3He Production Research Articles

The SPICE Project: Calibrated production rates of cosmogenic 3He and 21Ne in olivine and pyroxene from the 72 ka SP basalt flow, Arizona, USA

The formally named SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt flow that is preserved in the desert climate of northern Arizona, USA. The flow has an 40Ar/39Ar age of 72 ± 4 ka (2σ) and has undergone negligible erosion and/or burial, making its surface an ideal site for direct calibration of cosmogenic nuclide production rates. Production rates for cosmogenic 3He (3Hec) and 21Ne (21Nec) have been determined from SP flow olivine and pyroxene in this study. The error-weighted mean, sea-level, high latitude (SLHL) total reference production rates of 3He in olivine and pyroxene have identical values of 135 ± 8 at/g/yr (2σx‾; standard error) using time-independent Lal (1991)/Stone (2000) (St) scaling factors. These production rates decrease to identical values of 130 ± 8 at/g/yr (2σx‾; standard error) when 3He measurements are standardized to the CRONUS-P pyroxene standard. The St-scaled, error-weighted mean, total reference production rates of 21Ne in olivine and pyroxene are 48.4 ± 2.9 at/g/yr and 26.5 ± 1.7 at/g/yr (2σx‾; standard error), respectively, increasing to 49.3 ± 3.0 at/g/yr and 27.0 ± 1.7 at/g/yr (2σx‾; standard error), respectively, when standardized to the CREU-1 quartz standard. 3He and 21Ne production rates (St) overlap within 2σ uncertainty with other St-scaled production rates in the literature. SLHL 3He and 21Ne production rates in SP flow olivine and pyroxene are nominally lower if time-dependentLm and Sa scaling factors are used. Olivine and pyroxene both have identical, error-weighted mean SLHL production rates of 127 ± 8 at/g/yr (2σx‾; standard error) using Lm scaling factors and CRONUS-P standardized 3He measurements. These production rates decrease to identical values of 110 ± 7 at/g/yr (2σx‾; standard error) for olivine and pyroxene when using Sa scaling factors. The Lm-scaled, error-weighted mean, total reference production rates of 21Ne in olivine and pyroxene are 48.1 ± 2.8 at/g/yr and 26.4 ± 1.7 at/g/yr (2σx‾; standard error), respectively, when standardized to the CREU-1 quartz standard. The error weighted mean, local 21Ne/3He production rate ratio in olivine is 0.358 ± 0.009 (2σx‾; standard error), which increases to 0.378 ± 0.012 when using CREU-1 standardized 21Ne production rates and CRONUS-P standardized 3He production rates. The error weighted mean, local 21Ne/3He production rate ratio in pyroxene is 0.197 ± 0.006, or 0.208 ± 0.008 when 21Ne and 3He are standardized to CREU-1 and CRONUS-P, respectively. The updated, CREU-1 standardized 21Nec rate (St) in SPICE quartz is 16.5 ± 1.1 at/g/yr. Production of 21Ne in coexisting SPICE olivine (ol), pyroxene (px), and quartz (qz) (standardized to CREU-1; Fenton et al., 2019; this study) yields error-weighted mean, local production rate ratios of 3.00 ± 0.13 (2σx‾) and 1.64 ± 0.08 (2σx‾) for 21Neol/21Neqz and 21Nepx/21Neqz, respectively. This study suggests that production rates of 3He and 21Ne in SPICE olivine and pyroxene agree well with St- and Lm-scaled global mean production rates in the literature. It also indicates that CRONUS-P and CREU-1 standardizations yield production rates in even stronger agreement with these global mean rates.

Measurement of the production of (anti)nuclei in p–Pb collisions at [formula omitted

Measurements of (anti)proton, (anti)deuteron, and (anti)3He production in the rapidity range −1<y<0 as a function of the transverse momentum and event multiplicity in p–Pb collisions at a center-of-mass energy per nucleon–nucleon pair sNN=8.16TeV are presented. The coalescence parameters B2 and B3, measured as a function of the transverse momentum per nucleon and of the mean charged-particle multiplicity density, confirm a smooth evolution from low to high multiplicity across different collision systems and energies. The ratios between (anti)deuteron and (anti)3He yields and those of (anti)protons are also reported as a function of the mean charged-particle multiplicity density. A comparison with the predictions of the statistical hadronization and coalescence models for different collision systems and center-of-mass energies favors the coalescence description for the deuteron-to-proton yield ratio with respect to the canonical statistical model.

