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1. Time-temperature paths from intragrain oxygen isotope zoning

   Bonamici, C. E.; Kropf, G.; Borchers, B.; Parrish, G. (December 2019, AGU 2019 Fall Meeting)

   Oxygen isotopes are a well-known geochemical tool with applications to equilibrium thermometry, fluid tracing, and magma and ore petrogenesis. High-precision, high-spatial resolution oxygen isotope analysis by SIMS has also enabled the development of oxygen isotopes as a tool for geospeedometry. Here, we detail Fast Grain Boundary (FGB), an updated computational approach and software tool for determining time-temperature (T-t) histories through modeling of oxygen isotope diffusion. FBG models a rock system, rather than a single phase (cf. thermochronometry based on He, Ar, Pb), and has the potential to constrain continuous thermal histories over a wide range of temperatures, including at high temperature (500-800°C). The new FGB also allows for inversion of the FGB model to extract thermal histories from intragrain oxygen isotope zoning data using the Levenberg-Marquardt (LM) algorithm. Tests with synthetic datasets show that the LM algorithm is able to distinguish between simple linear cooling and more complex thermal histories containing, for instance, reheating events. Inversion of an actual oxygen isotope data set from titanite are consistent with the previously determined T-t path for the sample region, showing a brief period of >700 °C conditions, followed by cooling below 500 °C in <5 m.y.. However, the inversion suffers from a flat-bottomed minimum and does not produce a well-converged T-t path. These results point to analytical precision as a continuing challenge in recovering tightly constrained thermal histories for the real data set and emphasize the need for further development of high-precision microanalytical oxygen isotope standards. In the meantime, we use FGB modeling to explore sampling and analytical approaches that improve the resolution of inversion solutions for current analytical capabilities. For instance, inversion most successfully recovers a well constrained T-t path solution when SIMS analysis targets oxygen isotope gradients developed near grain rims, as opposed to oxygen isotope values in grain centers. Additional tests that probe the sensitivity of the inversion results to modal mineralogy and relative grain sizes suggest that careful targeting of samples in the field can enhance the recovery of unique T-t paths. 

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2. Garnet zoning patterns record multiple processes of chemical transfer during subduction

   https://doi.org/10.1016/j.epsl.2024.118634  

   George, Freya R.; Viete, Daniel R.; Ávila, Janaína; Seward, Gareth G.E.; Guice, George L.; Allen, Mark B.; Harrower, Michael J. (April 2024, Earth and Planetary Science Letters)

   In garnets from eclogites and blueschists formed within the subduction setting, fine-scale, oscillatory elemental zoning is a common feature, sometimes considered to record open-system fluid exchange during prograde metamorphism. We present oxygen isotope data for garnets with such zoning from five exhumed subduction zone complexes. Short length scale fluctuations in elemental and oxygen isotope zoning (which are themselves spatially decoupled) cannot be linked to open-system fluid exchange during garnet crystallization in all samples; these data do not provide evidence for a genetic relationship between elemental oscillations and fluid fluxing. However, garnets from one setting do provide clear evidence for syn-growth ingress of elementally and isotopically buffering fluids, a process that operated simultaneously with the formation of elemental oscillations. Our findings indicate multiple mechanisms of chemical transfer operate at the grain–rock scale during subduction, and that some subduction zone rocks may experience only limited interaction with external prograde fluids. These results are consistent with a picture of highly heterogenous volatile transfer during subduction, and suggest that some proportion of the fluid inventory inherited at shallow depths may be transferred to sub-arc depths. 

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3. Linking titanite U–Pb dates to coupled deformation and dissolution–reprecipitation

   Moser, A; Hacker, B; Gehrels, G; Seward, G; Kylander-Clark, ARC; Garber, Joshua M (March 2022, Contributions to Mineralogy and Petrology)

   Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains. 

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4. Linking titanite U–Pb dates to coupled deformation and dissolution–reprecipitation

   Moser, A; Hacker, B; Gehrels, G; Seward, G; Kylander‑Clark, R; Garber, J (March 2022, Contributions to Mineralogy and Petrology)

   Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mecha- nisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains. 

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5. Exploiting Thermochronology to Quantify Exhumation Histories and Patterns of Uplift Along the Margins of Tibet

   https://doi.org/10.3389/feart.2021.688374  

   Furlong, Kevin P.; Kirby, Eric; Creason, C. Gabriel; Kamp, Peter J.; Xu, Ganqing; Danišík, Martin; Shi, Xuhua; Hodges, Kip V. (June 2021, Frontiers in Earth Science)

   The utilization of thermal-chronological data to constrain mountain building processes exploits the links among rock uplift, exhumation, and cooling during orogenesis. Conceptually, periods of rapid uplift and associated denudation will lead to cooling of rocks as they approach Earth’s surface. The linkage between uplift and exhumation can be complex, but in practice exhumation is often assumed to directly track uplift. The reconstruction of temperature-time histories via thermochronologic systems provides a proxy method to relate the cooling of rock as it is exhumed toward the surface to orogenesis. For the rapid exhumation rates that can occur in active orogenic systems the thermal history will be complex as a result of heat advection, rates of propagation of thermal perturbations, and other processes that affect the cooling behavior. These effects become amplified as exhumation rates increase, and in regions experiencing exhumation rates greater than ∼0.2–0.3 mm/yr (0.2–0.3 km/Ma) simple assumptions of cooling through a constant geotherm will bias the subsequent interpretation. Here we explore, through a suite of generalized models, the impact of exhumation rate and duration on the resulting thermal history and apparent age results. We then apply lessons from these simple exhumation systems to data sets from the high-relief ranges along the eastern margin of the Tibetan Plateau to determine exhumation histories constrained by those data. The resulting exhumation histories provide constraints on the onset of Cenozoic exhumation, the subsequent pace of exhumation, and on the tectonic history of one of the major fault systems in the central Longmen Shan. 

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