Paleomagnetic study on a polarity transition and paleosecular variations from Saint Helena, South Atlantic

The first paleomagnetic investigations on Saint Helena Island (Engbers et al., 2020) discovered a profile of six lava flows recording a reversed-to-normal polarity transition with three intermediate directions. In this new study, the profile was resampled, extended by four lava flows and another parallel profile with twelve lavas was added. Mean characteristic remanent magnetization directions have been obtained from four to seven specimens per flow with alternating field or thermal demagnetizations. They are reasonably defined for sixteen of the sites with Fisher precision parameters k above 50 and α₉₅ confidence angles below 12.5° reproducing preliminary paleodirections measured by Engbers et al. (2020). For the majority of the flows, directions in the two parallel profiles were found to be very similar. Each profile yielded two or three intermediate directions with associated virtual geomagnetic pole latitudes between –18° and 29°. Therefore, the transitional nature of the lavas is well supported by two independent sampling campaigns and two parallel profiles. Another profile with 19 flows in the north of the island (Munden Hill) was also sampled but yielded no transitional directions. Hysteresis measurements reveal magnetic carriers of single or pseudo single domain grain size, while thermomagnetic analysis suggests titanomagnetites with both high and low Ti-content. Strong irreversibility of susceptibility with temperature suggests the presence of primary low-temperature oxidation or maghemitization. Paleointensity determination used the MT4 and IZZI protocols of the Thellier method with limited success. In agreement with previous paleointensity studies performed in the South Atlantic region, the reconstructed field was generally weak compared to the present day and the long-term average field. ⁴⁰Ar/³⁹Ar dating was attempted yielding approximate ages between 8 and 9 Ma, but accurate correlation to the geomagnetic polarity timescale is hindered by the imprecision introduced by the altered state of the samples.