EXPERIMENTAL DETERMINATION OF ZN, K AND RB DIFFUSION COEFFICIENTS IN OLIVINE AND THE ROLE OF DIFFUSION IN MODERATELY VOLATILE ELEMENT FRACTIONATION IN THE EARLY SOLAR SYSTEM

One of NASA's key strategic missions is the early solar system and its formation. At the beginning, a massive heating event occurred creating the material in which when cooled accreted to form chondrites, asteroids, and planetary bodies. Over the years, samples have been taken from these items and it has been found that when compared to the standards of the CI chondrite and Sun, other chondrites including enstatite, stony, and ordinary, differ up to one order of magnitude and planetary bodies such as, the Moon and the asteroid Vesta, up to 2 orders of magnitude when it pertains to moderately volatile elements (MVE). This study seeks to address this depletion of MVE in these different bodies in the solar system. We ask the question does diffusion play a role in the depletion in MVE once a solid phase is present. Here, we present the results of high-temperature diffusion experiments carried out with San Carlos olivine (Fo₉₀) and basalt melt (USGS BHVO-2 reference material) to determine the dependence of temperature, composition, and crystallographic orientation of Zn, K and Rb diffusion coefficient (D_zn, D_K, D_Rb) in olivine. We chose this composition because it is relevant to evaporation from small differentiated planetesimals, which may occur early in the solar system formation. Experiments were conducted in a Knudsen effusion cell chamber at vacuum covering a range in temperature from 900-1800°C with run duration from 12-24 hours. These temperatures will cover the basis of partial melted to fully melted basalt around the crystal which should allow for diffusion of certain elements into the olivine crystal. Additional experiments were carried out to quantitatively understand why Zn, Rb and K are depleted in planetary bodies. Using the same composition basalt, vacuum evaporation experiments from basalt were carried out at 1400°C and 1600°C to study their evaporation kinetics and isotopic fractionations. Preliminary conclusions suggest that Zn, Rb and K decrease from core to rim of the olivine with similar D_zn values of previous studies. These data suggest that when a solid phase is present during early planetary formation diffusion will likely inhibit MVE isotopic fractionation between the melt and the vapor.