Pete Burnard

High 3He/4He ratios in peridotite xenoliths from SW Japan revisited: Evidence for cosmogenic 3He released by vacuum crushing

[1]xa0In isotope studies of magmatic systems, magmatic He is preferentially extracted by crushing minerals under vacuum, whereas cosmogenic and/or radiogenic He isotopes are released by mineral melting as they are produced and remain in the minerals matrix. Interpretation of magma source composition or surface residence time depends on the assumption that vacuum crushing releases insignificant cosmogenic 3He. We reinvestigated the helium isotopic composition of olivines in xenoliths from Takashima and Kurose volcanoes, Japan, which were reported to have high 3He/4He, interpreted as mantle plume contributions [Sumino et al., 2000; Ikeda et al., 2001]. Combining vacuum crushing and heating protocols, we show that samples from both localities contain considerable concentrations of cosmogenic 3He. Significantly, up to 25% of the matrix-sited helium was extracted by prolonged crushing. It seems possible that previous high 3He/4He ratios measured in samples from Kurose and Takashima were the result of the high cosmogenic 3He concentrations, and do not imply a mantle plume beneath Japan. For Kurose lavas sampled about 1 m above present-day sea level, exposure ages computed from cosmogenic 3He accumulation are similar to K-Ar eruption ages of 1.13 ± 0.12 Ma, implying that erosion of the sampling site has been limited and that the samples have not been below sea level for prolonged periods during the last Myr. Helium extraction from the matrix by crushing cannot be explained by volume diffusion because the diffusivity of helium in olivine is slow at the crushing temperature and therefore requires an additional mechanism to enhance the release. A fracture-related extraction mechanism may be responsible for this phenomenon, and a semiquantitative model is developed assuming that the increase in specific surface area during crushing represents newly created fractures. According to this model, He released from about 10 unit cells of olivine from the fracture results in helium extraction of only 1%, but the spallation damage tracks along fractures may provide sufficient pathway for observed helium extraction of up to 25%.

Diffusive fractionation of noble gases and helium isotopes during mantle melting

The large differences in He and Ar diffusivities in silicate minerals could result in fractionation of the He/Ar ratio during melting of the mantle, producing He/Ar ratios in the primary mantle melts that are higher than those of the bulk mantle. Modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways (such as fractures or melt channels) suggests that significant (order of magnitude) He/Ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks. In addition, the 15% difference in 3He and 4He diffusivities could also produce isotopic fractionation between the melt and its solid source. Modeling the behavior of He and Ar during melting shows that small increases (few %) in 3He/4He should be correlated with larger variations (factor of 5) in 4He/40Ar. However, in order to test this hypothesis the effects of subsequent He–Ar fractionation that occur during degassing have to be corrected. I describe a scheme that can separate He/Ar variations in the primary melt from overprinted fractionation during magmatic degassing. Using the degassing-corrected data, there is a correlation between the primary melt’s 4He/40Ar and 3He/4He in mid-ocean ridge basalts (MORBs). The slope of the correlation is consistent with the models of preferential diffusion of 3He relative to 4He and of 4He relative to 40Ar from the solid mantle into the melt. Diffusive fractionation of noble gases during melting of the mantle can also account for low 4He/40Ar ratios commonly found in residual mantle xenoliths: preferential diffusion of He relative to Ar will produce some regions of the mantle with low 4He/40Ar, the complement of the high 4He/40Ar ratios in basalts. Diffusive fractionation cannot, however, account for differences between the He and Ne isotopic compositions of MORBs compared with ocean island basalts (OIBs); not only are the extremely high 3He/4He ratios of OIBs (up to 50 Ra) difficult to produce at reasonable mantle time and lengthscales, but also the Ne isotopic compositions of MORBs and OIBs do not lie on a single mass fractionation line, therefore cannot result from diffusive fractionation of a single mantle Ne source. If preferential diffusion of He from the solid mantle into primary melts is a significant process during generation of MORBs, then it is difficult to constrain the He concentration of the mantle: He concentrations in basalts and the He flux to the ocean essentially result from extraction of He from a larger (and unknown) volume of mantle than that that produced the basalts themselves. The He concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available.

