238U- and 232Th-series chronology of phonolite fractionation at Mount Erebus, Antarctica

Abstract

Uranium, thorium, radium, and barium abundances and 234U 238U and 230Th 232Th isotopic ratios determined by thermal ionization mass spectrometry and ( 228Th) ( 232Th ) activity ratios determined by alpha spectrometry are used to date anorthoclase growth and infer magma chamber residence times of phonolites erupted in 1984 and 1988 from Mount Erebus, Antarctica. The 1984 and 1988 glasses have slightly different ( 230Th) ( 232Th ) ratios but both have a 10% excess of ( 230Th) over ( 238U) and equilibrium ( 228Th) values. By comparing these data and Pb-isotopic data reported in Sun and Hanson (1975) to similar data for oceanic basalts, the duration of differentiation from basanite to phonolite is limited to less than 150,000 years. The anorthoclase separates have ( 230Th) ( 238U ) ratios exceeding those of the associated glasses but have ( 230Th) ( 232Th ) ratios like those of the glasses. Both glasses are depleted in 226Ra with respect to 230Th by about 25%, whereas associated anorthoclase separates have extreme excesses of 226Ra over 230Th and ( 228Th) ( 232Th) = 2.2. On a plot of ( 226Ra)/Ba vs. ( 230Th)/Ba, the glass-anorthoclase pairs produce isochrons averaging 2380 y, which represents the average age of anorthoclase growth in the shallow magma system at Erebus. The implied residence time of phonolite magmas in the shallow magma chamber system of Erebus is about 3000 y. Final crystal growth occurred after intrusion into the converting lava lake less than decades before eruption.