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

Th-230-U-238, Ra-226-Th-230, Pa-231-U-235 disequilibria, Th and Sr isotopes , trace and major elements were measured in a tholeiitic basalt series erup ted within a single week in November 1978 at the Ardoukoba Volcano (Asal ri ft, Djibouti). (Th-230/U-238) and (Pa-231/U-235) activity ratios are consta nt for all the lavas, within analytical uncertainties, and are equal to 1.4 2 and 2,20, respectively. (Ra-226/Th-230) activity ratios in the whole rock s decrease with Th content from 1.93 to 1.35. Trace elements measured in pl agioclase and groundmass show that all the lavas display a similar proporti on of accumulative plagioclase (similar to 30%). Once corrected for plagioc lase accumulation, the trace element variations can be modeled as a single fractional crystallization series which requires 30% crystallization (49% p lagioclase, 39% olivine and 12% clinopyroxene). However, this fractional cr ystallization cannot explain the whole range in (Ra-226/Th-230). Ra-226 rad ioactive 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 fr om (Ra-226/Th-230) data, two end-member models are proposed. The first one corresponds to several magma batches crystallizing continuously as closed s ystems. The second model corresponds to a zoned chamber crystallizing conti nuously, and regularly replenished with fresh parental magma. The fraction of crystallized magma per unit of time is 3.3 x 10(-4) yr(-1) for the open- system, and 3.6 x 10(-4) yr(-1) for the closed-system. Model ages of crysta llization of 1880 yr (+/-80 yr) and 870 yr (+/-30 yr) are inferred from the open- and the closed-system model, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.