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USGS1994

Re-Os isotopic evidence for an enriched-mantle source for the Noril'sk-type, ore-bearing intrusions, Siberia

Magmatic Cu-Ni sulfide ores and spatially associated ultramafic and mafic rocks from the Noril'sk I, Talnakh, and Kharaelakh intrusions are examined for Re-Os isotopic systematics. Neodymium and lead isotopic data also are reported for the ultramafic and mafic rocks. The Re-Os data for most samples indicate closed-system behavior since the ca. 250 Ma igneous crystallization age of the intrusions. There are small but significant differences in the initial osmium isotopic compositions of samples from the three intrusions. Ores from the Noril'sk I intrusion have γ Os values that vary from +0.4 to +8.8, but average +5.8. Ores from the Talnakh intrusion have γ Os values that range from +6.7 to +8.2, averaging +7.7. Ores from the Kharaelakh intrusion have γ Os values that range from +7.8 to +12.9, with an average value of +10.4. The osmium isotopic compositions of the ore samples from the Main Kharaelakh orebody exhibit minimal overlap with those for the Noril'sk I and Talnakh intrusions, indicating that these Kharaelakh ores were derived from a more radiogenic source of osmium than the other ores. Combined osmium and lead data for major orebodies in the three intrusions plot in three distinct fields, indicating derivation of osmium and lead from at least three isotopically distinct sources. Some of the variation in lead isotopic compositions may be the result of minor lower-crustal contamination. However, in contrast to most other isotopic and trace element data, Os-Pb variations are generally inconsistent with significant crustal contamination or interaction with the subcontinental lithosphere. Thus, the osmium and lead isotopic compositions of these intrusions probably reflect quite closely the compositions of their mantle source, and suggest that these two isotope systems were insensitive to lithospheric interaction. Ultramafic and mafic rocks have osmium and lead isotopic compositions that range only slightly beyond the compositions of the ores. These rocks also have relatively uniform ϵ Nd values that range only from −0.8 to + 1.1. This limited variation in neodymium isotopic composition may reflect the characteristics of the mantle sources of the rocks, or it may indicate that somehow similar proportions of crust contaminated the parental melts. The osmium, lead, and neodymium isotopic data for these rocks most closely resemble the mantle sources of certain ocean island basalts (OIB), such as some Hawaiian basalts. Hence, these data are consistent with derivation of primary melts from a mantle source similar to that of some types of hotspot activity. The long-term Re/Os enrichment of this and similar mantle sources, relative to chondritic upper mantle, may reflect 1. incorporation of recycled oceanic crust into the source more than 1 Ga ago, 2. derivation from a mantle plume that originated at the outer core-lower mantle interface, or 3. persistence of primordial stratification of rhenium and osmium in the mantle.

Siberia↗

USGS1994

The geochemical evolution of low-molecular-weight organic acids derived from the degradation of petroleum contaminants in groundwater

The geochemical evolution of low-molecular-weight organic acids in groundwater downgradient from a crude-oil spill near Bemidji, Minnesota, was studied over a five year period (1986–1990). The organic acids are metabolic intermediates of the degradation of components of the crude oil and are structurally related to hydrocarbon precursors. The concentrations of organic acids, particularly aliphatic acids, increase as the microbial alteration of hydrocarbons progresses. The organic-acid pool changes in composition and concentration over time and in space as the degradation processes shift from Fe(III) reduction to methanogenesis. Over time, the aquifer system evolves into one in which the groundwater contains more oxidized products of hydrocarbon degradation and the reduced forms of iron, manganese, and nitrogen. Laboratory microcosm experiments with aquifer material support the hypothesis that organic acids observed in the groundwater originate from the microbial degradation of aromatic hydrocarbons under anoxic conditions. The geochemistry of two other shallow aquifers in coastal plain sediments, one contaminated with creosote waste and the other with gasoline, were compared to the Bemidji site. The geochemical evolution of the low-molecular-weight organic acid pool in these systems is controlled, in part, by the presence of electron acceptors available for microbially mediated electron-transfer reactions. The depletion of electron acceptors in aquifers leads to the accumulation of aliphatic organic acids in anoxic groundwater.

