Temperature dependence of the cation distribution in nickel aluminate (NiAl2O4) spinel: a powder XRD study

NiAl₂O₄ is a largely inverse spinel, which in detail shows increasing randomisation with temperature of Ni and Al between the octahedral and tetrahedral cation sites of the spinel structure. We have used powder XRD to determine this cation distribution in various samples of NiAl₂O₄ quenched after annealing between 700 and 1400° C. The inversion parameter (x) can be measured with a precision of ± 0.004 (one standard deviation), and a comparison of different methods of synthesis, X-ray diffraction and refinement techniques, suggests a probable accuracy of better than 0.01. The results are supported by some preliminary single crystal refinements on flux-grown samples.Below 800° C the rate of cation ordering becomes very slow, and, despite reaching an apparently steady state, it is doubtful if our samples attained complete internal equilibrium. Above 1250° C the cation redistribution becomes so fast that the quenching method becomes unreliable. Between 800 and 1250° C inclusive, the degree of inversion changes smoothly from 0.87 at 800° C to 0.79 at 1250° C, and is accompanied by linear changes in u, the oxygen parameter, from 0.2555 to 0.2563 (±0.0002), and a₀, the lattice parameter, from 8.0462 to 8.0522 Å (±0.0002 Å).     

Elasticity of single-crystal aragonite by Brillouin spectroscopy

The elastic constants of natural single-crystal aragonite (CaCO₃) have been measured by Brillouin spectroscopy at ambient conditions. The elastic constants C₁₁, C₂₂, C₃₃, C₄₄, C₅₅, C₆₆, C₁₂, C₁₃ and C₂₃ are 171.1±1.0, 110.1±0.9, 98.4±1.2, 39.3±0.6, 24.2±0.4, 40.2±0.6, 60.3±1.0, 27.8±1.6 and 41.9±2.0 GPa, respectively, for aragonite. The linear compressibilities of the a-, b- and c-axis for aragonite at ambient conditions were derived from our measured data to be 3.0±0.2, 4.2±0.2 and 7.3±0.6×10^–3 GPa^–1, respectively. The aggregate bulk and shear moduli for aragonite using the Voigt-Reuss-Hill (VRH) scheme are thus calculated to be 68.9±1.4 and 35.8±0.2 GPa, respectively. The value of bulk modulus is in remarkable contrast to the literature value of 46.9 GPa measured almost a century ago. Our new datum, however, is closer to that derived from recent atomistic simulation and static compression studies.     

Hydration-dehydration behavior and thermodynamics of chabazite

Equilibrium in the chabazite-H₂O system was investigated by isothermal thermogravimetric analysis over a large range of temperatures (from 23 to 315°C) and H₂O-vapor pressures (from 0.03 to 28 mbar). Thermodynamic analysis of the phase-equilibrium data revealed the existence of three energetically distinct types of H₂O, referred to as S-1, S-2, and S-3. At 23°C and 26 mbar of H₂O-vapor pressure, chabazite has maximum H₂O occupancies of 8.2, 11.1, and 3.1 wt.% for S-1, S-2, and S-3, respectively. During dehydration, S-1 H₂O is lost first, followed by S-2 H₂O and then S-3 H₂O, with significant overlap for S-1 and S-2 as well as S-2 and S-3. The thermodynamics of chabazite-H₂O were modeled using three independent equilibrium formulations for S-1, S-2, and S-3. These formulations yielded standard-state molar Gibbs free energy of hydration of −21.8 ± 0.6, −52.1 ± 1.8, and −111.7 ± 6.7 kJ/mol for S-1, S-2, and S-3. Standard-state molar enthalpies of hydration for each type of H₂O are −65.6 ± 0.5, −100.1 ± 1.6, and −156.9 ± 6.2 kJ/mol, respectively. Integral molar values for the Gibbs free energy of hydration for each type of H₂O are −19.0 ± 0.7, −40.1 ± 2.1, and −76.9 ± 9.6 kJ/mol, respectively. Integral molar values for the enthalpy of hydration for each type of H₂O are −62.8 ± 0.6, −88.1 ± 1.9, and −122.2 ± 9.3 kJ/mol, respectively. Integration of the predicted total partial molar enthalpy of hydration for all three types of H₂O over the full H₂O content of chabazite gave an integral molar enthalpy of −39.65 ± 9.3 kJ/mol relative to liquid water. The thermodynamic data obtained for the hydration of natural chabazite were used to predict the hydration state of chemically similar chabazites under various temperatures and P_H2O, ranging from 25 to 400°C and from 10⁻⁵ to 10⁴ bars.     