Noble Gases in the Atmosphere

The atmosphere is the primary terrestrial reservoir of the heavy noble gases (Ne, Ar, Kr, Xe) and precise knowledge of the isotopic composition of atmospheric noble gases is important for many—if not all—fields of noble gas geochemistry. Air noble gases, including helium, are very commonly used as a running laboratory standard for calibrating instrumental discrimination and sensitivity (see Chap. 1), hence any potential temporal or spatial heterogeneities in the atmospheric noble gas composition could have consequences for the reliability and comparability of noble gas data. Metrological measurements such as the determination of Avogadro’s constant and the gas constant also depend on accurate determination of the isotopic composition (and isotopic masses) of atmospheric noble gases. However, absolute isotopic measurements are not straightforward and this section reviews both how absolute isotopic determinations have been made and assesses the temporal and spatial variability of the atmosphere at the present and in the recent (<2 Ka) past.

High-precision helium isotope measurements in air

Helium has two natural isotopes which have contrasted, and variable sources and sinks in the atmosphere (3He/4Heair = 1.382 ± 0.005 × 10−6). Variations in the atmospheric helium isotopic composition may exist below typical measurement precision thresholds (0.2 to 0.5%, 2σ). In order to investigate this possibility, it is necessary to be able to consistently measure helium isotopes in air with high precision (below 0.2% 2σ). We have created an air purification and measurement system that improves the helium isotope measurement precision. By purifying a large quantity of air at the start of a measurement cycle we can make rapid standard-bracketed measurements. Controlling the amount of helium in each measured aliquot minimizes pressure effects. With this method we improve the standard errors by 2× over measuring the same amount of gas in a single step. Individual measurements have standard errors of 0.2 to 0.3% (2σ), with three repeat samples needed to reach 0.1% or better errors. The long-term reproducibility of our calibration sample is 0.033% (2σ).

The Noble Gases as Geochemical Tracers: History and Background

This chapter describes the discovery of the noble gases and the development of the first instrumentation used for noble gas isotopic analysis before outlining in very general terms how noble gases are analysed in most modern laboratories. Most modern mass spectrometers use electron impact sources and magnetic sector mass filters with detection by faraday cups and electron multipliers. Some of the performance characteristics typical of these instruments are described (sensitivity, mass discrimination). Extraction of noble gases from geological samples is for the most part achieved by phase separation, by thermal extraction (furnace) or by crushing in vacuo. The extracted gases need to be purified and separated by a combination of chemical and physical methods. The principles behind different approaches to calibrating the mass spectrometers are discussed.

Neon isotopic measurements using high‐resolution, multicollector noble gas mass spectrometer: HELIX‐MC

[1]xa0We describe a new high-resolution, multicollection, noble gas mass spectrometer, the HELIX-MC, developed by GV instruments. The instrument uses a collector housing that holds five “MiniDual” detection systems, each detector containing two devices: a Faraday cup collector and channel electron multiplier. Each detector is 5.2 mm wide, which permits collecting adjacent beams separated by 1 atomic mass unit (amu) at mass 136. The HELIX-MC operates at high mass resolution (m/Δm > 1500), which allows the interference of 40Ar++ on 20Ne+ to be resolved. We report the results of some tests on the “MiniDual” collectors, in which we have achieved precise Ne abundance and isotopic measurements. After a correction for doubly ionized CO2 (0.37–1.5%), our results show that the HELIX-MC produces only very limited instrumental Ne isotopic mass fractionation (≈−0.5%.amu−1). The sensitivity of the HELIX-MC for neon measurement performed on 1012 ohms collector is around 1.4 × 10−3 A torr−1 for quantities of 20Ne analyzed comprised between 1.27 × 10−15 and 4.71 × 10−15 moles. Within this range, the reproducibility of the isotopic measurements of standard gas falls in the range between 0.5 and 0.8% (1σ) for 20Ne/22Ne and 1 and 1.5% for 21Ne/22Ne over a period of several days. The machine characteristics for measuring He are also reported.