The effect of aqueous diffusion on the fractionation of chlorine and bromine stable isotopes

Diffusive isotopic fractionation factors are important in order to understand natural processes and have practical application in radioactive waste storage and carbon dioxide sequestration. We determined the isotope fractionation factors and the effective diffusion coefficients of chloride and bromide ions during aqueous diffusion in polyacrylamide gel. Diffusion was determined as functions of temperature, time and concentration. The effect of temperature is relatively large on the diffusion coefficient (D) but only small on isotope fractionation. For chlorine, the ratio, D_³⁵Cl/D_³⁷Cl varied from 1.00128 ± 0.00017 (1σ) at 2 °C to 1.00192 ± 0.00015 at 80 °C. For bromine, D_⁷⁹Br/D_⁸¹Br varied from 1.00098 ± 0.00009 at 2 °C to 1.0064 ± 0.00013 at 21 °C and 1.00078 ± 0.00018 (1σ) at 80 °C. There were no significant effects on the isotope fractionation due to concentration. The lack of sensitivity of the diffusive isotope fractionation to anything at the most common temperatures (0 to 30 °C) makes it particularly valuable for application to understanding processes in geological environments and an important natural tracer in order to understand fluid transport processes.     

He diffusion systematics in minerals: Evidence from synthetic monazite and zircon structure phosphates

Helium diffusivity was measured in synthetic rare-earth-element orthophosphates with systematically varying properties to evaluate potential controls on He transport in minerals. In the zircon structure phosphates (in this study, the phosphates of Tb, Dy, Ho, Er, Tm, Yb, and Lu as well as synthetic xenotime, YPO₄), He diffusion is strongly anisotropic. Transport apparently proceeds preferentially through channels aligned with the c-axis. The activation energy for diffusion is almost the same (122 ± 6 kJ/mol) in all members of this family, but there is a monotonic decrease in D_o with atomic number from TbPO₄ (∼10⁵ cm²/s) to LuPO₄ (∼10 cm²/s). The c-parallel channels become increasingly constricted in the same sequence, likely accounting for the systematically decreasing diffusivity. The He closure temperature (r = 1 cm, dT/dt = 10 °C/Myr) increases with atomic number from 44 °C for TbPO₄ to 88 °C for LuPO₄. Diffusion of radiogenic helium from natural zircon and xenotime is much slower than these synthetic analogs predict, suggesting that coupled substitution of REE and P for Zr and Si and/or radiation damage profoundly modify the energetics of interstitial He diffusion. In particular, α-recoil may play a key role by damaging the continuity and integrity of the channels.Monazite structure phosphates (here La, Ce, Pr, Nd, Sm, and Gd phosphate) are far more He retentive than those of the zircon structure. Activation energies increase smoothly with atomic number from LaPO₄ (183 kJ/mol) to NdPO₄ (224 kJ/mol) then decrease again to GdPO₄ (198 kJ/mol). D_o values mimic this pattern, spanning a range from ∼10⁻¹ cm²/s (GdPO₄) to 10⁴ cm²/s (NdPO₄). Nevertheless, He closure temperatures increase monotonically with atomic number, from 300 °C in LaPO₄ to 410 °C in GdPO₄. No evidence was obtained bearing on diffusion anisotropy, but the monazite structure lacks through-going channels so it is not expected. Diffusion parameters for radiogenic helium in natural monazite are similar to those obtained on the synthetic analogs.Ionic porosity is not the primary control on He diffusion in the orthophosphates. Within a given structure and with limited elemental substitution, ionic porosity and He closure temperature are negatively correlated, as predicted. However, differences between crystal structures are far more important than ion packing density: at comparable ionic porosity the monazite structure phosphates have He closure temperatures ∼300 °C higher than the xenotime structure phosphates. Modifications to the structures by radiation damage likely play a similarly significant role in controlling He diffusion.     