The Blosseville Coast basalts of East Greenland: Their occurrence,composition and temporal variations

In the system CaO-MgO-A1₂O₃-SiO₂ the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatite_ss+sillimanite pyrope-grossular_ss+quartz (3), anorthite+enstatite_ss pyrope-grossular_ss+diopside_ss+quartz (2), anorthite+pyrope-grossular_ss+ quartz diopside_ss+kyanite (4) and anorthite+diopside_ss grossular-pyrope_ss +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En_0.5Fs_0.5). Enstatite and plagioclase (An_0.5Ab_0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.     

Compositional variations of volcanics along segments of recent spreading ridges

An investigation of glassy volcanics erupted within the last ten-million years along various segments of the mid-Atlantic Ridge and the East Pacific Rise has revealed major crustal compositional changes. The available data from the mid-Atlantic Ridge shows the existence of two petrological provinces: One, located between latitudes 33° and 53° N, is characterized by volcanics which have a tendency to be oversaturated ocean ridge basalts (OSORB) with respect to normative quartz; the second group of rocks, found between 25° S and 33° N, is generally composed of saturated ocean ridge basalts (SORB). In addition, the SORB volcanics have higher TiO₂ (1.7±0.3%), higher Na₂O (2.8±0.2%) and higher FeO^*/MgO (1.36±0.2) values than do the OSORB types (with 1.1±0.2%, 2.2±0.2% and 1.22±0.2 for the TiO₂, Na₂O, and FeO^*/MgO respectively), There is a correlation between the rate of crustal spreading and the compositional changes observed on the volcanics erupted along various segments of oceanic ridges. Slow-accreting plate boundaries having a total spreading rate of 2–3 cm/year are characterized by a low TiO₂ content (1.1±0.2%), low FeO^*/ MgO ratio (1.22±0.2) and a high an/an+ab ratio (0.62±0.05). Segments of fast-spreading ridges (total rate 11–13 cm/year) show a higher range of TiO₂ (2.1±0.4%) and FeO^*/MgO (1.6±0.4) and a lower range of the an/an + ab ratio (0.5±0.07). Ridge segments with a total spreading rate of 5–9 cm/year con sist of volcanics having intermediate values for the above parameters. Different degrees of partial melting of rising mantle material are suggested as a possible mechanism for explaining the compositional diversities encountered along oceanic ridge systems.Contribution n 677 du Département de Géophysique, Géologie, Géochimie Marines du C.O.B.     

Robust 24 ± 6 ka Ar/Ar age of a low-potassium tholeiitic basalt in the Lassen region of NE California

⁴⁰Ar/³⁹Ar ages on the Hat Creek Basalt (HCB) and stratigraphically related lava flows show that latest Pleistocene tholeiitic basalt with very low K₂O can be dated reliably. The HCB underlies ∼ 15 ka glacial gravel and overlies four andesite and basaltic andesite lava flows that yield ⁴⁰Ar/³⁹Ar ages of 38 ± 7 ka (Cinder Butte; 1.65% K₂O), 46 ± 7 ka (Sugarloaf Peak; 1.85% K₂O), 67 ± 4 ka (Little Potato Butte; 1.42% K₂O) and 77 ± 11 ka (Potato Butte; 1.62% K₂O). Given these firm age brackets, we then dated the HCB directly. One sample (0.19% K₂O) clearly failed the criteria for plateau-age interpretation, but the inverse isochron age of 26 ± 6 ka is seductively appealing. A second sample (0.17% K₂O) yielded concordant plateau, integrated (total fusion), and inverse isochron ages of 26 ± 18, 30 ± 20 and 24 ± 6 ka, all within the time bracket determined by stratigraphic relations; the inverse isochron age of 24 ± 6 ka is preferred. As with all isotopically determined ages, confidence in the results is significantly enhanced when additional constraints imposed by other isotopic ages within a stratigraphic context are taken into account.     

