| This research characterizes the geochemistry and textural attributes of titanite from five locations along modern fast-, slow-, and ultraslow-spreading mid-ocean ridges in an effort to understand its petrogenesis in ocean crust. We identified three types of oceanic titanite based on textural features, zoning in back-scattered electron (BSE) images, and geochemistry: 1) igneous titanite, 2) titanite formed during hydrothermal metamorphism, and 3) composite titanite that shows a mixed igneous/metamorphic history. Igneous titanite hosted in plagiogranite veins is typically euhedral, Fe-rich, and has seagull' shaped REE patterns with high concentrations (>10 3-104x chondrite). Metamorphic titanite is the most common type, hosted in Fe-Ti oxide gabbro or gabbro as either euhedral or anhedral grains, and is Al-rich with low concentrations of REEs (∼10-10 2x chondrite). Composite titanites show evidence of an igneous origin and a later phase of metamorphic growth. They have morphologies, zoning patterns, and/or geochemical characteristics similar to both igneous and metamorphic titanite, and may have up to four types of REE patterns within a sample.;Zr-in-titanite temperatures for samples with coexisting titanite and zircon range from 545 to 848°C (aTiO2 and aSiO2 ≈ 0.7 or 1.0, pressure ≈ 0.1 GPa), indicating that most titanite forms near or below the gabbro solidus. There is an overall negative correlation between Zr-in-titanite temperature and Eu/Eu* despite scattered data, while temperature plotted against Th/U ratios shows a continuous decrease in Th/U from late-magmatic to subsolidus temperatures for samples hosting composite titanite. The high variability in Eu/Eu*, Th/U, and REEs is due to several factors, including the initial composition of the parent melt, the influence of co-crystallizing minerals, and the composition of late-stage magmatic or metamorphic fluids that accompanied brittle deformation. In addition, titanite chemistry is highly sensitive to dynamic shifts in temperature (T), oxygen fugacity (fO2), and fluid conditions. We interpret the textural and geochemical characteristics of oceanic titanite, within the context of inferred crystallization and fluid processes in ocean crust.;Finally, we calculate distribution coefficients (DREE) for titanite hosted in evolved gabbroic rocks using measured titanite and coexisting zircon geochemistry and predicted melt compositions, in order to compare to DREE values from empirical studies and contribute data for future REE modeling. |
