Transformation of Refractory Oceanic Lithospheric Mantle by Reactive Melt Infiltration: An Experimental Study on the Roles of Temperature, Melt Volume and Ascent Velocity

Reactive melt infiltration critically modifies the physical and chemical properties of the oceanic lithospheric mantle (OLM). This process, involving melt‐rock reactions and in situ crystallization, exhibits substantial spatial and temporal variability driven by melt volume and ascent velocity. To explore how eclogite/pyroxenite‐derived plume melts transform refractory OLM, we conducted coupled reaction and in situ crystallization experiments at 2 GPa, systematically varying temperature, melt/rock ratio, and duration. We demonstrate that higher temperatures, lower melt/rock ratios, and shorter durations promote only orthopyroxene precipitation coupled with olivine dissolution, thereby preserving the starting harzburgitic lithology. In contrast, lower temperatures, higher melt/rock ratios, and longer durations facilitate clinopyroxene and orthopyroxene crystallization, converting refractory OLM harzburgite into fertile lherzolite or localized websterite. Beyond the well‐established influences of pressure and temperature, our study underscores the pivotal roles of melt volume and ascent velocity in governing metasomatism. This study reveals that clinopyroxene formed by high‐pressure (2 GPa) reactive melt infiltration explains clinopyroxene compositions in ocean island basalt–hosted mantle xenoliths, including high Na 2 O and TiO 2 and low Mg# and Cr#. Furthermore, these mineralogical transformations significantly alter OLM geophysical properties, reducing shear‐wave velocity by ∼2.2%–5.4% and increasing density by ∼1.4%–2.2%. We therefore propose that reactive melt infiltration explains shallow (50–60 km depth) low‐velocity anomalies observed in mature oceanic lithosphere (90–110 km thick) above a mantle plume. Concurrently, the associated density increase may induce delamination of metasomatized OLM, accompanied by buoyant ascent of underlying high‐temperature melting residues, ultimately driving vertical mantle convection within the plume.