Secular variations of helium and carbon isotopes at Galeras volcano, Colombia

We have measured3He/4He,4He/20Ne andCO2/4He ratios, and δ13C values of CO2 in eighteen samples from crater fumaroles of Galeras volcano, Colombia, and seven samples of bubbling gases of the Pandiaco mineral spring located about 8 km east of the volcano from April 1988 to July 1995. The3He/4He ratios of crater fumaroles, corrected for atmospheric contamination, increased significantly from 5.47 Ratm in December 1989 to 7.93 Ratm in August 1990 and reached a maximum of 8.84 Ratm in May 1992. The ratio then decreased gradually until July 1995. Overall variation of helium isotopes appears to be related to the current activity of the volcano. The3He/4He ratios of Pandiaco mineral spring were constant between April 1988 and November 1991 and increased in April 1993, although experimental error is large. The δ13C values of CO2 of crater fumaroles show negative correlation to the helium isotopes, while those of Pandiaco are relatively constant. Based on the δ13C values andCO2/3He ratios, it is possible to estimate fractions of upper mantle carbon in these samples. Calculated mantle fractions are 3–10% in crater fumaroles, while those in Pandiaco spring are negligibly small.