Hydrogen and carbon isotope studies on the graphite-bearing metapelites in the northern Kiso district of central Japan

Abstract

Measurements were made of the hydrogen isotope ratios of hydrous silicates (mica and amphibole) and whole rocks, and the carbon isotope ratios of graphite and carbonaceous matter in the metamorphic rocks from the northern Kiso district in central Japan.δD values of hydrous silicates in the graphite-bearing metapelites are always higher than those in graphite-free schists, even though the sample localities of the two rock-types are very close. Hydrogen isotopic equilibrium has been attained between the coexisting minerals.D/H ratios of water in the metamorphic fluids seem to depend strongly on the presence or absence of graphite and seem to be not constant throughout the district. The district is divided into three areas of low (metamorphic zones I, II), medium (zones IIIa–V) and highδ 13Cgr value (zones VIa–VII) areas. In the highδ 13Cgr values area, the carbon contents of the graphite-bearing rocks decrease slightly from zones VIa to VII, whereas theδ 13Cgr values increase sharply from the upper part of zone VIa to VIb. TheδD values of biotite in these graphite-bearing rocks are higher than those in the mediumδ 13Cgr area. This suggests that methane enriched inH and12C is produced and liberated by the devolatilization reactions between muscovite, graphite and water. The fluid produced is composed of water, methane and a subordinate amount of carbon dioxide, and its logfO2 value is deduced to be about 1.2 lower than that defined by the FMQ buffer. In the mediumδ 13Cgr area, theδ 13C values of graphite are nearly constant (−20.8‰), while the Fe2O3/(Fe2O3 + FeO) ratio of the graphite-bearing rocks apparently decreases with increasing metamorphic grade.δD differences in hydrous silicates between graphite-bearing and graphite-free rocks are observed. These facts are interpreted to mean that methane was produced in addition to water and carbon dioxide, and that its generation ( $$X_{{\text{CH}}_{\text{4}} } /X_{{\text{CO}}_{\text{2}} }$$ ratio of the fluid was about 2) had practically no isotope effect on the graphite. In the lowδ 13Cgr area, the carbon contents of the rocks decrease clearly from zones I to IIIa. TheδD andδ 13Cgr values of the non-metamorphosed shales are much lower than those of the low grade graphite-bearing metapelites. This suggests that methane is produced and liberated from the rocks even at the incipient stage of metamorphism.