FEASIBILITY OF THE U-PB HEMATITE CHRONOMETER IN SUPERIOR REGION MASSIVE IRON FORMATIONS

The development of U-Pb geochronology in zircon unlocked unprecedented insight into Earth's dynamics in deep time. However, in non-magmatic successions without syn-depositional zircons, such as most of the thick, iron-rich chemical sedimentary rocks that record Earth’s oxygenation history, key temporal data remains out of reach. Throughout the Precambrian, deposition of iron-oxides records the state and evolution of marine oxygenation, while post-depositional enrichment of iron-oxides informs models of ore formation. Recently, novel techniques and approaches like the U-Pb hematite chronometer enable the acquisition of mineral-scale datasets to test models of iron-oxide deposition and enrichment. Hematite incorporates substituting cations such as U in octahedral-coordination with expected closure temperature similar to rutile. Here we test the feasibility of applying the U-Pb chronometer in Precambrian massive iron deposits from the Superior region Trommald and Ironwood Iron formations, using coupled petrographic and LA-ICP-MS mapping data. LA-ICP-MS mapping is an ideal tool to identify targets for successful U-Pb dating via detection of hematite with the greatest homogeneity, high U (>5 ppm) and lowest Pbc. Such analyses can help set the stage for applications of the novel U-Pb hematite chronometer to depositional, deep ore, and surface ore iron oxides to tease apart the timing of primary precipitation and test enrichment mechanisms.