The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite

Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249(3-4), 148-161] revealing a correlation between radiogenic ⁴He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He diffusion kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U-Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage.Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic ³He used as a diffusant in step-heating diffusion experiments. In every sample, irradiation increased the activation energy (E_a) and the frequency factor (D_o/a²) of diffusion and yielded a higher He closure temperature (T_c) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in E_a and ln(D_o/a²) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites.To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural ⁴He as the diffusant. At temperatures above 290 °C a systematic change in T_c was observed, with values becoming lower with increasing temperature and time. For example, reduction of T_c from the starting value of 71 to ∼52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in T_c are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same E_a − ln(D_o/a²) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity.Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.     

Lithium isotopes in Guatemalan and Franciscan HP-LT rocks: Insights into the role of sediment-derived fluids during subduction

High-pressure, low-temperature (HP-LT) rocks from a Cretaceous age subduction complex occur as tectonic blocks in serpentinite mélange along the Motagua Fault (MF) in central Guatemala. Eclogite and jadeitite among these are characterized by trace element patterns with enrichments in fluid mobile elements, similar to arc lavas. Eclogite is recrystallized from MORB-like altered oceanic crust, presumably at the boundary between the down-going plate and overlying mantle wedge. Eclogite geochemistry, mineralogy and petrography suggest a two step petrogenesis of (1) dehydration during prograde metamorphism at low temperatures (<500 °C) followed by (2) partial rehydration/fertilization at even lower T during exhumation. In contrast, Guatemalan jadeitites are crystallized directly from low-T aqueous fluid as veins in serpentinizing mantle during both subduction and exhumation. The overall chemistry and mineralogy of Guatemalan eclogites are similar to those from the Franciscan Complex, California, implying similar P-T-x paths.Li concentrations (?90 ppm) in mineral separates and whole rocks (WR) from Guatemalan and Franciscan HP-LT rocks are significantly higher than MORB (4-6 ppm), but similar to HP-LT rocks globally. Li isotopic compositions range from −5‰ to +5‰ for Guatemalan HP-LT rocks, and −4‰ to +1‰ for Franciscan eclogites, overlapping previous findings for other HP-LT suites. The combination of Li concentrations greater than MORB, and Li isotopic values lighter than MORB are inconsistent with a simple dehydration model. We prefer a model in which Li systematics in Guatemalan and Franciscan eclogites reflect reequilibration with subduction fluids during exhumation. Roughly 5-10% of the Li in these fluids is derived from sediments.Model results predict that the dehydrated bulk ocean crust is isotopically lighter (δ⁷Li ? +1 ± 3‰) than the depleted mantle (∼+3.5 ± 0.5‰), while the mantle wedge beneath the arc is the isotopic complement of the bulk crust. A subduction fluid with an AOC-GLOSS composition over the full range of model temperatures (50-600 °C) gives an average fluid δ⁷Li (∼+7 ± 5‰ 1σ) that is isotopically heavier than the depleted mantle. If the lowest temperature steps are excluded (50-260 °C) as too cold to participate in circulation of the mantle wedge, then the average subduction fluid (δ⁷Li = +4 ± 2.3‰ 1σ, is indistinguishable from depleted mantle. Because of the relatively compatible nature of Li in metamorphic minerals, the most altered part of the crust (uppermost extrusives), may retain a Li isotopic signature (∼+5 ± 3‰) heavier than the bulk crust. The range of Li isotopic values for OIB, IAB and MORB overlap, making it is difficult to resolve which of these components may contribute to the recycled component in the mantle using δ⁷Li alone.     

Alpha thermochronology of carbonates

Step-heating experiments on 17 calcites from 11 different samples and 6 dolomites from 5 samples suggest a closure temperature of He in carbonates ∼70 ± 10 °C for a cooling rate of 10 °C/m.y. The bulk closure temperature in some samples may tend slightly higher due to the presence of diffusion domains larger (and therefore more retentive) than the sites in which the majority of He resides. The diffusivity of He in calcite is independent of the genesis of the mineral (igneous, sedimentary, metamorphic) or the source of the He (radiogenic, common, or laboratory induced) and in all samples analyzed the effective diffusion dimension for He is smaller than the size of the crystals investigated. Although calcite is a low-U mineral, this shortcoming can be overcome by analyzing large samples (>2 mm diameter) provided samples have a minimum of ∼0.3 ppm U; samples with smaller concentrations of alpha-producers are unlikely to produce enough radiogenic ⁴He sufficient to overwhelm He present in the crystals at the time they passed through their closure temperature.     

