Halogens (F, Cl and Br) at Oldoinyo Lengai volcano (Tanzania): Effects of magmatic differentiation, silicate–natrocarbonatite melt separation and surface alteration of natrocarbonatite

Halogen abundances of natrocarbonatites and their alteration products from Oldoinyo Lengai (Tanzania) are compared to those of associated silicate rocks and to various calcite carbonatites from eleven occurrences worldwide. Fresh natrocarbonatites are extremely enriched in F (up to 3.6 wt.%), Cl (up to 5.9 wt.%) and Br (up to 100 μg/g). During meteoric alteration, however, the major Cl- and Br-bearing minerals (mainly sylvite and gregoryite) are easily dissolved, leaving behind residual materials with relatively low Cl (≤ 1000 μg/g) and Br (≤ 15 μg/g) contents. The comparatively low Cl/Br ratios suggest preferential leaching of Cl relative to Br during alteration. At the same time, F is passively enriched as fluorite remains relatively stable during alteration. In the associated silicate rocks at Oldoinyo Lengai concentrations of all three halogens increase from primitive olivine melilitite to evolved combeite–wollastonite nephelinite, demonstrating their incompatible behavior during magmatic differentiation. Relatively constant Cl/Br ratios, but strongly decreasing F/Cl ratios, in these samples are explained by moderately incompatible behavior for F compared to pronounced and very similar incompatibilities for Cl and Br. Our data further imply that during silicate–natrocarbonatite melt separation all three halogens show a strong affinity for the natrocarbonatite. F and Cl are equally enriched in the natrocarbonatite while Cl and Br are slightly fractionated from each other with a preferential partitioning of Cl relative to Br into the natrocabonatite melt.