 concentrations and are significantly influenced by surface air temperature fluctuations, particularly in temperate latitudes. Previous monitoring studies of a well-ventilated, temperate-latitude cave (Westcave Preserve, central Texas) have documented seasonal variations in the oxygen isotope composition of calcite grown on glass substrates (with winter
15 August 2018
A simple role of coral-algal symbiosis in coral calcification based on multiple geochemical tracers
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Mayuri Inoue, Takashi Nakamura, Yasuaki Tanaka, Atsushi Suzuki, Yusuke Yokoyama, Hodaka Kawahata, Kazuhiko Sakai, Nikolaus Gussone Light-enhanced calcification of reef-building corals, which eventually create vast coral reefs, is well known and based on coral-algal symbiosis. Several controversial hypotheses have been proposed as possible mechanisms for connecting symbiont photosynthesis and coral calcification, including pH rise in the internal pool, role of organic matrix secretion, and enzyme activities. Here, based on the skeletal chemical and isotopic compositions of symbiotic and asymbiotic primary polyps of Acropora digitifera corals, we show a simple pH increase in the calcification medium as the predominant contribution of symbionts to calcification of host corals. We used the symbiotic and asymbiotic primary polyps reared for 10
15 August 2018
Contrasting evolution of iron phase composition in soils exposed to redox fluctuations
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Pauline Winkler, Klaus Kaiser, Aaron Thompson, Karsten Kalbitz, Sabine Fiedler, Reinhold Jahn Ferric iron (FeIII) solid phases serve many functions in soils and sediments, which include providing sorption sites for soil organic matter, nutrients, and pollutants. The reactivity of Fe solid phases depends on the mineral structure, including the overall crystallinity. In redox-active soils and sediments, repeated reductive dissolution with subsequent exposure to aqueous ferrous iron (Fe2+) and oxidative re-precipitation can alter Fe phase crystallinity and reactivity. However, the trajectory of Fe mineral transformation under redox fluctuations is unclear and has been reported to result in both increases and decreases in Fe phase crystallinity. Several factors such as water budget, organic matter input, redox dynamics as well as the initial Fe phase composition might play a role. The objective of our study was to examine if Fe minerals in soils that differ in porosity-dependent water leaching rate and initial Fe phase crystallinity, demonstrate distinct mineral transformations when subjected to redox fluctuations. We sampled paired plots of two soil types under similar management but with different water leaching rates and contrasting Fe oxide crystallinity an Alisol rich in crystalline Fe phases and an Andosol rich in short-range-ordered (SRO) Fe phases. The two soils were either exposed to several decades of redox fluctuations during rice paddy cultivation (paddy) or to predominantly oxic conditions in neighboring vegetable gardens (non-paddy). Paddy soils are uniquely suited for this type of study because they are regularly submerged and develop regular redox fluctuations. We also incubated the non-paddy soils in the laboratory for one year through eight anoxic/oxic cycles and monitored the aqueous soil geochemistry. M
15 August 2018
Carbon and oxygen isotope fractionation in the water-calcite-aragonite system
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Jens Fohlmeister, Jennifer Arps, Christoph Sp
15 August 2018
Timescales of collisional metamorphism from Sm-Nd, Lu-Hf and U-Pb thermochronology: A case from the Proterozoic Putumayo Orogen of Amazonia
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Mauricio Ibanez-Mejia, Elias M. Bloch, Jeff D. Vervoort The construction of mountain belts by continent-continent collision is a fundamental phase of the supercontinent cycle and an inevitable consequence of subduction-driven plate tectonics on Earth. Yet, quantitative estimation of the rate and duration of prograde orogenic metamorphism in high-grade collisional belts has proven elusive to constrain. Differences in the chemical partitioning and diffusive behavior of elements associated with the 147Sm-143Nd and 176Lu-176Hf isotopic systems in metamorphic garnet can provide valuable insights on the time scales over which this mineral grows and cools in regionally metamorphosed terranes. Here, we present new Sm-Nd and Lu-Hf isotopic results from multiple garnet and whole-rock aliquots from a granulite-grade metasedimentary rock from the Proterozoic Putumayo Orogen of South America. Using a combination of internal isochron dating by these two decay systems, combined with apatite U-Pb dating and pressure-temperature-time (P-T-t) constraints from mineral equilibria and chemical diffusion geospeedometry, we provide first-order insights into the time scales of a Proterozoic continent-continent collisional metamorphic event in Amazonia associated with its incorporation to the Supercontinent Rodinia. Our results indicate that these metasediments attained peak granulite-grade metamorphic conditions of nearly 0.62
