Paleointensity determinations on late Cretaceous – early Paleocene basalts from Walvis Ridge (IODP Exp. 391 and 397T)

A good knowledge of the long-term variations of geomagnetic paleofield intensity is essential for a complete description of the field history. However, we lack a complete description of the geomagnetic field over many time scales, especially beyond the last million years, making older measurements of great value. IODP Expeditions 391 and 397T cored igneous rocks from the Tristan-Gough-Walvis submarine seamount chain. Cores from Sites U1575, U1576 and U1577, drilled on Valdivia Bank, recovered late-Cretaceous basalts and related rocks while cores from Sites U1578 and U1585, drilled on Tristan and Center seamount chains,  recovered more recent Paleocene and related rocks. We selected 95 samples from these sites, mostly from massive lava flows and from some pillow lava flows, for paleointensity measurements. Hysteresis and FORC diagram measurements indicate that selected samples are single-domain, with the exception of those from Hole U1585A, which are pseudo-single-domain. Susceptibility vs. temperature curves indicate a wide variety of magnetic mineralogy in these rocks, ranging from reversible titanomagnetite with Curie temperatures varying between 150 and 550°C, to strongly irreversible assemblages of magnetic minerals.  Thellier-Thellier paleointensity experiments gave a success rate of 20/95. The most reliable results come from the high-Ti titanomagnetite samples. Corresponding VDM values averaged by cooling unit are generally low, ranging from 3.1 to 4.3×10²² Am², except for Hole U1577A, where the VDM is close to 7×10²² Am². These values are consistent with the few, low VDMs measured on whole rocks in the 62-85 Ma time interval, a period of low reversal frequency–less than 2/Myr. Our results do not seem to support an inverse relationship between field strength and reversal frequency for this period.