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

Abstract

We have measured 3He/ 4He, 4He/ 20Ne andCO 2/ 4He ratios, and δ 13C values of CO 2 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. The 3He/ 4He ratios of crater fumaroles, corrected for atmospheric contamination, increased significantly from 5.47 R atm in December 1989 to 7.93 R atm in August 1990 and reached a maximum of 8.84 R atm 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. The 3He/ 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 δ 13C values of CO 2 of crater fumaroles show negative correlation to the helium isotopes, while those of Pandiaco are relatively constant. Based on the δ 13C values andCO 2/ 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.