Kinetic partitioning of major-minor cations between olivine and Hawaiian tholeiitic basalt under variable undercooling and cooling rate conditions

Abstract

Listen

Translate article

In order to elucidate the kinetic partitioning of cations between olivine and basalt, we performed undercooling ( −ΔT ) and cooling rate ( CR ) experiments at atmospheric pressure and QFM-2 buffer. Starting from the superliquidus temperature of 1250 °C, a Hawaiian tholeiitic basalt was cooled at the rates of 4 ( CR4 ), 20 ( CR20 ), and 60 ( CR60 ) °C/h to the final target temperatures of 1175 °C ( −ΔT = 35 °C; −ΔT 35) and 1125 °C ( −ΔT = 85 °C; −ΔT 85). Results show that polyhedral olivine morphologies are obtained at -ΔT 35, whereas strong disequilibrium skeletal and/or dendritic textures form at -ΔT 85. The amount of forsterite in olivine decreases from to 85% to 78% with increasing both -ΔT and CR . A diffusive boundary layer also develops in the melt next to the olivine surface and its composition becomes progressively enriched in Ca, owing to its incompatible behavior with the lattice site. Residual melts are progressively depleted in silica and enriched in alkali from CR4 to CR60 , but silica-rich melts are observed with increasing -ΔT . In terms of Fe 2+ -Mg exchange, olivines obtained at -ΔT 35 are always in equilibrium with the diffusive boundary layer, comprising both the interface melt next to the olivine surface and the far-field melt where all chemical gradients cease. At -ΔT 85, however, the Fe 2+ -Mg exchange indicates two distinct equilibration stages between olivine core and far-field melt, and between olivine rim and interface melt. Partition coefficients (Kd) of Mg, Fe, Mn, Ca, and Cr calculated at the olivine-melt interface preferentially change as a function of -ΔT rather than CR . From -ΔT35 to -ΔT85 , Kd Mg , Kd Fe , Kd Mn , and Kd Cr remarkably increase, whereas the opposite applies to Kd Ca . Through the application of equilibrium partitioning models, we found that Mg, Fe, Mn, and Ca are incorporated into the olivine lattice site at near-equilibrium proportions. This generally good agreement with modeling data demonstrates that diffusive mass transport of cations in our experiments occurred under the conditions of local equilibrium at the olivine surface. In contrast, marked deviations from the expected equilibrium are found for K Cr in response to the major influence of crystal field stabilization energy on cation incorporation.