Abstract:
The mechanisms and kinetics of harzburgite reactive dissolution in basaltic liquids were examined using a combined experimental and numerical approach. Dissolution experiments were conducted at 1250–1290°C and 0.6–0.75 GPa using dissolution couples consisting of pre-synthesized rods of alkali basalt and harzburgite in graphite and platinum-lined molybdenum capsules. Reactive dissolution of harzburgite produces a melt-bearing, orthopyroxene-free dunite with a sharp mineralogical front at the dunite–harzburgite interface. The thickness of the dunite layer is proportional to the square root of experimental run time, and its growth rate is limited by the rates of diffusion of major components in the melt. Around the sharp mineralogical front there exists a broad composition boundary layer where major and trace element abundances in the interstitial melt and olivine vary systematically as a function of distance and time. The sharp mineralogical front and the broad composition boundary layer result from a combined effect of orthopyroxene and olivine dissolution at the dunite–harzburgite interface, olivine re-precipitation within the dunite, as well as diffusive exchange between the crystals and the melts. Based on experimental observations a simple model for harzburgite reactive dissolution was developed and used to extrapolate the experimentally measured dissolution rates and concentration profiles to conditions relevant to melt transport under the mid-ocean ridge. Model calculations demonstrate that diffusive dissolution alone is incapable of producing dunite dikes wider than a few meters within the time scale of mantle upwelling under the mid-ocean ridge. Prevalent melt percolation, hence large melt–rock ratios, is required in the formation of large dunite channels in the mantle. The composition of the reacting melt is of particular importance in determining the composition gradients in olivine across the dunite–harzburgite interface. By adjusting the reacting melt compositions we were able to produce concentration profiles broadly similar to those observed in our experiments as well as those reported across the dunite–harzburgite sharp contacts in the mantle sections of ophiolites and peridotite massifs around the world.