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.     

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.     

Source and movement of helium in the eastern Morongo groundwater Basin: The influence of regional tectonics on crustal and mantle helium fluxes

We assess the role of fracturing and seismicity on fluid-driven mass transport of helium using groundwaters from the eastern Morongo Basin (EMB), California, USA. The EMB, located ∼200 km east of Los Angeles, lies within a tectonically active region known as the Eastern California Shear Zone that exhibits both strike-slip and extensional deformation. Helium concentrations from 27 groundwaters range from 0.97 to 253.7 × 10⁻⁷ cm³ STP g⁻¹H₂O, with corresponding ³He/⁴He ratios falling between 1.0 and 0.26 R_A (where R_A is the ³He/⁴He ratio of air). All groundwaters had helium isotope ratios significantly higher than the crustal production value of ∼0.02 R_A. Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, in situ production within the aquifer, and extraneous fluxes (both crustal and mantle derived). All samples contained a mantle helium-3 (³He_m) flux in the range of 4.5 to 1351 × 10⁻¹⁴ cm³ STP ³He cm⁻² yr⁻¹ and a crustal flux (J₀) between 0.03 and 300 × 10⁻⁷ cm³ STP ⁴He cm⁻² yr⁻¹. Groundwaters from the eastern part of the basin contained significantly higher ³He_m and deep crustal helium-4 (⁴He_dc) concentrations than other areas, suggesting a localized source for these components. ⁴He_dc and ³He_m are strongly correlated, and are associated with faults in the basin. A shallow thermal anomaly in a >3,000 m deep graben in the eastern basin suggests upflow of fluids through active faults associated with extensional tectonics. Regional tectonics appears to drive large scale crustal fluid transport, whereas episodic hydrofracturing provides an effective mechanism for mantle-crust volatile transport identified by variability in the magnitude of degassing fluxes (³He_m and J₀) across the basin.     

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.     

Atmospheric helium isotope ratio: Possible temporal and spatial variations

The atmospheric ³He/⁴He ratio has been considered to be constant on a global scale, because the residence time of helium is significantly longer than the mixing time in the atmosphere. However, this ratio may be decreasing with time owing to the anthropogenic release of crustal helium from oil and natural gas wells, although this observation has been disputed. Here, we present the ³He/⁴He ratios of old air trapped in historical slags in Japan and of modern surface air samples collected at various sites around the world, measured with a newly developed analytical system. In air helium extracted from metallurgical slag found at refineries in operation between AD 1603 and 1907 in Japan, we determined a mean ³He/⁴He ratio of (5106 ± 108) × 10^-5 R_HESJ (where R_HESJ is the ³He/⁴He ratio of the Helium Standard of Japan), which is consistent with the previously reported value of (5077 ± 59) × 10^-5 R_HESJ for historical slags in France and United Arab Emirates and about 4% higher than that of average modern air, (4901 ± 4) × 10^-5 R_HESJ. This result implies that the air ³He/⁴He ratio has decreased with time as expected by anthropogenic causes. Our modern surface air samples revealed that the ³He/⁴He ratio increases from north to south at a rate of (0.16 ± 0.08) × 10^-5 R_HESJ/degree of latitude, suggesting that the low ³He/⁴He ratio originates in high-latitude regions of the northern hemisphere, which is consistent with the fact that most fossil fuel is extracted and consumed in the northern hemisphere.     

Helium, radon and radiocarbon studies on a regional aquifer system of the North Gujarat-Cambay region, India

