TRACKING MAGNETITE GROWTH DURING FLUID ROCK INTERACTIONS WITH (U-TH)/HE THERMOCHRONOLOGY (Invited Presentation)

Magnetite is a common secondary mineral that forms via fluid rock interactions in metamorphic, metasomatic, and igneous systems. Recently, magnetite (U-Th)/He thermochronology has been used to investigate the time, temperature, and alteration history of hydrothermal magnetite and their host rocks. When coupled with trace element and independent thermal history or thermometry data, magnetite (U-Th)/He dating can be used to link ‘time’ to processes such as fluid-flow, mineralization, faulting, or exhumation in mafic to ultramafic rocks. Here we use case studies from Syros, Greece and Wadi Fins, Oman to explore magnetite growth histories in altered ultramafic systems. These studies demonstrate how magnetite grain characterization is key for robust magnetite (U-Th)/He interpretation, including micro-computed tomography and trace element analysis to identify multiple growth generations and differentiate between magnetite growth/crystallization and cooling ages, even within the same sample. In Syros, Greece, magnetite (U-Th)/He data record both island-wide Miocene exhumation related cooling and discrete Pliocene fault-activated magnetite growth within serpentinites and chlorite schists from an exhumed high-pressure subduction complex. In Wadi Fins, Oman, magnetite (U-Th)/He records Miocene low-temperature growth within calcite veins that crosscut serpentinized peridotite in the mantle section of the eastern Samail Ophiolite. Although quite different from each other, both systems reveal how magnetite (U-Th)/He can be used to temporally constrain fluid rock interaction and vein mineralization histories over a range of pressure-temperature conditions.