Geochimica et Cosmochimica Acta↗

USGS1994

Experimental studies of alunite: I. 18 O- 16 O and D-H fractionation factors between alunite and water at 250-450°C

We have determined oxygen and hydrogen isotope fractionation factors between alunite and water over a temperature range of 250-450??C by reacting synthetic natroalunite with 0.7 m K2SO4 -0.1 to 0.65 m H2SO4 solutions to produce K-rich alunite. From 88 to 95% alkali and isotope exchange were observed in most of these experiments, and the partial equilibrium method was used to compute equilibrium fractionation factors. Least-squares fits of the data give 103 In ??alunite(so4)-H2O = 3.09 ( 106 T2 (K)) - 2.94 and 103 In ??alunite(OH)-H2O = 2.28 ( 106 T2 (K)) - 3.90. The intramineral 18O- 16O fractionation factor 103 In ??alunite(so4-OH site) is given by the expression 0.8 ( 106 T2 (K)) + 0.96. The alunite-water D-H fractionation factor ranges from -19 at 450??C to -6 at 250??C and does not appear to be strongly dependent on temperature. Runs with alkali exchange in the opposite direction were used to obtain 18O- 16O and D-H fractionation factors between natroalunite (mol% Na = 70-75) and water at 350-450??C. These indicate that mol% Na has negligible effect on the fractionation factors over this temperature range. Measured 18O-16O and D-H fractionation factors between alunite and 1.0 m KCl -0.5 m H2SO4 fluids also agree within 2?? with the values obtained from the K2SO4-H2SO4 fluids. However, experiments with alunite and distilled water at 400??C gave a value of 103 In ??alunite(SO4)-H2O of 0.0, compared with a value of 3.9 obtained at this temperature with K2SO4- and H2SO4-bearing fluids. This suggests that changes in fluid composition can affect alunite-water 18O-16O fractionation factors. Reconnaissance experiments with fine-grained natural natroalunite demonstrate that alunite-water D-H exchange can occur by hydrogen diffusion, although this process is generally not significant in the experiments with coarser grained synthetic alunites. ?? 1994.

Geochimica et Cosmochimica Acta↗

USGS1994

Experimental studies of alunite: II. Rates of alunite-water alkali and isotope exchange

Rates of alkali exchange between alunite and water have been measured in hydrothermal experiments of 1 hour to 259 days duration at 150 to 400°C. Examination of run products by scanning electron microscope indicates that the reaction takes place by dissolution-reprecipitation. This exchange is modeled with an empirical rate equation which assumes a linear decrease in mineral surface area with percent exchange (f) and a linear dependence of the rate on the square root of the affinity for the alkali exchange reaction. This equation provides a good fit of the experimental data for f = 17% to 90% and yields log rate constants which range from −6.25 moles alkali m −2 s −1 at 400°C to − 11.7 moles alkali m −2 s −1 at 200°C. The variation in these rates with temperature is given by the equation log k∗ = −8.17(1000/T(K)) + 5.54 (r 2 = 0.987) "> k∗ = −8.17(1000/T(K)) + 5.54 (r 2 = 0.987) which yields an activation energy of 37.4 ± 1.5 kcal/mol. For comparison, data from O'Neil and Taylor (1967) and Merigoux (1968) modeled with a pseudo-second-order rate expression give an activation energy of 36.1 ± 2.9 kcal/mol for alkali-feldspar water Na-K exchange. In the absence of coupled alkali exchange, oxygen isotope exchange between alunite and water also occurs by dissolution-reprecipitation but rates are one to three orders of magnitude lower than those for alkali exchange. In fine-grained alunites, significant D-H exchange occurs by hydrogen diffusion at temperatures as low as 100°C. Computed hydrogen diffusion coefficients range from −15.7 to −17.3 cm 2 s −1 and suggest that the activation energy for hydrogen diffusion may be as low as 6 kcal/mol. These experiments indicate that rates of alkali exchange in the relatively coarse-grained alunites typical of hydrothermal ore deposits are insignificant, and support the reliability of K-Ar age data from such samples. However, the fine-grained alunites typical of low temperature settings may be susceptible to limited alkali exchange at surficial conditions which could cause alteration of their radiometric ages. Furthermore, the rapid rate of hydrogen diffusion observed at 100–150°C suggests that fine-grained alunites are susceptible to rapid D-H re-equilibration even at surficial conditions.