15 August 2018
An isotopic, elemental and structural study of silicon nitride from enstatite chondrites
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): J. Leitner, C. Vollmer, T. Henkel, U. Ott, P. Hoppe We report an in-situ investigation of silicon nitride (Si3N4) grains from several enstatite chondrites of low petrologic types (EL3, EH3, and EH4). The grains occurred in various host phases, including Fe,Ni metal, schreibersite, sulfides, and also in the silicate fraction, and are of Solar System origin. Energy-dispersive X-ray spectroscopy (EDS) showed that carbon and oxygen are present in all investigated grains. Carbon- and N-isotopic compositions of 288 grains were measured by NanoSIMS. Nitrogen is isotopically light compared to terrestrial air, with an average
15 August 2018
Hydrogen isotope fractionation in the system brucite-water
15 August 2018
A unified Craig-Gordon isotope model of stable hydrogen and oxygen isotope fractionation during fresh or saltwater evaporation
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Roberto Gonfiantini, Leonard I. Wassenaar, Luis Araguas-Araguas, Pradeep K. Aggarwal Evaporation of water from the oceans and terrestrial environment governs the global water cycle and climate. Heavy isotope (2H,18O,17O) enrichment during evaporation led to the development of the well-known Craig-Gordon (C-G) model for quantifying evaporation. Several variables control the H and O isotopic composition of evaporating water including; the isotopic composition of the water, temperature, relative humidity, ambient vapor isotopic composition, diffusion and/or mixing at the water-air interface, and the thermodynamic activity (salinity) of water. Previous C-G modeling efforts considered these controlling variables separately; here we propose a newly unified C-G analytical model that allows for simultaneous quantification of all controlling variables in the evaporation of fresh and saline waters. Our unified model accurately predicted the results of laboratory water evaporation experiments conducted under a variety of molecular diffusion and turbulent diffusion conditions. We demonstrate the general applicability of the model by successfully predicting historical O and H isotope data for fresh and saltwater evaporation obtained from the scientific literature. The unified C-G model allows for improved estimates environmental parameters controlling the H and O isotope fractionation during water evaporation under natural conditions and can be used to better inform modeling efforts in regional and large-scale water balance studies.
15 August 2018
Synthetic fluid inclusions XXI. Partitioning of Na and K between liquid and vapor in the H2O-NaCl-KCl system at 600800
15 August 2018
Mg isotope response to dolomitization in hinterland-attached carbonate platforms: Outlook of
15 August 2018
Molecular simulations of hydrated phyllomanganates
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Aric G. Newton, Kideok D. Kwon Hydrated phyllomanganates are layered Mn-oxide minerals with interlayers that can possess variable water contents and react strongly with trace metals due to octahedral vacancies in the layer. The unique properties of phyllomanganates afford them a significant role in many environmental phenomena that affect soil and water composition mainly via cation exchange and adsorption of trace metals. Slight variations in the structural and chemical composition often result in a dramatic difference in the chemical reactivity of the minerals. Molecular simulations at the classical mechanical level of theory, which uses a simplified description of the interatomic potential energies, can provide an atomistic perspective of the relationship between the chemical composition and the bulk and interlayer structures. We introduce a set of interatomic potentials for hydrated phyllomanganates with variable vacancy and Mn3+ content and report the classical mechanical simulation results performed at standard temperature and pressure. The potentials we introduce provide not only a reasonable reproduction of the experimentally determined atomic structures of the chalcophanite group, but also new insights on similar phyllomanganate minerals with hexagonal symmetry and a range of vacancy contents. When a vacancy is protonated, Mn3+ is unstable in the hexagonal birnessite layer and occupies the interlayer as a cap on the associated vacancy. When a vacancy was charge-balanced by Mn3+ and Na+, considerable amounts of Mn3+ were incorporated into the hexagonal birnessite layer, but only at total Mn(III) contents greater than
15 August 2018
Theoretical calibration of the triple oxygen isotope thermometer