Results on light (anti)nuclei production in Pb–Pb collisions with ALICE at the LHC

The high collision energies reached at the LHC lead to significant production yields of light (anti)nuclei in proton-proton, p–Pb and Pb–Pb collisions. Light (anti)nuclei are identified using their specific energy loss (dE/dx), measured in the Time Projection Chamber, and their velocity using the Time-Of-Flight detector. The excellent tracking and particle identification capabilities of the ALICE experiment, as well as its low material budget, make this detector unique for measurements of these rarely produced particles. Results on (anti)deuteron, (anti)triton, (anti)3He and (anti) He production in Pb–Pb collisions at p sNN = 5.02 TeV, including their transverse momentum (pT) spectra, production yields and coalescence parameters BA, are presented. These results will be compared to the expectations of coalescence and statistical hadronization models to obtain information on the production mechanism of light (anti)nuclei in heavy-ion collisions. Furthermore, the first measurements of the d and 3He absorption cross section are shown.

Supernova versus cosmic ray origin for exotic nuclides in geomaterials: A test using 3He with 60Fe in marine sediments

We report 3He and 4He concentrations in 57 sediment samples from the southeast Indian Ocean where 60Fe excesses were previously identified in a subset of the same samples (Wallner et al., 2016). The coupled 60Fe-3He data allow further evaluation of two competing hypotheses: (1) a nearby supernova (SN) showered Earth with exotic radionuclides such as 60Fe during the last 3 million years, or (2) 60Fe in terrestrial archives was generated by reactions of galactic cosmic rays (GCRs) on micrometeorite grains that were irradiated for hundreds of millions of years in the interstellar medium, where 3He production by GCRs is larger than the solar wind 3He flux.Piston core ELT49-53 sediments show no correlation between 3He and 60Fe, and sedimentary 3He appears to be dominated by the presence of interplanetary dust particles (IDPs). Because 3He is not supplied in significant amounts by SN ejecta, the absence of a 3He-60Fe correlation provides additional, although circumstantial evidence for the supernova hypothesis. Large uncertainties in the relatively small number of sediment 60Fe measurements currently limit a firmer conclusion.The extraterrestrial 3He accumulation rate in ELT49-53 from 3.2 to 1.7 Ma was 0.88 ± 0.26 × 10−12 cm3 STP/g/kyr, similar to IDP 3He flux estimates from previous sedimentary and ice core records that span both shorter and longer time scales. 4He and 60Fe accumulation rates during this time interval were 0.11 ± 0.04 × 10−6 cm3 STP/cm2/kyr and 1.9 ± 0.5 × 104 atoms/cm2/kyr. Bulk sediment [4He] is strongly anti-correlated with sediment CaCO3 content, evidence for modulation of the terrigenous and cosmic sedimentary fractions primarily by changes in biogenic carbonate deposition. Although the dominant terrigenous source has not been uniquely identified at the Indian Ocean deposition site, it resembles eolian material from the continental interior of Australia, and shows a narrow range of 3He/4He (from 2 to 4 × 10−8, 0.015–0.030 RA) over the last ∼3 Myrs.

Subducting Indian Lithosphere Controls the Deep Carbon Emission in Lhasa Terrane, Southern Tibet

AbstractThe India‐Asia orogenic zone is considered to participate significantly in the Earth's geological carbon budget. However, the carbon cycling process at orogenic belts with ongoing continental subduction is poorly constrained. Here, new He‐C‐N systematics of hydrothermal gases in Lhasa terrane are reported to explore carbon cycling and the role of the subducting Indian slab. Except for samples where the CO2 content and C isotopic signature were modified by calcite precipitation, gas samples in Lhasa terrane display low 3He/4He ratios of 0.015–0.895 RA but with increasing mantle‐helium input from south to north. Our modeling results show that the continuous subduction of the Indian continental slab has caused high Th‐U contents and time‐integrated 4He production rates in the present mantle wedge beneath the Lhasa terrane. The N‐S helium isotopic trend can be interpreted as a northward‐decreasing mantle enrichment. Compared to volatiles showing crustal helium but mantle‐like δ13C ratios, mantle‐helium‐richer fluids display heavier δ13C ratios, higher CO2/3He and molar C/N values, indicating a substantial carbonate input. We propose that the heavier carbon isotopes are inherited from the carbonated‐enriched mantle (CEM) rather than the carbonate assimilation of the overlying continental crust. Combined with an eastward increase in dip angle of the Indian lithosphere, we suggest that the slab rollback has caused asthenosphere heat flow and consequent decarbonation in CEM beneath northern and eastern Lhasa terrane, leading to enhanced isotopically heavier carbon release. Our study highlights the controlling role of subducting Indian lithosphere on the regional distribution of volatiles in the Lhasa terrane.