Geochimica et Cosmochimica Acta↗

USGS1994

Cyclic variations of sulfur isotopes in Cambrian stratabound Ni-Mo-(PGE-Au) ores of southern China

Cyclic variations of δ 34 S values over a range of at least 48‰ in pyrite nodules from stratabound Ni-Mo-PGE(Au) ores of southern China are attributed to biogenic reduction of seawater sulfate in an anoxic, phosphogenic, and metallogenic basin. Cyclic introduction and mixing of normal seawater into typically stagnant basin waters led to extreme variations in δ 34 S values of aqueous sulfide species present at different times. Intermittent venting of metal-laden hydrothermal fluids into such a bacteriogenic sulfide-rich environment resulted in precipitation of metal sulfides as pseudomorphous replacements of organic debris and as sulfide sediments that record large δ 34 S CDT variations from −26 to +22‰. Apatite and silica dominated the replacement of the organic debris when metals were not being introduced into the basin. The combination of abundant organic debris, localized topographic basins for accumulation of the debris, bacterial production of sulfide species, and introduction of metal-bearing hydrothermal fluids provided the environment necessary to form these unusually rich Ni-Mo ores.

Geochimica et Cosmochimica Acta↗

USGS1995

Rhenium-osmium concentration and isotope systematics in group IIAB iron meteorites

Rhenium and osmium abundances, and osmium isotopic compositions were measured by negative thermal ionization mass spectrometry in thirty samples, including replicates, of five IIA and eight IIB iron meteorites. Concentrations in HA irons range from 4800 ppb Re and 66000 ppb Os (Negrillos) to 160 ppb Re and 800 ppb Os (Lombard). In the IIB subgroup, concentrations vary from 28 ppb Re and 180 ppb Os (Navajo) down to 0.8 ppb Re and 9 ppb Os (São Julião de Moreira and Santa Luzia). Log plots of Os vs. Re abundances for HA and IIB irons describe straight lines that approximately converge on Lombard, which has the lowest Re and Os abundances and highest 187 Re/ 188 Os measured in a IIA iron to date. The linear HA trend may be exactly reproduced by fractional crystallization with constant k Re and k Os , but is not well fitted using variable partition coefficients. The IIB iron trend, however, cannot be entirely explained by simple fractional crystallization. One explanation is that small amounts of Re and Os were added to the asteroid core during the final stages of crystallization. Another possibility is that diffusional enrichment of Os may have occurred in samples most depleted in Re and Os. The combined Re-Os isotopic data for HA irons give the following results: slope = 0.07803 ± 0.00076; intercept = 0.09609 ± 0.00045; age = 4584 ± 43 Ma (neglecting the uncertainty in the decay constant of ±3%). Four IIB iron meteorites (Mount Joy, Central Missouri, DRPA 78009, Santa Luzia) also plot within the analytical uncertainty of the HA isochron. These results are consistent with rapid (probably <50 Ma) core segregation, differentiation, and crystallization in the IIAB parent. Several IIB irons (Navajo, Sandia Mountains, Smithsonian Iron, and perhaps São Julião de Moreira) lie beyond analytical uncertainty above the IIA iron isochron, averaging 8 ± 2% higher in 187 Os/ 188 Os. These irons may have crystallized significantly after the HA irons and Mount Joy, but only if the 187 Re/ 188 Os of the melt was ≥2.2. There is no evidence for a IIA iron crystallizing in equilibrium with a melt having such a high ratio. Alternatively, the osmium isotopic systematics of these irons may have been slightly disturbed long after crystallization at ca. 3.3 Ga ago.

Geochimica et Cosmochimica Acta↗

USGS1995

A reconnaissance 40Ar/39Ar geochronologic study of ore-bearing and related rocks, Siberian Russia

40 Ar 39 Ar "> 40 Ar 39 Ar age spectra of biotite from a mineralized vein in the ore-bearing, Noril'sk I intrusion and from picritic-like gabbrodolerite from the weakly mineralized, Lower Talnakh intrusion show that these bodies were emplaced at 249 ± 2 Ma, which is not significantly different from the age of the Permian-Triassic boundary. The ore-bearing intrusions postdate the lower third of the flood-basalt sequence in the Noril'sk area and, on the basis of geochemistry, can best be correlated with lavas slightly younger than those which they cut. Thus, flood basalt was erupted at the time of the Permian-Triassic mass extinction event, although its role in this event is, as yet, ill defined. Additional new 40 Ar 39 Ar "> 40 Ar 39 Ar age data for a group of intrusive and extrusive rocks on the western margin of the Siberian craton indicate that mafic magmatism extended over a period of several tens of million years, whereas paleomagnetic data suggest that the bulk of the Siberian flood-basalt sequence near Noril'sk has been erupted in only a million years or so. 40 Ar 39 Ar "> 40 Ar 39 Ar ages of plagioclase from early flood-basalt flows are about 2% younger than those obtained for biotite from the crosscutting, Noril'sk I intrusion, probably because of slight alteration and Argon loss from the plagioclase.

