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 Loihi Seamount, Hawaii, collected during DSRV Pisces V dives in August and September 1992. Total dissolved CO2 and He are highly enriched over ambient seawater, with maximum measured concentrations of 190 mmol/kg CT and 0.193 mumol/kg He, and correlate linearly with dissolved Si concentration and sample temperature. Carbon dioxide deltaC-13 values range 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 source 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 extraction from the summit lavas. Comparison of the He-3/He-4 ratios of the vent waters with reported values for Loihi basalts suggests that the mantle-derived volatiles in the fluids are variably contaminated by radiogenic 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 remarkable temporal variation in the volatile content of the vent fluids: relative to fluid temperature, dissolved CO2 has decreased by approximately 30% compared to samples collected in 1987, whereas dissolved He concentrations 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 of magnitude greater than the value of 2.3 X 10(9) reported for 1987 samples. We suggest that these temporal changes reflect progressive degassing 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.