The study reports the age evolution of groundwater as it flows from the recharge area through a regional alluvial aquifer system in North Gujarat-Cambay region in western India. Radiocarbon (¹⁴C), ⁴He and ⁴He / ²²²Rn dating methods have been employed. Sediments from a drill core in the Cambay Basin were also analysed for uranium (U) and thorium (Th) concentrations and the measured values have been used to estimate the ⁴He and ²²²Rn production rate for groundwater age calculations. Additionally, factors controlling the distribution of ²²²Rn, ⁴He and temperature anomalies in groundwater, vis-à-vis their relation to the tectonic framework and lithology of the study area, have also been examined.The multi-isotope study indicated a reasonable correspondence in groundwater age estimates by the three methods employed. The groundwater ¹⁴C ages increased, progressively, in the groundwater flow direction: from the foothills of Aravalli Mountains in the east, and reached a value of ∼35 ka towards the region of lowest elevation, linking Little Rann of Kachchh (LRK)-Nalsarovar (NS)-Gulf of Khambhat (GK) in the western part of the study area. In this region, groundwater ages obtained for free flowing thermal wells and springs employing ⁴He and ⁴He / ²²²Rn systematics are in the order of million years. Such anomalous ages are possibly due to enhanced mobilisation and migration of ‘excess helium’ from hydrothermal circulation vents along deep-seated faults. Excluding such anomalous cases and considering all uncertainties, presently estimated ⁴He and ⁴He / ²²²Rn groundwater ages are in reasonable agreement with ¹⁴C age estimates in the Cambay Basin for helium release factor (Λ_He) value of 0.4 ± 0.3. The ⁴He method also indicated west-southwards progression of groundwater ages up to ∼100 ka beyond the Cambay Basin.Large ‘excess helium’ concentrations are also seen to be generally associated with anomalous groundwater temperatures (> 35 °C) and found to overlie some of the basement faults in the study area, particularly along the east and the west flanks of the Cambay Basin. Groundwater ²²²Rn activities in most of the study area are 800 ± 400 dpm/l. But, a thermal spring at Tuwa on the east flank of the Cambay Basin, having granitic basement at shallow depth, recorded the highest ²²²Rn activity (∼63,000 dpm/l).     

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.     

New insights into the carrier phase(s) of extraterrestrial He in geologically old sediments

To better understand the composition, characteristics of helium diffusion, and size distribution of interplanetary dust particles (IDPs) responsible for the long-term retention of extraterrestrial ³He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to ∼90 Myr. The leaching experiments suggest that the host phase(s) of ³He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al₂O₃, and SiC, but are consistent with extraterrestrial silicates, Fe-Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the ³He release profiles from the magnetic and non-magnetic components of the pelagic clays are remarkably similar. Because helium diffusion is likely to be controlled by mineral chemistry and structure, the stepped heating results suggest a single carrier that may be magnetite, or more probably a phase associated with magnetite. Furthermore, the stepped outgassing experiments indicate that about 20% of the ³He will be lost through diffusion at seafloor temperatures after 50 Myrs, while sedimentary rocks exposed on the Earth’s surface for the same amount of time would lose up to 60%. The absolute magnitude of the ³He loss is, however, likely to depend upon the ³He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50-100 μm in diameter are responsible for the extraterrestrial ³He in geologically old sediments [Stuart, F.M., Harrop, P.J., Knott, S., Turner, G., 1999. Laser extraction of helium isotopes from Antarctic micrometeorites: source of He and implications for the flux of extraterrestrial ³He flux to earth. Geochim. Cosmochim. Acta63, 2653-2665], our sieving experiment demonstrates that at most 20% of the ³He is carried by particles greater than 50 μm in diameter. The size-distribution of the ³He-bearing particles implies that extraterrestrial ³He in sediments record the IDP flux rather than the micrometeorite flux.     

Partitioning of Na between olivine and melt: An experimental study with application to the formation of meteoritic Na2O-rich chondrule glass and refractory forsterite grains

We report experimentally determined 1 atm olivine/melt D_Na partitioning data for low f_O2, a variety of melt compositions and a temperature range of 1325-1522 °C. We demonstrated that high-current electron microprobe analyses (EPMA, I = 500 nA, 600 s on the peak) allow quantitative determination of Na₂O in olivine down to ∼10 μg/g. The mean olivine/melt D_Na from 12 experimental runs is 0.0031 ± 0.0007 (1σ). This is the recommended value for low pressures and a wide range of natural compositions.This result is applied to the problem of the origin of alkalis in chondrules and the formation of chondritic refractory forsterite grains. The data on Semarkona (LL3.0) chondrules show that Na₂O is primordial and was present during olivine crystallization. For refractory forsterite grains from Murchison (CM2), we demonstrate that high CaO contents are not a result of equilibration with Na₂O-rich melts, but require high activities of CaO during their formation.     