Noril’sk-Talnakh Mining District, Siberia↗

USGS1995

Further evidence for a low U/Pb source in the moon: U-Th-Pb, Sm-Nd, and Ar-Ar, isotopic systematics of lunar meteorite Yamato-793169

The coarse-grained lunar meteorites, Yamato-793169 and Asuka-881757, represent a new type of low-Ti mare basalt. This paper reports the results of a U-Th-Pb, Sm-Nd, and Ar-Ar isotopic study of Yamato-793169 performed as part of a consortium studies of lunar basaltic meteorites. The isotopic study was carried out on a small sample (100 mg) so that only three density fractions could be separated. These fractions were leached with dilute acid in order to eliminate terrestrial Pb contamination. However, the leaching procedure did not completely remove this contamination in some fractions and also apparently caused a fractionation of elements (U, Th, Pb) due to preferential leaching effects, producing a secondary disturbance of the systematics. Furthermore, the Ar-Ar analyses indicate that the isotopic systematics in this meteorite might have been disturbed sometime later than 750 Ma. For these reasons, the ages obtained using different isotopic systems disagree with each other and a precise formation age could not be obtained for this meteorite. However, using what results there are, two reasonable interpretations can be made: (1) the Sm-Nd system yielded an age of 3.4 Ga that could be interpreted as the formation age, assuming that this system is possibly the least disturbed during the metamorphic event(s) that this meteorite experienced at least once, and that all other isotopic systems and their corresponding ages were disturbed, and (2) the U-Pb system yielded a nearly concordant age of 3.8 Ga that could be interpreted as the formation age and the Sm-Nd isotopic systematics were somehow disturbed. We prefer the second interpretation for the reasons discussed below. On a concordia diagram, the CDT (Cañon Diablo troilite)-corrected U-Pb isotopic data yield a discordia line similar to the lunar catastrophic array, indicating that the source of the meteorite formed during early lunar differentiation (∼4.4 Ga) and that the basalt was generated near 3.9 Ga. Total Ar-Ar age on plagioclase is 3.26 Ga, which seems to be too old if the formation age is 3.4 Ga, because low temperature fractions lost large amounts of radiogenic 40 Ar during the late thermal event. If we assume a formation age of 3.8 Ga, the estimated source 238 U/ 204 Pb (μ) is 21.6 ± 3.5 and ϵ Nd is 3.9 ± 0.3. These results indicate that the source of Yamato-793169 is more depleted than Apollo 12 and 15 LT basalts, but less depleted than Asuka-881757. Therefore, Yamato-793169 may represent a new type of LT- or VLT-like mare basalt that is different from Asuka-881757. The wide variety of lead and neodymium isotopic characteristics among LT and VLT mare basalts indicate that the lunar mantle was very heterogeneous with respect to trace element abundances.

Geochimica et Cosmochimica Acta↗

USGS1995

The geochemical cycling of stable Pb, 210Pb, and 210Po in seasonally anoxic Lake Sammamish, Washington, USA

The geochemical processes controlling the behavior of stable Pb, 210 Pb, and 210 po in seasonally anoxic Lake Sammamish, Washington were identified from water column distributions and box model calculations. Total (sum of dissolved and particulate) inventories of stable Pb, 210 Pb, and 210 Po increased in the whole lake during the latter part of the oxic stage of the lake and were attributed to diffusion from sediments. Large decreases in the total inventories of these elements occurred during the transition from oxic to anoxic conditions, and the lowest inventories were observed during the sulfidic stage of stratification. The cycling of stable Pb and 210 Pb during oxic periods appeared to be linked to Fe cycling while 210 Po cycling was more closely linked to the cycling of Mn. The behavior of stable and radioactive Pb and, possibly, 210 Po during anoxia was influenced by sulfur cycling. Thermodynamic calculations indicated that dissolved Pb concentrations might be controlled by PbS precipitation during anoxia. Flux balance calculations during stratification indicated that atmospheric deposition was the major source of both 210 Pb and 210 Po to the lake and fluvial input was more important for 210 Po than for 210 Pb. 210 Pb and 210 Po supplied by atmospheric deposition were scavenged and removed by sedimentation from the epilimnion. Residence times with respect to scavenging and sedimentation in the epilimnion were 1–3 and 617 days, respectively, for 210 Pb and 9–22 and 9–26 days for 210 Po. Model results in the hypolimnion indicated that at certain times the radionuclides diffused out of the sediments and at other times into the sediments. Residence times with respect to sedimentation of particulate nuclides in the hypolimnion were 2–124 days for 210 Pb and 8–48 days for 210 Po. The behavior of 210 Pb and 210 Po in Lake Sammamish was compared to that in B~' kford Reservoir, Massachusetts. Although the comparison indicated similarities (e.g., links to Fe and Mn cycling, larger scavenging rates for 210 Pb than for 210 Po), the major differences between the systems were that atmospheric inputs were greater than fluvial inputs, activities of the radionuclides were lower, and the presence of sulfide in the hypolimnion during anoxia affected the cycling of stable Pb, 210 Pb, and 210 Po in Lake Sammamish.

