Olivine Dissolution into MORB Melt: an Experimental and Theoretical Study

Olivine dissolution in a MORB melt experiments were conducted using a piston-cylinder apparatus at nominally 1250, 1350 and 1450°C temperatures and 0.5 and 1 GPa pressures. The experimental procedures follow those of Zhang et al. (1989). The goals of this work are to quantify diffusive crystal dissolution rates of olivine in basalt and to model convective crystal dissolution rates. From experiments, we (1) extract interface melt composition and examine the time scale for it to reach steady state concentration (Shaw, 2004); and (2) evaluate effective binary diffusion coefficient (EBDC) of the key dissolving component (MgO for olivine dissolution). At 0.5 GPa and 1250°C, interface melt composition changes in the first few minutes, and the e-folding time to reach a "steady state" MgO concentration is 171±71 s (1σ error). Within this time period, dissolution is controlled by both interface reaction and diffusion. Much beyond this time period, the dissolution is controlled by diffusion only. At 0.5 GPa but higher temperatures, the "steady state" MgO concentration is reached rapidly and the time scale (< 1 minute) cannot be resolved. It is hence inferred that the interface reaction rate increases more rapidly with temperature than MgO diffusivity, and olivine dissolution tends to be diffusion- controlled as temperature increases. Based on experimental data at 1250-1450°C and 0.5-1.0 GPa, both MgO diffusivity and the "steady-state" interface MgO concentration increase with increasing temperature and decrease with increasing pressure. Hence olivine dissolution rate in this melt also increases with temperature and decreases with pressure. From the results, we will model diffusive and convective dissolution rates in nature.