Cosmological constraints on heavy sterile neutrinos

We consider heavy sterile neutrinos νs , with mass in the range 10 MeV ∼ ms ∼ mπ ∼ 135 MeV, thermally produced in the early universe and freezing out after the QCD phase transition. The existence of these neutrinos may alter the value of the effective number of neutrino species N eff, measured by the cosmic microwave background (CMB), and the 4He production during the BBN. We provide a detailed account of the solution of the relevant Boltzmann equations and we identify the parameter space constrained by current Planck satellite data and forecast the parameter space probed by future Stage-4 ground-based CMB (CMB-S4) observations.

Determining the role of diffusion and basement flux in controlling 4He distribution in sedimentary basin fluids

The transport of helium from the crystalline continental basement and overlying Phanerozoic sedimentary formations to the near surface can be controlled by both diffusive and advective processes. The relative role of each is vital to helium resource prediction, and important in quantifying the residence times of fluids relevant to groundwater resources, hydrocarbon systems, geologic repositories for nuclear waste and carbon sequestration. The Williston Basin, North America, is a prominent sedimentary basin, providing an excellent natural laboratory to assess these processes. Here, we report noble gas isotopic and composition data for 28 gas samples from natural gas wells that sample different stratigraphic horizons down to the basement (Cretaceous to the Cambrian). Helium isotope ratios show a resolvable mantle 3He component (up to 4.7%) in most samples. Neon isotopic compositions of the Cambrian samples are consistent with a crystalline basement gas contribution. Both helium and neon isotopic observations provide evidence for the contribution of conservative noble gases from the crystalline basement or deeper into the overlying sedimentary basin. 4He groundwater concentrations in the sedimentary formations, calculated from 4He/20Ne values in gas samples, are in excess of in situ U+Th 4He production in some shallow units and depleted in others, providing further evidence of cross formation gas contributions.The highest 4He groundwater concentrations can be compared with the results obtained from a fully-coupled vertical scale transport model characterising diffusive-dominated transport through a static groundwater column. The model includes the 4He flux into the basin from the Precambrian basement and quantifies the apparent basement 4He flux to be between 0.8 - 1.6 × 10−6 mol 4He/m2 yr, comparable to the steady-state flux estimated for the average continental crust (1.47 × 10−6 mol 4He/m2 yr) (Torgersen, 2010). The lithologies in which 4He concentrations are significantly lower than the reference model predictions are consistent with a history of water flooding and produced water disposal in those formations over decades of hydrocarbon production. While an advective component cannot be ruled out, this work demonstrates the importance of both diffusion and the basin architecture development in controlling 4He flux into and out of different lithologies. The assumption of negligible 4He loss from the top surface of a lithology is often made when determining the 4He age of its groundwater. In the Williston Basin, this study shows that deeper lithologies may reach steady state at different stages of basin development, with shallower lithologies sometimes also showing significant 4He loss from their top surface. In the Williston Basin, 4He diffusive loss from the target lithology must be considered to accurately interpret 4He groundwater residence times and accumulation potential.

Study of total, absorption, and 3He and 3H production cross sections in 4He-proton collisions

Light ion breakup cross sections are important for studies of cosmic ray interactions in the inter-stellar medium or radiation protection considerations of energy deposition in shielding and tissues. Abrasion cross sections for heavy ion reactions have been modeled using the Glauber model in the large mass limit or Eikonal form of the optical potential model. Here we formulate an abrasion model for 4He fragmentation on protons using the Glauber model avoiding the large mass limit and include a model for final state interactions. Calculations of energy dependent total, absorption, elastic and breakup cross sections for 4He into 3He or 3H with proton targets are shown to be in good agreement with experiments for energies from 100 to 100,000 MeV/u. The Glauber model for light nuclei with and without a large mass limit approximation is shown to be in fair agreement above 300 MeV/u, however important differences occur at lower energies.

