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Abstract

The Fe-Ti oxide minerals, magnetite (Fe₃O₄) and ilmenite (FeTiO₃), are sensitive recorders of magmatic differentiation and subsolidus modification within ultramafic–mafic intrusions in the Earth’s crust. This study investigates the petrochemistry of oxide minerals from the Eocene Skaergaard layered intrusion of East Greenland, crystallized from a single batch of Fe-rich tholeiitic magma, and four Triassic–Jurassic Alaskan-type intrusions in British Columbia (Hickman, Polaris, Tulameen, Turnagain), representing the products of open-system primitive arc magmatism. Electronprobe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were used to determine the major, minor, and trace elemental compositions of oxide minerals. In Skaergaard, oxides are present throughout the magmatic stratigraphy with cumulus oxide appearing in rocks after ~60% crystallization of the magma. Oxidation-induced ilmenite exsolution is common in Skaergaard magnetite, occurring in variable abundances and textures. Systematic trends in compatible (e.g., Cr, Ni) and incompatible (e.g., Zr) elements in the Skaergaard oxides record progressive fractional crystallization of a closed magma reservoir. Textural and chemical evidence from the oxides also indicates the important role of silicate liquid immiscibility during late-stage differentiation. In the Alaskan-type intrusions, magnetite occurs interstitially in various lithologies and locally as massive layers and pods. Magnetite textures primarily reflect magmatic crystallization, later modified by subsolidus oxidation, local sulfide saturation, and fluid interaction. Magnetite compositions in Alaskan-type intrusions capture a range of magma differentiation processes, including fractional crystallization, magma recharge, and segregation of immiscible sulfide liquid. Vanadium systematics in magnetite provide qualitative constraints for the relatively oxidized conditions of these arc magmas compared to the more reduced Skaergaard magma. By comparing Fe-Ti oxide minerals from the Skaergaard and Alaskan-type intrusions, this study demonstrates how oxide petrochemistry reliably records fractionation, redox, and sulfide saturation processes, and underscores the importance of considering the effects of subsolidus re-equilibration (e.g., exsolution, alteration, hydrothermal overprinting) when interpreting primary signatures. The results of this study highlight the value of Fe-Ti oxide minerals as versatile petrogenetic indicators for assessing magma evolution in ultramafic–mafic intrusions across diverse tectonic settings, with broad applicability to oxide-bearing magmatic systems worldwide.