Deformation beneath Gakkel Ridge, Arctic Ocean: From mantle flow to mantle shear in a sparsely magmatic spreading zone

Abstract

Mantle deformation processes leading to seafloor spreading are often difficult to infer due to the highly serpentinized and weathered state of most abyssal peridotites. We investigated the development of high-temperature crystal-plastic deformation and lower temperature mylonitization processes in relatively fresh (<50% modal serpentine) and ultra-fresh (<1% serpentine) mantle peridotites derived from the heterogeneous mantle in the sparsely magmatic zone of ultraslow-spreading Gakkel Ridge system by analyzing 12 peridotites from two dredge sites (<1 km apart). Microstructurally, these 12 peridotites consist of seven high-T deformed samples and five mylonites. Modally, the 12 samples include harzburgites, lherzolites, an olivine websterite, and a plagioclase-bearing lherzolite. Based on their mineral major and trace element compositions, the lherzolites, harzburgites, and olivine websterite are residual peridotites. The lherzolites containing clinopyroxenes with flat REE patterns likely underwent refertilization with a high influx of melt. The plagioclase-bearing lherzolites probably formed by subsolidus reaction after the partial melting process. Microstructural observations support that high-T crystal-plastic deformation (most likely at temperatures exceeding 1000 °C) was active in the peridotites of the high-T deformation group, accommodating mantle flow beneath the Gakkel Ridge. The identified melt refertilization process may have contributed to the formation of [010]-fiber olivine fabrics in these peridotites. Mylonitic microstructures, decreasing fabric strength and grain-size reduction of olivine suggest that mylonitization occurred under relatively low-temperature mantle conditions (~800 °C) and probably accommodated strain localization. Water did not greatly affect the peridotites during the development of the shear zones, although amphibole with “dusty” zones developed in one mylonitic peridotite after mylonitization, indicating that late-stage metasomatism occurred locally within the shear zone. This low-T mylonitization is likely to have affected mantle peridotites of this region independently of petrogenetic processes. The development of these deformation processes in Gakkel Ridge suggests a shift from flow in the uppermost mantle to shear zone formation in the rift valley walls.