Ilmenite (high Mg,Mn,Nb) in the carbonatites from the Jacupiranga Complex, Brazil

Abstract

Ilmenite occurs rarely in the carbonatite plug (five separate intrusions) of the Jacupiranga Complex, in the jacupirangite near the contacts, and in the banded reaction zones between carbonatite and jacupirangite. Electron microprobe analyses reveal a wider range of ilmenite compositions than previously reported from carbonatites, as solid solutions in FeTiO_3-MgTiO_3-MnTiO_3-Fe2O_3, with high-Nb ilmenites containing up to 3.68 wt.% (Nb_2O_5 + Ta_2O_5). A group of discrete primary ilmenites from one carbonatite is distinguished by unusually high MnTiO_3 (MnO 15.1-19.6%); ilmenite inclusions in magnetites of the carbonatite intrusions are similar in composition to magnesian ilmenites previously reported; other ilmenites (a few discrete grains, but mainly lamellae with some granules associated with magnetite), have compositions that vary almost completely from FeTiO_3 to MgTiO_3, with less than 10% Fe_2O_3. Titanomagnetites (with Nb_2O_5 below detection limits) in jacupirangite have ilmenite lamellae near and within the reaction zones, with Nb_2O_5 increasing (Nb_2O_5 + Ta_2O_5 up to 0.95%) and Fe_2O_3 decreasing in ilmenite as carbonatite is approached. Pentavalent Nb and Ta replace Ti in ilmenite, causing cation deficiency. In coexisting magnetite and ilmenite, MgO and MnO partition preferentially into ilmenite; MgO shows a regular pattern of distribution between ilmenite and magnetite, whereas MnO is distributed irregularly. Coexisting magnetite and ilmenite (with low MgO and MnO) from the reaction rocks give equilibration temperatures of 570-595°C and oxygen fugacities of 10^(-18.5) to 10^(-19.5) atmospheres. The ilmenites are distinguished from ilmenites in kimberlites by high MnO, very low Cr_2O_3, and high (Nb_2O_5 + Ta_2O_5).

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