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Justin Hayles, Caihong Gao, Xiaobin Cao, Yun Liu, Huiming Bao The field of isotope geochemistry began with the study of oxygen isotope geothermometry, most notably for carbonates. For traditional oxygen isotope geothermometry only the relationship between one rare isotope, oxygen-18, and the common isotope, oxygen-16, is used because for most terrestrial processes the 17O-16O relationship scales with the 18O-16O relationship and is thought to not grant any new information. However, theoretical analysis predicts a small temperature-dependence of the equilibrium triple oxygen isotope relationship and instrumentation and techniques now allow for high-precision determination of the oxygen isotope composition for all three oxygen isotopes for a variety of sample types. To set the groundwork for triple oxygen isotope geothermometry, here we present new calibrations based on statistical thermodynamics and density functional theory for both the traditional two isotope and the recently introduced triple isotope thermometer for pairs of quartz, calcite, dolomite, fluorapatite, hematite, magnetite and liquid water. The results compare well with previous studies on 18O/16O fractionation where theoretical and experimental data are available. Of the models given here, pairs of quartz, calcite, dolomite and fluorapatite with water, hematite or magnetite show promising temperature sensitivities as triple isotope thermometers with acceptable uncertainties for surface and low-T hydrothermal environments.
15 August 2018
Effects of elevated CO2 on shell 13C and 18O content and growth rates in the clam Scapharca broughtonii
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Kozue Nishida, Masahiro Hayashi, Yuzo Yamamoto, Takahiro Irie, Yusuke Watanabe, Chiho Kishida, Yukihiro Nojiri, Mizuho Sato, Toyoho Ishimura, Atsushi Suzuki The effects of elevated CO2 levels on growth and stable carbon and oxygen isotope compositions (
15 August 2018
Contrasting meteoritic signatures within the Clearwater East and Clearwater West impact structures: The view from osmium isotopes
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): R. Terik Daly, Peter H. Schultz, John C. Lassiter, Staci W. Loewy, Lucy M. Thompson, John G. Spray Osmium isotopes provide a powerful tool for identifying meteoritic signatures in impactites. We apply the osmium isotope method to impact melt and country rocks from the Clearwater East and Clearwater West craters located in Quebec, Canada. Impact melts from Clearwater East have 187Os/188Os ratios of 0.12810.1285. These values indicate a significant meteoritic component, which exceeds that in impact melts at all terrestrial craters studied to date, except Morokweng. Such findings align with earlier results from chromium isotopes and platinum-group elements. In contrast, impact melts from Clearwater West have 187Os/188Os ratios between 6.604 and 59.12. These highly radiogenic ratios are indistinguishable from the 187Os/188Os ratios in country rocks. Hence, osmium isotopes provide no evidence for a meteoritic component in impact melts at Clearwater West. The Clearwater craters formed in almost identical targets. Therefore, target effects cannot readily explain the stark difference between the two Clearwater craters. If melt sheet heterogeneity is similar at the two craters, the probability that melts at Clearwater West host an undetected chondritic component is
15 August 2018
Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Mark A. Kendrick, Marco Scambelluri, J
15 August 2018
Stable isotope records across the Cretaceous-Paleogene transition, Stevns Klint, Denmark: New insights from the chromium isotope system
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Geoffrey J. Gilleaudeau, Andrea R. Voegelin, Nicolas Thibault, Julien Moreau, Clemens V. Ullmann, Robert M. Klaebe, Christoph Korte, Robert Frei Late Cretaceous to Early Paleogene time represents a critical transitional period in Earth history. Global events include short-term instability in climate and ocean circulation, large igneous province emplacement, and catastrophic extinction of marine and terrestrial fauna due at least in part to a bolide impact event. The response of the ocean system to Cretaceous-Paleogene (K-Pg) global events has been the subject of much research, yet fundamental questions remain regarding carbon cycling, climate, ocean mixing, and redox conditions. To help elucidate paleoceanographic changes across the K-Pg transition, we revisit the classic Maastrichtian to Danian chalk-limestone succession at Stevns Klint, Denmark with a suite of new geochemical data, including insights from the redox-sensitive chromium isotope system. We use carbon and oxygen isotope trends to form a well-constrained framework for the succession, before presenting strontium isotope, chromium isotope, and rare-earth element data for the units deposited both before and after the K-Pg event. Carbon isotopes show only minor perturbation across the K-Pg boundary and oxygen isotopes record a prominent positive excursion, with both of these trends standing in contrast to many K-Pg boundary sections worldwide. Strontium isotopes record a spike towards more radiogenic values, which is consistent with a globally-observed, short-term weathering pulse. The chromium isotope profile of Maastrichtian chalk begins with strongly positive values followed by a sharp negative excursion associated with a facies shift and initiation of bottom currents in the Chalk Sea.