Phase relations and mineral assemblages in the Ag-Bi-Pb-S system

Phase relations within the Ag-Bi-S, Bi-Pb-S, and Ag-Pb-S systems have been determined in evacuated silica tube experiments. Integration of experimental data from these systems has permitted examination and extrapolation of phase relations within the Ag-Bi-Pb-S quaternary system. — In the Ag-Bi-S system liquid immiscibility fields exist in the metal-rich portion above 597±3°C and in the sulfur-rich portion above 563±3°C. Ternary phases present correspond to matildite (AgBiS₂) and pavonite (AgBi₃S₅). Throughout the temperature range 802±2°C to 343±2°C the assemblage argentite (Ag₂S) + bismuth-rich liquid is stable; below 343°C this assemblage is replaced by the assemblage silver + matildite. — Five ternary phases are stable on the PbS-Bi₂S₃ join above 400°C — phase II (18 mol-% Bi₂S₃), phase III (27 mol-% Bi₂S₃), cosalite (33.3 mol-% Bi₂S₃), phase IV (51 mol-% Bi₂S₃), and phase V (65 mol-% Bi₂S₃). Phase IV corresponds to the mineral galenobismutite and is stable below 750±3°C. Phases II, III, and V do not occur as minerals, but typical lamellar and myrmekitic textures commonly observed among the Pb-Bi sulfosalts and galena evidence their previous existence in ores. Phase II and III are stable from 829±6°C and 816±6°C, respectively, to below 200°C; Phase V, stable only between 730±5°C and 680±5°C in the pure Bi-Pb-S system is stabilized to 625±5°C by the presence of 2% Ag₂S. Experiments conducted with natural cosalites suggest that this phase is stable only below 425±25°C in the presence of vapor. — In the Ag-Pb-S system the silver-galena assemblage is stable below 784±2°C, whereas the argentite + galena mineral pair is stable below 605±5°C. — Solid solution between matildite and galena is complete above 215±15°C; below this temperature characteristic Widmanstätten structure-like textures are formed through exsolution. Schematic phase relations within the quaternary system are presented at 1050°C, at 400°C, and at low temperature.

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Zusammenfassung Die Phasenbeziehungen in den Systemen Ag-Bi-S, Bi-Pb-S und Ag-Pb-S wurden durch Versuche in evakuierten Quarzglasröhrchen bestimmt. Die Auswertung aller experimentellen Daten gestattete eine Extrapolation der Phasenbeziehungen im quaternären System Ag-Bi-Pb-S. — Im System Ag-Bi-S besteht ein Zwei-Schemlzenfeld im metallreichen Teil über 597±3°C und im schwefelreichen Teil über 563±3°C. Die ternären Phasen entsprechen den Mineralien Schapbachit (AgBiS₂) und Pavonit (AgBi₃S₅). Zwischen 802±2°C und 343±2°C ist die Paragenese Silberglanz (Ag₂S) + Bi-reiche Schmelze stabil; unterhalb 343°C wird sie jedoch ersetzt durch die Paragenese Silber + Schapbachit. — Fünf ternäre Phasen sind stabil im Schnitt PbS-Bi₂S₃ oberhalb von 400°C: Phase II (18 Mol-% Bi₂S₃), Phase III (27 Mol-% Bi₂S₃), Cosalite (33.3 Mol-% Bi₂S₃), Phase IV (51 Mol-% Bi₂S₃) und Phase V (65 Mol-% Bi₂S₃). Phase IV entspricht dem Mineral Galenobismutit und ist stabil unterhalb 750±3°C. Die Phasen II, III und V kommen zwar nicht in der Natur vor, jedoch weisen typische myrmekitische und lamellare Gefüge, die man häufig in Pb-Bi-Sulfosalzen und deren Verwachsungen mit Bleiglanz beobachtet, auf die ehemalige Existenz solcher Phasen in diesen Erzen hin. Die Phasen II und III sind stabil von 829±6°C bzw. 816±6°C bis unter 200°C. Die Phase V, die im reinen System Bi-Pb-S zwischen 730±5°C und 680±5°C auftritt, wird in Gegenwart von 2% Ag₂S stabilisiert bis herab zu 625±5°C. Versuche mit natürlichen Cosaliten lassen darauf schließen, daß diese Phase nur unterhalb 425±25°C in Gegenwart einer Gasphase stabil ist. — Im System Ag-Pb-S ist die Paragenese Silber-Bleiglanz unterhalb von 784±2°C stabil, die Paragenese Silberglanz-Bleiglanz dagegen unterhalb 605±5°C. — Die Mischkristallreihe von Schapbachit und Bleiglanz ist vollständig oberhalb 215±15°C; unterhalb dieser Temperatur entstehen charakteristische Entmischungsgefüge ähnlich den Widmannstättenschen Figuren. Für das quaternäre System werden schematische Phasenbeziehungen für 1050°C, 400°C und eine noch tiefere Temperatur gegeben.

