GP008-0005
Evidence from Saturation Analysis and Thermal Fluctuation Tomography for Stress Dominated Magnetic Anisotropy in a Mid-Ocean Ridge Basalt
Tuesday, 15 December 2020
Poster
Fei Han1, Huapei Wang2, Tao Yang3, Yiliang Lv4, Lingxiao Zhao4, Duowen Zhu2, Junxiang Miao1, Shaochen Hu2, Jeffrey S Gee5 and Dennis V Kent6,7, (1)China University of Geosciences (Wuhan), School of Earth Sciences, Wuhan, China, (2)China University of Geosciences (Wuhan), School of Geophysics and Geomatics, Wuhan, China, (3)China University of Geosciences (Beijing), School of Geophysics and Information Technology, Beijing, China, (4)Huazhong University of Science and Technology, Wuhan National High Magnetic Field Center, Wuhan, China, (5)Scripps Institution of Oceanography, Geosciences Research Division, La Jolla, CA, United States, (6)Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, United States, (7)Rutgers University, Department of Earth and Planetary Sciences, Piscataway, NJ, United States
Abstract:
Mid-ocean ridge basalts (MORB) are thought to be ideal materials to record the strength of paleomagnetic field. Gee and Kent (1995) studied a zero-age MORB from Juan de Fuca Ridge. They found that the Mrs/Ms ratio of the chilled margin materials could reach up to 0.68 (saturation field=1 T), which they interpreted as cubic magnetocrystalline anisotropy. However, Fabian (2006) conducted near-saturation hysteresis measurements (7 T) on the same samples and found that the Mrs/Ms ratios were no more than 0.5, which he interpreted as stress (200 Mpa) dominated anisotropy. Here, we present our results from saturation and low-temperature rock magnetic measurements on the same MORB samples.
Our non-saturated hysteresis loops (up to 1.2 T or 2 T) show that the Mrs/Ms ratios (linear 70% slope paramagnetic correction) are 0.61, 0.54 and 0.37 for NF7a2a, NF7b2a and NF7g1b, respectively (Fig.1 b,e,c). However, according to “approach to saturation” analysis (Fabian, 2006), they are only ~90% saturated. Then, we conducted saturation hysteresis measurements (up to 15 T) on specimen NF7g1b and found the Mrs/Ms ratio dropped to 0.23 (~99% saturated). FORC diagrams show clear single domain behavior with micro-coercivity peaks from ~100 mT to ~15 mT (Fig.1 h,i,j). We also measured hysteresis loops and direct-current demagnetization (DCD) curves at lowered temperatures down to 10 K (Fig.1 d,g). Thermal fluctuation tomography (TFT) modeling indicates that our low-temperature DCD curves follow the predictions from samples dominated by stress anisotropy.
Our results strongly support that the internal stress dominates the magnetic anisotropy in these MORB samples, which may help us better understand the paleointensity reliability.
