RELATIONSHIP OF COEXISTING CARBONATE AND SILICATE MAGMA AT OL DOINYO LENGAI BASED ON FE AND U ISOTOPE RATIOS

Ol Doinyo Lengai (Tanzania) is the world’s only carbonatite volcano. It erupts both Na-carbonatite and alkali silicate rock (nephelinite), providing an ideal window through which to learn about: 1) the role of CO₂ in carbonate magma formation; 2) whether immiscibility leads to separate silicate and carbonatite magmas; 3) role of H₂O in differentiation by either crystal fractionation or temperature gradient processes in controlling isotopic fractionation in high temperature conditions.

In 2008, ODL explosively erupted nephelinite after many years of quiet Na-carbonatite lava flows. Our sample set includes 2 carbonatites, 2 mixed tephras and a sequence of 8 nephelinite tephras. We did both Fe and U isotope measurements on carbonatites and nephelinite tephras, aimed at understanding questions mentioned above. Fe isotope results showed 2 carbonatites are isotopically lighter than nephelinite tephras (δ⁵⁶Fe of 2 carbonatite are -0.35 and -0.55 ‰; δ⁵⁶Fe of nephelinite range from 0 to 0.2 ‰, respectively). However, U isotope results (δ²³⁸U) showed that 2 carbonatites, which gives -0.11 and -0.28 ‰, are isotopically heavier than 8 nephelinite tephras (range from -0.30 to -0.41 ‰).

Fe isotope results may reflect separation of silicate and carbonatite magma by immiscibility if the light isotope ⁵⁴Fe prefers going into carbonatite while heavier ⁵⁶Fe favors silicate magma. Fe isotope fractionation between Na-carbonatite and silicate rocks may also be caused by crystal fractionation—however fractionation factors for these processes have not been determined experimentally. For uranium, the distinctive isotopic signature could also be evidence for immisciblity. Furthermore, if there were a temperature gradient existing in the ODL system, one would predict positive correlation between the Fe and U isotope ratios which is not observed.

Future research measuring Fe and U isotope fractionation factors between Na-carbonatite and nephelinite will answer the questions on whether immiscibility causes carbonatite formation, as well as what are the roles of H₂O and CO₂ in associated magmatic processes.