40Ar/39Ar dating of the eruptive history of Mount Erebus, Antarctica: volcano evolution

Mt. Erebus, a 3,794-meter-high active polygenetic stratovolcano, is composed of voluminous anorthoclase-phyric tephriphonolite and phonolite lavas overlying unknown volumes of poorly exposed, less differentiated lavas. The older basanite to phonotephrite lavas crop out on Fang Ridge, an eroded remnant of a proto-Erebus volcano and at other isolated locations on the flanks of the Mt. Erebus edifice. Anorthoclase feldspars in the phonolitic lavas are large (similar to 10 cm), abundant (similar to 30 - 40%) and contain numerous melt inclusions. Although excess argon is known to exist within the melt inclusions, rigorous sample preparation was used to remove the majority of the contaminant. Twenty-five sample sites were dated by the 40Ar/39Ar method ( using 20 anorthoclase, 5 plagioclase and 9 groundmass concentrates) to examine the eruptive history of the volcano. Cape Barne, the oldest site, is 1,311+/-16 ka and represents the first of three stages of eruptive activity on the Mt. Erebus edifice. It shows a transition from sub-aqueous to sub-aerial volcanism that may mark the initiation of proto-Erebus eruptive activity. It is inferred that a further similar to 300 ky of basanitic/phonotephritic volcanism built a low, broad platform shield volcano. Cessation of the shield-building phase is marked by eruptions at Fang Ridge at similar to 1,000 ka. The termination of proto-Erebus eruptive activity is marked by the stratigraphically highest flow at Fang Ridge ( 758+/-20 ka). Younger lavas (similar to 550 - 250 ka) on a modern-Erebus edifice are characterized by phonotephrites, tephriphonolites and trachytes. Plagioclase-phyric phonotephrite from coastal and flank flows yield ages between 531+/-38 and 368+/- 18 ka. The initiation of anorthoclase tephriphonolite occurred in the southwest sector of the volcano at and around Turks Head ( 243+/-10 ka). A short pulse of effusive activity marked by crustal contamination occurred similar to 160 ka as indicated by at least two trachytic flows ( 157+/-6 and 166+/-10 ka). Most anorthoclase-phyric lavas, characteristic of Mt. Erebus, are less than 250 ka. All Mt. Erebus flows between about 250 and 90 ka are anorthoclase tephriphonolite in composition.