15 August 2018
Echinoid skeletal carbonate as archive of past seawater magnesium isotope signatures Potential and limitations
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Sylvia Riechelmann, Vasileios Mavromatis, Dieter Buhl, Martin Dietzel, Ren
15 August 2018
Equilibrium lithium isotope fractionation in Li-bearing minerals
Publication date: 15 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 235 Author(s): Shanqi Liu, Yongbing Li, Jie Liu, Yiwen Ju, Jianming Liu, Zhiming Yang, Yaolin Shi Lithium isotopes are important geochemical tracers for many geological processes. Knowledge of Li isotope fractionation factors is essential for understanding the behavior of Li isotopes. On the basis of density functional perturbation theory (DFPT), we calculate Li isotope fractionation parameters of
1 August 2018
Extensive CO2 degassing in the upper mantle beneath oceanic basaltic volcanoes: First insights from Piton de la Fournaise volcano (La R
1 August 2018
Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite: Implications for melt/fluid-rock reactions at Moho level beneath oceanic spreading centers
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): Mathieu Rospab
1 August 2018
Distinct chlorine isotopic reservoirs on Mars. Implications for character, extent and relative timing of crustal interactions with mantle-derived magmas, evolution of the martian atmosphere, and the building blocks of an early Mars
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): C.K. Shearer, S. Messenger, Z.D. Sharp, P.V. Burger, A.N. Nguyen, F.M. McCubbin The
1 August 2018
109Ag107Ag fractionation in fluids with applications to ore deposits, archeometry, and cosmochemistry
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): Toshiyuki Fujii, Francis Albarede Evidence of 109Ag/107Ag variability in ancient silver coins led us to calculate the reduced partition functions for 107Ag and 109Ag in various dissolved Ag species by ab initio methods in order to evaluate the extent of Ag fractionation in fluids and the potential of Ag isotopes to discriminate between different metal sources. We used a hybrid density functional implemented by the Gaussian 09 code and consisting of Bickleys three-parameter non-local hybrid exchange potential with Lee-Yang-Parr non-local functionals. The ratios ln
1 August 2018
Crystallization of the lunar magma ocean and the primordial mantle-crust differentiation of the Moon
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): Bernard Charlier, Timothy L. Grove, Olivier Namur, Francois Holtz We present crystallization experiments on silicate melt compositions related to the lunar magma ocean (LMO) and its evolution with cooling. Our approach aims at constraining the primordial internal differentiation of the Moon into mantle and crust. We used graphite capsules in piston cylinder (1.350.80 GPa) and internally-heated pressure vessels (<0.50 GPa), over 15801020
1 August 2018
Scale effect on the time dependence of mineral dissolution rates in physically heterogeneous porous media
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): Heewon Jung, Alexis Navarre-Sitchler Hydrologic heterogeneity complicates the time dependent behavior of mineral dissolution rates and introduces large uncertainty in effective surface area estimation. In order to resolve the coupled evolution of mineral dissolution rates and mineral surface area over time in various fluid flow conditions, this study presents multiple reactive transport simulations of anorthite dissolution over 3000
1 August 2018
