Understanding highly explosive basaltic eruptions: Evidence from olivine-hosted melt inclusions from Sunset Crater, AZ

Basaltic scoria cone volcanoes are the most abundant volcanic landform on Earth and occur in all tectonic settings. Basaltic magmas have lower viscosities, higher temperatures, and lower volatile contents than silicic magmas, and therefore generally have a lower potential for explosive activity. However, basaltic eruptions display great variability, from mild lava flows to more energetic explosions with large plumes. The mechanism controlling highly explosive basaltic eruptions, such as the ca. 1085 AD eruption of Sunset Crater, is poorly understood. Processes or conditions such as high volatile content in the source magma, injection of a compositionally distinct magma at depth, interaction with shallow magma reservoirs, or rapid crystallization and/or bubble nucleation in the shallow subsurface could increase explosivity of basaltic magmas. One method to test these hypotheses is melt inclusion analysis in order to constrain initial melt composition, volatile content and minimum storage depth. The San Francisco Volcanic Field (SFVF) in northern Arizona, active from 6 Ma-present, consists of over 600 volcanoes - mainly alkali basalt scoria cones along with five silicic centers. The eruption of Sunset Crater in the SFVF during the Holocene was an anomalously large basaltic explosive eruption, consisting of >8 explosive phases and 3 lava flows. Typical scoria cone-forming eruptions produce <0.1 km³ DRE of material, while the Sunset Crater tephra deposit is on the order of ~0.3 km³ DRE, with each phase characterized by volumes of 0.02-0.08 km³ DRE. The phases vary in size and style; the beginning stages of explosive activity (phases 1-2) were considerably smaller than phases 3-5, classified as subplinian. Because of the young age and desert setting of the volcano, the eruptive material is fresh and the deposit is well preserved. The bulk composition is an alkali basalt with Mg# 74. We studied 40 primary melt inclusions (MIs) hosted in 36 olivine crystals 0.5-2 mm in diameter from phase 3 of the eruption. Phase 3 was the largest phase by volume, distributed tephra over the greatest area, and its deposit has the highest proportion of preserved fine-grained (<0.5 mm) material. Almost all crystals with viable MIs contain multiple MIs, many containing 3 or more at varying distances from the surface of the crystal. A few crystals contain large populations of chromites. Measured MIs vary in size (~50-130 μm diameter) and shape, most are faceted, and all contain a vapor bubble. Major element analysis shows little compositional variability among the MIs, and indicates the melt is alkali basalt. The K₂O content varies between 0.90-1.10 wt.%, consistent with a small degree of crystal fractionation as also indicated by the relatively phenocryst-poor scoria. Sulfur and Chlorine contents are ~2000 and ~400 ppm, respectively. These results are similar to bulk rock and matrix glass compositions, and combined with the size of the eruption, suggest a relatively homogeneous melt storage zone. Future work will further constrain composition and volatile abundances for all phases of the eruption.