Chronology of serpentinization: Paleomagnetic evidence for 20 Ma serpentinization of the Oeyama ophiolite, Wakasa, Southwest Japan

Abstract

Serpentinization of mantle peridotite generates magnetite that is able to acquire a remanent magnetization parallel to the ambient geomagnetic field. The paleomagnetic direction of serpentine provides insights as to the timing of serpentinization. This study examines paleomagnetic properties of serpentine in the Paleozoic Oeyama ophiolite in Wakasa (35.39°N, 134.39°E), Southwest Japan. Reversed polarity magnetizations with clockwise deflections from the south were observed in the serpentine in the high temperature component (HTC) magnetization with an unblocking temperature (Tub) of ∼580°C. The mean reversed polarity direction was D = 226.0°, I=−37.8°, α95=5.9°, and n=30. The remaining samples with an HTC of Tub between 400-450°C show the present-day geomagnetic field direction. Rock magnetism reveals that the reversed direction is carried by single-domain (SD) grains of magnetite, whereas multi-domain (MD) magnetite grains carry the present-day geomagnetic field direction. Magnetite grains originated from olivine during serpentinization grew to SD size and acquired the chemical remanent magnetizations during a reversed polarity interval. As serpentinization progresses, SD magnetite grains increased to MD magnetite grains and/or were subjected to maghemitization. The mean reversed direction yielded a paleomagnetic pole at 47.4°N, 230.3°E (A95 = 5.7°), which overlaps with the 20 Ma segment of the apparent polar wander path for SW Japan. We conclude that serpentinization occurred in the Oeyama ophiolite at approximately 20 Ma during the clockwise rotation of SW Japan.