Calibration and application of silica-water triple oxygen isotope thermometry to geothermal systems in Iceland and Chile
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): Jordan A.G. Wostbrock, Zachary D. Sharp, Camilo Sanchez-Yanez, Martin Reich, Daniela B. van den Heuvel, Liane G. Benning Triple oxygen isotope analyses were made on geothermal fluids and precipitates from Chile and Iceland to calibrate the silica-water isotopic fractionation for abiotic silica formation at elevated temperatures and were used to evaluate potential fractionation effects of biogenic vs. abiogenic samples and polymorphism. Coexisting water and amorphous silica precipitated inside the heat exchanger of the Hellishei
1 August 2018
Trace element mobilization during incipient bioweathering of four rock types
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): Carmen I. Burghelea, Katerina Dontsova, Dragos G. Zaharescu, Raina M. Maier, Travis Huxman, Mary K. Amistadi, Edward Hunt, Jon Chorover Lithogenic trace element (TE) patterns of distribution, fate, and behavior in soils are influenced by plants and microorganisms. Our controlled mesocosm experiments quantified how incipient weathering of mineral-bound TEs (Be, V, Sr, Ba, Cr, Co, Ni, Mo, Cu, Zn, As, Se, Ag, Cd, Sn, Sb, Tl, and Pb) varies across four porous mineral substrates (basalt, rhyolite, granite, and schist), in the presence of buffalo grass (Bouteloua dactyloides), associated bacteria, and arbuscular mycorrhizal fungi (AM), a common plant symbiont. Particular focus was given to the net transfer of elements between the solid and solution phases, including chemical denudation (loss of element from the rock to the solution), plant TE uptake, and total mobilization (sum of denudation loss and uptake into plant biomass). Results revealed differences in TE denudation among rocks, basalt having the highest loss and schist the lowest. TE leaching in solution was time-dependent and it was likely influenced by variations in pH and DOC that were rock- and treatment-specific. The element with the highest rock-normalized release to the solution and highest enrichment in plant biomass was Mo across all rocks. Plants decreased denudation loss compared to abiotic controls for a large number of TEs in all substrates due to plant uptake, but for some elements increase in weathering due to plant activity resulted in increased denudation. Differences in TE patterns of behavior could be related to their Goldschmidt groups. Plant uptake was controlled by TE availability in solution, as well as plant physiological requirements. Plants and associated microbiota significantly enhanced mobilization for the majority of TEs across all rocks. Mycorrhiza significantly increased above-ground plant biomass production in rhyolite and concentrations in plant tissues for a high number of TEs in basalt. TE uptake into biomass was positively correlated with percent mycorrhizal infection, particularly in basalt and rhyolite. Mycorrhizal fungi also influenced TE denudation, rock-water fractionation, and total mobilization according to the rock type. Mycorrhizal activity was associated with a significant decrease in pH and increase in DOC fluxes in schist, supporting the idea that fungi enhance production of root exudates especially in substrates that are difficult to weather. Our results highlight the importance of incipient weathering at the plant-rock interface for patterns of TE cycling. They also indicate the importance of mycorrhiza in mineral dissolution, TE denudation, plant element uptake, and biomass growth.
