Cosmogenic Helium Research Articles

Dating geomorphologic surfaces using cosmogenic 3He

Cosmogenically produced 3He in rocks can be used to date geomorphologic surfaces. Using the Late Pleistocene Bonneville flood event, the subsequent development of the Provo shoreline, and associated volcanic rocks, a production rate of 432 atoms/g/yr in olivine is determined for 38°56′ N latitude (present geomagnetic latitude 46.5°) at 1445 m altitude integrated over the last 14,400 years. Measurements of cosmogenic helium in flood deposits, on river-scoured surfaces, and lava flows show that it is possible to date directly large-scale events such as volcanic eruptions and catastrophic floods. Olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3He method. However, in most cases, plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices.

Cosmic ray exposure dating with in situ produced cosmogenic 3He: Results from young Hawaiian lava flows

Cosmogenic helium contents in a suite of Hawaiian radiocarbon-dated lava flows were measured to study the use of the production rate of spallation-produced cosmogenic He-3 as a surface exposure chronometer. Basalt samples from the Mauna Loa and Hualalai volcanoes were analyzed, showing that exposure-age dating is feasible in the 600-13000 year age range. The data suggest a present-day sea-level production rate in olivine of 125 + or - 30 atoms/g yr.

Enhanced [formula omitted] ratios and cosmogenic helium in ultramafic xenoliths

Recently erupted ultramafic xenoliths generally have ratios of 3 He 4 He = (5–10) × R A , which are similar to the values characterizing mid-ocean ridge basalts (MORB). During the course of additional investigations of ultramafic xenoliths, samples from several locations have been found with 3 He 4 He ratios of either whole rock or mineral separates that are substantially higher than those found in MORB, up to over 400 × R A. These elevated ratios correspond to 3He excesses of between 8·10 −14 and 2·10 −12 cm 3 STP g −13He, assuming that the samples initially contained He with an isotopic composition similar to that of MORB and other xenoliths from the sample localities. These excesses are consistent with a derivation from in situ cosmic-ray spallation, considering the production rates for the particular latitudes, altitudes, and exposure ages involved. These results demonstrate that cosmogenic He identified in samples from Maui can be found in significant quantities in other geological samples that have been used to characterize mantle He.

In situ production of terrestrial cosmogenic helium and some applications to geochronology

The concentrations of cosmogenic He-3 have been measured in a series of basaltic drill core samples from Hawaiian volcanoes Haleakala and Mauna Loa. The He-3 concentration in the surface of a radiocarbon dated Mauna Loa flow (20,000 years) gives reasonable agreement with a theoretical production rate of 140 atoms/g/yr, and suggests that the uncertainty in this rate is roughly 10 percent. The results illustrate the feasibility of using He-3 to measure exposure ages of young basaltic lava flows and for measuring erosion rates. Erosion rates calculated from the three Haleakala cores range from 7 to 11 meters/million years. The drill core data demonstrate that accurate depth control is crucial to the use and evaluation of cosmogenic helium. Depth profiles from several of the older cores display a nonexponential depth dependence of He-3(c) below 170 g/sq cm, which is attributed to the contribution from Li-6(n, alpha)T, where the neutrons are from stopped muons. This has important implications for depth dependence of cosmogenic He-3 because muons are weakly attenuated compared to the nucleonic component that produces spallation.

Cosmogenic helium in a terrestrial igneous rock

High 3He/4He ratios in terrestrial samples are generally interpreted as indicating the presence of primitive or primordial gas1,2. Despite the ubiquitous presence of cosmogenic noble gases in meteorites, cosmogenic helium isotopic signatures have never been observed in terrestrial samples3 and have, for the most part, been considered to be insignificant on the Earth. I present here new helium isotopic measurements on samples from the Kula formation of Haleakala volcano (Hawaii) that are best explained by an in situ cosmogenic origin for a significant fraction of the 3He. Results from crushing and stepwise heating experiments, and consideration of the exposure age of the sample at the surface and the cosmic-ray fluxes strongly support this hypothesis. Although crustal cosmogenic helium has been proposed previously4,5, this represents its first unambiguous identification in a terrestrial sample.

