Cretaceous geomagnetic paleointensities: Thellier experiments on Pillow lavas and Submarine basaltic glass from the Ontong Java Plateau

Abstract

We present new Thellier paleointensity results on pillow lavas and pillow‐rim submarine basaltic glass (SBG) from 120 Ma basalts erupted on the Ontong Java Plateau. All pillow lavas have suffered low‐temperature alteration and are unsuitable for Thellier paleointensity experiments. In contrast, pillow‐rim SBG is virtually impermeable to seawater and therefore has suffered little or no low‐temperature alteration. Out of 21 selected SBG samples with magnetic moment larger than 10−10 Am2, 18 samples (from 12 chilled SBG margins) yielded paleointensity estimates of good technical quality. Rock magnetic measurements show the primary carrier of remanence is low‐Ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite. As suggested by a recent study, magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400–440°C, which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature. We suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength. For several pillow margins two to three samples were measured, and in such cases the data agree well. Altogether we obtain reliable estimates from 12 SBG pillow‐rims, yielding a mean paleointensity of 27.1 ± 13.4 μT and a corresponding virtual axial dipole moment of 5.8 ± 2.8 · 1022 Am2. This value is slightly higher than whole rock data from similar aged subaerial lava flows. The paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the Cretaceous Normal Superchron.