Washington↗

USGS1996

A glass spherule of questionable impact origin from the Apollo 15 landing site: Unique target mare basalt

A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 ??m-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al2O3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basalts. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the 40Ar/39Ar laser incremental heating technique, is 1647 ?? 11 Ma (2 ??); it is expressed as an age spectrum of seventeen steps over 96% of the 39Ar released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; 38Ar cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50??C s-1 is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile abundances, and the young age are perhaps all most consistent with an impact origin, but nonetheless not absolutely definitive. The 15434,28 glass is distinct from the common yellow impact glasses at the Apollo 15 landing site, in particular in its lower abundances of incompatible elements and much younger age. If we accept an impact origin, then the trace element relative abundances preclude both typical KREEP and the common Apollo 15 yellow impact glass from contributing more than a few percent of the incompatible elements to potential mixtures. The melted part of any target must have consisted almost entirely of a variety (or varieties) of mare basalt or glass distinct from any known mare basalts or glasses, including Apollo 15 yellow volcanic glass, or mixtures of them. However, the rind inclusions, similar to materials of local origin, do suggest a source near the Apollo 15 landing site. An impact melt cannot have dissolved much, if any, of such inclusions. A lack of regolith materials in the rind and in the melt component suggest an immature source terrain. Thus, even for an impact origin, there is the possibility (though not requirement) that the volcanic target is younger than most mare plains. The crater Hadley C, 25 km away, is a potential source. If the 15434,28 glass is instead directly of volcanic origin, it represents an extremely young mare magma of a type previously undiscovered on the Moon.

Geochimica et Cosmochimica Acta↗

USGS1996

Hydrogen isotope systematics of phase separation in submarine hydrothermal systems: Experimental calibration and theoretical models

Hydrogen isotope fractionation factors were measured for coexisting brines and vapors formed by phase separation of NaCl/H2O fluids at temperatures ranging from 399-450??C and pressures from 277-397 bars. It was found that brines are depleted in D compared to coexisting vapors at all conditions studied. The magnitude of hydrogen isotope fractionation is dependent on the relative amounts of Cl in the two phases and can be empirically correlated to pressure using the following relationship: 1000 ln ??(vap-brine) = 2.54(??0.83) + 2.87(??0.69) x log (??P), where ??(vap-brine) is the fractionation factor and ??P is a pressure term representing distance from the critical curve in the NaCl/H2O system. The effect of phase separation on hydrogen isotope distribution in subseafloor hydrothermal systems depends on a number of factors, including whether phase separation is induced by heating at depth or by decompression of hydrothermal fluids ascending to the seafloor. Phase separation in most subseafloor systems appears to be a simple process driven by heating of seawater to conditions within the two-phase region, followed by segregation and entrainment of brine or vapor into a seawater dominated system. Resulting vent fluids exhibit large ranges in Cl concentration with no measurable effect on ??D. Possible exceptions to this include hydrothermal fluids venting at Axial and 9??N on the East Pacific Rise. High ??D values of low Cl fluids venting at Axial are consistent with phase separation taking place at relatively shallow levels in the oceanic crust while negative ??D values in some low Cl fluids venting at 9??N suggest involvement of a magmatic fluid component or phase separation of D-depleted brines derived during previous hydrothermal activity.

