DISSOLUTION RATES OF UPPER MANTLE MINERALS IN AN ALKALI BASALT MELT AT HIGH PRESSURE: AN EXPERIMENTAL STUDY AND IMPLICATIONS FOR ULTRAMAFIC XENOLITH SURVIVAL

Abstract

The dissolution rates of the major upper mantle minerals olivine, orthopyroxene, clinopyroxene, spinel, and garnet have been determined in an alkali basalt melt at superliquidus temperatures and 5, 12, and 30 kb. At low pressure where olivine is the liquidus phase of the basalt, olivine has a slower dissolution rate than clinopyroxene; however, at higher pressure where clinopyroxene is the liquidus phase, clinopyroxene has a slower dissolution rate than olivine. The relative rates of dissolution of olivine and clinopyroxene at each pressure are, therefore, governed by their relative stabilities in the melt and hence by the structure of the melt. As the degree of superheating above the liquidus increases at each pressure, the dissolution rates of olivine and clinopyroxene converge, suggesting that the melt undergoes temperature-induced structural changes.Orthopyroxene has a dissolution rate similar to olivine at high pressure and similar to clinopyroxene at low pressure. Spinel has the slowest dissolution rate at each pressure. Garnet dissolves very rapidly at 12 kb and at a comparable rate of olivine at 30 kb. The dissolution rates determined in the experiments vary from 9.21 × 10 -9cm s -1 for spinel at 5 kbar and 1250°C to 3.83 × 10 -5cm s -1 for garnet at 30 kb and 1500°C.Textures produced during the dissolution experiments are related to mineral stability in the melt at each pressure and are independent of the degree of superheating. The mineral phases that are stable on or near the liquidus exhibit no reaction; whereas complex reaction textures and crystallization characterize dissolution of minerals that are relatively unstable in the melt.Concentration profiles in the melt adjacent to the same crystal for different experimental durations are identical, indicating that dissolution is time-independent and a steady-state process. However, cation diffusion coefficients calculated for single-component oxides in the melt reveal that dissolution may not be completely controlled by diffusion of cations away from the crystal/melt interface. The apparent diffusivities positively correlate with the dissolution rate, which suggests that the stability of the mineral is an important factor to consider when deriving diffusion coefficients from these experiments. Other factors that may be involved are multi-component effects and the nature of the diffusing species in the melt.A simple model has been constructed that predicts the survival of ultramafic xenoliths in alkali basalt magmas as a function of xenolith radius, magma ascent time and superheating. The results of the model suggest that the relative proportions of peridotite and pyroxenite xenoliths brought to the surface in alkali basalts are generally representative of their proportions as constituents of the upper mantle. Further experiments using different melt compositions are required to extend the model.