Contrasting oxygen diffusion in nepheline, diopside and other silicates and their relevance to isotopic systematics in meteorites

Abstract

Oxygen self-diffusion coefficients have been determined in natural nepheline Na 3(Na, K)[Al 4Si 4O 16] and synthetic diopside CaMg(SiO 3) 2, at one atmosphere, by monitoring the rate of 18O exchange between CO 2 gas and minerals of pre-determined grain size (10, 12.5, and 15 μm radius). Oxygen self-diffusion in nepheline between 1000° and 1300°C fits a linear Arrhenius relation of the form: D = 5.9 × 10 −9 exp[(−25.0 ± 2.5) × 10 3 cal RT ] ; and for diopside between 1150° and 1350°C: D = 6.3 exp[(−96.7 ± 5.8) × 10 3 cal RT ] . The activation energy for oxygen diffusion in nepheline is similar to that of melilite (31.9 ± 0.1 kcal/mol), but the oxygen diffusivity is lower by two orders of magnitude. The diffusion of oxygen in diopside is similar to that of nepheline at 1375°C; however, the activation energy is almost four times higher. This contrast suggests a difference in the crystal chemical and structural controls of the minerals. Both nepheline and melilite are characterized by strongly distorted alkali sites within their frameworks, which could account for their relatively low activation energies and high diffusivities. More importantly, nepheline and melilite have sheet-like structures and high anionic porosities which may provide ready migration paths for oxygen. The concentration of open space between the silicate sheets in melilite may explain the greater diffusivity of oxygen in melilite than in nepheline. Diopside, forsterite, anorthite, spinel are characterized by high activation energies (80 to 100 kcal) and are refractory with very stable bonds and low anion porosities. Furthermore, these minerals are much denser and closer packed than nepheline or melilite making it difficult for oxygen to migrate within the crystalline structure and have access to the cation-oxygen bonds. Oxygen diffusion studies constrain interpretations of post-solidus oxygen exchange among minerals and may be used to explain the stable isotope systematics observed in Ca-Al rich inclusions of carbonaceous chondrites. The slow exchange and diffusion of oxygen in diopside suggests that the isotopic composition of clinopyroxenes in Ca-Al rich inclusions of carbonaceous chondrites has not changed appreciably since their incorporation into the meteorite. However, minerals such as nepheline and melilite, which readily exchange oxygen, record the isotopic composition of the last nebular gas to which they were exposed.