Mg diffusion in anorthite: implications for the formation of early solar system planetesimals

Abstract

We have measured the self diffusion coefficients of Mg, Ca, and Sr in anorthitic plagioclase in order to assess the potential of Mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism. Diffusion couples were constructed from polished single crystals of natural anorthite and synthetic, isotopically enriched anorthite glass. Couples were annealed at atmospheric pressure and 1200–1400°C and isotopic concentration profiles were measured with an ion microprobe. The results show that Mg diffusion in anorthite is surprisingly fast, with D Mg being over 2 orders of magnitude greater than D Sr. This indicates that the diffusion coefficient of Mg in anorthite cannot be approximated with that for Sr. Mg diffusion in the c-direction is also slightly faster than in the b-direction, while Ca and Sr diffusion appear to be isotropic. The results provide important constraints on the thermochronological history of anorthite-bearing mineral assemblages that preserve radiogenic 26Mg excesses. In a planetesimal heated by the decay of 26Al, the temperature at any point depends on the planetesimal size, time of formation, thermal conductivity, and depth within the planetesimal. Given sufficient heating, 26Mg heterogeneities produced by the in-situ decay of 26Al in Ca-, Al-rich inclusions (CAIs) and chondrules will be erased by diffusive equilibration. Using the self diffusion coefficient for Mg in anorthite measured in this study, we show that the common occurrence of 26Mg excesses in these inclusions requires that they must be stored in small (≲15 km) bodies or the outermost rims of larger bodies for the first 1–2 million years of the solar system's history. For early formed bodies larger than 15 km, most of the mass will have been heated sufficiently for any radiogenic 26Mg to have been diffusively homogenized in the Mg-rich planetary environment.