Geochimica et Cosmochimica Acta↗

USGS1996

Kinetic and mineralogic controls on the evolution of groundwater chemistry and 87Sr/86Sr in a sandy silicate aquifer, northern Wisconsin, USA

Substantial flowpath-related variability of 87Sr/86Sr is observed in groundwaters collected from the Trout Lake watershed of northern Wisconsin. In the extensive shallow aquifer composed of sandy glacial outwash, groundwater is recharged either by seepage from lakes or by precipitation that infiltrates the inter-lake uplands. 87Sr/86Sr of groundwater derived mainly as seepage from a precipitation-dominated lake near the head of the watershed decreases with progressive water chemical evolution along its flowpath due primarily to enhanced dissolution of relatively unradiogenic plagioclase. In contrast, 87Sr/86Sr of groundwater derived mainly from precipitation that infiltrates upland areas is substantially greater than that of precipitation collected from the watershed, due to suppression of plagioclase dissolution together with preferential leaching of Sr from radiogenic phases such as K-feldspar and biotite. The results of a column experiment that simulated the effects of changing residence time of water in the aquifer sand indicate that mobile waters obtain relatively unradiogenic Sr, whereas stagnant waters obtain relatively radiogenic Sr. Nearly the entire range of strontium-isotope composition observed in groundwaters from the watershed was measured in the experimental product waters. The constant mobility of water along groundwater recharge flowpaths emanating from the lakes promotes the dissolution of relatively unradiogenic plagioclase, perhaps due to effective dispersal of clay mineral nuclei resulting from dissolution reactions. In contrast, episodic stagnation in the unsaturated zone along the upland recharge flowpaths suppresses plagioclase dissolution, perhaps due to accumulation of clay mineral nuclei on its reactive surfaces. Differences in redox conditions along these contrasting flowpaths probably enhance the observed differences in strontium isotope behavior. This study demonstrates that factors other than the calculated state of mineral saturation must be considered when attempting to simulate chemical evolution along flowpaths, and that reaction models must be able to incorporate changing contributions from reacting minerals in the calculations.

Geochimica et Cosmochimica Acta↗

USGS1996

Comparison of the partitioning behaviours of yttrium, rare earth elements, and titanium between hydrogenetic marine ferromanganese crusts and seawater

In order to evaluate details of the partitioning behaviours of Y, rare earth elements (REEs), and Ti between inorganic metal oxide surfaces and seawater, we studied the distribution of these elements in hydrogenetic marine ferromanganese (Fe-Mn) crusts from the Central Pacific Ocean. Nonphosphatized Fe-Mn crusts display shale-normalized rare earths and yttrium (REY SN ) patterns (Y inserted between Dy and Ho) that are depleted in light REEs (LREEs) and which show negative anomalies for Y sn , and positive anomalies for La SN , Eu SN , Gd SN , and in most cases, Ce sn . They show considerably smaller Y/ Ho ratios than seawater or common igneous and clastic rocks, indicating that Y and Ho are fractionated in the marine environment. Compared to P-poor crusts, REY SN patterns of phosphatized Fe-Mn crusts are similar, but yield pronounced positive Y sn anomalies, stronger positive La SN anomalies, and enrichment of the HREEs relative to the MREEs. The data suggest modification of REY during phosphatization and indicate that studies requiring primary REY distributions or isotopic ratios should be restricted to non-phosphatized (layers of) Fe-Mn crusts. Apparent bulk coefficients, K d m , describing trace metal partitioning between nonphosphatized hydrogenetic Fe-Mn crusts and seawater, are similar for Pr to Eu and decrease for Eu to Yb. Exceptionally high values of K D Ce , which are similar to those of Ti, result from oxidative scavenging of Ce and support previous suggestions that Ce (IV) is a hydroxide-dominated element in seawater. Yttrium and Gd show lower K D values than their respective neighbours in the REY series. Results of modelling the exchange equilibrium between REY dissolved in seawater and REY sorbed on hydrous Fe-Mn oxides corroborate previous studies that suggested the surface complexation of REY can be approximated by their first hydroxide binding constant. Negative “anomalies” occur for stabilities of bulk surface complexes of Gd, La, and particularly Y. The differences in inorganic surface complex stability between Y and Ho and between Gd and its REE neighbours are similar to those shown by the stabilities of complexes with aminocarboxylic acids and are significantly larger than those shown by stabilities of complexes with carboxylic acids. Hence, sorption of Y and REEs onto hydrous Fe-Mn oxides may contribute significantly to the positive Y SN and Gd SN anomalies in seawater.

