Paleomagnetic and rock magnetic study of baked clays by spontaneous combustion

Under certain conditions, buried rocks rich in organic matter can undergo spontaneous combustion if their temperature is raised to a certain ignition threshold and oxygen is supplied. Although spontaneous combustion is not very common, it has been described by several authors (Alastuey et al., 1993; Svensen et al., 2003; Abad et al., 2019). In the geological record, these phenomena can be observed due to the pyrometamorphic processes they generate, characterised by very high temperature and low pressure. When the altered rocks are clays, the result are natural clinkers. In this work we carry out a palaeomagnetic and rock magnetism study of these very rare materials. Specifically, we study stratiform bodies of baked clays by spontaneous combustion from 1) the Lower Cretaceous Escucha Formation near Utrillas (Iberian Ranges) and 2) lake sediments from the upper Miocene Molinicos Basin (Betic Cordillera).

Rock magnetism analysis indicates that natural baked clays are characterised by the presence of a very rare mineral phase of high coercivity, low unlocking temperature and thermal stability so-called HCSLT. This phase has only been described in controlled laboratory conditions (Petersen et al., 1987) and in some well-heated archaeological materials (McIntosh 2007). Recently, it has been shown that HCSLT is carried by ε-Fe2O3, a rare metastable polymorph of Fe2O3 with ferrimagnetic behaviour at room temperature (López-Sánchez et al., 2019; Dejoie et al., 2014). In the samples analysed, this phase coexists with magnetite, hematite or maghemite. The palaeomagnetic study indicates that the NRM is carried by the HCSLT phase with the same palaeomagnetic direction as the other high-temperature magnetic phases. The analysis of the directions of the characteristic component provides information about the timing of the spontaneous combustion that generated the carrier minerals and their magnetization.

References:

Abad, I., Sánchez-Gómez, M., Reolid, M., Sánchez-Vizcaíno, V.L. (2019). Minerals, 2019, 9(12), 748

Alastuey, A., Bastida, J., Fernández-Turiel, J.L., Querol, X., Signes, M. (1993). Cuadernos de Geología Ibérica, 17, 171–184.

Dejoie, C., P. Sciau, W. Li, L. Noé, A. Mehta, K. Chen, H. Luo, M. Kunz, N. Tamura, and Z. Liu (2014).  Sci. Rep., 4, 4941

López-Sánchez, J., McIntosh, G., Osete, M.L., del Campo, A., Villalaín, J.J., Pérez, L., Kovacheva, M., Rodríguez de la Fuente, O. (2017). Geochemistry, Geophysics, Geosystems, 18 (7), pp. 2646-2656.

McIntosh, G., M. Kovacheva, G. Catanzariti, M. L. Osete, and L. Casas (2007). Geophys. Res. Lett., 34, L21302.

Petersen, N., N. Schembera, E. Schmidbauer, and H. Vali (1987). Phys. Chem. Miner., 14, 118–121.

Svensen, H.; Dysthe, D.G.; Bandlien, E.H.; Sacko, S.; Coulibaly, H.; Planke, S. (2003). Geology, 31, 581–584.Díaz García, F. (1993). Revista de la Sociedad Geológica de España 6, 105-114.