The composition of fluids stored in the central Mexican lithospheric mantle: Inferences from noble gases and CO2 in mantle xenoliths

We present the first isotopic (noble gases and CO2) characterization of fluid inclusions coupled to Raman microspectroscopy analyses in mantle xenoliths from Central Mexico, a geodynamically complex area where the Basin and Range extension was superimposed on the Farallon subduction (terminated at 28 Ma). To characterize the isotopic signature of the Central Mexican lithospheric mantle, we focus on fluid inclusions entrapped in mantle xenoliths found in deposits of the Joya Honda maar (JH), a Quaternary monogenetic volcano belonging to the Ventura Espiritu Santo Volcanic Field (VESVF) in the state of San Luis Potosí (central Mexico). Thirteen ultramafic plagioclase-free xenoliths were selected, all exhibiting a paragenesis Ol > Opx > Cpx > > Sp, and being classified as spinel-lherzolites and harzburgites. All xenoliths bring textural evidence of interstitial glass veins bearing dendritic trails of secondary melt and fluid inclusions (composed of silicate glass ± CO2 ± Mg-Ca carbonates ± pyrite). These are related to pervasive mantle metasomatism driven by carbonate-rich silicate melt. The Ar and Ne systematics reflect mixing between MORB-like upper mantle and atmospheric fluids, the latter interpreted as reflecting a recycled air component possibly inherited from the Farallon plate subduction. The 3He/4He ratios vary between 7.13 and 7.68 Ra, within the MORB range (7–9 Ra), and the 4He/40Ar* ratios (0.4–3.11) are similarly close to the expected range of the fertile mantle (1–5). Taken together, these pieces of evidence suggest that (i) either the mantle He budget was scarcely modified by the Farallon plate subduction, and/or (ii) that any (large) crustal contribution was masked by a later metasomatism/refertilization episode, possibly during the subsequent Basin and Range extension. A silicate melt-driven metasomatism/refertilization (revealed by the association between glass veins and fluid inclusions) is consistent with calculated helium residence time for the Mexican lithospheric mantle (20 to 60 Ma) that overlaps the timing of the above geodynamic events. We propose that, after the refertilization event (e.g., over the last ~20 Ma), the lithospheric mantle has evolved in a steady-state, becoming slightly more radiogenic. We also estimated 3He fluxes (0.027–0.080 mol/g), 4He production rates (340–1000 mol/yr), and mantle CO2 fluxes (3.93 × 107 mol/yr to 1.18 × 108 mol/yr) using the helium isotopic values measured in JH mantle xenoliths. Finally, the JH xenoliths exhibit CO2/3He ratios comparable to those of the upper mantle (from 3.38 × 108 to 3.82 × 109) but more positive δ13C values (between −1.0 and − 2.7‰), supporting the involvement of a crustal carbonate component. We propose that the metasomatic silicate melts recycled a crustal carbonate component, inherited by the Farallon plate subduction.

Green Bank Telescope Observations of 3He+: Planetary Nebulae

Abstract We use the Green Bank Telescope to search for 3He+ emission from a sample of four Galactic planetary nebulae: NGC 3242, NGC 6543, NGC 6826, and NGC 7009. During the era of primordial nucleosynthesis, the light elements 2H, 3He, 4He, and 7Li were produced in significant amounts, and these abundances have since been modified primarily by stars. Observations of 3He+ in H ii regions located throughout the Milky Way disk reveal very little variation in the 3He/H abundance ratio—the “3He Plateau”—indicating that the net effect of 3He production in stars is negligible. This is in contrast to much higher 3He/H abundance ratios reported for some planetary nebulae. This discrepancy is known as the “3He Problem.” We use radio recombination lines observed simultaneously with the 3He+ transition to make a robust assessment of the spectral sensitivity that these observations achieve. We detect spectral lines at ∼1–2 mK intensities, but at these levels, instrumental effects compromise our ability to measure accurate spectral line parameters. We do not confirm reports of previous detections of 3He+ in NGC 3242 nor do we detect 3He+ emission from any of our sources. This result calls into question all reported detections of 3He+ emission from any planetary nebula. The 3He/H abundance upper limit we derive here for NGC 3242 is inconsistent with standard stellar production of 3He and thus requires that some type of extra-mixing process operates in low-mass stars.

