284-5 Dating Deformation with Titanite: An Example from High-Temperature, High-Pressure Deformation Experiments

Session: The Deformation-Metamorphism-Fluid Triplet Governing Plate Boundaries and Orogens

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Directly dating deformation is critical for understanding the timing and rates of many geologic processes, including plate boundary evolution, fluid­–rock reactions, and the formation of critical mineral resources. Studies from exhumed ductile shear zones demonstrate that titanite U-Pb petrochronology is a powerful tool for directly dating deformation. However, feedbacks among deformation, fluid flow, and other metamorphic processes (e.g., neocrystallization and volume diffusion) make it challenging to discern the extent that dates represent the timing of deformation in exhumed ductile shear zones. We performed a series of high-temperature, high-pressure titanite deformation experiments to determine the role that dislocation creep and mechanical twinning play in resetting titanite U-Pb dates. Experimental samples consisted of vacuum hot-pressed titanite and orthoclase powders (in a ratio of 30:70 by weight). General shear experiments were completed at T=750­–950 °C, P_c= 1.5 GPa, strain rate of ~10^-5s^-1, and either nominally dry conditions or with ~0.2 wt% DI H₂O. The deformation phase of experiments typically lasted ~2­–3 days, with finite shear strains reaching ~2­–4. The resulting titanite deformation microstructures are similar to those from titanite deformed in nature: mechanical twins are pervasive; gradual changes in lattice orientation are consistent with the operation of dislocation creep; and micron-scale euhedral titanite grains form aggregates and tails at the margins of larger titanite grains. Titanite U-Pb analyses (acquired via secondary ion mass spectrometry) from the undeformed starting material yielded an inverse isochron age of 998 ± 5 Ma (MSWD = 1.3). The titanite deformed at 950 °C and nominally dry conditions yielded systematically younger U-Pb dates, producing a weighted mean age of 963 ± 4 Ma, and a higher MSWD (3.9). Experiments run at 750 °C and 850 °C yielded U-Pb dates indistinguishable from the undeformed starting material. This suggests that temperature plays a key role in facilitating Pb loss during deformation, either directly through diffusion or indirectly through the operation of different deformation mechanisms and slip systems at varying temperatures. Our results support the notion that plastic deformation facilitates Pb loss, and that U-Pb dates may be fully reset in plastically deformed titanite over geologic timescales. As the precise mechanisms and timescales required for date resetting are unclear; it remains critical to consider additional context to interpret dates as deformation ages.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7235

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