An experimental study of the kinetics of lherzolite reactive dissolution with applications to melt channel formation

The kinetics of lherzolite dissolution in an alkali basalt and a basaltic andesite was examined experimentally at 1,300 degrees C and 1 GPa using the dissolution couple method. Dissolution of lherzolite in basaltic liquids produces either the melt-bearing dunite-harzburgite-lherzolite (DHL) sequence or the melt-bearing harzburgite-lherzolite sequence depending on whether the reacting melt is or close to olivine saturation (alkali basalt) or olivine + orthopyroxene saturation (basaltic andesite). The dunite in the DHL sequence is pyroxene-free and the harzburgites in both sequences are clinopyroxene-free. The melt fraction and olivine grain size in the dunite are larger than those in the harzburgite. The olivine grain size in the dunite and harzburgite in the DHL sequence also increases as a function experimental run time. Across the sharp dunite-harzburgite and harzburgite-lherzolite interfaces, systematic compositional variations are observed in the reacting melt, interstitial melt, olivine, and to a lesser extent, pyroxenes as functions of distance and time. The systematic variations in lithology, grain size, mineral chemistry, and melt compositions are broadly similar to those observed in the mantle sections of ophiolites. The processes of lherzolite dissolution in basaltic liquids involve dissolution, precipitation, reprecipitation, and diffusive transport in the interstitial melts and surrounding minerals. Preferential dissolution of olivine and clinopyroxene and precipitation of orthopyroxene in the basaltic andesite produces the melt-bearing harzburgite-lherzolite sequence. Preferential dissolution of clinopyroxene and orthopyroxene and precipitation of olivine results in the melt-bearing DHL sequence. Preferential mineral dissolution can also affect the composition of the through-going melt in a dunite channel or harzburgite matrix. Systematic variations in melt fraction and mineral grain size in the peridotite sequences are likely to play an important role in the development of channelized or diffuse porous melt flow in the mantle.