     

Paleogene paleoclimate reconstruction using oxygen isotopes from land and freshwater organisms: the use of multiple paleoproxies

Understanding past climate change is critical to the interpretation of earth history. Even though relative temperature change has been readily assessed in the marine record, it has been more difficult in the terrestrial record due to restricted taxonomic distribution and isotopic fractionation. This problem could be overcome by the use of multiple paleoproxies. Therefore, the δ¹⁸O isotopic composition of five paleoproxies (rodent tooth enamel, δ¹⁸O_Phosphate = +17.7 ± 2.0‰ n = 74 (VSMOW); fish scale ganoine δ¹⁸O_Phosphate = +19.7 ± 0.7‰ n = 20 (VSMOW); gastropod shell δ¹⁸O_Calcite = −1.7 ± 1.3‰ n = 50 (VPDB); charophyte gyrogonite δ¹⁸O_Calcite = −2.4 ± 0.5‰ n = 20 (VPDB); fish otolith δ¹⁸O_Aragonite = δ¹⁸O = −3.6 ± 0.6‰ n = 20 (VPDB)) from the Late Eocene (Priabonian) Osborne Member (Headon Hill Formation, Solent Group, Hampshire Basin, UK) were determined. Because diagenetic alteration was shown to be minimal the phosphate oxygen component of rodent tooth enamel (as opposed to enamel carbonate oxygen) was used to calculate an initial δ¹⁸O_{Local water} value of 0.0 ± 3.4‰. However, a skewed distribution, most likely as a result of the ingestion of evaporating water, necessitated the calculation of a corrected δ¹⁸O_{Local water} value of −1.3 ± 1.7‰ (n = 62). This δ¹⁸O_{Local water} value corresponds to an approximate mean annual temperature of 18 ± 1°C. Four other mean paleotemperatures can also be calculated by combining the δ¹⁸O_{Local water} value with four independent freshwater paleoproxies. The calculated paleotemperature using the fish scale thermometry equations most likely represents the mean temperature (21 ± 2°C) of the entire length of the growing season. This should be concordant with the paleotemperature calculated using the Lymnaea shell thermometry equation (23 ± 2°C). The lack of concordance is interpreted to be the result of diagenetic alteration of the originally aragonitic Lymnaea shell to calcite. The mean paleotemperature calculated using the charophyte gyrogonite thermometry equation (21 ± 2°C), on the other hand, most likely represents the mean temperature of a single month toward the end of the growing season. The fish otolith mean paleotemperature (28 ± 2°C) most likely represents the mean temperature of the warmest months of the growing season. An approximate mean annual temperature of 18 ± 1°C, in addition to a mean growing season paleotemperature of 21 ± 2°C (using fish scale only) with a warmest month temperature of 28 ± 2°C, and high associated standard deviations suggest that a subtropical to warm temperate seasonal climate existed during the deposition of the Late Eocene Osborne Member.     

A comparison of isotope fractionation of carbon and hydrogen from paddy field rice roots and soil bacterial enrichments during CO2/H2 methanogenesis

To better understand the isotope biogeochemistry of paddy field CH₄, we investigated carbon and hydrogen isotope fractionation during CO₂ reduction by a methanogenic community enriched from California paddy field soil and rice plants. Results from analyses of terminal restriction fragment length polymorphism (T-RFLP) and sequences of the archaeal small-subunit (SSU) rRNA-encoding genes (rDNA) showed a difference in methanogenic community structure between the soil (dominated by Methanobacteriaceae) and roots (dominated by Methanospirillaceae) which was essentially the same for sampling dates 15 and 99 days after flooding (DAF). CO₂/H₂ methanogenesis by these microbial communities produced CH₄ with different isotope ratios and fractionation factors (α factors). The carbon isotope α factors in an open system with a continuous supply of 0.5% H₂ were 1.050 ± 0.002 and 1.057 ± 0.001 for soil and root enrichment cultures at 15 DAF, and 1.052 ± 0.0.002 and 1.059 ± 0.002 for soil and root enrichment cultures at 99 DAF, respectively. These α factors are similar to, but distinct from values previously obtained from cultures of mesophilic methanogens and are larger than calculated values (1.045) for paddy soil. Fractionation of hydrogen isotopes was also studied in a closed system under 80% H₂. The difference in α factors between soil and root enrichment cultures remained clear. The hydrogen isotope fractionations between culture water and the product CH₄ were −327 ± 14‰ and −319 ± 18‰ for soil enrichments, and −389 ± 17‰ and −382 ± 21‰ for root enrichments at 15 DAF and 99 DAF, respectively.     

