CALCIOCARBONATITE DYKES AT OLDOINYO LENGAI, TANZANIA: THE FATE OF NATROCARBONATITE

Abstract

Carbonatite dykes are described for the first time from the active natrocarbonatite volcano Oldoinyo Lengai, Tanzania. The dykes are fine grained, with a microporphyritic texture of polycrystalline, granular calcite microphenocrysts. The porous texture is indicative of solution and leaching processes. Groundmass phases are fluorite with 2.1-3.9 wt.% Sr, Mn-rich magnetite (up to 10.2 wt.% MnO), REE- and Si-rich fluorapatite (up to 20.6 wt.% REE2O3 and 9.9 wt.% SiO2) and a Mn-Ba hydroxide. Chemically, the dykes are Ca-carbonatitic, with high concentrations of F, Sr, Fe and Mn. Total alkalis are below 2 wt.%. Compared with fresh natrocarbonatite, the REE, Y, Zr, Th, Nb and Pb are enriched, whereas Rb, Cs, Ba, Cl and S are markedly depleted. Stable isotope ratios are δ13C -1.95 ‰ PDB and δ18O +24.12 ‰ SMOW, well outside the compositional field of primary Lengai natrocarbonatites, indicating extensive atmospheric exchange and re-equilibration, in particular of oxygen. These dykes are interpreted as alteration products of the typical Lengai natrocarbonatite. From the evidence at Oldoinyo Lengai, we believe that calcite carbonatites may have different origins. In contrast to the widespread primary magmatic crystallization of calciocarbonatitic melts, alteration and replacement exchange of original natrocarbonatites of Oldoinyo Lengai produce secondary calcite carbonatites. Using the criteria presented here, a distinction between calcitized natrocarbonatites and primarily calciocarbonatitic extrusive rocks is possible, on the basis of textural evidence mimicking the porphyritic texture of natrocarbonatite, and on the polycrystalline nature of calcite pseudomorphs after nyerereite. Noteworthy in the secondary calcite are the high Sr contents (2.2-3.5 wt.% SrO), and variable Ba contents (0.3-2.2 wt.% BaO). A prominent primary feature in the calciocarbonatite of the dykes is the sheaf-like intergrowth texture of fluorite, which is typical of fresh natrocarbonatites from Oldoinyo Lengai. Stable isotope ratios are independent criteria indicating extensive alteration and secondary exchange.