EARLY MAGMATIC AND REACTION-INDUCED TRENDS IN MAGNETITE FROM THE CARBONATITES OF KERIMASI, TANZANIA

We examined the mode of occurrence, pattern of zoning and composition of magnetite and associated spinel-group minerals in three types of calciocarbonatite from the Kerimasi volcano, in Tanzania. In all samples, magnetite is one of the earliest phases to have crystallized, and shows an appreciable compositional variation. The majority of compositions correspond to magnetite with low to moderate proportions of magnesioferrite and ulvospinel components (10-28 and 2-28 mol.%, respectively) and < 15 mol.% of spinel and jacobsite. One sample of pyroclastic carbonatite also contains crystals of Mn-rich magnesioferrite (15-17 mol.% MnFe2O4). The two trace elements consistently present in appreciable amounts are V (400-2000 ppm) and Zn (700-3300 ppm); the abundances of other trace elements are much lower and very variable (<= 15 ppm Cr, 170 ppm Ni, 220 ppm Co, 490 ppm Zr, 14 ppm Hf, 95 ppm Nb, 3 ppm Ta, and 80 ppm Ga). Magnetite is thus a minor host of Zr, Hf, Nb and Ta in carbonatites. The composition of magnetite crystallizing from carbonatitic magma evolves by becoming depleted in Mg and Ti, whereas its Al content inversely correlates with the V content and, thus, is sensitive to variations in f(O-2). The compatibility of V is interpreted to decrease, and that of Mn to increase, with increasing f(O-2). Covariation between the Mn and Zn contents suggests that the partitioning behavior of Zn is controlled by the coupled substitution Zn2+ Mn3+ Fe-12+Fe-13+. The Mg-Ti depletion trend is accompanied by a decrease in Zr and Ta contents at constant or decreasing levels of Nb and Hf, which has implications for the partitioning behavior of high-field-strength elements in carbonate melts. In addition to the magmatic evolutionary trend, the Kerimasi magnetite exhibits a previously unrecognized trend arising from a reaction of the magnetite with the carbonatitic magma. This trend involves enrichment of the peripheral parts of magnetite crystals in Mg, Al, Mn, Zr and Nb, and their mantling by Fe-rich spinel. This trend requires that a(Mg2+) and a(Al3+) in the magma increase with evolution, whereas a( SiO2) remains low to impede the precipitation of Mg-Al silicates.