Composition of rhodizite

Summary A chemical analysis of rhodizite from Manjaka, Madagascar, establishes the new formula CsAl₄Be₄B₁₁(OH)₄O₂₅. Space group P43m; a₀ 7.317±0.001 Å; density 3.44±0.01 (meas.), 3.47 (calc.); Z=1. The index of refraction, 1.693±0.001 (Na), and the unit cell dimension are identical within the limits state for material from lithia-pegmatites at Manjaka, Antsongombato, Antandrokomby and Ambalalehifotsy, in Madagascar. Hardness 81/2.

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Zusammenfassung Eine chemische Analyse von Rhodizit an Material von Manjaka, Madagaskar, liefert die neue Formel CsAl₄Be₄B₁₁(OH)₄O₂₅. Raumgruppe P43m, a₀=7,317±0,001 Å; Dichte 3,44±0,01 (exp.), 3,47 (ber.); Z=1. Der Brechungsindex, 1,693±0,001 (Na), und die Gitterkonstante sind innerhalb der angegebenen Grenzen für Material von den Lithium-Pegmatiten bei Manjaka, Antsongombato, Antandrokomby und Ambalalehifotsy auf Madagaskar gleich. Härte 81/2.

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Dedicated to ProfessorF. Machatschki on the occasion of his 70th birthday.     

D/H ratios of atmospheric H2 in urban air: results using new methods for analysis of nano-molar H2 samples

We present the results of a study of the concentration and D/H ratio of molecular hydrogen from air in the Los Angeles Basin and adjacent San Gabriel Mountains. These data define a mixing relationship in dimensions of D/H ratio vs. 1/(H₂) which constrains the δD_VSMOW of unpolluted winter air in this region to be ca. +100 to +125 ‰ and that of urban H₂ sources to be ca. −270 ‰. This study makes use of a new method for measuring the deuterium content of molecular hydrogen in small samples (∼100 to 500 cc) of air, which we describe in detail. The method consists of an off-line separation of H₂ followed by introduction to the mass spectrometer in a continuous flow of He. Off-line, all components of an atmospheric gas sample, with the exception of He, H₂, and Ne are condensed by exposure to a cold-trap held at 30 Kelvin. This separation is followed by cryo-transfer of non-condensable gases to a small volume molecular sieve finger, with assist from a mercury piston pump. At the mass spectrometer, the sample is put in line with a continuous flow of He where it is focused on to an additional column of molecular sieve before subsequent introduction into the ion source. Analyses of DH/H₂ ratio have accuracy and precision of ±4 to 7 per mil. Comparison of sample peak area to peak areas of standards of known size allows for determination of H₂ concentration with accuracy and precision of ∼±5%, relative. The method reduces sample size and processing time by several orders of magnitude compared to previous methods, allowing for sampling at proportionately higher spatial and temporal resolution.     

Thermochemistry of yavapaiite KFe(SO4)2: Formation and decomposition

Yavapaiite, KFe(SO₄)₂, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO₄)₂, ΔH°_f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol⁻¹.K⁻¹ and ΔG°_f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe₂O₃ + H₂O has been calculated, at pH₂O = 1 atm, the phase boundary lies at 219 ± 2°C.     

Mercury sources and transformations in a man-perturbed tidal estuary: The Sinnamary Estuary, French Guiana

The distribution, partition and speciation of mercury (Hg) were studied along the redox gradient of an anthropogenically perturbed tropical estuary, the Sinnamary Estuary in French Guiana. This system is a partially mixed estuary characterized by an anoxic freshwater end-member, while the marine end-member consists of the Amazon Plume.The set up of an artificial oxygenation system in the anoxic freshwater end-member generates sharp gradients of major chemical species (iron, sulfides, etc.) coupled with intense organic matter (OM) turnover. The coexistence of oxygenated waters and dissolved sulfides in an organic rich environment depicts the Upper Sinnamary Estuary (USE: part of Sinnamary Estuary under the tidal influence but upstream of the salt intrusion) as a potential site for Hg methylation. The concentrations of all mercury compounds (HgT) in the unfiltered samples (HgT_UNF), in the dissolved (HgT_D) and particulate (HgT_P) phases of the USE average 11 ± 3, 6 ± 2 and 5 ± 3 (i.e. 600 ± 200 pmol g⁻¹) pmol L⁻¹, respectively. Average concentrations of monomethylmercury (MMHg) in the unfiltered (MMHg_UNF), dissolved (MMHg_D) and particulate (MMHg_P) phases were 3.7 ± 1.0, 2.0 ± 0.9 and 1.8 ± 1.2 (i.e. 220 ± 130 pmol g⁻¹) pmol L⁻¹, respectively. Water oxygenation and sulfides concentrations emerged to play a critical role in controlling MMHg levels. Additionally, iron cycling, acid-base mechanisms, and redox-dependent processes were involved in the MMHg partitioning between phases.Overall, the USE constitutes a biogeochemical reactor that gathers partitioning and methylation processes. The permanent MMHg inputs from the anoxic freshwater end-member combined with the intense endogenous Hg methylation ensures high-MMHg levels in both dissolved and particulate phases. To illustrate, the USE exports 60 ± 20% more MMHg_UNF than it imports: 5.5 ± 0.7 vs. 3.5 ± 1.2 kg year⁻¹.     