Empirical evidence for cosmogenic 3He production by muons

Cosmic ray muons penetrate deeply into rock where they interact with atoms to produce cosmogenic nuclides. Incorporation of the muon contribution to the production rates of cosmogenic nuclides increases the accuracy of exposure dates, burial ages, and erosion rates inferred from measured nuclide concentrations. In the absence of empirical evidence, it is generally assumed that muons do not produce 3He, a cosmogenic nuclide commonly used for exposure dating. Here we assess whether muons produce 3He by measuring He isotope concentrations in pyroxene and ilmenite from a ∼300 m deep drill core and other subsurface samples of the mid-Miocene Columbia River Basalt in Washington, USA. 3He concentrations in our samples exhibit an exponential decline with depth with an e-folding length of 32.4 m, which corresponds to an attenuation length for 3He production of 8780 g cm−2. The deeply penetrating exponential is diagnostic of 3He production by cosmic ray muons. Assuming no erosion, we constrain the minimum surface muonogenic production rate to be 0.23 atom g−1 pyroxene yr−1, whereas when incorporating erosion the production rate is 0.45 atom g−1 pyroxene yr−1. 3He concentrations in samples deeper than ∼100 meters are consistent with model-based estimates of depth-independent nucleogenic production from the capture by 6Li of neutrons produced by alpha particle reactions on light elements. Measurements in other subsurface samples indicate that muon-produced 3He is prevalent across the Columbia Plateau. The penetration depth of muonogenic 3He production is substantially deeper, and the ratio of muon- to spallation-produced 3He is substantially lower, than found for other cosmogenic nuclides. Our results provide the first definitive empirical evidence for 3He production by muons, which has several implications for quantifying the timing and rates of Earth surface change and interpreting He isotope ratios. Importantly, despite the low production rates, landforms in the Channeled Scablands, which were formed by incision of the Columbia River Basalt by the late-Pleistocene Missoula floods, have high concentrations of 3He inherited from post-Miocene muon exposure. Hence 3He production by muons must be considered, particularly when dating rapid erosional events in old bedrock. Our findings indicate samples with less than several tens of meters of shielding by overlying rock will contain cosmogenic 3He that elevates 3He/4He ratios. Hence caution should be used when using 3He/4He ratios from samples at shallower depths to infer mantle sources of basalt.

Constraining the production mechanism of light (anti-)nuclei in small systems with ALICE at the LHC: Parallel Session - Small systems III

The large samples of high-quality data collected at the LHC by the ALICE Collaboration allow for a systematic study of light (anti-)nuclei production in pp collisions at s=5,7and13TeV and in p–Pb collisions at sNN=5.02TeV. The excellent performance of the Inner Tracking System, Time-Projection Chamber and Time-Of-Flight detectors provide a clear identification and separation of primary produced light (anti-)nuclei from secondaries. The results on deuteron and 3He production as a function of multiplicity in pp and p–Pb collisions are shown. The goal is to study whether the production of (anti-)nuclei in small systems is better described by the coalescence model or by the statistical hadronisation model. The main observables for testing the production mechanisms are the coalescence parameter BA and the ratio between the transverse-momentum integrated yields of nuclei and protons, d/p, 3He/p and 3H/p. These observables are studied as a function of the event multiplicity and compared with the predictions of theoretical models.

Characterization of flow dynamics around highly-utilized agricultural wells in a fractured-rock aquifer: Assessment of uncertainties lying on groundwater age-dating

When environmental tracers are used to determine groundwater age distributions, various factors including measurement error, model structural error, and sample contamination can affect the outcome. In this study, we tried to untangle and explain the uncertainties associated with the inference of age distribution for a set of highly-utilized production wells in a fractured-rock aquifer system. A total of 17 samples were collected at various depths (22–150 m) from production wells. These samples were analyzed for six age tracers, including 3H, tritiogenic 3He, radiogenic 4He, CFC-11, CFC-12, and CFC-113. A Bayesian approach was used for evaluating age distribution for the samples based on several presumed lumped parameter models (LPM)s and shape free models. It was found that the measured data in most wells could be explained by traditional LPMs reasonably well. However, for five of the 17 wells it was found that the simple LPMs were not adequate and the more complex shape-free approaches resulted better based on the Deviance Information Criteria (DIC) measure. This looked attributed to 1) competition between densely located wells, 2) a thick vadose zone layer possibly causing preferential flows 3) fractures allowing a localized production. Furthermore, it was shown that three other factors including tracer contamination, non-uniform 4He production, and the heterogeneity in the travel time in vadose zone added uncertainties to groundwater age-dating. Overall results showed that the combination of Bayesian approach with DIC criterion can help to identify uncertainties associated with the age distribution for hydrologically complicated systems.

Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments

Fluids residing in fractures in Precambrian Shield environments can contain gases with concentrations up to 30% 4He, 6% 40Ar, and more than 50% H2 depending on the location (expressed as % volume of the free gas phase post-exsolution). These gases are produced through two distinct mechanisms; 1. Radioactive decay of naturally occurring U, Th, and K which produces H2 via radiolysis of water and the noble gases (e.g. 4He and 40Ar) and, 2. Hydration reactions with mafic and ultramafic minerals in the host rock that produce H2. He production is economically important as known reservoirs of 4He and 3He, resources integral to medical applications and different manufacturing sectors, are rapidly depleting. H2 meanwhile has recently been posited as an emerging, clean, energy source for a post-hydrocarbon energy future. 40Ar, though more common, also has important applications in research and industry. The accumulation of 4He and 40Ar can also be used to provide vital constraints on fluid residence times, crucial to understanding planetary processes. In both planetary science and astrobiology there is also keen interest in H2 as an energy source supporting subsurface microbial metabolisms and/or abiotic organic synthesis. For both H2 and 4He, innovative exploration strategies are required to understand their production and accumulation in the deep crust. At present few studies have specifically focussed on establishing the relationship between these two reactive and conservative elements in the deep subsurface, despite their cogenetic nature.We investigate the dependence of H2, 4He, and 40Ar production rates on rock type and geologic setting. Through collating and combining previously published dissolved gas data with substantial new data for specific localities, conservative estimates of H2 production are generated for a range of sites across the Canadian Shield. The results support the prevailing theory that H2 production dominantly depends on the associated lithology. This study also identifies broadly homogeneous 4He and 40Ar production rates based on a relatively homogeneous range of U, Th, and K concentrations in the source rocks. As a consequence, H2/4He ratios are relatively low in felsic environments where H2 is mostly produced radiolytically, whereas they are elevated in mafic environments due to additional H2 production through hydration reactions (e.g. serpentinization). Extrapolating to Precambrian terrains globally this study estimates the total annual production of 4He and 40Ar from these environments specifically to the worldwide budget and reports here a global production in these environments comparable to both production in the entire oceanic crust and to the total net outgassing of these elements at Mid-Ocean Ridges.

Experimental determination of intragranular helium diffusion rates in boron carbide (B4C)

Boron carbide, B4C, is widely used as a neutron absorber in nuclear reactors. Since neutron absorption by 10B leads to 4He production, it appears necessary to study He behaviour and its possible effects on the B4C ceramic. In this study, the diffusion characteristics of ion-implanted He in B4C (500 keV, fluences from 1 x 10+13 to 2 x 10+15 He.cm−2) were investigated mainly by Thermo-Desorption Spectroscopy (TDS) from 600 °C up to B4C melting point. The experiments were done on dense B4C samples having large grains (30–60 μm) to render grain boundaries effects on He outward diffusion ineffective and thus to access intragranular He diffusion kinetics. From controlled temperature ramp experiments, it was notably observed that He release was realized in two main stages. A first He population was able to exit the material at moderate temperatures by interstitial diffusion. Then a second population was quantitatively released only over 1150 °C. This was attributed to He atoms that, in their initial interstitial diffusion course at moderate temperatures from their implantation sites, got trapped in defect aggregates and/or He bubbles. As the nucleations of both these traps are expected to be related to helium and irradiation defect concentrations, the ratio of the two He populations was indeed found to be correlated with the implantation fluence. From the obtained He release curves, the apparent activation energies (Ea) of He intragranular diffusion in B4C was determined (2.6–3.1 eV) in the 800–1100 °C temperature range. This value appears slightly higher than the ones determined at lower temperatures, hinting that a change in diffusion mechanism may occur around 800 °C. The apparent Ea of He detrapping from He bubbles (∼2.5 eV) and from defect aggregates (∼4 eV) were also determined for temperatures within 1200–1500 °C.

Cosmogenic 3He production rate in ilmenite and the redistribution of spallation 3He in fine-grained minerals

Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of ^3He in olivine or pyroxene. However, geochemical investigations using ^3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic ^3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic ^3He production rate in ilmenite by measuring ^3He concentrations in basalts with fine-grained (∼20 μm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic ^3He production rate of 93.6 ± 7.7 atom g^(−1) yr^(−1) that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and ^3He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic ^3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of ^3He from the surrounding, higher ^3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation ^3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ∼10% when grain radii are <150 μm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ∼20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite.

