Domain structure of volcanics’ ferromagnetic grains: A case study of the Saatly ultradeep well, Azerbaijan

Abstract

Various magnetic properties of magnetic minerals show strong grain-size depen-dence. This dependency occurs because these parameters are influenced by the magnetic grain-size (domain state) of the samples. Natural rocks often contain a mixture of different magnetic minerals with different grain-size (domain states). Thus determination of the domain state is important. The Saatly ultradeep well’s volcanics were used as a research material. In the depth intervals of 3540—8126 m the volcanics had been revealed by the borehole. According to data of the petrographic and microprobe analysis the volcanogenics of the ultradeep borehole is conditionally divided into a two parts: 1) the upper part (an interval of depths of 3540—5500 m); 2) the bottom part (an interval of depths of 5500—8126 m). In the upper part of section according to microprobe analysis of volcanogenic rocks ore grains are presented by two generations: 1) large grain of titanomagnetites (Ti-poor), homogeneous or with a decay structures; 2) small-sized usually well faced grain of practically stoichiometric magnetite. So, homogeneous small-sized grains or fragments of decay large grains (which are strongly changed chemically) can have properties of single-domain particles. Here the ferromagnetic grains (especially in the depth interval 3540—5000 m) are often noted as a pseudo-single domain. In the bottom part of the section the presence of multidomain grains of the Ti-poor titano-magnetite is noted. And since depths of 7000 m the oxidized magnetite is also observed. The pseudo-single-domain grains are fixed much less than in the upper section. And, since a depth mark of 7300 m and deeper, they are absent wholly. In the result of rese-arch it turned out that the ferromagnetic is presented by basically multidomain grains. It should be noted that Lauri—Fuller’s Criterion is not applicable at identification of a domain structure of the complex ensemble of the ferromagnetic grains.