Contrasting isotopic signatures between anthropogenic and geogenic Zn and evidence for post-depositional fractionation processes in smelter-impacted soils from Northern France

Zinc isotopes have been studied along two smelter-impacted soil profiles sampled near one of the largest Pb and Zn processing plants in Europe located in northern France, about 50 km south of Lille. Mean δ⁶⁶Zn values along these two soil profiles range from +0.22 ± 0.17‰ (2σ) to +0.34 ± 0.17‰ (2σ) at the lowest horizons and from +0.38 ± 0.45‰ (2σ) to +0.76 ± 0.14‰ (2σ) near the surface. The δ⁶⁶Zn values in the lowest horizons of the soils are interpreted as being representative of the local geochemical background (mean value +0.31 ± 0.38‰), whereas heavier δ⁶⁶Zn values near the surface of the two soils are related to anthropogenic Zn. This anthropogenic Zn occurs in the form of franklinite (ZnFe₂O₄)-bearing slag grains originating from processing wastes at the smelter site and exhibiting δ⁶⁶Zn values of +0.81 ± 0.20‰ (2σ). The presence of franklinite is indicated by EXAFS analysis of the topsoil samples from both soil profiles as well as by micro-XANES analysis of the surface horizon of a third smelter-impacted soil from a distant site. These results indicate that naturally occurring Zn and smelter-derived Zn exhibit significantly different δ⁶⁶Zn values, which suggests that zinc isotopes can be used to distinguish between geogenic and anthropogenic sources of Zn in smelter-impacted soils. In addition to a possible influence of additional past sources of light Zn (likely Zn-sulfides and Zn-sulfates directly emitted by the smelter), the light δ⁶⁶Zn values in the surface horizons compared to smelter-derived slag materials are interpreted as resulting mainly from fractionation processes associated with biotic and/or abiotic pedological processes (Zn-bearing mineral precipitation, Zn complexation by organic matter, and plant uptake of Zn). This conclusion emphasizes the need for additional Zn isotopic studies before being able to use Zn isotopes to trace sources and pathways of this element in surface environments.     

Low-temperature anisotropic diffusion of helium in zircon: Implications for zircon (U-Th)/He thermochronometry

In the last decade the zircon (U-Th)/He (ZHe) thermochronometer has been applied to a variety of geologic problems. Although bulk diffusion coefficients for He in zircon are available from laboratory step-heating experiments, little is known about the diffusion mechanism(s) and their dependence on the crystallographic structure of zircon. Here, we investigate the diffusion of He in perfectly crystalline zircon using atomistic simulation methods that provide insights into the structural pathways of He migration in zircon. Empirical force fields and quantum-mechanical calculations reveal that the energy barriers for He diffusion are strongly dependent on structure. The most favorable pathway for He diffusion is the [0 0 1] direction through the open channels parallel to the c-axis (, activation energy for tracer diffusion of a He atom along [0 0 1]). In contrast, energy barriers are higher in other directions where narrower channels for He diffusion are identified, such as [1 0 0], [1 0 1], and [1 1 0] (ΔE^∗ of 44.8, 101.7, and 421.3 kJ mol⁻¹, respectively). Molecular dynamics simulations are in agreement with these results and provide additional insight in the diffusion mechanisms along different crystallographic directions, as well as the temperature dependence. Below the closure temperature of He in zircon [T_c ∼ 180 °C, Reiners P. W., Spell T. L., Nicolescu S., and Zanetti K. A. (2004) Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with Ar-40/Ar-39 dating. Geochim. Cosmochim. Acta68, 1857-1887], diffusion is anisotropic as He moves preferentially along the [0 0 1] direction, and calculated tracer diffusivities along the two most favorable directions differ by approximately five orders of magnitude (D_[001]/D_[100] ∼ 10⁵, at T = 25 °C). Above this temperature, He atoms start to hop between adjacent [0 0 1] channels, along [1 0 0] and [0 1 0] directions (perpendicular to the c-axis). The diffusion along [1 0 0] and [0 1 0] is thermally activated, such that at higher temperatures, He diffusion in zircon becomes nearly isotropic (D_[001]/D_[100] ∼ 10, at T = 580 °C). These results suggest that the anisotropic nature of He diffusion at temperatures near the closure temperature should be considered in future diffusivity experiments. Furthermore, care should be taken when making geologic interpretations (e.g., exhumation rates, timing of cooling, etc.) from this thermochronometer until the effects of anisotropic diffusion on bulk ages and closure temperature estimates are better quantified.     

