Effect of Water in Melt at Elevated Pressures on Melt–Peridotite Reaction: An Experimental Study with Application to Garnet Orthopyroxenite Formation in Orogenic Mantle

Abstract Garnet pyroxenite provides crucial lithological and geochemical evidence for understanding mantle properties and processes. It can be formed via reaction of peridotites with silicate melts at moderate to high pressures. However, the formation mechanisms of garnet pyroxenites in the orogenic mantle and the origins of variations in their mineral compositions are unclear. To better understand the effects of water on the lithology and mineral chemistry of garnet pyroxenites, we carried out experiments of reactions between a hydrous basaltic andesite (4 wt % water) and a spinel lherzolite at 2–3 GPa and 1300–1375 °C using the reaction couple method. Control experiments without water in the melt were also conducted under the same conditions. The hydrous melt–peridotite reaction produced melt-bearing orthopyroxenites. In particular, a melt-bearing garnet orthopyroxenite and garnetite were formed at the melt–rock interface and on the melt side of the reaction couple, respectively, and the lherzolite transformed to garnet lherzolite at 3 GPa. In contrast, the control experiments produced garnet and clinopyroxene layers at 2 GPa and an eclogite at 3 GPa. Presence of water in melt stabilizes garnet relative to clinopyroxene on the liquidus, and intensifies the assimilation of reacting melt by the peridotite partial melt, which are the effects leading to the formation of garnetite and garnet orthopyroxenite, respectively. The thickness of garnet orthopyroxenite in the 3-GPa hydrous reaction experiments increases with temperature (from 1325 to 1375 °C) and run duration; garnet-rich and garnet-poor sections were generated at the higher temperature. Compared with previous hydrous melt–peridotite reaction experiments conducted at relatively high pressures (≥3 GPa) and low temperatures (≤1200 °C), high temperature is needed for the reaction to form and to develop garnet orthopyroxenite. The orthopyroxenite–peridotite assembly produced in our experiments is similar to garnet orthopyroxenite dikes in orogenic peridotites, which highlights the importance of hydrous melt–peridotite reactions in the formation of such dikes. The comparison of mineral chemistry between experimental products and natural samples suggests that subsolidus re-equilibration plays a significant role in causing the variations in mineral composition. Re-equilibration via intergranular diffusion between the constituent minerals can explain the low Mg# [atomic 100 × Mg/(Mg + Fe)] values of garnet and high Mg# values of orthopyroxene, as well as the low temperatures calculated using geothermometers. The processes of melt–peridotite reaction at high temperatures and subsolidus re-equilibration also govern trace element distributions among minerals in garnet orthopyroxenites from the orogenic massifs.