Crystal and magma residence at Kilauea Volcano, Hawaii: ~(230)Th-~(226)Ra dating of the 1955 east rift eruption

摘要

Previous estimates of crystal storage time of magmas at Kilauea Volcano, Hawaii, range from a few years to a few thousand years, leading to considerable uncertainty in the time scales of processes of magmatic storage and differentiation. We present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using ~(226)Ra-~(230)Th disequilibria, and apply this approach to the early phase of the 1955 east rift eruption at Kilauea. When fractionation of Ra from Ba (a proxy for initial Ra in the crystals) during crystal growth is considered along with the effects of inclusions in the minerals, the data are consistent with co-precipitation of plagioclase and clinopyroxene from a melt represented by the groundmass at a mean age of 1000_(-400)~(+300) a. Unless a significant fraction (>30%) of the crystals are remnants from an earlier batch of evolved magma in the system, these data constrain the minimum magmatic residence time to be ～550 yr, considerably longer than most previous estimates of storage time at Kilauea as well as those for some other basaltic systems. For the temperature interval of augite + plagioclase growth in the early 1955 magma, a maximum constant cooling rate of 0.1 ℃/yr (1 * 10~(-5) ℃/h) is derived from the minimum magmatic residence time of 550 yr. The total magma storage time would be >2500 yr if this cooling rate applied to the entire thermal history of the magma, although a more complex cooling history where cooling rates were more rapid early in the storage history is permissive of a total residence time which is not much longer than 550 yr. The disparate estimates of magma residence at Kilauea may reflect the uncertainties in the methods of estimation in addition to true variations in storage time for different batches of magma. More work is necessary in order to determine whether a long residence time is characteristic of rift zone lavas and/or of Kilauean lavas in general.