15 July 2018
Electron backscatter diffraction (EBSD) study of seven heavily metamorphosed chondrites: Deformation systematics and variations in pre-shock temperature and post-shock annealing
Publication date: 1 August 2018
Source:Geochimica et Cosmochimica Acta, Volume 234 Author(s): Alex M. Ruzicka, Richard C. Hugo We used electron backscatter diffraction (EBSD) methods to study the crystallography of olivine and other minerals in seven heavily metamorphosed (petrographic type 6 or 6/7) but variably shocked ordinary chondrites from the H (Kernouv
15 July 2018
Sulfur isotope signatures of eucrites and diogenites
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Nanping Wu, James Farquhar, James W. Dottin, Nivea Magalh
15 July 2018
Kinetic D/H fractionation during hydration and dehydration of silicate glasses, melts and nominally anhydrous minerals
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): M. Roskosz, E. Deloule, J. Ingrin, C. Depecker, D. Laporte, S. Merkel, L. Remusat, H. Leroux The distribution of hydrogen isotopes during diffusion-driven aqueous processes in silicate glasses, melts and crystals was investigated. Hydration/dehydration experiments were performed on silica glasses at 1000
15 July 2018
Barium isotopic composition of the upper continental crust
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Xiao-Yun Nan, Hui-Min Yu, Roberta L. Rudnick, Richard M. Gaschnig, Juan Xu, Wang-Ye Li, Qun Zhang, Zhang-Dong Jin, Xian-Hua Li, Fang Huang The upper continental crust (UCC) is an important reservoir of Ba within the Earth. We report high precision (
15 July 2018
The precipitation of gypsum, celestine, and barite and coprecipitation of radium during seawater evaporation
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Yoav Oved Rosenberg, Ziv Sade, Jiwchar Ganor While the precipitation paths of the major ions (i.e., Ca2+, Na+, K+, Cl
15 July 2018
Cadmium cycling in the water column of the Kuroshio-Oyashio Extension region: Insights from dissolved and particulate isotopic composition
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Shun-Chung Yang, Jing Zhang, Yoshiki Sohrin, Tung-Yuan Ho We measured dissolved and particulate Cd isotopic composition in the water column of a meridional transect across the Kuroshio-Oyashio Extension region in a Japanese GEOTRACES cruise to investigate the relative influence of physical and biogeochemical processes on Cd cycling in the Northwestern Pacific Ocean. Located at 3050N along 165E, the transect across the extension region possesses dramatic hydrographic contrast. Cold surface water and a relatively narrow and shallow thermocline characterizes the Oyashio Extension region in contrast to a relatively warm and highly stratified surface water and thermocline in the Kuroshio Extension region. The contrasting hydrographic distinction at the study site provides us with an ideal platform to investigate the spatial variations of Cd isotope fractionation systems in the ocean. Particulate samples demonstrated biologically preferential uptake of light Cd isotopes, and the fractionation effect varied dramatically in the surface water of the two regions, with relatively large fractionation factors in the Oyashio region. Based on the relationship of dissolved Cd concentrations and isotopic composition, we found that a closed system fractionation model can reasonably explain the relationship in the Kuroshio region. However, using dissolved Cd isotopic data, either a closed system or steady-state open system fractionation model may explain the relationship in the surface water of the Oyashio region. Particulate
15 July 2018
Dissolved and colloidal copper in the tropical South Pacific
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Saeed Roshan, Jingfeng Wu Copper (Cu) as a bioactive trace metal in the ocean has widely been studied in the context of chemical speciation. However, this trace metal is extremely understudied in the context of physical speciation (i.e., size- or molecular weight-partitioning), which may help in characterizing dissolved Cu species. In this study, we determine total dissolved Cu (<0.2
15 July 2018
Diffusive exchange of trace elements between alkaline melts: Implications for element fractionation and timescale estimations during magma mixing
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Diego Gonz
15 July 2018
Effects of particle composition on thorium scavenging in the North Atlantic
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Paul Lerner, Olivier Marchal, Phoebe J. Lam, Andrew Solow The dependence of thorium scavenging by particles on particle composition is examined at selected stations of the U.S. GEOTRACES North Atlantic Section (GA03). Scavenging is here described by the apparent, first-order rate constant of Th adsorption onto particles ( k 1 ), as estimated from an inversion of Th radioisotope and radioactive parent data. Our k 1 estimates are regressed against particle phase data using two different models. Model I considers biogenic particles (POC
15 July 2018
A new type of oxidized and pre-irradiated micrometeorite
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Carole Cordier, Bastian Baecker, Ulrich Ott, Luigi Folco, Mario Trieloff This paper investigates the mineralogy and noble gas composition of a unique micrometeorite from the Transantarctic Mountains, #45c.29. The magnetite rim and the particle interior with olivine, pyroxene and magnetite relict grains (30250