Partitioning of Nb and Ta between rutile and felsic melt and the fractionation of Nb/Ta during partial melting of hydrous metabasalt

In order to fully assess the role of rutile in fractionation of Nb/Ta during partial melting of hydrous metabasalt, we have measured rutile - felsic melt partition coefficients (D values) for Nb and Ta with tonalitic to trondhjemitic compositions at 1.5-3.5 GPa, 900-1350 °C and ∼5.0-20 wt% H₂O. D_Nb, D_Ta and D_Nb/D_Ta range from 17 ± 1 to 246 ± 13, 34 ± 2 to 232 ± 25 and 0.51 ± 0.04 to 1.06 ± 0.13, respectively. For the compositions investigated, melt composition appears to have no observable effect on the partitioning; the effect of pressure is also slight; whereas temperature and H₂O have marked effects. D_Nb, D_Ta and D_Nb/D_Ta increase with decreasing temperature and H₂O content, showing a reversal of D_Nb/D_Ta from <1.0 to >1.0. Using the data that approached equilibrium and obeyed Henry’s law, expressions describing the dependences of D_Nb, D_Ta and D_Nb/D_Ta on temperature, pressure and melt H₂O content were obtained:

(1)     

Iron and sulfide oxidation within the basaltic ocean crust: implications for chemolithoautotrophic microbial biomass production

Microbial processes within the ocean crust are of potential importance in controlling rates of chemical reactions and thereby affecting chemical exchange between the oceans and lithosphere. We here assess the oxidation state of altered ocean crust and estimate the magnitude of microbial biomass production that might be supported by oxidative and nonoxidative alteration. Compilations of Fe₂O₃, FeO, and S concentrations from DSDP/ODP drill core samples representing upper basaltic ocean crust suggest that Fe³⁺/ΣFe increases from 0.15 ± 0.05 to 0.45 ± 0.15 within the first 10-20 Myr of crustal evolution. Within the same time frame 70 ± 25% of primary sulfides in basalt are oxidized. With an annual production of 4.0 ± 1.8 × 10¹⁵ g of upper (500 ± 200 m) crust and average initial concentrations of 8.0 ± 1.3 wt% Fe and 0.125 ± 0.020 wt% S, we estimate annual oxidation rates of 1.7 ± 1.2 × 10¹² mol Fe and 1.1 ± 0.7 × 10¹¹ mol S. We estimate that 50% of Fe oxidation may be attributed to hydrolysis, producing 4.5 ± 3.0 × 10¹¹ mol H₂/yr.Thermodynamic and bioenergetic calculations were used to estimate the potential chemolithoautotrophic microbial biomass production within ridge flanks. Combined, aerobic and anaerobic Fe and S oxidation may support production of up to 48 ± 21 × 10¹⁰ g cellular carbon (C). Hydrogen-consuming reactions may support production of a similar or larger microbial biomass if iron reduction, nitrate reduction, or hydrogen oxidation by O₂(aq) are the prevailing metabolic reactions. If autotrophic sulfate reduction or methanogenesis prevail, the potential biomass production is 9 ± 7 × 10¹⁰ g C/yr and 3 ± 2 × 10¹⁰ g C/yr, respectively. Combined primary biomass production of up to ∼1 × 10¹² g C/yr may be similar to that fueled by anaerobic oxidation of organic matter in deep-seated heterotrophic systems. These estimates suggest that water-rock reactions may support significant microbial life within ridge flank hydrothermal systems, These estimates suggest that water-rock reactions may support significant microbial life within ridge flank hydrothermal systems.     

