CARBON-DIOXIDE AND HELIUM IN HYDROTHERMAL FLUIDS FROM LOIHI SEAMOUNT,HAWAII, USA - TEMPORAL VARIABILITY AND IMPLICATIONS FOR THE RELEASE OF MANTLE VOLATILES

Dissolved CO2, deltaC-13, He, and He-3/He-4 were determined in warm (< 30-degrees-C) hydrothermal fluids from Pele's Vents on the summit of L oihi Seamount, Hawaii, collected during DSRV Pisces V dives in August and September 1992. Total dissolved CO2 and He are highly enriched ove r ambient seawater, with maximum measured concentrations of 190 mmol/k g CT and 0.193 mumol/kg He, and correlate linearly with dissolved Si c oncentration and sample temperature. Carbon dioxide deltaC-13 values r ange from -5.5 to -1.7 parts per thousand (PDB) and corrected He-3/He- 4 ratios range from 21.7 to 27.0R(a), indicating a primary magmatic so urce for both gases. The high concentrations of these volatiles in the vent waters relative to reported bulk concentrations in Loihi basalts suggest that both gases are most likely introduced into the fluids by direct degassing from a magma body, rather than hydrothermal extracti on from the summit lavas. Comparison of the He-3/He-4 ratios of the ve nt waters with reported values for Loihi basalts suggests that the man tle-derived volatiles in the fluids are variably contaminated by radio genic He, probably due to assimilation of basement rock by the source magma. Such a mechanism could also be responsible for the range of He- 3/He-4 ratios observed in lavas from Loihi. The data demonstrate remar kable temporal variation in the volatile content of the vent fluids: r elative to fluid temperature, dissolved CO2 has decreased by approxima tely 30% compared to samples collected in 1987, whereas dissolved He c oncentrations are roughly one-twentieth the 1987 values; the CO2/He-3 ratios of the 1992 samples are approximately 17-27 X 10(9), an order o f magnitude greater than the value of 2.3 X 10(9) reported for 1987 sa mples. We suggest that these temporal changes reflect progressive dega ssing from a magmatic intrusion with significant fractionation Of CO2 and He, a process which may also account for the range in C/He ratios of Loihi lavas.