Reaction Processes between Tholeiitic Melt and Residual Peridotite in the Uppermost Mantle: an Experimental Study at 0{middle dot}8 GPa

Abstract

We present the results of a series of anhydrous piston cylinder experiments that illustrate the mechanisms and implications of reaction between tholeiitic melt and depleted peridotite in the uppermost mantle. To simulate infiltration^reaction processes we have applied a three-layer setup in which a layer of primitive basaltic powder (‘melt layer’) is consecutively overlain by a ‘peridotite layer’ and a layer of vitreous carbon spheres (‘melt trap’). The peridotite layer is mixed from pure separates of orthopyroxene, clinopyroxene and spinel (Balmuccia peridotite), and San Carlos olivine.Two tholeiitic melt compositions, respectively with compositions in equilibrium with lherzolitic (ol, opx, cpx) and harzburgitic (ol, opx) residues after partial melting at 1·5 GPa, were employed. Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0·8 GPa with peridotite of variable grain size, in the temperature range 1200^13208C and for run durations of 10 min to 92 h. In this P^T range, representing conditions encountered in the transition zone between the thermal boundary layer and the top of the asthenosphere below oceanic spreading centers, the melt is subjected to fractionation and the peridotite is partially melting (Ts � 12608C). Modal observations indicate a strong dependence between phase relations in the melt layer and changes in the modal abundances of the peridotite layer, as a function of both temperature and melt composition. Textural and compositional evidence, as well as modeling of Fe^Mg profiles in olivine, demonstrates that reaction between percolating melt and peridotite occurs by a combination of dissolution^reprecipitation and solid-state diffusion. Dissolution^reprecipitation leads to well-equilibrated phases whereas diffusional equilibration introduces zoning at experimental timescales.We discuss the observed reaction mechanisms and the consequent compositional changes in the light of local chemical equilibria and reaction kinetics. The results have direct implications for melt migration in upper-mantle thermal boundary layers.