An experimental investigation of the diffusive infiltration of alkalis into partially molten peridotite: Implications for mantle melting processes

Abstract

When a silica‐undersaturated melt is juxtaposed with partially molten peridotite, alkali elements rapidly diffuse into the peridotite in a process referred to as the diffusive infiltration of alkalis (DIA). Four types of piston cylinder experiments were performed providing constraints on the DIA process: (1) simple phase relation experiments, (2) melt‐melt diffusion couples, (3) experiments examining the reaction between powders of basanite and spinel lherzolite (direct mixture experiments), and (4) diffusive infiltration‐reaction couples between basanite and partially molten peridotite. Melt‐melt diffusion couples constrain effective binary diffusion coefficients (EBDC) for all major elements except Na to be in the 10−6 to 10−7 cm2/s range at 1450°C and 0.9 GPa. The Na concentration profile is not binary being coupled to gradients in SiO2. EBDCs for Cl, Li, Rb, Sr, Ba, and La are in the 10−6 cm2/s range while Nb and Zr are in the 10−7 cm2/s range. A time series of direct mixture experiments shows that basanite reacts with lherzolite to reduce the orthopyroxene mode to a constant value within 15 min at 1300°C and 0.9 GPa. Quench modified melts in the direct mixture experiments have 50–51 wt.%. SiO2, independent of the basanite/lherzolite ratio. In contrast, infiltration‐reaction experiments show that sodium diffuses from basanite into partially molten peridotite in 10–30 min, resulting in quench modified melt pools with up to 64 wt.% SiO2 within the peridotite. Modal analysis shows that addition of alkalis causes orthopyroxene to incongruently break down to olivine plus silica rich melt. In experiments using a basanite with enriched Cl concentrations, a distinct boundary zone appears between the basalt‐peridotite interface and the peak in SiO2 concentration. Melts within this boundary zone have elevated CaO and Cl concentrations relative to both the basanite and melts within the peridotite beyond the boundary zone. The repeated formation of this boundary zone may indicate an important aspect of the DIA process, possibly responsible for the formation of anorthitic plagioclase and CaO rich melts.