[S-CG52] Ocean Floor Geoscience

[SCG52-P14] Absolute paleointensity estimated from Central Indian Ridge basalts and its implication for dating

*Yutaka Yoshimura¹, Masakazu Fujii^2,3 (1.Faculty of Social and Cultural Studies, Kyushu University, 2.National Institute of Polar Research, 3.The Graduate University for Advanced Studies)

Keywords:Central Indian Ridge basalts, Absolute paleomagnetic field intensity (paleointensity), Paleointensity dating

Volcanism in mid-ocean ridges (MORs) plays a fundamental role in the Earth system. Because eruption intervals in MORs could be very short and hydrothermal activities are complicated, high-resolution dating of the seafloor is essential to estimate seafloor spreading rate and evaluate hydrothermal activities. Previous studies have been conducted to distinguish new and old lava flows using structural observations and chemical composition analysis, but their information provides just relative ages, not absolute ages. In addition, it is difficult obtain certain eruption age if rock samples have a large heterogeneity or homogeneity of chemical composition. In order to deal with these problems, a paleomagnetic method has been proposed to measure the absolute paleomagnetic intensity (paleointensity) of submarine lava to detect the unique pattern of each age as an age indicator. However, previously reported absolute paleointensities derived from submarine basaltic glasses collected in the East Pacific Rise are highly variable, which potentially indicate a problem with the sample properties and/or the paleointensity estimation method. Here, we conducted absolute paleointensity measurement experiments for submarine basalts using the Tsunakawa-Shaw method, a technique that is expected to provide more accurate absolute paleointensities than conventional methods. Studied basaltic rock samples were collected from the Central Indian Ridge by using the submersible SHINKAI 6500 dive 6K#926 operated during the R/V Yokosuka YK05-16 cruise. After conducting the experiments on 18 specimens of the eight lava sites, six specimens from two lava sites (R07 and R08) passed the statistical acceptance criteria of the Tsunakawa-Shaw method, showing precise absolute paleointensities of 33.0 ± 2.0 μT and 35.8 ± 3.4 μT (2σ standard deviations). Absolute paleointensities estimated from the two lavas are consistent within 2σ standard deviation although they have different morphologies, pillow lava and sheet lava, respectively. Therefore, we concluded that reliable absolute paleointensities can be estimated from the MOR basalts. Our results also indicate that the two lavas erupted at the same time. Our results are ~0.7 times weaker than the geomagnetic intensity at the diving site (45.8 μT, IGRF-13). Based on the geomagnetic model “CALS3K.4”, the geomagnetic field strength at the diving site was below 35 µT at times between 800 and 1600 CE, so data in this study infer that the lava erupted 1600 CE or before.