Geochemistry And Mineralogy Of The Phonolite Lava Lake, Erebus Volcano, Antarctica: 1972-2004 And Comparison With Older Lavas

摘要

Mount Erebus, Antarctica, is a large (3794 m) alkaline open-conduit stratovolcano that hosts a vigorously converting and persistently degassing lake of anorthodase phonolite magma. The composition of the lake was investigated by analyzing glass and mineral compositions in lava bombs erupted between 1972 and 2004. Matrix glass, titanomagnetite, olivine, clinopyroxene, and fluor-apatite compositions are invariant and show that the magmatic temperature (～1000℃) and oxygen fugacity (△logFMQ=-0.9) have been stable. Large temperature variations at the lake surface (～400-500℃) are not reflected in mineral compositions. Anorthoclase phenocrysts up to 10 cm in length feature a restricted compositional range (An_(10.3-22.9)Ab_(62.8-68.1)Or_(11.4-272) with complex textural and compositional zoning. Anorthoclase textures and compositions indicate crystallization occurs at low degrees of effective undercooling. We propose shallow water exsolution causes crystallization and shallow convection cycles the anorthoclase crystals through many episodes of growth resulting in their exceptional size. Minor variations in eruptive activity from 1972 to 2004 are decoupled from magma compositions. The variations probably relate to changes in conduit geometry within the volcano and/or variable input of CO_2-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 lava bomb compositions and demonstrate that Erebus volcano has been erupting lava and tephra from the summit region with the same bulk composition for ～ 17 ka.