FLUID-INCLUSION EVIDENCE FOR PAST TEMPERATURE-FLUCTUATIONS IN THE KILAUEA EAST RIFT-ZONE GEOTHERMAL AREA, HAWAII

Heating and freezing data were obtained for fluid inclusions in hydrot hermal quartz, calcite, and anhydrite from several depths in three sci entific observation holes drilled along the lower East Rift Zone of Ki lauea volcano, Hawaii. Compositions of the inclusion fluids range from dilute meteoric water to highly modified sea water concentrated by bo iling. Comparison of measured drill-hole temperatures with fluid-inclu sion homogenization-temperature (Th) data indicates that only about 15 % of the fluid inclusions could have formed under the present thermal conditions. The majority of fluid inclusions studied must have formed during one or more times in the past when temperatures fluctuated in r esponse to the emplacement of nearby dikes and their subsequent coolin g. The fluid-inclusion data indicate that past temperatures in SOH-4 w ell were as much as 64 degrees C hotter than present temperatures betw een 1000 and 1500 m depth and they were a maximum of 68 degrees C cool er than present temperatures below 1500 m depth. Similarly, the data s how that past temperatures near the bottoms of SOH-1 and SOH-2 wells w ere up to 45 and 59 degrees C, respectively, cooler than the present t hermal conditions; however, the remainder of fluid-inclusion Th values for these two drill holes suggests that the temperature's of the trap ped waters were nearly the same as the present temperatures at these s lightly shallower depths. Several hydrothermal minerals (erionite, mor denite, truscottite, smectite, chlorite-smectite, chalcedony, anhydrit e, and hematite), occurring in the drill holes at higher temperatures than they are found in geothermal drill holes of Iceland or other geot hermal areas, provide additional evidence for a recent heating trend.