SHORT-LIVED DECAY SERIES DISEQUILIBRIA IN THE NATROCARBONATITE LAVAS OF OLDOINYO LENGAI, TANZANIA - CONSTRAINTS ON THE TIMING OF MAGMA GENESIS

The 1988 natrocarbonatite lavas from Oldoinyo Lengai volcano, Tanzania have been analysed for Th-232, Th-230, Th-228, U-238, Ra-228 and Ra-226. These lavas are unique, in showing disequilibria between Th-228 and Th-232, and between Ra-228 and Th-232. Aa and pahoehoe lavas have a mean (Th-228/Th-232) activity ratio of 5.5 +/- 0.6, and one lava has (Ra-228/Ra-226) = 0.11 +/- 0.01. The lavas have (Th-230/U-238) approximately 0.1-0.2, and [U/Th] weight ratios of 2.0-3.2. Late-stage samples, extruded from the lavas on cooling and interpreted as extreme fractionates of the original lavas are highly enriched in U and Ra relative to Th. These samples have measured [U/Th] weight ratios of 5.6-6.4, and a calculated (Ra-228/Th-232) activity ratio of 108 +/- 5. Disequilibria between U-238-Th-230-Ra-226 are consistent with an origin by immiscibility of 4-22wt% natrocarbonatite from nephelinite magma. Disequilibria between Th-232-Ra-228-Th-228 are consistent with either of two endmember models: (1) instantaneous separation of magma at depth, with eruption 20 +/- 1 years later; (2) recharging of a steady-state magma chamber below Oldoinyo Lengai with a maximum volume of 1.5 +/- 0.2 x 10(7) m3 of carbonatite, and a mean magma residence time of 81 +/- 9 years. The total time between natrocarbonatite generation and eruption is between 20 and 81 years.