High 3He/4He ratios in lower East Rift Zone steaming vents precede a new phase of Kilauea 2018 eruption by 8 months

摘要

On May 1, 2018, a magnitude 5.0 earthquake heralded the collapse of the Pu’u O’o Vent on the middle East Rift Zone (ERZ) of Kilauea Volcano, active since 1983. Increased seismicity was recorded on the middle to lower ERZ from April 30 until May 2, 2018. The active lava lakes within both Pu’u O’o Vent and Halema’uma’u Crater began to drain and the summit caldera began to deflate, with the summit collapse ending on August 2, 2018 and lower ERZ eruptive lava activity ending by 4 September 2018. Herein we report on elevated ³He/⁴He ratios in steaming vents in the lower ERZ from samples collected in early September 2017. Gas isotopic measurements were made with a new, field-portable He isotope detector capable of sub-daily monitoring of the ³He/⁴He ratio. When corrected for air contamination, these values exceed those previously reported for Kilauea by nearly twofold, resembling a purer hotspot plume signature, such as those measured directly over the mantle plume at Loihi Seamount to the SE of Hawaii Island, and in older basalt flows when Kilauea and its sister Hawaiian shield volcanoes were located more directly over the plume. The discovery, which presages the eruption there by more than eight months, suggests that we either sampled a ³He/⁴He rich magma already in place in the lower ERZ or a shallow groundwater reservoir in the lower ERZ (Puna district) with anomalously low values of ⁴He relative to their ³He/⁴He ratio, similar to previous findings there and suggestive of a previously unknown He isotopic fractionation.