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Short-lived decay series disequilibria in the natrocarbonatite lavas of Oldoinyo Lengai, Tanzania: constraints on the timing of magma genesis

1991; Elsevier BV; Volume: 105; Issue: 4 Linguagem: Inglês

10.1016/0012-821x(91)90179-l

1385-013X

David M. Pyle, J. B. Dawson, Μ. Ivanovich,

earthquake and tectonic studies

The 1988 natrocarbonatite lavas from Oldoinyo Lengai volcano, Tanzania have been analysed for232Th,230Th,228Th,238U,228Ra/226Ra. These lavas are unique, in showing disequilibria between228Th/232Th, and between228Ra and232Th. Aa and pahoehoe lavas have a mean (228Th232Th) activity ratio of 5.5 ± 0.6 , and one lava has (228Ra/226Ra) = 0.11 ± 0.01 . The lavas have (230Th/238U) ∼ 0.1–0.2 , and [ UTh] 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 [ UTh] weight ratios of 5.6–6.4, and a calculated (228Ra/232Th) activity ratio of 108 ± 5 . Disequilibria between238U&z.sbnd;230Th&z.sbnd;226Ra are consistent with an origin by immiscibility of 4–22wt% natrocarbonatite from nephelinite magma. Disequilibria between232Th&z.sbnd;228Ra&z.sbnd;228Th 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 × 107 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.