FLUID-FLOW AND WATER-ROCK INTERACTION IN THE EAST RIFT-ZONE OF KILAUEA-VOLCANO, HAWAII

The East Rift Zone of Kilauea Volcano in Hawaii represents a major are a of geothermal activity. Fluid inclusion and stable isotope analyses of secondary hydrothermal minerals in core samples from three scientif ic observation holes (SOH) drilled into the rift zone indicate that th e geothermal system is dominated by meteoric waters to depths of as mu ch as 1500 m below sea level. Calculated delta(18)O and delta D values for fluids on the north side of the rift zone indicate that the deep meteoric fluids may be derived from precipitation on the upper slopes of Mauna Loa Volcano. In the interior of the rift zone, recharge is do minated by seawater mixed with local meteoric water. Water/rock ratios in the rift area are approximately 2, but strongly O-18-enriched flui ds in the deeper parts of the SOH-2 and SOH-4 drill holes (on the nort h side of the rift) indicate that the fluids underwent extensive inter action with rocks prior to reaching this part of the rift zone. Marine carbonates at the subaerial to submarine transition (between 1700 and 1780 m depth) in SOH-4 have not fully equilibrated with the fluids, s uggesting that the onset of hydrothermal activity in this area was rel atively recent (<2000 years). This may represent increased volcanic ac tivity along the rift after the end of the Ai La'au phase of eruptive activity at the Kilauea summit approximately 1000 years ago, or it may reflect progressive evolution ofthe hydrothermal system in response t o southward migration of intrusive activity within the rift.