KĪLAUEA VOLCANO’S INTERMITTENT OPEN-SYSTEM BEHAVIOR: PETROLOGIC EVIDENCE AND IMPLICATIONS FOR ERUPTION STYLE

Kīlauea (Hawaiʻi, USA) has intermittently operated as an open-system volcano throughout its ~200 years of recorded history. The most obvious open-system behavior is convecting lava lake activity within Kīlauea’s summit caldera (e.g., from 1823–1894, 1906–1924, 2008–2018). Another open-system phenomenon at Kīlauea is lava drain-back, where ponded lava returns to the deeper magmatic system by flowing down through fissures or cracks. Lava drain-back has been observed during a number of Kīlauea eruptive episodes, including the 1959 Kīlauea Iki eruption (Richter et al. 1970) and most recently in the early weeks of the 2020 summit eruption. H₂O and sulfur (as SO₂) significantly degas from Kīlauea magmas only at very low pressures (<200 m depth) and lava lake convection and lava drain-back are therefore key processes that recycle shallowly-degassed, H₂O- and sulfur-depleted melts back into Kīlauea’s plumbing system.

Primary Kīlauea magmas are estimated to contain 0.5–0.7 wt% H₂O and 1300–1600 ppm S (e.g., Anderson and Brown 1993; Edmonds et al. 2013), but mixing with recycled volatile-depleted melts could cause significant variations in volatile concentrations between magma batches. H₂O- and sulfur-poor submarine glasses from the Puna Ridge, which erupted at depths where hydrostatic pressure would have prevented most degassing, provide strong evidence for the recycling of shallowly-degassed magma (Dixon et al. 1991). Additionally, melt inclusions from the 2018 Lower East Rift Zone eruption are commonly sulfur-poor (400–800 ppm S) and have isotopically light sulfur signatures (0 to -2‰ δ³⁴S) consistent with sulfur-loss by shallow SO₂ degassing (Lerner et al. 2021). Many of these sulfur-depleted melt inclusions are hosted in Fo_86-89 olivine grains, indicating that at times in the past, highly primitive Kīlauea magmas (1200–1300 °C) reached very shallow depths and underwent substantial degassing before being recycled back into the magmatic system.

We propose that Kīlauea’s long history of lava lake activity and lava drain-back events have recycled substantial volumes of degassed magma, including primitive magmas, into the shallow magmatic system. Hybridization between fresh magmas and recycled volatile-depleted magmas may control important eruption properties such as fire fountain height and lava viscosity.