Geochemical analyses of volcanic bombs erupted from Mount Erebus

This data set consists of geochemical analyses of volcanic bombs erupted from
Mount Erebus between 1972 and 2004 and selected older lava samples from the
summit area. Detailed electron microprobe analyses of minerals in the volcanic
bombs are given in Appendix.

Mount Erebus, Ross Island, Antarctica, is a large (3794 m) alkaline
open-conduit stratovolcano that hosts a persistently convecting and degassing
lake of anorthoclase-phyric phonolite magma. The composition of the lava
(magma) lake over the last 31 years was investigated by examining glass and
mineral compositions in lava bombs erupted between December 1972 and January
2004. Olivine, clinopyroxene, titanomagnetite, fluor-apatite, and matrix glass
compositions are invariant and show that the magmatic temperature (~1000°C) and
oxygen fugacity (ΔlogFMQ = -0.9) have been constant. Anorthoclase
phenocrysts up to 10 cm in length show complex textural and compositional
zoning that reflects growth during convection in a PH2O gradient under
isothermal conditions. Bomb textures and anorthoclase compositions suggest
that crystallization occurs at low degrees of effective undercooling and is
controlled by shallow degassing of water in the conduit.

Minor variations in eruptive activity in the last 31 years are decoupled from
magma compositions. The variations probably relate to changes in conduit
geometry within the volcano and/or variable input of CO₂-rich
volatiles into the upper-level magma chamber from deeper in the system. Eleven
bulk samples of phonolite lava from the summit plateau that range in age from 0
± 4 ka to 17 ± 8 ka were analyzed for major and trace elements. Small
compositional variations are controlled by anorthoclase content. The lavas are
indistinguishable from modern bulk tephra compositions and demonstrate that Mt.
Erebus has been erupting lava and tephra from the summit region with the same
bulk composition for ~17 ka.