Ar/Ar dating of hydrothermal activity, biota and gold mineralization in the Rhynie hot-spring system, Aberdeenshire, Scotland

This study presents a new high-precision ⁴⁰Ar/³⁹Ar age for the Devonian hot-spring system at Rhynie. Hydrothermal K-feldspar sampled from two veins that represent feeder conduits and a hydrothermally altered andesite wall rock, date the hydrothermal activity, the fossilised biota, and syn - K-feldspar gold mineralization at 403.9 ± 2.1 Ma (2σ). Oxygen isotope data for the parent fluid (−4‰ to 2‰) show that the K-feldspar was precipitated from a dominantly meteoric fluid, which mixed with magmatic fluids from a degassing magma chamber.The ⁴⁰Ar/³⁹Ar age (403.9 ± 2.1 Ma [2σ]) when recalculated (407.1 ± 2.2 Ma [2σ]) with respect to the astronomically tuned age for Fish Canyon sanidine (28.201 ± 0.023 Ma [1σ]), also provides a robust marker for the polygonalis-emsiensis Spore Assemblage Biozone within the Pragian-?earliest Emsian. Furthermore, the age identifies the Devonian pull-apart volcano-sedimentary basins of the British and Irish Caledonides (and their root zones), as specific targets for future gold exploration.     

Silicon isotopes in meteorites and planetary core formation

The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ³⁰Si differences are resolved between chondrites (δ³⁰Si = −0.49 ± 0.15‰, 2σ_SD) and achondrites (δ³⁰Si = −0.47 ± 0.11‰, 2σ_SD), although enstatite chondrites are consistently lighter (δ³⁰Si = −0.63 ± 0.07‰, 2σ_SD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ³⁰Si_{BSE−meteorite∗} = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ³⁰Si_{BSE−meteorite∗} value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.     

Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model

Helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal, a quantity that varies over the lifetime of the apatite. Using recently published laboratory data we develop and investigate a new kinetic model, the radiation damage accumulation and annealing model (RDAAM), that adopts the effective fission-track density as a proxy for accumulated radiation damage. This proxy incorporates creation of crystal damage proportional to α-production from U and Th decay, and the elimination of that damage governed by the kinetics of fission-track annealing. The RDAAM is a version of the helium trapping model (HeTM; Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249, 148-161), calibrated by helium diffusion data in natural and partially annealed apatites. The chief limitation of the HeTM, now addressed by RDAAM, is its use of He concentration as the radiation damage proxy for circumstances in which radiation damage and He are not accumulated and lost proportionately from the crystal.By incorporating the RDAAM into the HeFTy computer program, we explore its implications for apatite (U-Th)/He thermochronometry. We show how (U-Th)/He dates predicted from the model are sensitive to both effective U concentration (eU) and details of the temperature history. The RDAAM predicts an effective He closure temperature of 62 °C for a 28 ppm eU apatite of 60 μm radius that experienced a 10 °C/Ma monotonic cooling rate; this is 8 °C lower than the 70 °C effective closure temperature predicted using commonly assumed Durango diffusion kinetics. Use of the RDAAM is most important for accurate interpretation of (U-Th)/He data for apatite suites that experienced moderate to slow monotonic cooling (1-0.1 °C/Ma), prolonged residence in the helium partial retention zone, or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss. Under common circumstances the RDAAM predicts (U-Th)/He dates that are older, sometimes much older, than corresponding fission-track dates. Nonlinear positive correlations between apatite (U-Th)/He date and eU in apatites subjected to the same temperature history are a diagnostic signature of the RDAAM for many but not all thermal histories.Observed date-eU correlations in four different localities can be explained with the RDAAM using geologically reasonable thermal histories consistent with independent fission-track datasets. The existence of date-eU correlations not only supports a radiation damage based kinetic model, but can significantly limit the range of acceptable time-temperature paths that account for the data. In contrast, these datasets are inexplicable using the Durango diffusion model. The RDAAM helps reconcile enigmatic data in which apatite (U-Th)/He dates are older than expected using the Durango model when compared with thermal histories based on apatite fission-track data or other geological constraints. It also has the potential to explain at least some cases in which (U-Th)/He dates are actually older than the corresponding fission-track dates.     

