Ra-Th disequilibria systematics: timescale of carbonatite magma formation at Oldoinyo Lengai volcano, Tanzania

Carbonatite magma can form and erupt within 7 to 18 years, and the event seems associated with prior volcanic eruptions. This determination of magma age is possible because the carbonatite lava and ash which were erupted in 1960-66 from Oldoinyo Lengai volcano, Tanzania, have the most extreme disequilibria between U and Th series nuclides yet measured in volcanic rocks. At the time of eruption: (228Ra)/(232Th) equal approx. 27 and (226Ra)/(230Th) > 60;(238U)/(232Th) >10, while (230Th)/(232Th) = 1.0; and (210Pb)/(226Ra) equals approx. 0.3. Three end-member models are presented which enable interpretation of these disequilibria. If the disequilibrium formed instantaneously, the event occurred about 7 years before initial eruption, and just before the last preceding but small eruption of Oldoinyo Lengai. If, instead, the disequilibrium formed continuously, the process must have begun 15 to 18 years before initial eruption, just after the last preceding major eruption. The disequilibria data confirm that the carbonatites are not fused trona, but do not distinguish between other genetic options (mantle fusion, selective assimilation, liquid immiscibility). However, the shortness of magma-formation time together with mass-balance considerations suggest formation due to the continuous exsolution of 2 to 20% of carbonatite from nephelinite which was itself Ra-enriched. (author)