Geochimica et Cosmochimica Acta↗

USGS1996

Osmium isotopes and silver as tracers of anthropogenic metals in sediments from Massachusetts and Cape Cod bays

High concentrations of osmium (Os) and silver (Ag) and low 187Os/186Os ratios in Boston sewage make these elements sensitive tracers of the influence of sewage on marine sediments in Massachusetts and Cape Cod bays. Pristine marine sediments have Ag concentrations more than 200 times lower than sewage sludge, Os concentrations 10-40 times lower, and 187Os/186Os ratios six times higher. Surface sediments from both Massachusetts and Cape Cod bays exhibit both high Ag concentrations and low 187Os/186Os ratios indicating the influence of sewage particles on marine sediments in this region extends some 70 km from the point of sewage release. In detail, the distribution of Os and Ag do not support a model of simple physical mixing of sewage particles with normal marine sediments. Deviations from the mixing model may be the result of fractionation of Os and Ag in the marine environment, and [or] independent temporal variations in the Os and Ag content of the waste stream. The results of this investigation suggest that osmium isotopes may be widely applicable as tracers of the influence of sewage on sediments in estuarine environments and that subtle variations in the isotopic composition of Os in the waste stream may help constrain the sources of Os and other metals delivered to the environment.

Cape Cod Bay, Massachusetts Bay↗

USGS1996

A minimum U-Pb age for Siberian flood-basalt volcanism

Establishing an accurate and precise age for Siberian flood-basalt volcanism is of great importance in evaluating causes for the unequaled mass extinction of flora and fauna at the Permian-Triassic boundary. We report a new, minimum U-Pb age obtained from zircon and baddeleyite from the mineralized Noril'sk I intrusion that cuts the lower third of this rapidly deposited, 3500-m-thick volcanic sequence near Noril'sk. This 251.2 ?? 0.3 (2??) Ma age is within analytical error of the SHRIMP U-Pb age for zircon from the Permian-Triassic boundary at Meishan, South China [251.1 ?? 3.6 Ma (2??)], and confirms Siberian basaltic volcanism as a possible contributor to the mass extinction.

Geochimica et Cosmochimica Acta↗

USGS1996

Mechanism of molybdenum removal from the sea and its concentration in black shales: EXAFS evidence

Molybdenum K-edge EXAFS (extended X-ray absorption fine structure) spectra yield new structural information about the chemical environment of Mo in high-Mo black shales and sediments. Two spectral types are found. The less common one, associated with Mo ores developed in shale in China, is that of a MoS2 phase, possibly X-ray amorphous jordisite. The other, associated with Cretaceous deep sea sediments and with other black shales, is characterized by short Mo-O distances (1.69-1.71 A??), by Mo-S distances of 2.30-2.38 A??, and in some cases by second shell Mo and Fe interactions, which suggests that some Mo resides in transition metal-rich phases. EXAFS spectra of synthetic amorphous materials, prepared by scavenging Mo from HS solutions with Fe(II), FeOOH, and humic acid, suggest that the second spectral type arises from Mo present chiefly in two forms. One is a compact, Mo-Fe-S "cubane" type compound with Mo-S distances of ???2.36 A?? and Mo-Fe distances of ???2.66 A??, while the other is probably an organic form containing some Mo-O double bonds (???1.69 A??). Laboratory products, that were prepared by scavenging dissolved Mo from sulfidic solutions with humic acid, yield spectra quite similar to the second spectral type observed in shales and sediments, including unexpected indications of Mo-Fe interactions. Molybdenum L-edge spectra indicate that the mean oxidation state in the sediments and shales lies between IV and VI. This work demonstrates the merit of EXAFS for obtaining structural information on natural materials containing X-ray amorphous components which defeat conventional mineralogical characterization. The implications of these findings regarding Mo scavenging from sulfidic natural waters are considered. We introduce the concept of a geochemical switch, in which HS- transforms the marine behavior of Mo from that of a conservative element to that of a particle reactive element. The action point of the HS- switch is calculated to be, aHS- = 10-3.6 - 10-4.3. When aHS- approaches the action point, Mo becomes reactive to particles containing transition metals (e.g., Fe). We conjecture that thiols, including humic-bound thiol groups, also switch Mo behavior. In contrast to previous ideas, our model for Mo scavenging deemphasizes the role of reduction from Mo(VI) to Mo(V) as the initial step in scavenging; instead, we emphasize the ease with which Mo forms covalent bonds to transition metals and organic molecules via S bridges.

