OLIVINE GEOCHEMISTRY AND SERPENTINIZATION FROM FE-TI OXIDE-BEARING ULTRAMAFIC INTRUSIONS IN THE DULUTH COMPLEX, MN

The Duluth Complex consists of a series of mafic intrusions that formed 1.1 billion years ago as part of the Midcontinent Rift System. The western margin of the complex hosts 12 different Fe-Ti oxide-bearing ultramafic intrusions (OUIs). Little is known about the genesis of these OUIs, which contain significant domestic Ti resources. However, there are minerals present that can act as proxies to discover their genesis, such as olivine. In this study, we use olivine geochemistry and mineralogy of serpentinized olivine to unravel characteristics of the parent melt and to understand lower temperature alteration that impacted the system. Two OUIs, Titac and Longnose, were studied using a variety of tools to understand olivine textures and geochemistry, including petrographic microscopy, scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy. Olivine is heavily serpentinized in both OUIs, with Titac olivine showing tubular, fractured textures and Longnose showing striated olivine, with bulges indicating deformation. Compositionally, the olivine forsterite content of both Titac and Longnose (~Fo₆₀ to ~Fo₇₀) is greater than for olivine from the host troctolite of both intrusions (~Fo₃₆ to ~Fo₆₂), with Longnose olivine having a higher Fo content than Titac. The higher Fo content of the OUI rocks (pyroxenite, peridotite, massive/semi-massive oxides) in both OUIs can be attributed to the abundance of Fe-Ti oxides, which leach Fe from the area, imparting olivine with a higher Fo content. The higher Fo content in Longnose indicates that it was formed by a relatively more primitive magma than that which formed Titac. From our Raman results, we found that olivine reacted with a hydrothermal fluid to form lizardite, antigorite, magnetite, and cronstedtite. There is no trend with depth and lizardite is the predominant serpentine group mineral of both OUIs. Lizardite and cronstedtite form at lower temperatures than antigorite, and we hypothesize that there were at least two separate serpentinization events which occurred, a widespread event at a lower temperature (~400 ℃) and another, less widely distributed, at a higher temperature (~600 ℃) that impacted Longnose to a greater degree. These analyses are part of a larger study of the OUIs, and will fit into the larger picture to understand how they formed.