Design and construction of a 1.8 K superfluid 4He system with a G-M cryocooler

This paper describes a superfluid 4He (He II) production system with a G-M cryocooler developed to supply continuous cooling capacity for cryogenic devices working below 2 K, such as the sealed-cell lambda-point devices and the photon detectors. Unlike the existing cooling method that injecting and pumping liquid 4He (He I) to cool the devices, high purity 4He gas stored in gas cylinder is firstly liquefied by a G-M cryocooler in the condenser, and then cooled down under 2.17 K at the He II pot through a Joule-Thomson valve along with a dry vacuum pump. This design allows a continuous supplement of He II below 2.17 K by using 4He at ambient temperature, which avoids the temperature oscillation to the devices being cooled when refilling He I. Due to the removing of cryogen injection, no additional cryogenic apparatus and professional operation are required. In this system, the He II pot can reach a temperature below 1.8 K for continuous operation.

U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages

The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, α-implantation, α-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71 Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.

Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids

Fibrous diamond growth zones often contain abundant high-density fluid (HDF) inclusions and these provide the most direct information on diamond-forming fluids. Noble gases are incompatible elements and particularly useful in evaluating large-scale mantle processes. This study further constrains the evolution and origin of the HDFs by combining noble gas systematics with δ13C, N concentrations, and fluid inclusion compositions for 21 individual growth zones in 13 diamonds from the Finsch (n = 3), DeBeers Pool (n = 7), and Koffiefontein (n = 3) mines on the Kaapvaal Craton. C isotope compositions range from −2.8 to −8.6‰ and N contents vary between 268 and 867 at.ppm, except for one diamond with contents of <30 at.ppm N. Nine of the thirteen studied diamonds contained saline HDF inclusions, but the other four diamonds had carbonatitic or silicic HDF inclusions. Carbonatitic and silicic HDFs yielded low He concentrations, R/Ra (3He/4Hesample/3He/4Heair) values of 3.2–6.7, and low 40Ar/36Ar ratios of 390–1940. Noble gas characteristics of carbonatitic-silicic HDFs appear consistent with a subducted sediment origin and interaction with eclogite. Saline HDFs are characterised by high He concentrations, with R/Ra mostly between 3.9 and 5.7, and a wide range in 40Ar/36Ar ratios (389–30,200). The saline HDFs likely originated from subducted oceanic crust with low He but moderate Ar contents. Subsequent interaction of these saline HDFs with mantle peridotite could explain the increase in He concentrations and mantle-like He isotope composition, with the range in low to high 40Ar/36Ar ratios dependent on the initial 36Ar content and extent of lithosphere interaction. The observed negative correlation between 4He contents and R/Ra values in saline HDFs indicates significant in situ radiogenic 4He production.

Establishing a Bayesian approach to determining cosmogenic nuclide reference production rates using He-3

Production rates are a cornerstone of in situ cosmogenic nuclide applications, including surface exposure dating, erosion rate/denudation rate estimates, and burial dating. The most common approach for estimating production rates is to measure cosmogenic nuclide samples from sites with independently well-constrained exposure histories. In addition, while researchers attempt to minimize the effects of erosion through careful site and sample selection, it can be present at some unknown level in certain sites. We present a general Bayesian methodology for combining information from the nuclide concentrations, the exposure history, and the possibility of erosion, to determine the production rate at a given site. Then, we use another Bayesian approach to combine the results from the various sites.Cosmogenic 3He is an ideal test-bed for our Bayesian approach. It has the most calibration sites of the commonly measured cosmogenic nuclides, and there is evidence for the effect of erosion on some of the sites. Our approach largely reconciles previous discrepancies between sites of widely varying age, even at latitudes where geomagnetic effects are significant. With the canonical Lal/Stone scaling scheme, we derive a global sea level high latitude 3He production rate of 118 ± 2 atoms g−1 yr−1 when considering olivine and pyroxene together. Using the Lifton–Sato–Dunai scaling scheme yields a similar rate of 121 ± 2 atoms g−1 yr−1. Uncertainties associated with these values are improved over previous studies, due to both reduced scatter among the sites and an approach to combining sites which deemphasizes outliers.