Production rates of cosmogenic helium isotopes in iron meteorites

The production rates, P, of cosmogenic 3He and 4He in iron meteorites can be described by introducing empirically determined effective mass difference, ΔA, terms in the general formula for the total isobaric yield of cosmogenic nuclides of mass A: P(A) = k 1 ′ · (ΔA + 4) −k 2 ′ , ΔA = 56 − A, covering A = 54 − 20. It is found that: P( 3He) = 16.7 · k 1 ′ · (22) −k 2 ′ and P( 4He) = 18.3 · k 1 ′ · (14) −k 2 ′ .

Cosmogenic and radiogenic helium, neon, and argon in amphoteric chondrites

Cosmogenic and radiogenic rare gases were determined by mass spectrometry in 16 amphoterites. The radiation, U, Th-He, and K-Ar ages were calculated. Radiation ages of amphoterites vary from 0.9 to 58 m.y. They resemble the age distribution of the hypersthene chondrites in that few amphoterites have ages in the interval 2–6 m.y. The amphoterites Appley Bridge, Hamlet, and Soko-Banja have the same fall date, October 13, but very different radiation ages—1, 31, and 58 m.y.—indicating that they did not belong to a periodic meteorite shower or stream. The same seems to be true also of Olivenza and Vavilovka, both of which fell on June 19. The Ar40 contents of 4 amphoterites differ significantly from previous determinations by other authors, a finding that supports earlier conclusions that the K abundance in some amphoterites seems to be nonuniform. Although 2 of the amphoterites had U, Th-He ages as short as 1 b.y., most others are indeed quite old. Seventy-five per cent have ages greater than 3.0 b.y., whereas only 20% of the hypersthene chondrites have ages above this limit. It is concluded that differences in chemistry and mineralogy cannot account for the systematic differences in gas-retention ages of bronzite, hypersthene, and amphoteric chondrites. Most, if not all, amphoterites seem to have escaped the reheating episode that hypersthene chondrites underwent late in their history.

Rare gases in tektites

Tektites from a number of localities have been examined for their contents of potassium, atmospheric argon, radiogenic argon, atmospheric neon, and possible cosmogenic neon. At 300°K, the coefficient of diffusion for helium in tektite glass from Kalgoorlie, Australia is 4 × 10 −10 cm 2/sec. The activation energy for helium diffusion is 0.27 electron volts. The helium solubility constant is 0.005. Corresponding quantities for neon diffusion are 2 × 10 −15 cm 2/sec, 0.61 electron volts and 0.005. The helium diffusion rate is so high that tektites retain no radiogenic or cosmogenic helium. The recent group of tektites, however, should retain a substantial amount of any cosmogenic neon contained at time of fall. In no case was any excess Ne 21 detected. From the upper limits of original Ne 21 content, a maximum “flight time” since last melting has been computed for each tektite. For a tektite from Kalgoorlie, this is 28,000 years. This result is in apparent disagreement with the finding by Ehmann and Kohman of radioactive Al 26 in australites. Potassium-argon ages have been computed for each tektite. These range from 0 to 32 m.y. and are usually in agreement with the stratigraphie age of the formations on which the tektites lie, according to presently accepted time scales. Data in this paper do not support a recent suggestion that the moldavites of Czechoslovakia and the recently discovered tektites from Georgia U.S.A. are part of the same fall.

Production of Helium in Iron Meteorites by the Action of Cosmic Rays

The helium distribution in a slice from the iron meteorite, Grant, has been measured and plotted in the form of contour maps. The contours of constant helium show a minimum helium content and isotopic ratio, ${\mathrm{He}}^{3}$/${\mathrm{He}}^{4}$, near the center of the slice, the isotopic ratio varying from 0.26 near the center to 0.30 at the surface. A cosmogenic helium production rate equation was fitted to the data giving a ${\mathrm{He}}^{3}$/${\mathrm{He}}^{4}$ production ratio by primary cosmic rays of 0.50 and by secondary particles of 0.14. Primary and secondary particle interaction cross sections were found to be 540 mb and 720 mb, respectively. The ratio of the average postatmospheric radius to the pre-atmospheric radius of Grant was calculated to be 0.65.