Diffusion of Ar in muscovite

Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced ⁴⁰Ar^∗ loss that is revealed in ⁴⁰Ar/³⁹Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate ⁴⁰Ar^∗ loss from the grains that remained intact during hydrothermal treatment at 10 kbar permitting calculation of diffusion coefficients in the temperature range 730-600 °C. Diffusion data generated in this manner yield an activation energy (E) of 63 ± 7 kcal/mol and frequency factor (D_o) of 2.3  cm²/s. Experiments at 20 kbar yield diffusivities lower by about an order of magnitude and correspond to an activation volume of ∼14 cm³/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature (T_c) of 425 °C for a 100 μm radius grain cooling at 10 °C/Ma at 10 kbar (T_c = 405 °C at 5 kbar. Age and log (r/r_o) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous ⁴⁰Ar/³⁹Ar thermochronology to low grade, regionally metamorphosed terranes.     

Experimental measurement of the solubility of bismuth phases in water vapor from 220 °C to 300 °C: Implications for ore formation

Preliminary measurements were carried out of the solubility of the O_2-buffering assemblage bismuth + bismite (Bi₂O₃) in aqueous liquid–vapor and vapor-only systems at temperatures of 220, 250 and 300 °C. All experiments were carried out in Ti reaction vessels and were designed such that the Bi solids were contained in a silica tube that prevented contact with liquid water at any time during the experiment. Two blank (no Bi solids present) liquid–vapor experiments at 220 °C yielded Bi concentrations (±1σ) in the condensed liquid of 0.22 ± 0.02 mg/L, whereas the solubility measurements at this temperature yielded an average value of approximately 6 ± 9 mg/L, with replicate experiments ranging from 0.3 to 26 mg/L. Although the 6 mg/L value is associated with a considerable degree of uncertainty, the experiments do indicate transport of Bi through the vapor phase. Measured Bi concentrations in the condensed liquid at 250 °C were in the same range as those at 220 °C, whereas those at 300 °C were significantly lower (i.e., all below the blank value). Vapor-only experiments necessarily contained much smaller initial volumes of water, thereby making the results more susceptible to contamination. Single blank runs at 220 and 300 °C yielded Bi concentrations of 82 and 16 mg/L, respectively. Measured concentrations (±1σ) of Bi in the vapor-only solubility experiments at 220 °C were 235 ± 78 mg/L for an initial water volume of 0.5 mL, and at 300 °C were 56 ± 30 mg/L and 33 ± 21 for initial water volumes of 1 and 2 mL, respectively, suggesting strong preferential partitioning of Bi into the vapor. The results indicate a negative dependence of Bi solubility on temperature, but are inconclusive with respect to the dependence of Bi solubility on water density or fugacity.     

