Studies of lithospheric mantle from (ultra)slow spreading ridges have shown that melt
extraction at mid-ocean ridges may be incomplete, producing metasomatism/refertilization
of the shallow lithospheric mantle. However, it remains unclear whether similar processes
operate off axis and could affect the cooling lithosphere. Here, we report the discovery of a
garnet xenocryst in a petit-spot lava sampled on the top of the downgoing Pacific plate in
front of Japan. The trace-element composition of this garnet xenocryst, in particular the
low chromium, excludes a peridotitic origin, while the flat mid– to heavy rare earth element
pattern does not support direct crystallization from melt percolating through the oceanic
lithosphere. Garnet formation is therefore interpreted as formed by a subsolidus reaction
of a plagioclase-bearing cumulate during the progressive off-axis cooling of the lithosphere.
Combining lithosphere cooling models and the specific physical conditions required for subsolidus
formation of garnet in tholeiitic systems (0.7–1.2 GPa) indicates that melt percolation
to produce plagioclase-bearing cumulate occurs >150 km off axis. These conditions support
that some low-degree melts produced off axis are not collecting to form mid-oceanic ridge
basalt (MORB), but percolate and crystallize during the cooling and thickening of the lithospheric
mantle. The demonstration of mantle refertilization affecting Pacific lithosphere off
axis is critical because such a process could explain the presence of metasomatic domains
with distinct physical and chemical properties in the depleted oceanic lithosphere.