The possibility of dating minerals by the accumulation of ^4He from U and Th decay
has been recognized for many years (e.g., Strutt 1905), but in the century since the idea
was first conceived, the method has rarely been applied successfully. After several
investigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957;
Fanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973;
Leventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding
unreliable and usually low ages, presumably as a result of diffusive He loss possibly
associated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the
method by proposing that in the case of apatite, He ages might be meaningfully
interpreted as ages of cooling through very low temperatures. Laboratory diffusion data
presented by these authors indicated a closure temperature of about 100oC, a value
supported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al.
1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages
increase systematically with sample elevation in a mountain range, as expected for
exhumation-induced cooling through a low closure temperature. Based on the strength of
these results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997;
House et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has
focused on applications of apatite He thermochronometry. There is also renewed interest
in He dating of other U- and Th-bearing minerals both for dating mineral formation and
for thermochronometry. For example, Lippolt and coworkers have undertaken detailed
studies of He diffusion and dating of various phases, most notably hematite formed in
hydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et
al. 1993; Wernicke and Lippolt 1994a,b).
Here I present an overview of recent techniques, calibrations, and applications of the
(U-Th)/He dating method; Hurley (1954) provides an excellent summary of earlier work in
this field. Much of this paper focuses on apatite, because the He behavior and requisite
analytical techniques are better established for this phase than for other target minerals,
such as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior
required for thermochronometric applications, yet recent work is also considering applications
to direct dating, for example, of young tephras (Farley et al. 2001).

(U-Th)/He Dating: Techniques, Calibrations, and Applications

U-Th-He dating of apatite: A potential thermochronometer

Diffusion of cosmogenic3He in olivine and quartz: implications for surface exposure dating

Glaciers around the globe provide a unique medium for investigating the temporal and spatial variation of dust deposition. Here we investigate the spatial and seasonal distribution of eolian dust deposition in central Asia through the analysis of concentrations and size distributions of insoluble particles (over the size range 1–22 μ m diameter) in snow and ice samples collected from eight glaciers in the mountains of central Asia. The sample locations cover a wide geographical area and fall within 4 distinct landscape/climate zones: the southern slopes of the eastern Himalaya; the Karakoram; south eastern Tibetan Plateau; northern/western Tibetan Plateau. Snow samples from each of the 4 landscape/climatic zone displays distinct dust deposition characteristics which are dependent upon the physiographic location of the sampling site with respect to source regions of dust and atmospheric transport pathways. The southern slopes of the eastern Himalaya show very low particle concentrations and low annual dust deposition. This region is relatively free from the influence of Asian dust. The Karakoram appears to be a major sink for particles less than 2 μ m in diameter which have been transported long distances with the westerly jet stream. Glaciers in the south eastern Tibetan Plateau record the influx of dust generated in the arid and semi-arid regions to the west. Glaciers on the northern and western margin of the Tibetan Plateau lie adjacent to vast arid and semi-arid regions of western China and provide dust depostion records mainly of local significance. Our results suggest that high elevation glaciers in the eastern′ Himalaya, Karakoram and the south eastern Tibetan Plateau are those most likely to provide longer term dust deposition records which reflect regional to hemispheric signals. DOI: 10.1034/j.1600-0889.1994.t01-2-00005.x

Modern eolian dust deposition in central Asia

The glacial regions of central Asia considered in our study are influenced by tropical monsoons and western extratropical cyclones. Isotopic δ18O and δD data were obtained over 3 years in three climatic regions: Gongga massif of southeastern Tibet (windward slope of summer monsoon), Xixibangma massif on the northern slope of the Himalayas (leeward of summer monsoon), and the massifs of Pobeda-Khan Tengry in the central Tien Shan (exposed to western airstreams). The survey reported here provides information from atmospheric precipitation, snow pits and a 23 m ice core. The significant differences of oxygen isotopic ratios (from −25.1 per mil to −9.5 per mil) indicate that the Indian and Pacific Oceans as well as the Atlantic Ocean are sources of moisture on the northern slope of the Himalayas and southeastern Tibet. Sharply changing isotopic ratios in precipitation corresponded to changing wind direction and were associated with different sources of air masses on the northern slope of the Himalayas and southeastern Tibet. Steady isotopic ratios in atmospheric precipitation and the absence of changing δ18O composition in a 23 m ice core suggest only one source of moisture in central Tien Shan. The relatively heavier oxygen isotopic ratios in atmospheric precipitation and ice core indicate the moisture from which the precipitation was derived originated over the Caspian or Mediterranean seas. Within the Eurasian continent, air masses developed over the Atlantic Ocean advance farther than those from the Pacific and Indian Oceans.

Isotopic measurements of precipitation on central Asian glaciers (Southeastern Tibet, northern Himalayas, central Tien Shan)

The tritium content of Mt. Everest ice (Khumbu glacier) shows the annual accumulation to average some 1.7 meters of water equivalent. A layer structure exists in the glacier. The layers down to 20 meters were sampled (Dyhrenfurth expedition), and the tritium peaks due to the nuclear weapons test series Castle (1954), Hardtack (1958), and the U.S.S.R. tests of 1961 and 1962 were identified. The samples were not taken deep enough to reach the pretest level for measurement of the natural tritium content, which was the original intent. The layers occur about two per year.

Tritium in Mt. Everest ice—Annual glacier accumulation and climatology at great equatorial altitudes

Comparison of K-Ar and ages based on U-Th-He on young basalts from the San Francisco Volcanic Field near Flagstaff, Arizona, shows that approximately one fifth of the He is retained and thus U-Th-He ages are too young. Leaching with weak acid produces large losses of U in only a few minutes, which suggests that He is lost from surficial positions on minerals from radiation damage sites and from interconnecting crystal defects. One sample of large plagioclase phenocrysts showed excess He. It seems unlikely that the U-Th-He method can be used to obtain reliable ages on young basalts. (auth)

Joel S. Leventhal

Papers

Tritium in Mt. Everest ice—Annual glacier accumulation and climatology at great equatorial altitudes

An evaluation of the uranium‐thorium‐helium method for dating young basalts