揮発性元素同位体による火山化学・環境化学の研究(1998年度日本地球化学会賞受賞記念論文)

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This paper describes a study on environmental and igneous geochemistry using volatile elemental isotopes such as ^3He/^4He and ^<40>Ar/^<36>Ar rations, and δ^<13>C and δ^<15>N values. Helium escapes from the Earth's atmosphere to inter-planetary space because of its low atomic mass. In contrast it is degassed from the solid Earth into the air. Degassing rate of helium, hereafter called helium flux, may provide useful information on the average concentration of uranium and thorium in the crust and generation of heat through radioactive decay. We have estimated helium flux from a continental land area based on the gradient of the ^3He/^4He ratios down a natural gass well. The observed flux of 2-3×10^6atoms/cm^2s agrees well with a theoretical flux calculated by a correlation between terrestrial heat flow and production of helium via a-decay of uranium and thorium. Extensive mining of fossil fuels such as natural gas and petroleum may release radiogenic helium with a low ^3He/^4He ratio accumulated in the crust. In order to check the anthropogenic release of radiogenic helium, we have measured the secular variation of atmospheric ^3He/^4He ratio. The ratio decreases with time and the rate of change suggests that anthropogenic helium flux yields 5×10^<15> cm^3 STP/year, which is significantly larger than the natural flux of 1×10^<13>cm^3 STP/year. The estimated flux is consistent with annual production of natural gas and petroleum and their helium/carbon ratios. In addition to anthropogenic release, CO_2 may also be degassing from the solid Earth through volcanic and hydrothermal activity. We have measured δ^<13>C values and CO_2/^3He ratios of high temperature fumaroles in cir-cum Pacific volcanic regions. Based on the simple mixing equation of three components, the upper mantle, organic sediment and limestone, the orgin of the carbon in the sample is assessed. Contribution of mantle-derived carbon is about 20% and a major part is attributable to recycled limestone carbon in the subduction zones. Global volcanic flux of carbon, 1.8×10^<12> mol/year, is estimated by using CO_2/^3He ratios and ^3He flux from literature. It is significantly smaller than the anthropogenic flux of 5×10^<14> mol/year, but is not negligible. The flux, if accumulated over 4.5 billion years of geological time, amounts to 8.3×10^<21> mol which agrees well with the 9×10^<21> mol of the present inventory of carbon on the Earth's surface. We have investigated the origin of nitrogen in volcanic gases in island arc and back-arc basin basalt glasses based on the δ^<15>N values and N_2/^<36>Ar ratios. Contribution of mantle-derived nitrogen is about 15% in subduction zones and the major fraction is derived from recycled sedimentary nitrogen. Global volcanic flux of nitrogen, 2.4×10^9mol/year, is estimated by corrected N_2/^3He ratios for elemental fractionation and the ^3He flux from literature. The flux, if accumulated again over 4.5 billion years, yields 1.3×10^<19> mol, which is one order of magnitude smaller than the present inventory of nitrogen on the Earth's surface, consistent with a catastrophic degassing of the atmopshere.
日本地球化学会の論文
2001-03-20

揮発性元素同位体による火山化学・環境化学の研究(1998年度日本地球化学会賞受賞記念論文)

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