Controlled cooling-rate experiments on olivine-hosted melt inclusions: chemical diffusion and quantification of eruptive cooling‐rates on Hawaii and Mars

Abstract

Controlled cooling‐rate experiments were conducted on olivine‐hosted melt inclusions to characterize the development of compositional zoning observed in natural inclusions. All of the experimentally cooled inclusions are zoned due to olivine crystallization on the inclusion wall and diffusive exchange between the boundary layer adjacent to the growing olivine and the inclusion centers. Experimentally cooled inclusions are characterized by lower MgO and FeO and higher SiO₂, Al₂O₃, and Na₂O (and other incompatible oxides) near the inclusion wall relative to the inclusion center. The compositions at the centers of inclusions are susceptible to modification by diffusion, particularly for small inclusions and those subjected to low cooling rates. Uphill diffusion is evident in every oxide and is recognized by local extrema along a diffusion profile. CaO exhibits the most extreme manifestation of uphill diffusion, and a model attributes the diffusion behavior in CaO to solution nonideality in the boundary layer liquid. MgO profiles from experimentally cooled inclusions were fit with a diffusion model by varying the cooling rate. The cooling rates that resulted in the best fit models were always within a factor of 2 and typically within ±10% of the experimental cooling rates, which ranged from 70 to 50,000 °C/hr. The model was applied to MgO profiles across natural glassy olivine‐hosted melt inclusions from Hawaii and the shergottite Yamato 980459. Cooling rates from zoned melt inclusions in Yamato 980459 range from 85 to 1,047 °C/hr (mean = 383±43 °C/hr, 1σ, n=8) and support the hypothesis that the sample erupted at or near the Martian surface.