Grain boundary partitioning of Ar and He

An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. ^(37)Ar and ^(4)He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt). These data yield values of GB-area-normalized partitioning, K¯^(Ar)_(ITM), with units of (Ar/m^3 of solid)/(Ar/m^2 of GB) of 6.8 x 10^3 – 2.4 x 104 m^(-1). Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness (δ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient (K^(Ar)_(GB) = X^(Ar)_(crystals)/X^(Ar)_(GB) may be determined. For reasonable values of δ, K^(Ar)_(GB) is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with K¯^(He)_(ITM) between 4 x 10^3 and 4 x 10^4 cm^(-1), similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e., <0.1% fluid). Heterogeneities in grain size within dry equilibrated systems will correspond to significant differences in bulk rock noble gas content

Tectonic control on southern Sierra Nevada topography, California

In this study we integrate the apatite (U-Th)/He thermochronometric technique with geomorphic, structural, and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late Cenozoic faulting in the southern Sierra Nevada. The geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low-relief landscape, little affected by internal deformation, to a more complex landscape affected by numerous faults. Regionally, the relict landscape surface is readily resolved by age-elevation relationships of apatite He ages coupled to geomorphology. These relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting. On the basis of 63 new apatite He ages and stratigraphic data from proximal parts of the San Joaquin basin we resolve two sets of normal faults oriented approximately N–S and approximately NW. Quaternary west-side-up normal faulting along the N–S Breckenridge–Kern Canyon zone has resulted in a southwest step over from the Owens Valley system in the controlling structure on the regional west tilt of Sierran basement. This zone has also served as a transfer structure partitioning Neogene-Quaternary extension resulting from normal displacements on the NW fault set. This fault system for the most part nucleated along Late Cretaceous structures with late Cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region

A Framework for Evaluating Variation in (U-Th)/He Datasets

This paper presents a framework for evaluating variation in (U-Th)/He datasets. The framework is objective, repeatable, and based on compatibility of thermal histories derived from individual (U-Th)/He dates. The structure of this new method includes three fundamental steps. First, the allowable thermal history of each individual grain is quantitatively constrained with a model. Second, the thermal histories of all grains from a sample are visualized on the same axes. Third, the compatibility of the allowable thermal histories of each individual grain is evaluated. This allows a user to assess whether variation among single grain dates can plausibly be explained (referred to here as legitimate) or not (illegitimate). Additionally, this methodology allows for accurate representation of the impact that illegitimate variation has on the thermal history constraints of a sample. We demonstrate the application of this new framework using a variety of examples from the literature, as well as with synthetic data. Modeling presented here is executed using the modeling software QTQt (version 5.6.0) and the He diffusion kinetics based on the radiation damage accumulation and annealing model, but the framework is designed to be easily adaptable to any modeling software and diffusion parameters

Uplift and erosion of the San Bernardino Mountains associated with transpression along the San Andreas fault, California, as constrained by radiogenic helium thermochronometry

Apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the San Andreas fault. By documenting aspects of the low-temperature (40°–100°C) thermal history of the tectonic blocks of the San Bernardino Mountains in southern California, we have placed new constraints on the magnitude and timing of uplift. Old helium ages (64–21 Ma) from the large Big Bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the Late Cretaceous period. These ages imply that the surface of the plateau may have been exposed in the late Miocene and was uplifted only ∼1 km above the Mojave Desert in the last few Myr by thrusting on the north and south. A similar range in helium ages (56–14 Ma) from the higher San Gorgonio block to the south suggests that its crest was once contiguous with that of the Big Bear block and that its greater elevation represents a localized uplift that the Big Bear plateau did not experience. The structure of the San Gorgonio block appears to be a gentle antiform, based on the geometry of helium isochrons and geologic constraints. Young ages (0.7–1.6 Ma) from crustal slices within the San Andreas fault zone indicate uplift of a greater magnitude than blocks to the north. These smaller blocks probably experienced ≥3–4 km of uplift at rates ≥1.5 mm/yr in the past few Myr and would stand ≥2.5 km higher than the Big Bear plateau if erosion had not occurred. The greater uplift of tectonic blocks adjacent to and within the San Andreas fault zone is more likely the result of oblique displacement along high-angle faults than motion along the thrust fault that bounds the north side of the range. We speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike-slip system. Transpression thus appears to have been accommodated by both vertical displacement within the San Andreas fault zone and thrusting on adjacent structures

Helium distributions in ocean island basalt olivines revealed by X-ray computed tomography and single-grain crushing experiments

