Mineralogy and Origin of Exsolution In Ti-Rich Magnetite From Different Magmatic Fe-Ti Oxide-Bearing Intrusions

Titanium-rich magnetite from different magmatic Fe-Ti oxide-bearing intrusions consists of host magnetite (nearly pure Fe 3 O 4 ) and different types of exsolution. The exsolution of Ti-rich magnetite is indicative of the physical and chemical conditions at which the host intrusions formed. However, the mechanisms to form these types of exsolution have been debated. To examine the formation mechanisms of the different types of exsolution, multiple microanalytical techniques, such as electron microprobe analysis, micro Raman spectroscopy, and micro X-ray diffraction, were used to examine the chemical composition, morphology, and crystalline structure of exsolved minerals in Ti-rich magnetite from different Fe-Ti oxide-bearing intrusions in China. The characterization results indicate that pleonaste and ilmenite are the two dominant exsolution phases in the Ti-rich magnetite, whereas the ulvospinel phase is barely found. The ilmenite exsolution may have different origins: (1) the cloth-texture lamellae primarily exsolved as ulvospinel, which was oxidized to ilmenite later at 600 °C; (3) some ilmenite lamellae exsolved directly from cation-deficient solid solution due to the substitution of Ti 4+ + □ → 2Fe 2+ ; (4) sub-solidus re-equilibration of coexisting Fe-Ti oxides can also lead to ilmenite exsolution in Ti-rich magnetite and magnetite exsolution in ilmenite.