Petrologic Insights into Rift Zone Magmatic Interactions under Kīlauea's Nāpau Crater (1922-2011)

Abstract

The high frequency of historical eruptions at Kīlauea Volcano presents an exceptional opportunity to address fundamental questions related to the transport, storage, and interaction of magmas within rift zones. In particular, the Nāpau Crater area on Kīlauea’s East Rift Zone (ERZ) has experienced frequent historical fissure eruptions (nine within 50 years). Repeated intrusion and eruption in this region suggests that the Nāpau Crater area serves as a magma mixing ‘depot’ within the ERZ. Chemical analyses of lavas from fissure eruptions near Nāpau Crater in 1983, 1997, and 2011 have been interpreted to represent mixing between rift-zone-stored magmas and new, summit-derived magma (e.g., Garcia et al., 1992; Thornber et al., 2003; Orr et al., 2015). To better understand potential mixing processes within the ERZ, whole-rock, glass, and mineral compositions were determined for a newly collected suite of samples from the 1963, 1965, 1968, 1983, 1997, and 2011 eruptions. These data help elucidate the pre-eruptive history and temporal evolution of lavas from the most recent eruptive activity near Nāpau Crater in March 2011 (Puʻu ʻŌʻō episode 59). Whole-rock XRF data reveal two geochemically distinct magma batches for the 2011 eruption: one less evolved (6.5-6.6 wt. % MgO) than the other (6.1-6.3 wt. % MgO). Olivine crystals from both batches show normal and reverse zoning, indicating mixing prior to episode 59. Least squares regression calculations document the possibility of mixing between summit-derived magma and residual magma from the 1983 and 1997 eruptions, but surprisingly, show no indication of involvement of earlier magmas from the 1960s. Two physically and chemically distinct pods of stored magma interacted with the episode 59 dike to create the compositions seen in whole-rock analyses of episode 59 lavas. Like the two batches of eruptive products, one storage pod is geochemically more evolved than the other. Least squares regression calculations are consistent with the less evolved pod having contained a fractionated equivalent of 1983 lava and the more evolved pod having contained a hybrid of compositions similar to 1983 and 1997 lavas. Both pods reside at depths ranging from 1.6 to 3.0 km under the Nāpau Crater area. These results clarify the spatial and chemical relations of stored magmas in one of the most frequently active portions of the ERZ and provide insight into their fractionation and mixing histories.