The East Rift Zone of Kilauea Volcano in Hawaii represents a major area
of geothermal activity. Fluid inclusion and stable isotope analyses of
secondary hydrothermal minerals in core samples from three scientific
observation holes (SOH) drilled into the rift zone indicate that the
geothermal system is dominated by meteoric waters to depths of as much
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
dominated by seawater mixed with local meteoric water. Water/rock ratios
in the rift area are approximately 2, but strongly O-18-enriched fluids
in the deeper parts of the SOH-2 and SOH-4 drill holes (on the north
side of the rift) indicate that the fluids underwent extensive
interaction 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,
suggesting that the onset of hydrothermal activity in this area was
relatively recent (<2000 years). This may represent increased volcanic
activity 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 to
southward migration of intrusive activity within the rift.