U-decay series and trace element systematics in the 1978 eruption of Ardoukoba, Asal rift: timescale of magma crystallization

Abstract

230Th– 238U, 226Ra– 230Th, 231Pa– 235U disequilibria, Th and Sr isotopes, trace and major elements were measured in a tholeiitic basalt series erupted within a single week in November 1978 at the Ardoukoba Volcano (Asal rift, Djibouti). ( 230Th/ 238U) and ( 231Pa/ 235U) activity ratios are constant for all the lavas, within analytical uncertainties, and are equal to 1.42 and 2.20, respectively. ( 226Ra/ 230Th) activity ratios in the whole rocks decrease with Th content from 1.93 to 1.35. Trace elements measured in plagioclase and groundmass show that all the lavas display a similar proportion of accumulative plagioclase (∼30%). Once corrected for plagioclase accumulation, the trace element variations can be modeled as a single fractional crystallization series which requires 30% crystallization (49% plagioclase, 39% olivine and 12% clinopyroxene). However, this fractional crystallization cannot explain the whole range in ( 226Ra/ 230Th). 226Ra radioactive decay must also be taken into account. Consequently, this basaltic series also represents an age series evolving from the older magma (#544) to the most recent magma (#546). To constrain magma crystallization ages from ( 226Ra/ 230Th) data, two end-member models are proposed. The first one corresponds to several magma batches crystallizing continuously as closed systems. The second model corresponds to a zoned chamber crystallizing continuously, and regularly replenished with fresh parental magma. The fraction of crystallized magma per unit of time is 3.3×10 −4 yr −1 for the open-system, and 3.6×10 −4 yr −1 for the closed-system. Model ages of crystallization of 1880 yr (±80 yr) and 870 yr (±30 yr) are inferred from the open- and the closed-system model, respectively.