Diffuse gas emissions at the Ukinrek Maars,Alaska: Implications for magmatic degassing and volcanic monitoring

Diffuse CO₂ efflux near the Ukinrek Maars, two small volcanic craters that formed in 1977 in a remote part of the Alaska Peninsula, was investigated using accumulation chamber measurements. High CO₂ efflux, in many places exceeding 1000 g m⁻² d⁻¹, was found in conspicuous zones of plant damage or kill that cover 30,000–50,000 m² in area. Total diffuse CO₂ emission was estimated at 21–44 t d⁻¹. Gas vents 3-km away at The Gas Rocks produce 0.5 t d⁻¹ of CO₂ that probably derives from the Ukinrek Maars basalt based on similar δ¹³C values (∼−6‰), ³He/⁴He ratios (5.9–7.2 R_A), and CO₂/³He ratios (1–2 × 10⁹) in the two areas. A lower ³He/⁴He ratio (2.7 R_A) and much higher CO₂/³He ratio (9 × 10¹⁰) in gas from the nearest arc-front volcanic center (Mount Peulik/Ugashik) provide a useful comparison. The large diffuse CO₂ emission at Ukinrek has important implications for magmatic degassing, subsurface gas transport, and local toxicity hazards. Gas–water–rock interactions play a major role in the location, magnitude and chemistry of the emissions.     

An extraterrestrial He-based timescale for the Paleocene-Eocene thermal maximum (PETM) from Walvis Ridge, IODP Site 1266

In the deep-sea, the Paleocene-Eocene Thermal Maximum (PETM) is often marked by clay-rich condensed intervals caused by dissolution of carbonate sediments, capped by a carbonate-rich interval. Constraining the duration of both the dissolution and subsequent cap-carbonate intervals is essential to computing marine carbon fluxes and thus testing hypotheses for the origin of this event. To this end, we provide new high-resolution helium isotope records spanning the Paleocene-Eocene boundary at ODP Site 1266 in the South Atlantic. The extraterrestrial ³He, ³He_ET, concentrations replicate trends observed at ODP Site 690 by Farley and Eltgroth (2003). By assuming a constant flux of ³He_ET we constrain relative changes in accumulation rates of sediment across the PETM and construct a new age model for the event. In this new chronology the zero carbonate layer represents 35 kyr, some of which reflects clay produced by dissolution of Paleocene (pre-PETM) sediments. Above this layer, carbonate concentrations increase for ∼165 kyr and remain higher than in the latest Paleocene until 234 ⁺⁴⁸/₋₃₄ kyr above the base of the clay. The new chronology indicates that minimum δ¹³C values persisted for a maximum of 134 ⁺²⁷/₋₁₉ kyr and the inflection point previously chosen to designate the end of the CIE recovery occurs at 217 ⁺⁴⁴/₋₃₁ kyr. This allocation of time differs from that of the cycle-based age model of Röhl et al. (2007) in that it assigns more time to the clay layer followed by a more gradual recovery of carbonate-rich sedimentation. The new model also suggests a longer sustained δ¹³C excursion followed by a more rapid recovery to pre-PETM δ¹³C values. These differences have important implications for constraining the source(s) of carbon and mechanisms for its subsequent sequestration, favoring models that include a sustained release of carbon after an initial pulse.     

Compressibility of molten Apollo 17 orange glass and implications for density crossovers in the lunar mantle

We performed density measurements on a synthetic equivalent of lunar Apollo 17 74,220 “orange glass”, containing 9.1 wt% TiO₂, at superliquidus conditions in the pressure range 0.5-8.5 GPa and temperature range 1723-2223 K using the sink/float technique. In the lunar pressure range, two experiments containing pure forsterite (Fo₁₀₀) spheres at 1.0 GPa and 1727 K, and at 1.3 GPa-1739 K, showed neutral buoyancies, indicating that the density of molten orange glass was equal to the density of Fo₁₀₀ at these conditions (3.09 ± 0.02 g cm⁻³). A third tight sink/float bracket using Fo₉₀ spheres corresponds to a melt density of 3.25 ± 0.02 g cm⁻³ at ∼2.8 GPa and ∼1838 K.Our data predict a density crossover for the molten orange glass composition with equilibrium orthopyroxene at ∼2.8 GPa, equivalent to a depth of ∼600 km in the lunar mantle, and a density of ∼3.25 g cm⁻³. This crossover depth is close to the orange glass multiple saturation point, representing its minimum formation depth, at the appropriate oxygen fugacity (2.8-2.9 GPa). A density crossover with equilibrium olivine is predicted to fall outside the lunar pressure range (>4.7 GPa), indicating that molten orange glass is always less dense than its equilibrium olivines in the Moon. Our data therefore suggest that that lunar liquids with orange glass composition are buoyant with respect to their source region at P < ∼2.8 GPa, enabling their initial rise to the surface without the need for additional external driving forces.Fitting the density data to a Birch-Murnaghan equation of state at 2173 K leads to an array of acceptable solutions ranging between 16.1 and 20.3 GPa for the isothermal bulk modulus K₂₁₇₃ and 3.6-8 for its pressure derivative K′, with best-fit values K₂₁₇₃ = 18.8 GPa and K′ = 4.4 when assuming a model 1 bar density value of 2.86 g cm⁻³. When assuming a slightly lower 1 bar density value of 2.84 g cm⁻³ we find a range for K₂₁₇₃ of 14.4-18.0 and K′ 3.7-8.7, with best-fit values of 17.2 GPa and 4.5, respectively.     

