Magmatic Plumbing Samples a Smorgasbord: At Least Four Components in Lava Erupted From Fissure 17, Kilauea 2018

In 2018, a chemically complex lava erupted from Fissure 17 during the Lower East Rift Zone (LERZ) eruption of Kilauea, Hawaii. Fissure 17 was active from May 13-25 of 2018, and was more explosive than other vents, which produced a more fluid basalt. Mixing of two different magmas is visible in hand specimen, with undeformed crystalline inclusions in a microcrystalline groundmass that sheared around them. As previously noted (Gansecki et al. 2019 Science 366: eaaz0147), Fissure 17 lavas plot on a linear mixing trend between basalt and icelandite end-members, and the basalt is itself likely a mixture of a juvenile mafic end-member and a high-Ti end-member that was already present in the rift zone and dominant in phase 1 of the eruption. Our preliminary hypothesis to explain the observed chemistry and textures was that fluid basaltic lava encountered a cool, evolved, crystal-rich mush pocket (icelandite) from a previous magmatic episode, which would have formed the inclusions. However, the three inclusions analyzed were basaltic and very crystalline (and phenocryst-rich), more so than reported for early phase 1 or any other phase of 2018, while the microcrystalline groundmass was more felsic. Calorimetry was used to determine the glass content of the groundmass (~50% glass) and the basaltic inclusions (no detectable glass). The most felsic samples are very phenocryst-poor, similar to the low crystallinity of the Puna dacite encountered by drilling nearby in 2005. The basaltic inclusions show little to no evidence for disaggregation and chemical interaction with the intermediate host, and they match the composition of 1955 lavas more closely than they match with the "basalt" end of the mixing line. The conclusion is that, although Fissure 17 lava is visibly heterogeneous, with numerous inclusions, the inclusions are not the major source of the geochemical variation in the suite. Most of the mixing occurred between two crystal-poor magmas, icelandite and basalt (itself already the product of mixing between two basaltic end-members). This mixing is now texturally invisible, but detectable with geochemistry. The fourth component comprising the erupted lava was crystal-rich basalt, which was rheologically incompatible and thus not thoroughly mixed, but preserved as inclusions.