Diffusion of lead in plagioclase and K-feldspar: an investigation using Rutherford Backscattering and Resonant Nuclear Reaction Analysis

Chemical diffusion of Pb has been measured in K-feldspar (Or₉₃) and plagioclase of 4 compositions ranging from An₂₃ to An₉₃ under anhydrous, 0.101 MPa conditions. The source of diffusant for the experiments was a mixture of PbS powder and ground feldspar of the same composition as the sample. Rutherford Backscattering (RBS) was used to measure Pb diffusion profiles. Over the temperature range 700–1050°C, the following Arrhenius relations were obtained (diffusivities in m²s^-1):Oligoclase (An₂₃): Diffusion normal to (001): log D = ( – 6.84 ± 0.59) – [(261 ± 13 kJ mol ^–1)/2.303RT]Diffusion normal to (010): log D = ( – 3.40 ± 0.50) – [(335 ± 11 kJ mol ^–1)/2.303RT]Andesine (An₄₃): Diffusion normal to (001): log D = ( – 6.73 ± 0.54) – [(266 ± 12 kJ mol ^–1)/2.303RT] Diffusion normal to (010) appears to be only slightly slower than diffusion normal to (001) in andesine.Labradorite (An₆₇): Diffusion normal to (001): log D = ( – 7.16 ± 0.61) – [(267 ± 13 kJ mol ^–1)/2.303RT] Diffusion normal to (010) is slower by 0.7 log units on average.Anorthite Diffusion normal to (010): log D = ( – 5.43 ± 0.48) – [(327 ± 11 kJ mol ^–1)/2.303RT]K-feldspar (Or₉₃): Diffusion normal to (001): log D = ( – 4.74 ± 0.52) – [(309 ± 16 kJ mol ^–1)/2.303RT] Diffusion normal to (010): log D = ( – 5.99 ± 0.51) – [(302 ± 11 kJ mol ^–1)/2.303RT]In calcic plagioclase, Pb uptake is correlated with a reduction of Ca, indicating the involvement of PbCa exchange in chemical diffusion. Decreases of Na and K concentrations in sodic plagioclase and K-feldspar, respectively, are correlated with Pb uptake and increase in Al concentration (measured by resonant nuclear reaction analysis), suggesting a coupled process for Pb exchange in these feldspars. These results have important implications for common Pb corrections and Pb isotope systematics. Pb diffusion in apatite is faster than in the investigated feldspar compositions, and Pb diffusion rates in titanite are comparable to both K-feldspar and labradorite. Given these diffusion data and typical effective diffusion radii for feldspars and accessory minerals, we may conclude that feldspars used in common Pb corrections are in general less inclined to experience diffusion-controlled Pb isotope exchange than minerals used in U-Pb dating that require a common Pb correction.     

Changes in the composition of organic matter from prodeltaic sediments after a large flood event (Po River, Italy)

The Po River (Italy) experienced a 100-year flood in October 2000. Surface sediments (0-1 cm) from cross-shelf transects were collected in the Po prodelta area (Adriatic Sea) in December 2000, in order to describe the distribution of organic matter (OM) along the main sediment dispersal system immediately after the flood event. Stations were subsequently reoccupied in October 2001 and April 2002. This sampling program provided a special opportunity to characterize the initial surficial flood deposit and the evolution of its associated OM over the course of 2 years. CuO oxidation, elemental, δ¹³C, Δ¹⁴C, and grain-size analyses were carried out to characterize the source, age, and spatial variability of sedimentary OM. Statistical analysis (PERMANOVA) was then applied to investigate temporal changes in different portions of the Po prodelta area. Isotopic and biomarker data suggest that the sedimentary OM in the flood deposit was initially dominated by aged (Δ¹⁴C_Dec-00 = −298.7 ± 56.3‰), lignin-poor OM (Λ_Dec-00 = 1.96 ± 0.33 mg/100 mg OC), adsorbed on the fine material (clay_Dec-00 = 72.1 ± 4.8%) delivered by the flood. In the 2 years following the flood, post-depositional processes significantly increased the content of lignin (Λ_Oct-01 = 2.19 ± 0.51 mg/100 mg OC; Λ_Apr-02 = 2.61 ± 0.63 mg/100 mg OC); and coarse material (silt and sand), while decreasing the contributions from aged OC (Δ¹⁴C_Oct-01 = −255.7 ± 32.8‰; Δ¹⁴C_Apr-02 = −213.2 ± 30.4‰) and fine fraction (clay_Oct-01 = 54.8 ± 9.5%; clay_Apr-02 = 44.6 ± 13.3%). The major changes were observed in the northern and central portions of the prodelta.     