Noble gas composition of the solar wind as collected by the Genesis mission

We present the elemental and isotopic composition of noble gases in the bulk solar wind collected by the NASA Genesis sample return mission. He, Ne, and Ar were analyzed in diamond-like carbon on a silicon substrate (DOS) and ^84,86Kr and ^129,132Xe in silicon targets by UV laser ablation noble gas mass spectrometry. Solar wind noble gases are quantitatively retained in DOS and with exception of He also in Si as shown by a stepwise heating experiment on a flown DOS target and analyses on other bulk solar wind collector materials. Solar wind data presented here are absolutely calibrated and the error of the standard gas composition is included in stated uncertainties. The isotopic composition of the light noble gases in the bulk solar wind is as follows: ³He/⁴He: (4.64 ± 0.09) × 10⁻⁴, ²⁰Ne/²²Ne: 13.78 ± 0.03, ²¹Ne/²²Ne: 0.0329 ± 0.0001, ³⁶Ar/³⁸Ar 5.47 ± 0.01. The elemental composition is: ⁴He/²⁰Ne: 656 ± 5, and ²⁰Ne/³⁶Ar 42.1 ± 0.3. Genesis provided the first Kr and Xe data on the contemporary bulk solar wind. The preliminary isotope and elemental composition is: ⁸⁶Kr/⁸⁴Kr: 0.302 ± 0.003, ¹²⁹Xe/¹³²Xe: 1.05 ± 0.02, ³⁶Ar/⁸⁴Kr 2390 ± 150, and ⁸⁴Kr/¹³²Xe 9.5 ± 1.0. The ³He/⁴He and the ⁴He/²⁰Ne ratios in the Genesis DOS target are the highest solar wind values measured in exposed natural and artificial targets. The isotopic composition of the other noble gases and the Kr/Xe ratio obtained in this work agree with data from lunar samples containing “young” (∼100 Ma) solar wind, indicating that solar wind composition has not changed within at least the last 100 Ma. Genesis could provide in many cases more precise data on solar wind composition than any previous experiment. Because of the controlled exposure conditions, Genesis data are also less prone to unrecognized systematic errors than, e.g., lunar sample analyses. The solar wind is the most authentic sample of the solar composition of noble gases, however, the derivation of solar noble gas abundances and isotopic composition using solar wind data requires a better understanding of fractionation processes acting upon solar wind formation.     

Solubility of cyclooctasulfur in pure water and sea water at different temperatures

The solubility of cyclooctasulfur in water and sea water at various temperatures in the range between 4 and 80 °C was determined. Cyclooctasulfur in equilibrium with rhombic sulfur reacted with hot acidic aqueous potassium cyanide to form thiocyanate anion which was measured by anion chromatography. Sulfur solubility in pure water was found to increase with temperature by more than 78 times: from 6.1 nM S₈ at 4 °C to 478 nM S₈ at 80 °C. The following thermodynamic values for solubilisation of S₈ in water were calculated from the experimental data: K° = 3.01 ± 1.04 × 10⁻⁸, ΔG_r° = 42.93 ± 0.73 kJ mol⁻¹, ΔH_r° = 47.4 ± 3.6 kJmol⁻¹, ΔS_r° = 15.0 ± 11.7 J mol⁻¹ K⁻¹). Solubility of cyclooctasulfur in sea water was found to be 61 ± 13% of the solubility in pure water regardless of the temperature.     

Ar loss in experimentally deformed muscovite and biotite with implications for Ar/Ar geochronology of naturally deformed rocks

The effects of deformation on radiogenic argon (⁴⁰Ar^∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO₂) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating ⁴⁰Ar^∗ loss of 0-35% in muscovite and 2-3% ⁴⁰Ar^∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation ⁴⁰Ar/³⁹Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% ⁴⁰Ar^∗ loss relative to the undeformed muscovite. The in situ UV laser ablation ⁴⁰Ar/³⁹Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% ⁴⁰Ar^∗ loss. No spatial correlation is observed between in situ⁴⁰Ar/³⁹Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed ⁴⁰Ar^∗ loss in the experimentally treated muscovite can be utilized to predict average ⁴⁰Ar^∗ diffusion dimensions. Maximum ⁴⁰Ar/³⁹Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for ⁴⁰Ar^∗ diffusion ?700 μm. The UV laser ablation and IR laser incremental ⁴⁰Ar/³⁹Ar ages indicating ⁴⁰Ar^∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ?100 μm. These results support a hypothesis of grain-scale ⁴⁰Ar^∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for ⁴⁰Ar^∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both ⁴⁰Ar^∗ retention and intragrain distribution in deformed mica. Intragrain ⁴⁰Ar/³⁹Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.     

