Authigenic biotite from hydrothermally altered terrigenous sediments of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean, hole ODP 1038B)

Abstract

For the first time, authigenic dispersed biotite was discovered in Pleistocene terrigenous sediments of the Central Hill, located in the Escanaba Trough in the southern part of the Gorda Ridge (northeastern sector of the Pacific Ocean), which accounts for almost the entire content of fine fractions 1 μm of some samples from ODP 1038B hole. The authigenic nature of biotite is associated with the metasomatic effect of hydrothermal solution on terrigenous clay minerals after intrusion of basaltic magma in the form of laccolith with a temperature of ~1200°C. The mineral composition of fine fractions of sediments was studied using complex analytical methods, including modeling of their diffraction patterns. It has been established that the dispersed micaceous mineral (biotite) is trioctahedral, high-iron, does not swell when saturated with glycol, but contracts after heating at 550°C. It is shown that in its structure there are no foreign layers, the height and composition of which differ from the micaceous layers. The decrease in the height of micaceous layers upon heating biotite to 550°C is mainly due to a decrease in the height of 2:1 octahedra due to the difference in the Fe2+–O and Fe3+–O bond lengths as a result of the oxidation of Fe2+ cations It has been established a limit value for the coefficient of variation CV, characterizing the absence of mixed-layering in a regular structure, which should not exceed a value of 0.10.