Helium isotopes in soil gas: the surficial indicators of the ‘leaking mantle’

Abstract:

By re-defining ‘excess helium’ as isotopic abundance of both helium isotopes normalized to those of the air, helium-3 and helium-4 in the soil gas stand out as two independent proxies of mantle and crustal degassing in the Yilan Plain, Taiwan. The spatial distribution of soil gas helium-3 correlates well with the locations of both shallow and deep structures; and its extent of anomaly indicates the depth of faults. Helium-3 also points out the locations of the dyke intrusion and the initial depression center of the Plain. Soil gas helium-4, on the other hand, does not reflect structure locations as well as helium-3. We suggest that the faults might have activated long ago so that crustal helium-4 had released to the atmosphere, while helium-3 still has constant supply from the mantle. One prominent feature–obvious abundance difference–shows up in spatial distribution of both helium-3 and helium-4 at longitude 121.75°. This feature could only be explained by the appearance of deep tectonic structures since helium-3 is a primordial gas from the deep earth. We believe that it is the location of the sharp plate boundary as proposed by Ustaszewski et al. (2012). However, we suggest that the location should be in the Plain rather than in the mountain area. The distribution of soil helium-4 also implies that the crustal delamination depth is ranged around 3-14 km, where the major heat-producing elements in the upper crust are heated, and thus enhance crustal degassing. With the new definition of excess helium, we found the interpretation of helium isotopes in soil gas is no longer affected by air contamination. Even soil gas samples are taken from surficial crust; helium isotopes are still useful to trace mantle and crustal degassing and could shed light on tectonic settings.