15 July 2018
Effects of atmospheric composition on apparent activation energy of silicate weathering: I. Model formulation
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Yoshiki Kanzaki, Takashi Murakami We have developed a weathering model to comprehensively understand the determining factors of the apparent activation energy of silicate weathering in order to better estimate the silicate-weathering flux in the Precambrian. The model formulates the reaction rate of a mineral as a basis, then the elemental loss by summing the reaction rates of whole minerals, and finally the weathering flux from a given weathering profile by integrating the elemental losses along the depth of the profile. The rate expressions are formulated with physicochemical parameters relevant to weathering, including solution and atmospheric compositions. The apparent activation energies of silicate weathering are then represented by the temperature dependences of the physicochemical parameters based on the rate expressions. It was found that the interactions between individual mineral-reactions and the compositions of solution and atmosphere are necessarily accompanied by those of temperature-dependence counterparts. Indeed, the model calculates the apparent activation energy of silicate weathering as a function of the temperature dependence of atmospheric CO2 (
1 July 2018
Comments on The impact pseudotachylitic breccia controversy: Insights from first isotope analysis of Vredefort impact-generated melt rocks by Reimold et al. 2017 (GCA 214, 266282)
Publication date: 15 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 233 Author(s): Adam A. Garde, Martin B. Klausen
1 July 2018
Simulating Donnan equilibria based on the Nernst-Planck equation
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): Thomas Gimmi, Peter Alt-Epping Understanding ion transport through clays and clay membranes is important for many geochemical and environmental applications. Ion transport is affected by electrostatic forces exerted by charged clay surfaces. Anions are partly excluded from pore water near these surfaces, whereas cations are enriched. Such effects can be modeled by the Donnan approach. Here we introduce a new, comparatively simple way to represent Donnan equilibria in transport simulations. We include charged surfaces as immobile ions in the balance equation and calculate coupled transport of all components, including the immobile charges, with the Nernst-Planck equation. This results in an additional diffusion potential that influences ion transport, leading to Donnan ion distributions while maintaining local charge balance. The validity of our new approach was demonstrated by comparing Nernst-Planck simulations using the reactive transport code Flotran with analytical solutions available for simple Donnan systems. Attention has to be paid to the numerical evaluation of the electrochemical migration term in the Nernst-Planck equation to obtain correct results for asymmetric electrolytes. Sensitivity simulations demonstrate the influence of various Donnan model parameters on simulated anion accessible porosities. It is furthermore shown that the salt diffusion coefficient in a Donnan pore depends on local concentrations, in contrast to the aqueous salt diffusion coefficient. Our approach can be easily implemented into other transport codes. It is versatile and facilitates, for instance, assessing the implications of different activity models for the Donnan porosity.

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1 July 2018
Trace element diffusion and kinetic fractionation in wet rhyolitic melt
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): Megan E. Holycross, E. Bruce Watson Piston-cylinder experiments were run to determine the chemical diffusivities of 21 trace elements (Sc, V, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Th and U) in hydrous rhyolitic melts at 1
1 July 2018
pH-dependence of production of oxidants (Cu(III) and/or HO) by copper-catalyzed decomposition of hydrogen peroxide under conditions typical of natural saline waters
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): Guowei Xing, A. Ninh Pham, Christopher J. Miller, T. David Waite The oxidation and reduction kinetics of copper in natural aquatic systems determines its prevailing redox state, with implication to its toxicity, bioavailability and solubility. When reduced copper(I) co-occurs with ubiquitous hydrogen peroxide (H2O2), their reaction forms an oxidizing intermediate able to affect redox transformation of copper, other transition metals and organic carbon. In this work the kinetics and mechanism of reactions of nanomolar concentrations of copper (Cu) with H2O2 in bicarbonate-buffered 0.7
1 July 2018
O, Mg, and Si isotope distributions in the complex ultrarefractory CAI Efremovka 101.1: Assimilation of ultrarefractory, FUN, and regular CAI precursors
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): J
1 July 2018
Evolution of atmospheric xenon and other noble gases inferred from Archean to Paleoproterozoic rocks
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): G. Avice, B. Marty, R. Burgess, A. Hofmann, P. Philippot, K. Zahnle, D. Zakharov We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3