Geotechnical characterisation of stratocone crater wall sequences, White Island Volcano, New Zealand

Geotechnical characterisation is undertaken for 3 broad units comprising the bulk of the stratigraphy identified on White Island Volcano, Bay of Plenty, New Zealand, an active island stratovolcano. Field and laboratory measurements were used to describe rock mass characteristics for jointed lava flow units, and ring shear tests were undertaken to derive residual strength parameters for joint infilling materials within the lavas. Rock Mass Rating (RMR) and Geological Strength Index (GSI) values were calculated and converted to Mohr-Coulomb strength parameters using the Hoek-Brown criterion. Backanalysis of known landslide scarps was used to derive strength parameters for brecciated rock masses and hydrothermally altered rock masses. Andesite lava flows have high intact strength (σ_ci = 184 ± 50 MN m^− 2; γ = 24.7 ± 0.3 kN m^− 3) and typically 3 wide, infilled joint sets, one parallel to flow direction and two steeply inclined, with spacings of 0.3-1.7 m. Joints are rough, with estimated friction angles for clean joints of ?_j = 42-47°. Joint infill materials are clayey silts derived from weathering of wall rocks and primary volcanic sources; they have low plastic (54%) and liquid (84%) limits and residual strength values of c_r = 0 kN m^− 2 and ?_r = 23.9 ± 3.1°. RMR values range from 70 to 73, giving calculated strength parameters of c′ = 1161-3391 kN m^− 2 and ?′ = 50.5-62.3°. Backanalysis suggests brecciated rock masses have c′ = 0 kN m^− 2 and ?′ = 35.4°, whereas GSI observations in the field suggest higher cohesion (c′ = 306-719 kN m^− 2) and a range of friction angles bracketing the backanalysed result (?′ = 30.6-41.7°). Hydrothermally altered rock masses have c′ = 369 kN m^− 2 and ?′ = 14.9°, indicating considerable loss of strength, especially frictional resistance, compared with the fresh lava units. Values measured at outcrop scale in this study are in keeping with other published values for similar volcanic edifices; backanalysed data suggest weaker rock mass properties than those determined at outcrop. This is interpreted as a scale issue, whereby rock mass characteristics of a large rock mass (crater wall scale) are weaker than those of small outcrops, due in part to the overestimation of friction angle from measurements on small exposures.     

Combined U-Th/He and Ar/Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the ⁴⁰Ar/³⁹Ar and U-Th/He methods. The objective of the study is to compare the recently demonstrated U-Th/He age method, which uses basaltic olivine phenocrysts, with ⁴⁰Ar/³⁹Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U-Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U-Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2σ uncertainties), which are in general equal to or younger than ⁴⁰Ar/³⁹Ar ages. Basalt from the Polulu sequence on Kohala gives a U-Th/He age of 354 ± 54 ka and a ⁴⁰Ar/³⁹Ar age of 450 ± 40 ka. All of the U-Th/He ages, and all but one spurious ⁴⁰Ar/³⁹Ar ages conform to the previously proposed stratigraphy and published ¹⁴C and K-Ar ages. The ages also compare favorably to U-Th whole rock-olivine ages calculated from ²³⁸U-²³⁰Th disequilibria. The U-Th/He and ⁴⁰Ar/³⁹Ar results agree best where there is a relatively large amount of radiogenic ⁴⁰Ar (>10%), and where the ⁴⁰Ar/³⁶Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U-Th/He and ⁴⁰Ar/³⁹Ar ages do not agree within uncertainty. U-Th/He and ⁴⁰Ar/³⁹Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U-Th/He ages were unattainable while ⁴⁰Ar/³⁹Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U-Th/He and ⁴⁰Ar/³⁹Ar methods are complimentary for basalts with ages of order 100-500 ka.     

Calibration of the calcite-water oxygen-isotope geothermometer at Devils Hole, Nevada, a natural laboratory

The δ¹⁸O of ground water (−13.54 ± 0.05 ‰) and inorganically precipitated Holocene vein calcite (+14.56 ± 0.03 ‰) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 °C of 1.02849 ± 0.00013 (1000 ln α_{calcite-water} = 28.09 ± 0.13). Using the commonly accepted value of ∂(α_{calcite-water})/∂T of −0.00020 K⁻¹, this corresponds to a 1000 ln α_{calcite-water} value at 25 °C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 °C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a δ¹⁸O of water, from which the calcite precipitated, that is too negative by 1.5 ‰ using a temperature of 33.7 °C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite-water oxygen isotopic fractionation factor may be in order.Assuming the Devils Hole oxygen isotopic value of α_{calcite-water} represents thermodynamic equilibrium, many marine carbonates are precipitated with a δ¹⁸O value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches ¹⁶O in the solid carbonate over ¹⁸O, feigning oxygen isotopic equilibrium.     

