INSIGHTS INTO THE MAGMATIC ASCENT RATES OF KIMBERLITES FROM FE-MG INTERDIFFUSION RATES IN ILMENITE

The diffusion rates of Fe, Mg, and Mn in ilmenite at high-temperature conditions are key to understanding the ilmenite disequilibria preserved in volcanic rocks. Ilmenite and coexisting oxides are often used to estimate pre-eruptive temperatures and oxygen fugacities. However, these methods are only reliable when coexisting phases are in equilibrium. Evidence for ilmenite disequilibria exists in rapidly cooled volcanic rocks as disequilibrium pairs of ilmenite and titanomagnetite, and also as compositional zoning in ilmenite grains. We conducted diffusion-annealing experiments in a 0.5” piston cylinder apparatus to investigate the diffusivity of Fe²⁺, Mg²⁺, and Mn²⁺ in ilmenite solid solutions between 800°C and 1200°C. Synthetic polycrystalline ilmenite was juxtaposed against either an oriented, synthetic geikielite (MgTiO₃) crystal, or a synthetic polycrystalline Mn-bearing ilmenite, in a “diffusion-couple” geometry. Geikielite single crystals were oriented to control the diffusion direction (either perpendicular or parallel to the crystallographic c-axis). Diffusion profiles were obtained by electron microprobe analysis.To demonstrate the utility of cation diffusion rates in ilmenite solid solutions, we have applied our cation interdiffusion coefficients to disequilbrium textures in kimberlitic ilmenite. Kimberlitic ilmenite megacrysts often exhibit 100 to 500 µm thick rims of Mg enrichment that form during high-temperature exchange between the megacryst suite and the kimberlite magma. Using our experimentally determined Fe-Mg interdiffusion rates, we have calculated the time for Mg to diffuse 100-500 µm into the ilmenite rims as a function of temperature. Our results suggest the kimberlitic melt and ilmenite megacrysts were in contact for 5-30 days prior to eruption.