Strontium Isotope Studies of Alkalic Rocks: The Active Carbonatite Volcano Oldoinyo Lengai, Tanzania
Sr isotopic ratios as well as Rb, Sr, and K abundances are reported for 16 samples of lava and 12 ejected plutonic blocks from the active carbonatite volcano Oldoinyo Lengai, northern Tanzania. Lava and plutonic blocks together exhibit a range of Sr 87 /Sr 86 ratios from 0.7033 to 0.7096. Phonolitic lava exhibits the same range as nephelinite, while two specimens of the sodium carbonatite lava have ratios of 0.7059 and 0.7061, similar to many of the silicate lavas. The Sr 87 /Sr 86 ratios of the lava cannot be correlated with any other geochemical parameter or with their position in the extrusion cycle. Ejected plutonic blocks have isotopic ratios similar to or slightly lower than the lava samples and generally contain smaller amounts of K, Rb, and Sr. Similarity in Sr isotopic composition and K/Rb ratios between the phonolite and nephelinite suggests that the more silicic phonolite is not rheomorphic fenite. Correlation coefficients between several major oxides and trace elements indicate the presence of two groups, one high in Al 2 O 2 , SiO 2 , and Th, and another enriched in CaO, K 2 O, Na 2 O, P 2 O 5 , and Sr. Sr isotopic ratios of four specimens of evaporites from Lake Natron and Lake Magadi vary between 0.7045 and 0.7114 and are, within limits of experimental error, similar to the values from the carbonatite and silicate lavas. These are discussed in terms of the overall petrogenesis of the Oldoinyo Lengai lavas.