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

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

Aug 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.105..378p&link_type=abstract

Earth and Planetary Science Letters, Volume 105, Issue 4, p. 378-396.

Physics

13

Scientific paper

The 1988 natrocarbonatite lavas from Oldoinyo Lengai volcano, Tanzania have been analysed for 232Th, 230Th, 228Th, 238U, 228Ra and 226Ra. These lavas are unique, in showing disequilibria between 228Th and 232Th, and between 228Ra and 232Th. Aa and pahoehoe lavas have a mean (228Th/232Th) 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 [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 (228Ra/232Th) activity ratio of 108 +/- 5.
Disequilibria between 238U-230Th-226Ra are consistent with an origin by immiscibility of 4-22wt% natrocarbonatite from nephelinite magma. Disequilibria between 232Th-228Ra-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.

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