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