University of Oregon, United States

A rock magnetic study has been performed on rock samples recovered at Ocean Drilling Program Leg 192 sites on the Ontong Java Plateau in the western Pacific. Igneous rocks from the five Leg 192 sites displayed variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Titanomagnetite and titanomaghemite are present in the Ontong Java rocks. Samples with titanomagnetite exhibit Verwey transitions in the vicinity of 120K. Low-temperature curves for samples with multiple magnetic phases do not clearly show the Verwey transition. The inversion of titanomaghemite to a strongly magnetized magnetite is shown by the irreversible thermomagnetic-cooling curve. Despite the geographically widespread locations of the drill sites, variations in rock magnetic properties closely resemble each other, consistent with the fundamental results of the leg that the basement rocks were derived from homogeneous Kwaimbaita-type magma with a single age of approximately 120 Ma. The rock magnetic investigation provides constraints to evaluate the fidelity of the natural magnetic memory in the basalt rocks and corroborates the palaeomagnetic palaeolatitudes determinations for the Ontong Java Plateau. The generally good quality of rock magnetic data exhibited by Leg 192 rocks supports the inference that the characteristic directions of magnetization isolated from the Cretaceous Ontong Java Plateau sites were acquired near the onset of the Cretaceous Long Normal Superchron about 120 Ma. The portion of the Pacific plate containing the Leg 192 sites was in the southern hemisphere during the mid-Cretaceous volcanism.