Formation enthalpy of ThSiO4 and enthalpy of the thorite → huttonite phase transition

The standard enthalpy of formation of thorite and huttonite and the enthalpy of the phase transition between these polymorphs were determined using high-temperature oxide melt solution calorimetry and transposed temperature drop calorimetry. Standard enthalpies of formation of thorite and huttonite are reported for the first time and are −2117.6 ± 4.2 kJ/mol and −2110.9 ± 4.7 kJ/mol, respectively. Based on our measurements, thorite and huttonite are metastable relative to SiO₂ (quartz) and ThO₂ (thorianite) at standard conditions, but are presumably stabilized at high temperature by the entropy contribution. Based on the measured enthalpy of the thorite-huttonite phase transition of 6.7 ± 2.5 kJ/mol, a dP/dT slope for the transformation was calculated as −1.21 ± 0.45 MPa/K.     

Experimental phase relations of hydrous peridotites modelled in the system H2O-CaO-MgO-Al2O3,-SiO2

The hydration of peridotites modelled by the system H₂O-CaO-MgO-Al₂O₃-SiO₂ has been treated theoretically after the method of Schreinemakers, and has been investigated experimentally in the temperature range 700°–900° C and in the pressure range of 8–14 kbar. In the presence of excess forsterite and water, the garnet- to spinel-peridotite transition boundary intersects the chlorite dehydration boundary at an invariant point situated at 865±5° C and 15.2±0.3 kbar. At lower pressures, a model spinel lherzolite hydrates to both chlorite- and amphibole-bearing assemblages at an invariant point located at 825±10° C and 9.3±0.5 kbar. At even lower pressures the spinel-to plagioclase-peridotite transition boundary intersects the dehydration curve for amphibole+forsterite at an invariant point estimated to lie at 855±10° C and 6.5±0.5 kbar.Both chlorite and amphibole were characterized along their respective dehydration curves. Chlorite was found to shift continuously from clinochlore, with increasing temperature, to more aluminous compositions. Amphibole was found to be tremolitic with a maximmum of 6 wt.% Al₂O₃.The experimentally determined curves in this study were combined with the determined or estimated stability curves for hydrous melting, plagioclase, talc, anthophyllite, and antigorite to obtain a petrogenetic grid applicable to peridotites, modelled by the system H₂O-CaO-MgO-Al₂O₃-SiO₂, that covers a wide range of geological conditions. Direct applications of this grid, although quite limited, can be made for ultramafic assemblages that have been extensively re-equilibrated at greenschist to amphibolite facies metamorphism and for some highgrade ultramafic assemblages that display clear signs of retrogressive metamorphism.     

Hydrogen isotopes in volcanic plumes: Tracers for remote temperature sensing of fumaroles

In high-temperature volcanic fumaroles (>400 °C), the isotopic composition of molecular hydrogen (H₂) reaches equilibrium with that of the fumarolic H₂O. In this study, we used this hydrogen isotope exchange equilibrium of fumarolic H₂ as a tracer for the remote temperature at volcanic fumaroles. In this remote sensing, we deduced the hydrogen isotopic composition (δD value) of fumarolic H₂ from those in the volcanic plume. To ascertain that we can estimate the δD value of fumarolic H₂ from those in a volcanic plume, we estimated the values in three fumaroles with outlet temperatures of 630 °C (Tarumae), 203 °C (Kuju), and 107 °C (E-san). For this we measured the concentration and δD value of H₂ in each volcanic plume, along with those determined directly at each fumarole. The average and maximum mixing ratios of fumarolic H₂ within a plume’s total H₂ were 97% and 99% (at Tarumae), 89% and 96% (at Kuju), and 97% and 99% (at E-san). We found a linear relationship between the depletion in the δD values of H₂, with the reciprocal of H₂ concentration. Furthermore, the estimated end-member δD value for each H₂-enriched component (−260 ± 30‰ vs. VSMOW in Tarumae, −509 ± 23‰ in Kuju, and −437 ± 14‰ in E-san) coincided well with those observed at each fumarole (−247.0 ± 0.6‰ in Tarumae, −527.7 ± 10.1‰ in Kuju, and −432.1 ± 2.5‰ in E-san). Moreover, the calculated isotopic temperatures at the fumaroles agreed to within 20 °C with the observed outlet temperature at Tarumae and Kuju. We deduced that the δD value of the fumarolic H₂ was quenched within the volcanic plume. This enabled us to remotely estimate these in the fumarole, and thus the outlet temperature of fumaroles, at least for those having the outlet temperatures more than 400 °C. By applying this methodology to the volcanic plume emitted from the Crater 1 of Mt. Naka-dake (the volcano Aso) where direct measurement on fumaroles was impractical, we estimated that the δD value of the fumarolic H₂ to be −172 ± 16‰ and the outlet temperature to be 868 ± 97 °C. The remote temperature sensing using hydrogen isotopes developed in this study is widely applicable to many volcanic systems.