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 areas1–4, but those from continental areas are sparse5–7. 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.