1 July 2018
Effect of silicon on activity coefficients of siderophile elements (Au, Pd, Pt, P, Ga, Cu, Zn, and Pb) in liquid Fe: Roles of core formation, late sulfide matte, and late veneer in shaping terrestrial mantle geochemistry
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): K. Righter, K. Pando, M. Humayun, N. Waeselmann, S. Yang, A. Boujibar, L.R. Danielson Earths core contains
1 July 2018
Barite recrystallization in the presence of 226Ra and 133Ba
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): Frank Heberling, Volker Metz, Melanie B
1 July 2018
The influence of magmatic fluids and phase separation on B systematics in submarine hydrothermal vent fluids from back-arc basins
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): Frederike K. Wilckens, Eoghan P. Reeves, Wolfgang Bach, Anette Meixner, Jeffrey S. Seewald, Andrea Koschinsky, Simone A. Kasemann The composition of submarine hydrothermal vent fluids is affected by a variety of processes, such as interaction of heated seawater with rocks and sediments, addition of magmatic fluids, as well as phase separation and segregation. How these processes specifically affect the vent fluid composition is still poorly understood. In particular, the relative role of phase separation and magmatic degassing, which is common in arc/back-arc hydrothermal systems, is not well known. To provide new insights into these processes, we analysed B contents and isotope ratios in hydrothermal vent fluids and volcanic rocks from the Manus Basin, Papua New Guinea, and Nifonea volcano, New Hebrides back-arc. These fluids show a range of salinities, gas contents, acidities, and host rock compositions; many of them are influenced by phase separation and by addition of magmatic volatiles (both CO2 and SO2). Previous studies of hydrothermal vents in arc/back-arc settings suggest that B contents and isotopic composition of vent fluids are controlled by interactions between seawater, basement and sediments, and propose that phase separation and magmatic fluids play only a subordinate role. In our study, we demonstrate that vent fluids with minor magmatic input indeed reflect the interaction between seawater and oceanic crust. In contrast, the low-salinity Nifonea fluids and some of the acid-sulphate fluids from the Manus Basin have higher B contents as expected, whereas other volatile-rich fluids from the Manus Basin show B depletions. The lack of correlation between B contents and the intensity of magmatic fluid influx (CO2 and SO2) may indicate that magma degassing is not responsible for the B enrichments or depletions in these vent fluids. B enrichments might be related to preferential partitioning of B into the vapour phase during phase separation under PT-conditions well above the two-phase curve and critical line (i.e. T

Reaction-induced porosity fingering: Replacement dynamic and porosity evolution in the KBr-KCl system
Publication date: 1 July 2018
Source:Geochimica et Cosmochimica Acta, Volume 232 Author(s): Nicolas Beaudoin, Andrea Hamilton, Daniel Koehn, Zoe Kai Shipton, Ulrich Kelka In this contribution, we use X-ray computed micro-tomography (X-CT) to observe and quantify dynamic pattern and porosity formation in a fluid-mediated replacement reaction. The evolution of connected porosity distribution helps to understand how fluid can migrate through a transforming rock, for example during dolomitization, a phenomenon extensively reported in sedimentary basins. Two types of experiment were carried out, in both cases a single crystal of KBr was immersed in a static bath of saturated aqueous KCl at room temperature and atmospheric pressure, and in both cases the replacement process was monitored in 3D using X-CT. In the first type of experiment a crystal of KBr was taken out, scanned, and returned to the solution in cycles (discontinuous replacement). In the second type of experiment, 3 samples of KBr were continuously reacted for 15, 55 min and 5.5 h respectively, with the latter being replaced completely (continuous replacement). X-CT of KBr-KCl replacement offers new insights into dynamic porosity development and transport mechanisms during replacement. As the reaction progresses the sample composition changes from KBr to KCl via a K(Br, Cl) solid solution series which generates porosity in the form of fingers that account for a final molar volume reduction of 13% when pure KCl is formed. These fingers form during an initial and transient advection regime followed by a diffusion dominated system, which is reflected by the reaction propagation, front morphology, and mass evolution. The porosity develops as fingers perpendicular to the sample walls, which allow a faster transport of reactant than in the rest of the crystal, before fingers coarsen and connect laterally. In the continuous experiment, finger coarsening has a dynamic behaviour consistent with fingering processes observed in nature. In the discontinuous experiment, which can be compared to rock weathering or to replacement driven by intermittent fluid contact, the pore structure changes from well-organized parallel fingers to a complex 3D connected network, shedding light on the alteration of reservoir properties during weathering.
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