Formation of the P-Fe-Ti-REE Kodal deposit, Norway: the melt inclusion record

Abstract

In the southern part of the Permian Oslo Rift (South Norway), the Kodal deposit is an exceptional Fe-Ti oxide and apatite mineralization, consisting of a succession of lenses over 2 km long and 20 m wide. Discovered in the early 18th century, this Fe-Ti-P and REE (Rare Earth Elements) ore is the largest occurrence hosted within alkaline monzonitic rocks. Several studies have been undertaken on the Kodal deposit to determine its origin, resulting in multiple petrogenetic models, but no consensus has been reached yet. The aim of this study is to investigate whether silicate liquid immiscibility played a role in the formation of the remarkable Kodal deposit. To do this, we examined the polycrystalline inclusions hosted within the apatite crystals. The inclusions were re-homogenized at high temperatures, reaching 1100°C, using a piston cylinder apparatus. It appeared that the majority of the inclusions retained residual phases interpreted as solid inclusions trapped along with the liquid. The melt compositions display a narrow compositional range; 29–45.1 wt.% SiO_2 and 6.23–2.43 wt.% FeO_tot. This interval corresponds to Fe-rich melts when compared to immiscible pairs highlighted in natural rocks and experimental studies. Despite the disparity in the colors of the polycrystalline inclusions, which range from light to dark tints, the complete absence of Si-rich melts diminishes the likelihood of emulsification of two liquids during the formation of the Kodal deposit. Furthermore, Kodal melt inclusions exhibit a concordant evolution of the major elements with the first basalt sequences extruded in the Oslo Rift.