New Paleointensity Results at 30Ma Obtained From Ethiopian Large Igneous Province (LIP) Using Tsunakawa-Shaw Method

We estimated paleointensities from the Ethiopian Large Igneous Province (LIP) by the Tsunakawa-Shaw method [Tsuunakawa and Shaw, 1994; Yamamoto et al., 2003] and the pseudo-Thellier method [e.g. Tauxe et al., 1995] to understand geomagnetic field behavior in about 30 Ma, the Oligocene. The main part of this LIP erupted in a short time span at 30 ± 0.4 Ma according to Hofmann et al. [1997] and Rochette et al. [1998]. A few paleointensities from the Ethiopian LIP were previously reported by Riisager et al. [1999], providing VDMs ranging from 3.0 to 10.5×10²² Am² from only 8 lava flows by the Thellier method with pTRM check, whose average is (4.6 ± 1.9) ×10²² Am². In order to clarify variations in geomagnetic field intensity in more detail, we analyzed samples collected from 93 lava flow units sampled along the Lima Limo section, whose direction was already measured by Ahn et al. [in preparation]. We obtained paleointensities from 29 lava flows by the Tsunakawa-Shaw method. Our VDM data range from 0.3 to 10.2 ×10²² Am² and the mean is (4.2 ± 2.6) ×10²² Am². The mean VDM is little larger than the last 5 Myr ((3.6 ± 2.1) ×10²² Am²) [Yamamoto and Tsunakawa, 2005] but consistent with it by its 1σ standard deviation. The large variations of our VDMs suggest that the geomagnetic field around 30 Ma has variability similar to that of the present field represented by relative paleointensity curves such as the PISO-1500 [Channell et al., 2009]. Paleointensities were also estimated from 48 lava flows by the pseudo-Thellier method. The relative paleointensity variations were same as those obtained by the Tsunakawa-Shaw method, which provides us a prospective that non-heating estimation of paleointensity may be useful and practical.