Helium flux in a continental land area estimated from ³He/⁴He ratio in northern Taiwan

Abstract

The helium flux and its isotopic composition may provide useful information not only on the helium budget in the atmosphere but also on the uranium and thorium contents of the rocks in the Earth's crust and mantle and the generation of heat from radioactive decay of these elements. Data have been available from oceanic areas^1–4, but those from continental areas are sparse^5–7. We report here the ³He and ⁴He fluxes derived from measurement of the ³He/⁴He ratio of natural gases in the northern part of Taiwan. The estimated ³He and ⁴He fluxes are 3.9 and 2.7 × 106 atoms cm⁻² s⁻¹ at the Chinshui site, and 7.2 and 2.4 × 106 atoms cm⁻² s⁻¹ at the Chuhuangkeng site. The ³He fluxes are in approximate agreement with the global flux estimated from excess ³He in sea water¹. The ⁴He fluxes are comparable to the ⁴He flux⁶ for the continents deduced from terrestrial heat flow data and the heat/He flux ratio, but are significantly larger than those reported for the ocean floor. The relationship observed between the He isotope ratio and CO₂ and CH₄ concentrations suggests that there is a mixing trend between the high ³He/⁴He-high CO₂-low CH₄ component and the low ³He/⁴He-low CO₂-high CH₄ component. The former may be derived from the upper mantle and the latter produced in a sedimentary environment.