The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ¹⁷O = 0.95 * δ¹⁸O − 3.24; R² = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo_99.6-99.8; δ¹⁸O = −7.2‰ to −5.5‰; δ¹⁷O = −9.6‰ to −8.2‰) are more ¹⁶O-rich than the isolated olivine grains (Fo_39.6-86.8; δ¹⁸O = 3.1‰ to 5.1‰; δ¹⁷O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo_97.2-99.8) with oxygen-isotope compositions (δ¹⁸O = −5.2‰ to 5.9‰; δ¹⁷O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ¹⁷O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with ¹⁶O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R² = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM_slope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with ¹⁶O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.     

Thermal history of the unsaturated zone at Yucca Mountain,Nevada, USA

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (T_h) up to ∼80 °C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite δ¹⁸O values, and depositional timing is constrained by the ²⁰⁷Pb/²³⁵U ages of chalcedony or opal in the deposits. Fluid inclusion T_h from 50 samples of calcite and four samples of fluorite range from ∼35 to ∼90 °C. Calcite δ¹⁸O values range from ∼0 to ∼22‰ (SMOW) but most fall between 12 and 20‰. The highest T_h and the lowest δ¹⁸O values are found in the older calcite. Calcite T_h and δ¹⁸O values indicate that most calcite precipitated from water with δ¹⁸O values between −13 and −7‰, similar to modern meteoric waters.     

The LaPaz Icefield 04840 meteorite: Mineralogy, metamorphism, and origin of an amphibole- and biotite-bearing R chondrite

The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ∼13% ferri-magnesiohornblende and ∼0.4% phlogopite by volume. Other minerals include olivine (Fo₆₂), orthopyroxene (En₆₉Fs₃₀Wo₁), albite (An₈Ab₉₀Or₂), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe³⁺, as determined by Mössbauer spectrometry and electron microprobe chemical analyses. Fe³⁺/Fe_tot values are olivine ?5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (^ANa + ^IVAl ↔ ^A□ + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based on the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60 °C from a amphibole-plagioclase thermometer; P_H2O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; P_CO2 unconstrained; f_O2 at QFM + 0.5 log units; . The hydrogen in LAP 04840 is very heavy, an average δD value of +3660 ± 75‰ in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.     

Environmental memory and a possible seasonal bias in the stable isotope composition of (U-Th)/He-dated goethite from the Canadian Arctic