X-ray computed tomography of individual olivine crystals in basalts from Ofu and Olosega islands, American Samoa, reveals that a small fraction of the olivines contain the vast majority of the fluid inclusions. Single-grain crushing experiments demonstrate that He and CO_2 reside primarily in these inclusions. Low CO_2 pressures in most grains, corresponding to depths of less than 1 km, provide evidence of ubiquitous decrepitation and associated pressure reduction in the fluid inclusions. Even so, the olivines with the highest inclusion volumes yielded sufficient He to obtain precise He concentrations and isotopic compositions. Within analytical uncertainty, ^3He/^4He ratios are homogeneous among the olivines from each basalt, but among basalts, the ratios range from 21 to 35 Ra. The total range in C/^3He ratio within the analyzed olivines is from 3.6 × 10^7 to 1.5 × 10^(10), and varies by nearly an order of magnitude within the olivines from each basalt. We postulate that this wide range of C/^3He ratios is caused by grain-scale decoupling of C and ^3He due to extensive He diffusion out of fluid inclusions through the olivine lattice during magma ascent and cooling. If so, primary Ofu-Olosega magmas probably had C/^3He ratios less than 4 × 10^8, which is lower than previous estimates for hotspot magmas

Evidence for >5 Ma paleo-exposure of an Eocene–Miocene paleosol of the Bohnerz Formation, Switzerland

We obtained (U–Th)/He formation ages and cosmogenic ^3He concentrations for pisoliths from a paleosol of the Bohnerz Formation (Siderolithic) of Central Europe. The paleosol is exposed in the Almenbühl quarry near Lohn, Canton Schaffhausen, Switzerland. The paleosol consists of red clay of 3–4 m thickness developed on deeply weathered Jurassic limestone and overlain by Early Miocene conglomerates. The (U–Th)/He formation ages of the pisoliths are between 50 Ma and 8 Ma, with most ages being older than 17 Ma. There is a sharp decline in the frequency of ages at the time of burial of the paleosol at 17 Ma. These ages are inconsistent with the previous assumption that the Bohnerz Formation formed in a Cretaceous to Early Eocene laterite in a tropical climate. We propose that the Bohnerz Formation more closely resembles Terra Rossa soils, which do not require a tropical climate to form. The ^3He concentration in the pisoliths is roughly constant with depth throughout the paleosol at 300 Matoms/g. We interpret this as the result of soil convection during cosmic ray exposure. The minimum exposure duration at the surface of the paleosol is ∼5 Ma. A simple model of soil convection shows that the true exposure duration of the paleosol is approximately 10–20 Ma. These results indicate that the clay soils of the Bohnerz formation were continuously exposed at the surface for millions of years. Since the paleosol was covered by conglomerate since 17 Ma, the ^3He measured here was produced by cosmic ray exposure before burial. Cosmogenic ^3He concentrations measured in fine-grained soil iron-oxides (<1 μm) are similar to those measured in pisoliths. This might indicate that fine-grained iron-oxides are retentive to helium and might be used for studying the formation and cosmic ray exposure of modern soils and paleosols

U-loss associated with laser-heating of hematite and goethite in vacuum during (U-Th)/He dating and prevention using high O₂ partial pressure

Single-aliquot (U-Th)/He dating of hematite has been used to study iron-oxide precipitation in various environments, but we show there is an important challenge to the method: highly retentive hematite samples require temperatures of > 1000° C to be completely degassed, whereas the temperature for major U-loss is ∼980° C. This leads to erroneously high (U-Th)/He ages. Through the analysis of U, Th, and Sm of hematite and goethite samples, we show the degree of U-loss at this temperature and demonstrate that prolonged heating at temperatures of 950° C can lead to U-loss. We show that loss of U in goethite and hematite samples is associated with phase change from hematite to magnetite as Fe is reduced. The onset temperature of vacuum reduction of hematite can be increased from about 800-900° C in vacuum to approximately 1250° C in an oxygen partial pressure of 100 mbar. We show that samples can be outgassed to extract helium at 1150° C without U-loss in an O₂-rich atmosphere during heating, which does not increase the analytical blanks. We describe our implementation and automation of the procedure. An average age calculated on a reference hematite sample from replicate aliquots (n=12), which were analyzed using this procedure, has a relative uncertainty of 2% (1σ), and is within uncertainty of the previously measured two-aliquot age. We suggest this O₂ degassing procedure as a way to precisely and reproducibly determine single-aliquot hematite and goethite (U-Th)/He ages

Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia

New structural, paleomagnetic, and apatite (U-Th)/He results from the continental margin inboard of the Queen Charlotte fault (~54°N) delineate patterns of brittle faulting linked to transform development since ~50 Ma. In the core of the orogen, ~250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ~50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until <9 Ma. Local tilting in the core of the orogen is associated with glacially driven, post-4 Ma exhumation. Integration with previous results shows that post-50 Ma dextral and normal faulting affected a region ~250 km inboard of the transform and ~300 km along strike. Initially widespread, the zone of active extension narrowed and migrated toward the transform ~25 Ma after initiation of the transform, while dextral faulting continued throughout the region. Differential amounts of post-50 Ma extension created oroclines at the southern and northern boundaries of the deformed region. This region approximately corresponds to continental crust that was highly extended just prior to transform initiation. Variation in Neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid-Cretaceous plutons. Similarities to the Gulf of California suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops