揮発性元素同位体による火山化学・環境化学の研究  [in Japanese] Environmental and igneous geochemistry of volatile element isotopes  [in Japanese]

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Author(s)

    • 佐野 有司 SANO Yuji
    • 東京大学海洋研究所海洋環境研究センター Center for Environmental Research, The Ocean Research Institute, The University of Tokyo

Abstract

This paper describes a study on environmental and igneous geochemistry using volatile elemental isotopes such as <sup>3</sup>He/<sup>4</sup>He and <sup>40</sup>Ar/<sup>36</sup>Ar rations, and δ<sup>13</sup>C and δ<sup>15</sup>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 <sup>3</sup>He/<sup>4</sup>He ratios down a natural gass well. The observed flux of 2-3×10<sup>6</sup>atoms/cm<sup>2</sup>s 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 <sup>3</sup>He/<sup>4</sup>He ratio accumulated in the crust. In order to check the anthropogenic release of radiogenic helium, we have measured the secular variation of atmospheric <sup>3</sup>He/<sup>4</sup>He ratio. The ratio decreases with time and the rate of change suggests that anthropogenic helium flux yields 5×10<sup>15</sup> cm<sup>3</sup> STP/year, which is significantly larger than the natural flux of 1×10<sup>13</sup>cm<sup>3</sup> 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<sub>2</sub> may also be degassing from the solid Earth through volcanic and hydrothermal activity. We have measured δ<sup>13</sup>C values and CO<sub>2</sub>/<sup>3</sup>He 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<sup>12</sup> mol/year, is estimated by using CO<sub>2</sub>/<sup>3</sup>He ratios and <sup>3</sup>He flux from literature. It is significantly smaller than the anthropogenic flux of 5×10<sup>14</sup> mol/year, but is not negligible. The flux, if accumulated over 4.5 billion years of geological time, amounts to 8.3×10<sup>21</sup> mol which agrees well with the 9×10<sup>21</sup> 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<sup>15</sup>N values and N<sub>2</sub>/<sup>36</sup>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<sup>9</sup>mol/year, is estimated by corrected N<sub>2</sub>/<sup>3</sup>He ratios for elemental fractionation and the <sup>3</sup>He flux from literature. The flux, if accumulated again over 4.5 billion years, yields 1.3×10<sup>19</sup> 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.

Journal

  • Chikyukagaku

    Chikyukagaku 35(1), 43-59, 2001

    The Geochemical Society of Japan

References:  63

Codes

  • NII Article ID (NAID)
    110008680016
  • NII NACSIS-CAT ID (NCID)
    AN00141280
  • Text Lang
    JPN
  • Article Type
    ART
  • ISSN
    0386-4073
  • NDL Article ID
    5711965
  • NDL Source Classification
    ZM41(科学技術--地球科学)
  • NDL Call No.
    Z15-645
  • Data Source
    CJP  NDL  NII-ELS  J-STAGE 
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