Goethite (Ax-2) from Axel Heiberg Island (∼80°N) on the margin of the Arctic Ocean is the dominant mineral in a sample of “petrified” Eocene wood, but U, Th, and He measurements suggest that the goethite (α-FeOOH) crystallized in the latest Miocene/Pliocene (ca. 5.5 to 2.8 Ma). Measured δD and δ¹⁸O values of Ax-2 are −221 (±6)‰ and −9.6 (±0.5)‰, respectively. The inferred δD and δ¹⁸O values of the ancient water were about −139‰ and −18.6‰, respectively, with a calculated temperature of crystallization of 3 (±5)°C, which compares with the modern summer (J-J-A) temperature of 3 °C and contrasts with a modern MAT of −19 °C. Published results from various biological proxies on nearby Ellesmere Island indicate a Pliocene (∼4 Ma) MAT of either −6 or −0.4 °C and corresponding seasonal amplitudes of about 18 or 13 °C. A conductive heat flow model suggests that a temperature of 3 °C could represent goethite crystallization at depths of ∼100-200 cm (for MAT = −6 °C) or ∼250-450 cm (for MAT = −0.4 °C) over seasonally restricted intervals of time.The δ¹⁸O value of the Ax-2 water (−18.6‰) is more positive than the modern J-J-A precipitation (−22‰). In combination, the paleotemperatures and δ¹⁸O values of ancient waters (from Ax-2 and published results from three Eocene or Pliocene proxy sites on Axel Heiberg and Ellesmere Islands) are consistent with a warm season bias in those isotopic proxies. The results are also consistent with higher proportions of J-J-A precipitation in the annual total. If so, this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates, and suggests that the Arctic hydrologic cycle, as expressed in the seasonal distribution and isotopic composition of precipitation (perhaps modified by a warmer Arctic Ocean), differed from modern.The δ¹³C value of the Fe(CO₃)OH component in the Ax-2 goethite is +6.6‰, which is much more positive than expected if crystallizing goethite incorporated CO₂ derived primarily from oxidation of relict Eocene wood with δ¹³C values of about −24‰. This apparent paradox may be resolved if the goethite is a product of oxidation of ¹³C-rich siderite, which had previously replaced wood in an Eocene methanogenic burial environment. Thus, the goethite retains a carbon isotope “memory” of a diagenetic Eocene event, but a δD and δ¹⁸O record of the latest Miocene/Pliocene Arctic climate.     

Searching for parental kimberlite melt

Constraining the composition of primitive kimberlite magma is not trivial. This study reconstructs a kimberlite melt composition using vesicular, quenched kimberlite found at the contact of a thin hypabyssal dyke. We examined the 4 mm selvage of the dyke where the most elongate shapes of the smallest calcite laths suggest the strongest undercooling. The analyzed bulk compositions of several 0.09-1.1 mm² areas of the kimberlite free from macrocrysts were considered to be representative of the melt. The bulk analyses conducted with a new “chemical point-counting” technique were supplemented by modal estimates, studies of mineral compositions, and FTIR analysis of olivine phenocrysts. The melt was estimated to contain 26-29.5 wt% SiO₂, ∼7 wt% of FeO_T, 25.7-28.7 wt% MgO, 11.3-15 wt% CaO, 8.3-11.3 wt% CO₂, and 7.6-9.4 wt% H₂O. Like many other estimates of primitive kimberlite magma, the melt is too magnesian (Mg# = 0.87) to be in equilibrium with the mantle and thus cannot be primary. The observed dyke contact and the chemistry of the melt implies it is highly fluid (η = 10¹-10³ Pa s at 1100-1000 °C) and depolymerized (NBO/T = 2.3-3.2), but entrains with 40-50% of olivine crystals increasing its viscosity. The olivine phenocrysts contain 190-350 ppm of water suggesting crystallization from a low SiO₂ magma (a_SiO2 below the olivine-orthopyroxene equilibrium) at 30-50 kb. Crystallization continued until the final emplacement at depths of few hundred meters which led to progressively more Ca- and CO₂-rich residual liquids. The melt crystallised phlogopite (6-10%), monticellite (replaced by serpentine, ∼10%), calcite rich in Sr, Mg and Fe (19-27%), serpentine (29-31%) and minor amounts of apatite, ulvöspinel-magnetite, picroilmenite and perovskite. The observed content of H₂O can be fully dissolved in the primitive melt at pressures greater than 0.8-1.2 kbar, whereas the amount of primary CO₂ in the kimberlite exceeds CO₂ soluble in the primitive kimberlite melt. A mechanism for retaining CO₂ in the melt may require a separate fluid phase accompanying kimberlite ascent and later dissolution in residual carbonatitic melt. Deep fragmentation of the melt as a result of volatile supersaturation is not inevitable if kimberlite magma has an opportunity to evolve.     

