Oxygen isotope composition of carbonates, silicates, and oxides in selected carbonatites: Constraints on crystallization temperatures of carbonatite magmas

Oxygen isotope compositions and fractionations between calcite (Cc) and magnetite (Mt), diopside-rich clinopyroxene (Di), monticellite (Mnt), kimzeyite-rich garnet (Gt), and biotite (Bt) were measured for carbonatites from Oka (Canada), Magnet Cove (USA), Jacupiranga (Brazil), and Essonville (Canada), to obtain crystallization temperatures and explore the crystallization history of carbonatites. The highest isotopic temperatures are obtained from Cc-Mt fractionations from Oka (745-770°C) and Cc-Mnt fractionations from Magnet Cove (700 and 760°C). Cc-Mt temperatures for very coarse-grained, euhedral magnetite phenocrysts and calcite from Jacupiranga are 700 °C. In samples that contain diopside and magnetite, the Cc-Mt temperatures are always higher than Cc-Di temperatures. This difference is consistent with crystallization of magnetite before diopside, minor retrograde resetting of magnetite isotopic compositions, and the order of crystallization inferred from inclusions of Mt in Di. Cc-Mt, Cc-Di, and Cc-Mnt fractionations are thus interpreted to represent those established during crystallization at rapid cooling rates (10³-10⁴ °C/my). Diffusion model calculations indicate that at slower post-crystallization cooling rates (10-10² °C/my), magnetite compositions should experience significant isotopic resetting by diffusional exchange with Cc, Bt, and apatite, and yield lower temperatures than Cc-Di. Cc-Bt fractionations correspond to the lowest temperatures (440-560 °C). Although some of these are relatively high isotopic temperatures for biotite, they most likely represent those established during subsolidus retrograde exchange between biotite and calcite during rapid subsolidus cooling.