Geochimica et Cosmochimica Acta↗

USGS1996

Reduction of aqueous transition metal species on the surfaces of Fe(II)-containing oxides

Experimental studies demonstrate that structural Fe(II) in magnetite and ilmenite heterogeneously reduce aqueous ferric, cupric, vanadate, and chromate ions at the oxide surfaces over a pH range of 1–7 at 25°C. For an aqueous transition metal m , such reactions are 3 [ Fe 2 + Fe 2 3 + ] O 4 (magnetite) + 2 / n m z → 4 [ Fe 2 3 + ] O 3 (magnetite) + Fe 2 + + 2 / n m z − n "> 3[Fe2+Fe23+]O4(magnetite)+2/nmz→4[Fe23+]O3(magnetite)+Fe2++2/nmz−n and 3 [ Fe 2 + Ti ] O 3 (ilmenite) + 2 / n m z → Fe 2 3 + Ti 3 O 9 (pseudorutile) + Fe 2 + + 2 / n m z − n , "> 3[Fe2+Ti]O3(ilmenite)+2/nmz→Fe23+Ti3O9(pseudorutile)+Fe2++2/nmz−n, where z is the valance state and n is the charge transfer number. The half cell potential range for solid state oxidation [Fe(II)] → [Fe(III)] is −0.34 to −0.65 V, making structural Fe(II) a stronger reducing agent than aqueous Fe 2+ (−0.77 V). Reduction rates for aqueous metal species are linear with time (up to 36 h), decrease with pH, and have rate constants between 0.1 and 3.3 × 10 −10 mol m −2 s −1 . Iron is released to solution both from the above reactions and from dissolution of the oxide surface. In the presence of chromate, Fe 2+ is oxidized homogeneously in solution to Fe 3+ . X-ray photoelectron spectroscopy (XPS) denotes a Fe(III) oxide surface containing reduced Cr(III) and V(IV) species. Magnetite and ilmenite electrode potentials are insensitive to increases in divalent transition metals including Zn(II), Co(II), Mn(II), and Ni(II) and reduced V(IV) and Cr(III) but exhibit a log-linear concentration-potential response to Fe(III) and Cu(II). Complex positive electrode responses occur with increasing Cr(VI) and V(V) concentrations. Potential dynamic scans indicate that the high oxidation potential of dichromate is capable of suppressing the cathodic reductive dissolution of magnetite. Oxide electrode potentials are determined by the Fe(II)/Fe(III) composition of the oxide surface and respond to aqueous ion potentials which accelerate this oxidation process. Natural magnetite sands weathered under anoxic conditions are electrochemically reactive as demonstrated by rapid chromate reduction and the release of aqueous Fe(III) to experimental solution. In contrast, magnetite weathered under oxidizing vadose conditions show minimum reactivity toward chromate ions. The ability of Fe(II) oxides to reduce transition metals in soils and groundwaters will be strongly dependent on the redox environment.

Geochimica et Cosmochimica Acta↗

USGS1996

Wide angle X-ray scattering (WAXS) study of "two-line" ferrihydrite structure: Effect of arsenate sorption and counterion variation and comparison with EXAFS results

Wide angle X-ray scattering (WAXS) measurements have been made on a suite of “two-line” ferrihydrite (FHY2) samples containing varying amounts of coprecipitated arsenate. Samples prepared at pH 8 with counter ions chloride, nitrate, and a mixture of both also were examined. The raw WAXS scattering functions show that “two-line” ferrihydrite actually has a large number of non-Bragg (i.e., diffuse scattering) maxima up to our observation limit of 16 Å −1 . The type of counter ion used during synthesis produces no significant change in this function. In unarsenated samples, Radial Distribution Functions (RDFs) produced from the scattering functions show a well-defined Fe-O peak at 2.02 Å in excellent agreement with the mean distance of 2.01 Å from extended X-ray absorption fine structure (EXAFS) analysis. The area under the Fe-O peak is consistent with only octahedral oxygen coordination about iron, and an iron coordination about oxygen of 2.2, in agreement with the EXAFS results, the sample composition, and XANES measurements. The second peak observed in the RDFs is clearly divided into two populations of correlations, at 3.07 and 3.52 Å, respectively. These distances are close to the EXAFS-derived Fe-Fe subshell distances of 3.02–3.05 and 3.43–3.46 Å, respectively, though this is misleading as the RDF peaks also include contributions from O-Fe and O-O correlations. Simulated RDFs of the FeOOH polymorphs indicate how the observed RDF structure relates to the EXAFS pair-correlation function, and allow comparisons with an ordered ferrihydrite structure.

Geochimica et Cosmochimica Acta↗