Seawater temperature proxies based on DSr, DMg, and DU from culture experiments using the branching coral Porites cylindrica

In order to investigate the incorporation of Sr, Mg, and U into coral skeletons and its temperature dependency, we performed a culture experiment in which specimens of the branching coral (Porites cylindrica) were grown for 1 month at three seawater temperatures (22, 26, and 30 °C). The results of this study showed that the linear extension rate of P. cylindrica has little effect on the skeletal Sr/Ca, Mg/Ca, and U/Ca ratios. The following temperature equations were derived: Sr/Ca (mmol/mol) = 10.214(±0.229) − 0.0642(±0.00897) × T (°C) (r² = 0.59, p < 0.05); Mg/Ca (mmol/mol) = 1.973(±0.302) + 0.1002(±0.0118) × T (°C) (r² = 0.67, p < 0.05); and U/Ca (μmol/mol) = 1.488(±0.0484) − 0.0212(±0.00189) × T (°C) (r² = 0.78, p < 0.05). We calculated the distribution coefficient (D) of Sr, Mg, and U relative to seawater temperature and compared the results with previous data from massive Porites corals. The seawater temperature proxies based on D calibrations of P. cylindrica established in this study are generally similar to those for massive Porites corals, despite a difference in the slope of D_U calibration. The calibration sensitivity of D_Sr, D_Mg, and D_U to seawater temperature change during the experiment was 0.64%/°C, 1.93%/°C, and 1.97%/°C, respectively. These results suggest that the skeletal Sr/Ca ratio (and possibly the Mg/Ca and/or U/Ca ratio) of the branching coral P. cylindrica can be used as a potential paleothermometer.     

Constraints on the loss of matrix-sited helium during vacuum crushing of mafic phenocrysts

Vacuum crushing is an efficient technique to selectively release the mantle-derived helium component trapped within olivine and pyroxene phenocrysts. However, contrary to previous assumptions, recent studies have shown that this method may liberate significant matrix-sited cosmogenic ³He (³He_c) or radiogenic ⁴He (⁴He^∗). Because this loss may bias both the determination of magmatic ³He/⁴He ratios and the accuracy of ³He_c measurements, it is essential to understand what mechanism is responsible and under what conditions matrix helium loss is manifest. To address this question, olivines and pyroxenes with various amounts of matrix-sited ³He (from 10⁷ to 10¹¹ at. g⁻¹) were crushed in air or in vacuum using several crushing devices. Sample temperature was controlled during each crushing experiment, and ranged from 25 to 325 °C. The resulting powders were then sieved to obtain several homogeneous grain fractions ranging between <10 and >300 μm. The ³He_c concentrations measured in each fraction clearly show that significant ³He_c loss (>20%) affects only the finest fraction (<10 μm) and, importantly, only under hot conditions (here T ?300 °C). Even the smallest fractions (<10 μm) quantitatively retain matrix-sited ³He_c when crushed under cold conditions (T ?25 °C), regardless of the duration and energy of crushing. These results invalidate the model previously proposed by (Yokochi R., Marty B., Pik R. and Burnard P. (2005) High ³He/⁴He ratios in peridotite xenoliths from SW Japan revisited: evidence for cosmogenic ³He released by vacuum crushing. Geochem. Geophys. Geosyst.6, doi:10.1029/2004GC000836) that involved spallation tracks and implied that the magnitude of loss was mainly controlled by the grain size. Moreover, new diffusion experiments were carried out to constrain the diffusivity of matrix-sited helium in crushed olivines. When used to model diffusive ³He_c loss as a function of grain size during crushing, these new data predict the observed release fairly well. Therefore, we conclude that temperature-enhanced volume diffusion is one of the main mechanisms controlling the release of ³He_c during crushing. For future applications, special attention should thus be paid to control the grain size, the crushing duration, and the temperature of the sample.