ABSTRACT

Carbon dioxide and graphitic carbon in olivine crystals from dunite nodules in the 1801 lava flow of the Hualalai Volcano, Hawaii were extracted by three methods, i.e., the pyrolysis under oxygen atmosphere, the ball-milling and the dissolution with hydrochloric acid. It is concluded that most of CO₂ extracted by the oxidative pyrolysis was derived from fluid inclusions, by comparing the isotopic ratios of CO₂ released by the oxidative pyrolysis and the ball-milling. Graphitic carbon was recovered from the evaporation residue after olivine crystals were dissolved in hydrochloric acid. The averaged concentration of CO₂ extracted by the oxidative pyrolysis was 0.87μmole/g and δ¹³C value was -3.2‰. These of graphitic carbon were 0.51μmole/g and -26.9‰, respectively, No appreciable amount of hydrogen-bearing species such as H₂, H₂O, CH₄, higher hydrocarbons and H₂S was detected in olivine crystals by both the oxidative pyrolysis and the ball-milling. Assuming all the extracted carbon is present in fluid inclusions in olivine crystals and the disproportionation reaction of CO (2CO → CO₂ + C) has occurred in inclusions, we conclude that the major carbon-bearing species in the mantle beneath the Hawaiian Islands are CO and CO₂, and the CO/CO₂ ratio is estimated to be about 3 on the basis of the experimental data and that the δ¹³C value of the carbon in upper mantle below the Hawaiian Islands is around -12‰. Some additional data on Japanese olivines suggest that the δ¹³C value of the mantle carbon may not be unique throughout the mantle.