O¹⁸/O¹⁶ and C¹³/C¹² Ratios of Coexisting Minerals in the Oka and Magnet Cove Carbonatite Bodies

Abstract

Oxygen isotope analyses were obtained for five magnetites (δ = +1.1 to +2.9), three biotites (+4.2 to +5.2), two melilites (+4.7 to +5.2), one apatite (+5.6), one niocalite (+4.2) and one pyrochlore (+1.2) from five samples of okaite and carbonatite in the Oka, Quebec, and Magnet Cove, Arkansas, complexes. Calcites from these and three other samples were analyzed for both O¹⁸/O¹⁶ and C¹³/C¹² (δO¹⁸ = +6.6 to +7.6, δC¹³ = -5.8 to -5.2). The order of increasing O¹⁸ concentration among minerals is consistent in all samples. Calculated whole-rock δO¹⁸ values (+4.7 to +5.9) are similar to those of mafic and ultramafic igneous rocks, and the δO¹⁸ and δC¹³ values of the calcites fall within the range previously suggested as being characteristic of primary igneous carbonatites. Crystallization temperatures, calculated from O¹⁸ fractionations for the pair calcite-magnetite, demonstrate that the okaites and carbonatites crystallized at about the same temperatures as, respectively, gabbros and "granitic" igneous rocks.