Storage conditions and longevity of rift zone magmas at Kīlauea Volcano, Hawai'i: melt inclusion insights from the 2018 Lower East Rift Zone eruption

Rift zones radiating from the summit of shield volcanoes collect and distribute part of the magma supplied towards the surface. They are responsible for a substantial fraction of the total volumetric lava output, and likely add to the volcano's long term magma storage capacity. At Kīlauea, their vertical structure has been inferred to extend down to at least 10 km depth, but little is still known about (1) the parts of the rift zones that are activated during an eruption, (2) the depths at which older residual magma pockets reside, and (3) the longevity of these reservoirs.

The 2018 Lower East Rift Zone (LERZ) eruption was dominated by olivine tholeiite (MgO~7 wt.%) that likely originated from the summit reservoir(s), but the eruption sequence started with partial emptying of at least two older (>55 years) rift zone magmas: an evolved basalt (MgO~4.5 wt.%) and an andesite (MgO~2.5 wt.%), both stored under/near Leilani Estates.

A series of 17 samples spanning key eruption phases was selected for melt inclusion (MI) work to fingerprint the chemical attributes of various end-members involved, as well as the depth(s) of magma storage using CO₂ and H₂O contents. Microprobe analyses of MIs confirm findings from bulk rock and matrix glass data: at least three magma end-members are required to explain compositional variability. A potential fourth dacite end-member (MgO~1.5 wt.%) was identified within plagioclase and pyroxene-hosted MIs, and a fifth, much more primitive magma (MgO>12 wt.%) is preserved within high Fo (> Fo₈₇) olivine. The remarkable diversity of magmas involved is leveraged to extract a range of possible storage pressures for each end-member. MicroRaman analysis of MI bubbles within olivine from the mafic endmember yield minimum trapping pressures up to ~1800 bars (~6-7km depth). FTIR analyses of H₂O and CO₂ in these MIs will be used to complement microRaman work and reconstruct total volatile content.

Ranges in storage pressures extracted can be used as inputs for numerical models of conductive cooling within rift zone magma bodies to investigate their longevity (time before reaching solidus) given vertical extent and length along the rift. Preliminary results from these models show that thick (~200 m) dike-shaped bodies can survive for more than a century and preserve a significant fraction of eruptible melt.