Regional and temporal variations in CO2/He,He/He and δC along the North Anatolian Fault Zone,Turkey

New He and C relative abundance, isotope and concentration results from nine geothermal locations situated along an 800-km transect of the North Anatolian Fault Zone (NAFZ), Turkey, that were monitored during the period November 2001–November 2004, are reported. The geothermal waters were collected every 3–6 months to study possible links between temporal geochemical variations and seismic activity along the NAFZ. At the nine sample locations, the He isotope ratios range from 0.24 to 2.3R_A, δ¹³C values range from −4.5 to +5.8‰, and CO₂/³He ratios range from 5 × 10⁹ to 5 × 10¹⁴. The following geochemical observations are noted: (1) the highest ³He/⁴He ratios are found near the Galatean volcanic region, in the central section of the NAFZ, (2) at each of the nine sample locations, the ³He/⁴He ratios are generally constant; however, CO₂/³He ratios and He contents both show one order of magnitude variability, and δ¹³C values show up to ∼4‰ variability, and (3) at all locations (except Re?adiye), δ¹³C values show positive correlations with CO₂ contents. The results indicate that at least three processes are necessary to account for the geochemical variations: (1) binary mixing between crustal and mantle-derived volatiles can explain the general characteristics of ³He/⁴He ratios, δ¹³C values, and CO₂/³He ratios at the nine sample locations; (2) preferential degassing of He from the geothermal waters is responsible for variations in CO₂/³He values and He contents at each sample location; and (3) CO₂ dissolution followed by calcite precipitation is responsible for variations in CO₂ contents and δ¹³C values at most locations. For each of the geochemical parameters, anomalies are defined in the temporal record by values that fall outside two standard deviations of average values at each specific location. Geochemical anomalies that may be related to seismic activity are recorded on June 28, 2004 at Yalova, where a M = 4.2 earthquake occurred 43 days earlier at 15 km distance from the sample location, and on April 7, 2003 at Efteni, where a M = 4.0 earthquake occurred 44 days later at a distance of 12 km. At both locations, the sampling periods containing geochemical anomalies were preceded by an increase in M ? 3 earthquakes occurring within 60 days and less than 40 km distance.     

Chlorine isotopic composition in seafloor serpentinites and high-pressure metaperidotites. Insights into oceanic serpentinization and subduction processes

Bulk-rock chlorine content and isotopic composition (δ³⁷Cl) were determined in oceanic serpentinites, high-pressure metaperidotites and metasediments in order to gain constraints on the global chlorine cycle associated with hydrothermal alteration and subduction of oceanic lithosphere. The distribution of insoluble chlorine in oceanic serpentinites was also investigated by electron microprobe. The hydrothermally-altered ultramafic samples were dredged along the South West Indian Ridge and the Mid-Atlantic Ridge. The high-pressure metamorphic samples were collected in the Western Alps: metaperidotites in the Erro-Tobbio unit and metasediments in the Schistes Lustrés nappe.Oceanic serpentinites show relatively large variations of bulk-rock Cl contents and δ³⁷Cl values with mean values of 1105 ± 596 ppm and −0.7 ± 0.4‰, respectively (n = 8; 1σ). Serpentines formed after olivine (meshes) show lower Cl content than those formed after orthopyroxene (bastites). In bastites of two different samples, Cl is positively correlated with Al₂O₃ and negatively correlated with SiO₂. These relationships are interpreted as reflecting preferential Cl-incorporation into the bastite structure distorted by Al (substituted for Si) rather than different alteration conditions between olivine and orthopyroxene minerals. High-pressure metaperidotites display relatively homogeneous Cl contents and δ³⁷Cl values with mean values of 467 ± 88 ppm and −1.4 ± 0.1‰, respectively (n = 7; 1σ). A macroscopic high-pressure olivine-bearing vein, formed from partial devolatilization of serpentinites at ∼2.5 GPa and 500-600 °C, shows a Cl content and a δ³⁷Cl value of 603 ppm and −1.6‰, respectively. Metasediments (n = 2) show low whole-rock Cl contents (<15 ppm Cl) that did not allow Cl isotope analyses to be obtained.The range of negative δ³⁷Cl values observed in oceanic serpentinites is likely to result from water-rock interaction with fluids that have negative δ³⁷Cl values. The homogeneity of δ³⁷Cl values from the high-pressure olivine-bearing vein and the metaperidotite samples implies that progressive loss of Cl inherited from oceanic alteration throughout subduction did not significantly fractionate Cl isotopes. Chlorine recycled in subduction zones via metaperidotites should thus show a range of δ³⁷Cl values similar to the range found in oceanic serpentinized peridotites.     

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.