Magnetisk susceptibilitet och magnetisk mineralogiav alkaliska bergarter från Lake Natron-EngarukaMonogenetic Volcanic Field (Tanzania)

The Lake Natron-Engaruka Monogenetic Volcanic Field (LNE-MVF) in northern Tanzania is a region of great scientific value due to its unique geology. This area is located adjacent to Oldoinyo Lengai, the world’s only active (natro-)carbonatite volcano. Volcanic landforms associated with the LNE-MVF are scattered across the rift and comprise more than 150 craters dating back to the Pleistocene, with magmas primarily consisting of melilitites and nephelinites. Despite extensive studies, the volcanic subsurface systems and magma source regions remain not fully understood, leading to ongoing research into the system’s mineralogical and geophysical aspects. This work builds on previous studies and aims to analyze the magnetic susceptibility and mineralogy of melilitites and nephelinites from the LNE-MVF. The thesis also focuses on investigating whether bulk susceptibility could potentially serve as a rapid indi-cator to determine the composition of local rocks in the volcanic field. In this study, magnetic susceptibility measurements, identification of ferromagnetic minerals through Curie-temperature analysis, and optical microscopy were used to analyse 49 samples from the LNE-MVF area. Regarding the chemical composition, the dominant elements did not show a clear lin-ear relationship with susceptibility. For the melilitites, susceptibility appeared to be more associated with the solid solution phases of present ferromagnetic minerals, such as magnetite. In samples with higher magnetic susceptibility, it was challenging to discern the mineral composition using microscopy, as they consisted of opaque lapilli in a calcite matrix. Nephelinites displayed a small to moderate cor-relation between magnetic susceptibility and chemical composition. In the analysed samples, higher susceptibility coincided with more mafic minerals such as clinopyroxene and, particularly, olivine. Some of the phenocryst phases in the nephelinite samples showed signs of discoloration, likely due to fluid alteration. Based on the relationship between susceptibility and chemical content, both the nephelinites and melilitites demonstrated a positive correlation between magnetic susceptibility and chemical prop-erties such as the content of iron oxides, actinides, and other elements like phosphorus and manganese. Additionally, mineralogical properties, such as the degree of fractional crystallization and the presence of mafic minerals, showed a potential relationship with magnetic susceptibility. In summary, these char-acteristics are considered to indicate an observable correlation between chemical and mineralogical composition and magnetic susceptibility in the LNE-MVF samples. However, these observed relation-ships did not follow simple or linear patterns that could be fully explained by magnetic susceptibility.